Craniofacial sequelae of lesions to facial and trigeminal motor nuclei in growing rats.код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 7687-103 (1988) Craniofacial Sequelae of Lesions to Facial and Trigeminal Motor Nuclei in Growing Rats KENNETH E. BYRD Department ofBasic Sciences, University ofSouthern California School of Dentistry, Uniciersity Park, MC-0641, Los Angeles, California 90089-0641 KEY WORDS Motoneuron, Growth, Form, Neuromuscular, Craniofacial ABSTRACT Unilateral electrolytic lesions were made in the left-sidefacial motor nucleus (FMNu) of six Sprague-Dawleyrats at 35 days of age in order to correlate craniofacial sequelae with changed motoneuron function. Experimental and control rats were killed at 22, 32, 42, and 52 days postoperatively to provide muscle weight, brain histology, and dry skull preparations for analyses. Dissection, muscle weight, motoneuron count, and osteometric data revealed that lesion-side facial and masticatory muscles were affected by the lesions. Paired t-tests indicated that significant differences existed between weights of experimental lesion- and nonlesion-side anterior digastric, temporalis, masseteric complex, and medial pterygoid muscles, numbers of facial and trigeminal motoneurons, and several skeletal dimensions of the skull. Basicranial dimensions of experimental animals were least affected by the lesion, whereas zygomatic arch, dorsal facial region, and mandibular condyle dimensions were most affected. Statistical analyses also detected significant differences between experimental and control groups for several skeletal dimensions of the skull. Data indicated that damage to the trigeminal motor nucleus (TMNu) was secondary to the primary lesion in the FMNu. Motoneurons within the facial and trigeminal neuromuscular complexes (FNC and TNC) play an important role in craniofacial growth and development. Craniofacial form has been important to students of human variation and evolution at least since the days of Blumenbach (1795). Despite the theoretical framework offered by Wolff s law (1892), no significant experimental studies addressing the mechanical effects of muscle upon skull form were accomplished until Washburn (1946,1947) and his contemporaries (Horowitz and Shapiro, 1951, 1955; Avis, 1959, 1961). Muscles of facial expression (those innervated by cranial nerve VII,the facial nerve) have been demonstrated to be of major importance in human craniofacial growth (Delaire and Chateau, 1977; DeIaire, 1978). In addition, total paralysis of these muscles combined with facial-nerve degeneration may occur in 17%(Alberti and Biagioni, 1972) to 25%(Manning and Adour, 1972) of the total pediatric Bell’s palsy patient population and require surgical intervention before long- 0 1988 ALAN R. LISS, INC term morphological and physiological sequelae manifest themselves (Jenkins et al., 1985). Previous studies have concentrated either on resecting branches of the facial nerve and studying morphologicallphysiological sequelae (Huber and Hughson, 1926; Washburn, 1946; Jarabak et al., 1949; Cammermeyer, 1963) or on correlating facial neuromuscular changes with morphological alterations (Miller and Chierici, 1982;Miller et al., 1982; Vargervik et al., 1984). A major problem has been to isolate the effects of the facial neuromuscular complex (FNC) on craniofacial growth from those caused by sensory loss, Received April 3, 1987; revision accepted September 21, 1987. Address correspondence to Dr. Kenneth E. Byrd, Department of Basic Sciences, USC School of Dentistry, University Park, MC-0641, Los Angeles, CA 90089-0641. 88 K.E. BYRD tissue scarring, and circulatory interruptions (Johnston, 1976) during actual facial-nerve resections. This problem can be essentially solved by electrolytically lesioning (Gardner et al., 1980; Byrd, 1984) the facial motor nucleus (FMNu) within the brainstem. The purpose of this investigation was to determine the effects of alteration of the FNC on craniofacial postnatal development by lesioning facial rnotoneurons (FMNe) within the brainstem of actively growing rats. These data therefore should approximate the craniofacial morphological sequelae resulting from facial motor nucleus (FMNu) aplasia (Walker, 1961) and/or progressive facial hemiatrophy (Chusid, 1979). Detailed physiologic data (EMG and mandibular movement tracking a s in Byrd, 1984) complementary to this study will be published elsewhere. MATERIALS AND METHODS Six experimental and six control male Sprague-Dawley rats underwent stereotaxic surgery at 35 days of age. The animals were induced with a n intramuscular injection of “rat cocktail” (mixture of ketamine 100 mgl ml, rompun 20 mglml, and acepromazine 10 mglml) in the amount of 0.01 ml per 10 g of body weight. After induction, the animals were positioned in a Kopf model 900 stereotaxic unit and maintained under general anesthesia with halothane. Surgical protocol was identical to that described in Gardner et al. (1980) and Byrd (1984), in that a midline incision was made over the sagittal suture; integument and periosteum were reflected, and a small (4mm-diameter) hole drilled through the bone to expose the dura mater. Both experimental and control animals had a hole drilled directly over the left-side facial motor nucleus (FMNu), according to general stereotaxic coordinates provided by Paxinos and Watson (1982). Control animals had only their dura exposed, and no lesions were produced in the manner described by Gardner et al. (1980) and Byrd (1984). Experimental animals had unilateral electrolytic lesions (2 mA, 15 sec) placed in their left-side FMNu in the manner described in Byrd (1984) for the trigeminal motor nucleus (TMNu). After piercing the dura, a Rhodes NEX-100 concentric bipolar electrode was slowly lowered while continuously stimulating at 1 pulse per sec at between 2 to 0.1 volts. The electrode descended in a n axis 1.9 mm to the left and 1.9 mm posterior to stereotaxic “ear bar zero.” FMNu targets were located at a mean minimum stimulation threshold of 0.41 & 0.31 volts with no trigeminal motor nucleus (TMNu) stimulation. At no time during the electrode stimulation procedure for locating the FMNu target did the trigeminal innervated muscles of mastication move; only facial muscles supplied by the FMNu contracted during this procedure. Electrodes were then placed a n additional 0.5 mm below that position eliciting the lowest threshold voltage causing facial muscle response and lesions were produced with a Grass direct-current lesion maker (2 mA, 15 sec). Immediately after wound closure and treatment with Betadine, both lesioned and nonlesioned animals were given intraperitoneal injections of chloramphenicol sodium succinate (0.25 ml) and were administered a n oral suspension of chloromycetin palmitate (1.0 ml) daily for 1 week postoperatively. In order to document relative growth rates, both experimental and control animals were weighed to the nearest 0.1 g with Ohaus dial scales on the day of lesioning and at 22, 32, 42, and 52 days postoperatively. Means and standard deviations were calculated for these data. Experimental and control animals were killed at 22, 32 (two rats), 42, and 52 (two rats ) days postoperative; transcardiac perfusion with 10% buffered formalin preceded by a flush with physiological saline occurred a t that time. Brains of lesioned and nonlesioned animals were removed after transcardiac perfusion for histological documentation of lesion extent. Two additional rats were killed and perfused immediately after lesioning in order to confirm the locus of the initial FMNu lesion. In order to document FMNu lesion effects on the muscles of facial expression, these muscles were dissected out en mass by means of a n inverted “skin mask.” The following muscles for both experimental and control groups were then dissected free and weighed to the nearest 0.01 g with Mettler dial scales after storage in 70% ethanol: both left- and right-side anterior digastric; posterior digastric; temporalis; masseteric complex = combined superficial, deep, maxillomandibularis, and zygomaticomandibularis portions (Turnbull, 1970); and medial pterygoid. Unfortunately, lateral pterygoid proved impossible to remove intact for weighing purposes and LESIONED FACIAL AND TRIGEMINAL MOTOR NUCLEI 89 TABLE 1. Osteological measurements used in this studv Cranial measurements 1) Midline frontonasal suture to premaxillonasal suture (E-I, E-17 2) Maxillozygomatic suture to premaxillonasal suture (C-I,C’-I‘) 3) Maxillozygomatic suture to bregma (C-F, C‘-F) 4) Zygomaticosquamosal suture to premaxillofrontomaxillary suture (D-J, D’-J’) 5) Zygomaticosquamosal suture to midline frontonasal suture (D-E, D’-E) 6 ) Anteriormost incisoprernaxillary contact to anterior first molar (K-M, K‘-M’) 7) Anteriormost incisopremaxillary contact to occipitobullobasisphenoid contact (K- R, K‘-R’) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) Premaxillomaxillary suture to anterior first molar (P-M, P’-M’) Premaxillomaxillary suture to maxillozygomatic suture (P-C, P’-C’) Anterior first molar to maxillozygomatic suture ( M E , M‘-C’) Anterior first molar to zygomaticosquamosal suture (M-D, M‘-D’) Posterior third molar to maxillozygomatic suture (N-C, N’-C’) Posterior third molar to zygomaticosquamosal suture (N-D, N’-D‘) Posterior third molar to occipitobullobasisphenoidcontact (N-R, N’-R‘) Posterior palatal spine to maxillozygomatic suture 1Q-C, Q-C’) Posterior palatal spine to occipitobullobasisphenoidcontact (Q-R, Q-R’) Occipitobullobasisphenoid contact to maxillozygomatic suture (R-C, R’-C‘) Mandibular measurements 18) Gonial angle to superiormost incisorcorpus contact (S-T, S‘-T’) 19) Gonial angle to posterior third molar (S-W, S’-W‘) 20) Gonial angle to anteriormost condylar articular surface (S-Z, S’-Z’) 21) Superiormost incisorcorpus contact to posterior third molar (T-W, T’-W‘) 22) Superiormost incisorcorpus contact to superiormost coronoid process (T-Y, T’-Y’) 23) Superiormost incisorcorpus contact to anteriormost condylar articular surface (TZ, T‘-Z’) 24) Inferiormost incisorcorpus contact to superiormost coronoid process (U-Y, U’-Y’) 25) Inferiormost incisorcorpus contact to anteriormost condylar articular surface (Uz , U’-Z’) 26) Posterior third molar to anteriormost condylar articular surface (W-Z, W’-Z’) 27) Maximum mandibular height (MMH, MMH’) 28) Maximum condylar width (MCW, MCW‘) 29) Maximum condylar length (MCL, MCL’) was not used. More superficial facial muscles, such a s the nasolabialis and dilator nasi, were also not weighed because of their complex anatomy and delicate structure, which prevented their intact removal for weighing purposes. After removal of the previously described muscles, the remaining tissue was cleaned and dry skull preparations were made of both lesioned and nonlesioned animals. A series of 17 cranial and 12 mandibular measurements of the experimental and control groups (see Table 1 and Fig. 1) were made for both left and right sides, to the nearest 0.1 mm with a Helios needle-point dial caliper. The landmarks for three mandibular measurement points (S, Y, and Z in Fig. l) require further clarification: (1) point S (gonial angle) was the posteriormost point of the mandibular angle, (2) point Y (superiormost coronoid process) was the highest point of the coronoid process when the mandible was in norma lateralis, and (3) point Z (anteriormost condylar articular surface) was the most anterior contact of the condylar articular facet1 surface with the nonarticular surface of the mandibular head (see Fig. 1). In addition, maximum mandibular height (MMH) was measured between two parallel planes: the first, at the contact surfaces of the inferior aspect of the mandible in norma lateralis; the second, at point Y (superiormost coronoid process). Maximum condylar length (MCL) was measured between the anteriormost and posteriormost articular surfaces of the mandibular condyle (anterior and posterior contacts between articular and nonarticular surfaces of mandibular head). Maximum condylar width (MCW was measured between the lateralmost and medialmost articular surfaces of each mandibular condyle. Histological documentation of lesion extent was accomplished by first embedding the desired section of rat b r a i n (pons) after trans- 90 K.E. BYRD G I B A Y I Z --s w u Fig. 1. Osteometric points used for this study. Primed (9 points refer to right-side measurements. Refer to Table 1for complete description of points. The purpose of motoneuron-count data was to document differences between the left (lesioned) and right sides of both experimental and control groups. To that end, actual motoneuron counts were made with the Optomax IV image analysis system and were based on contrast differences between facial/ trigeminal motoneurons and their surrounding glial cells. The Optomax contrast detector setting was kept at a constant value of 83 during motoneuron-count procedures, and the frame circle (area) counted was kept constant between sides of the same histological section. Although every fourth 10-pm paraffin section was mounted to facilitate data analysis for this study, a total of 374 facial and 294 trigeminal motoneuron counts using the Optomax system provided sufficient data for statistical analyses, as shown in Tables 2 and 3. Statistical analyses of muscle weight, motoneuron count, and osteometric data consisted of means and standard deviations calculated prior to tests of significance by paired t-tests. Paired t-tests (Sokal and Rohlf, 1969) were used to test differences between (1)left-lesioned and right-nonlesioned sides and (2) experimental-lesioned and controlnonlesioned groups. These data are shown in Tables 2-6. Thirty-five-day-old rats were used instead of younger animals because of their ability to better recover from stereotaxic surgery. Although 35-day-old rats are beyond their period of most rapid craniofacial growth, sufficient craniofacial growth continues, particularly in their snout and facial regions (Baer et al., 19831, to make analyses worthwhile. For this study, the assumption was made that any differences between the craniofacial growth rates of animals killed a t 57, 67, 77, and 87 days after birth were minimal. RESULTS cardiac perfusion and stabilization in 70% ethanol. 10-pm-thick paraffin sections were then cut, and every fourth section was mounted and stained with Cresyl-violet stain (LaBossiere and Glickstein, 1976). An estimate of both left- and right-side facial motoneurons (FMNe) and trigeminal motoneurons (TMNe) was made with a n Optomax IV image analysis system. Both lesioned and nonlesioned groups had their FMNu and TMNu evaluated in this manner. BehavioraUphyswlogical sequelae Postlesion testing of experimental animals revealed no lesion-side (1)vibrissae (whisker) movement, (2) ear movement, or (3) nose movement; all three are indicative of FMNu damage. In addition, no lesion-side ear tremble or eye blink reflex (Fig. 2) could be elicited; lesion-side trigeminal sensory supply was largely intact, as demonstrated by touching the lesion-side vibrissae of the experimental animals and eliciting responses 91 LESIONED FACIAL AND TRIGEMINAL MOTOR NUCLEI TABLE 2. Numbers of facial motoneurons @MNe) detected in experimental and control groups (X f S.D.) Days postop. Experimentals 22 32 42 52 Controls Left Right Left Right 0 (n = 19) 0.69 f 1.95 (n = 26) 4.74 k 7.98 (n = 31) 53.08 f 20.05 (n = 13) 83.21 t 30.66* (n = 19) 69.12 f 28.66* (n = 26) 72.81 rt 31.12* (n = 31) 87.62 5 37.15* (n = 13) 66.95 f 29.78 (n = 22) 55.94 f 31.14 (n = 32) 43.88 f 18.28 (n = 25) 39.05 f 27.73 (n = 19) 68.68 f 31.59 (n = 22) 55.31 f 38.44 (n = 32) 43.88 f 18.24 (n = 25) 41.74 f 26.06 (n = 19) *Left significantly different from right P 6 .01 TABLE 3. Numbers of trigeminal motoneuron_s(TMNe) detected in experimental and control groups (X f S.D.) Days postop. 22 32 42 52 Left Experimentals Right 7.90 f 6.68 (n = 20) 6.11 t 6.96 (n = 19) 0.16 f 0.62 (n = 25) 25.28 & 11.89 (n = 18) 58.20 f 38.36* (n = 20) 58.47 f 30.07* (n = 19) 47.04 f 22.12* (n = 25) 73.22 f 20.17* (n = 18) *Left significantly different from right P Controls Left Right 31.06 15.08 (n = 17) 29.77 f 16.90 (n = 22) 25.75 f 10.05 (n = 8 ) 27.06 f 16.31 (n = 18) 32.65 f 15.35 (n = 17) 30.50 f 18.95 (n = 22) 27.75 f 9.94 (n = 8) 23.67 rt 17.99 (n = 18) < .01. from them. These behaviors were present in experimental animals immediately after surgery and were present throughout the course of the study. Despite lesion-side (left) “mouth droop” and nasal deviation to the nonlesion side (right), experimental rats fed themselves efficiently immediately after recovering from surgery. Within 3 to 4 days postlesion, however, it was observed that the experimental rats appeared to have difficulty in eating their norma1 rat pellets. Both experimental and control rats were then provided water-softened pellets and a paste made of powdered pellets and water. Approximately 13 days postlesion, hypereruption of maxillary and mandibular incisors of the experimental rats necessitated their reduction so that they could continue to feed and gain weight. Both experimental and control rats were fed and watered ad libitum, but experimental rats were not a s rapid in their weight gain, cornpared with their control counterparts. The body weights (x k sod-)for experimental and Control groups a t the time of lesion- ing were, respectively, 153.1 +_ 7.1 g and 150.2 f 7.7 g. Presacrifice body-weight gains of experimental rats at 22, 32, 42, and 52 days postoperatively were 28.7 f 27.7 g, 68.5 f 16.9 g, 116.5 22.1 g, and 175.9 f 14.9 g, respectively. Presacrifice body-weight gains of control animals a t 22, 32,42, and 52 days postlesion were 134.8 & 9.4 g, 161.6 f 6.2 g, 180.9 k 6.7 g, and 280.6 f 22.2 g, respectively. These weight-gain differences between experimental and control groups reflect the physiological and morphological sequelae of the FMNu lesions in the lesioned rats. + Facial muscles Lesion+ide facial muscles were pale and less robust than their nonlesion-side counterparts. The lesion-side nasolabialis muscle et al., 1983, which whiskers, nose, and upper lip, was much smaller than the nonlesion-side muscle, as was the lesion-side zygomaticus, platysma, matson 92 K.E. BYRD terior digastric of the control animals were significantly ( P < .01) heavier than their experimental antimeres (Table 4). Left (lesion-side) temporalis, anterior digastric, masseteric complex, and medial pterygoid were significantly ( P < .05, < .01) lighter than right ones in the experimental animals (Table 4).Experimental posterior digastric was not significant ( P < 20). No significant left-right differences were detected in the control animals (Table 4).Despite its apparent atrophy, as shown in Figure 5, posterior digastric did not show any significant weight difference between lesioned and nonlesioned sides of experimental animals. Left-side posterior digastric in control animals was significantly (P < .01)heavier than its experimental counterpart, however. The facts that all five left-side control muscles were significantly heavier than their ex- Fig. 2. Eye blink reflex test in FMNu-lesioned animal at 52 days postoperative. Note normal blink reflex for nonlesion-side eye (top) and absent blink reflex for lesion-side eye (bottom). and mentalis. Lesion-side orbicularis oris was also paler and less robust than on the nonlesion-side. Deeper dissections prior to dry skull preparations also revealed a significant color difference between lesion- and nonlesion-side buccinator muscles in the experimental animals (Fig. 3). Lesion-side buccinators appeared both paler and thinner than their nonlesion-side counterparts. Masticatory muscles Figures 4 and 5 demonstrate that there was not only atrophy of lesion-side posterior digastric (innervated by facial nerve), but also considerable wasting of lesion-side muscles of ast tic at ion (innervated by trigeminal nerve). Table 4 indicates statistically significant differences between experimental and controlmuscleweights for left and right ternp o r a b anterior digastric, and Illasseteric complex. Left-side medial pterygoid and pos- Fig. 3. Temporalis and buccinator muscles in FMNulesioned animal at 32 days postoperative. Contrast appearance of nonlesion-side muscles (top) with lesion-side temporalis and buccinator muscles (bottom).Buccinator muscles are indicated by arrows. Note (1)robust temporalis on nonlesion side (top) and (2) pale buccinator on lesion side (bottom). LESIONED FACIAL AND TRIGEMINAL MOTOR NUCLEI TABLE 4. Muscle weights of exnerimental and control eroum lX Muscle Ant. digastric Post. digastric Temporalis Masseteric complex Medial pterygoid Experimentals Left Right 0.12 k 0.02 (n = 6) 0.13 k 0.01 (n = 6) 0.29 f 0.22 (n = 6) 0.76 i 0.56 (n = 6) 0.18 +_ 0.06 (n = 5) 0.16 k 0.02** (n = 6) 0.14k 0.02 (n = 6) 0.53 0.19** (n =6) 1.14 ir 0.37* (n = 6) 0.25 +_ 0.04** (n = 5) *Left significantly different from right P < .05. **Left significantly different from right P < .01. +ControlSignificantly different from experimentals P 93 + S.D.P ) Controls Left Right 0.20 f 0.02'~ (n = 6) 0.15 f O.Olr (n = 6) 0.91 & 0.18+ (n = 6) 1.80 0.33$ (n = 6) 0.29 & 0.07T (n = 6) 0.21 & 0.Olt (n = 6) 0.15 f 0.02 (n = 6) 0.90 k 0.17t (n = 6) 1.81 i 0.35' (n = 6) 0.28 f 0.07 (n = 6) < .01. perimental antimeres and that only three right-side control muscles were significantly heavier than their experimental counterparts suggest that the lesion's effect was greater on left-side muscles of the experimental animals (see Table 4). In addition, the significant differences between left- (lesion) and right-side temporalis, anterior digastric, masseteric complex, and medial pterygoid muscles in the experimentals point to a major effect caused by the lesion. Facial and trigeminal motoneurons Histological sections of the two rats killed and perfused immediately after lesioning revealed (1) the accurate placement of the electrolytic lesion into the FMNu and (2) no apparent damage t o the TMNu. Although the FMNu was indeed the primary lesion site in the experimental animals (see Figs. 6 and 71, TMNu damage was substantial at 22 and 32 days postoperatively (Fig. 7 and Table 3). The mean size of the primary FMNu lesion (Fig. 6) for all six experimental animals was approximately 645 ( f131) pm wide by 1438 (+257) pm high. No lesion-side FMNe were detected at 22 days postoperatively in the experimental group and few were detected a t 32 and 42 days postoperatively (Table 2). However, very few lesion-side TMNe were detected a t 42 days postoperatively (Table 3). ~ ~ ~ iF M~N ~~ and -T M~ N ~iof experimend ~ tal animals were si@ificantly ( p .O1) fewer than nonlesion-side motoneurons, but no significant differences were detected between sides of control animals (Tables 2 and 3). The greater left-side FMNe and TMNe counts for experimental animals at 52 days postoperatively probably reflect a relatively poor lesion for those particular animals; their Fig. 4. Masticatory muscles of FMNu-lesionedanimal at 22 days postoperative. Note robust temporalis and masseter muscles of nonlesion side (top), compared with atrophied appearance of lesion-side muscles (bottom). histological sections showed most gliosis dorsal and slightly caudal to the FMNu proper. However, statistical analyses of these motoneuron-count data strongly suggest that the 94 K.E. BYRD Fig, 5, Superior (top) and inferior (bottom) views of animal shown in Fig. 4.Notice relative atrophy of lesionside temporalis (top) and posterior digastric muscles (bottom), as indicated by arrows. major effect of the unilateral FMNu lesion was on the lesioned (left) side. Osteometric analyses Figures 8 and 9 show the extent of skeletal alterations due to the effects of the lesion. Notable cranial alterations in the experimental animals included (1)deviation of nasal and premaxillary bones to the nonlesioned (right) side, (2) bone loss in the nonlesion-sideinfraorbital region, and (3) less robust lesion-side zygomatic arches (see Fig. 8). Mandibular alterations of experimental animals included (1)reduced size of lesionside condyles, coronoid processes, and gonial angles and (2) smaller lesion-side mandibular corpus (see Figs. 8 and 9). Tables 5 and 6 reveal that the greatest effect of the unilateral FMNu lesion upon experimental animals occurred for cranial measurement C-I and mandibular measurements W-Z, maximum condylar width, and maximum condylar length (refer to Fig. 1 for orientation). Paired t-tests detected no significant differences between left and right sides in the control animals for either cranial or mandibular measurements. However, controls were significantly ( P < .05, < .01) different from experimentals for all cranial and mandibular dimensions except left-side C-I and T-Z and right-side D-J, D-E, and T-Z. Only mandibular dimension T-Z displayed no significant differences between experimental and control groups (see Fig. 1, Tables 5 and 6). Several cranial measurements that provided growth vector information for the basicranium (K-R, P-M, P-C, M-C, and M-D) did not display any significant differences between lesioned and nonlesioned sides of experimental animals (Table 5). The cranial dimension C-I is of particular interest because the experimental nonlesion-side(right) was significantly ( P < .01) greater than its control counterpart; C-I was the only cranial or mandibular dimension in which the experimental mean value was greater than the control value. Mandibular dimension W-Z (Table 6) was unusual in that the significant difference ( P < .O1) between right-side and Control samples was €Water than that between left-side experimental and control groups ( P < .05). The significantly ( P < .01) greater value of the experimental left-side W-Z dimension, compared with its experimental right-side, probably reflects the shorter condylar-length value shown for the experimental left-side maximum condylar length (MCL)dimension (see Fig. 1,Table 6). The fact that both lesion-sidecondylar width (MCW) and length (MCL) dimensions were significantly ( P < .01) smaller than nonlesion-side ones in experimental animals suggests a major effect of the lesion on condylar growth. Summary of osteometric analyses In general, control animals displayed significantly greater dimensions than did experimentals for (1)cranial measurements except C-I and (2) all mandibular measurements, with one exception: T-Z. The only significant differences between left and right sides of experimental animals occurred for (1) dimen- LESIONED FACIAL AND TRIGEMINAL MOTOR NUCLEI 95 Fig. 6. Nonlesion-side (large arrow) and lesion-side to lesion-side facial motoneurons (FMNe), compared with (small arrow) facial motor nuclei (FMNu) of lesioned nonlesion-side FMNe. animal at 32 days postoperative. Notice massive damage Fig. 7. Nonlesion-side (large arrow) and lesion-side (small arrow) trigeminal motor nuclei (TMNu) of animal shown in Fig. 6; section is approximately 1,320 pm anterior to that shown in Fig. 6 . Notice (1)the fewer lesion- side trigeminal motoneurons (TMNe), compared with their nonlesion-side counterparts, and (2) the absence of massive damage to lesion-side TMNu, contrasted with that shown in Fig. 6 for the FMNu. 96 K.E. BYRD (Figs. 6 and 7, Tables 2 and 3) indicated that the FMNu was the primary lesion site; TMNu damage, although substantial, was apparently secondary to the initial lesion produced in the FMNu. The anteroposterior distance between histological sections of the same animal shown in Figures 6 and 7 is approximately 1,320 pm. As documented in Figure 48 of Paxinos and Watson (1982), the anteroposterior distance between caudalmost TMNu and the FMNu lesion site used in this study (1.9 mm lateral and 1.9 mm posterior to ear bar zero) is a t least 1,200 pm. In addition, at the coordinates of 1.9 mm lateral to ear bar zero, inferiormost TMNu is approximately 900 pm Fig. 8. Superior views of crania (top) and mandibulae (bottom) of FMNu-lesioned animals at 22, 32, and 42 days postoperative (left to right). Notice progressive reduction of bone in infraorbital region (arrows in top plate), slight rostra1 deviation toward nonlesion (right) side, and progressive reduction in size of lesion-side (left) mandibular condyles (arrows in bottom plate). sions involving the zygomatic arch and dorsal facial region in the cranium (C-I) and (2) dimensions involving the articular surfaces of the mandibular condyle (W-Z, maximum condylar width, maximum condylar length). DISCUSSION Lesion effectiveness Data presented here indicate that unilateral lesions of the FMNu caused significant alterations of the craniofacial complex. The damage to the TMNu and the muscles of mastication was unexpected because (1) no observable stimulation of TMNe occurred during the lesioning process and (2) a small (2-mA, 15-sec) lesion was used. Histological documentation of lesion damage Fig. 9. Superior (top) and inferior (bottom) views of mandible of FMNu-lesioned animal a t 42 days postoperative. Notice lesion-side (left) condylar reduction, incisor deviation, and mandibular corpus reduction relative to nonlesion (right) side. 97 LESIONED FACIAL AND TRIGEMINAL MOTOR NUCLEI TABLE 5. Cranial dimensions ofexperimental and control animals Measurement E-I c-I C-F D-J D-E K-M K-R P-M P-C M-C M-D N-C N-D N-R Q-C Q-R R-C 13.35 & 0.88 (n = 6) 19.95 f 1.68 (n = 6) 13.07 & 0.87 (n = 3) 17.32 f 1.75 (n = 6) i8.03 L 1.48 (n = 6) 13.82 f 0.76 (n = 6) 31.52 f 2.11 (n = 6) 5.52 f 0.55 (n = 6) 14.92 f 1.40 (n = 6) 9.92 f 0.99 (n = 6) 15.28 f 1.07 (n = 6) 6.87 f 1.17 (n = 6) 9.67 f 1.11 (n = 6) 10.25 f 0.92 (n = 6) 10.02 f 1.12 (n = 6) 8.70 f 0.98 (n = 6) 13.42 f 1.36 (n = 61 13.33 f 0.98 (n = 6) 22.12 f 1.81* (n = 6) 13.47 f 1.25 (n = 3) 18.13 f 1.85 (n = 6) i8.83 f 1.62 (n = 6) 13.72 f 0.78 (n = 6) 31.57 f 2.25 (n = 6) 5.63 & 0.67 (n = 6) 14.93 f 1.39 (n = 6) 10.20 f 0.98 (n = 6) 15.55 f 1.05 (n = 6) 6.90 f 0.89 (n = 6) 9.68 k0.68 (n = 6) 10.27 f 1.08 (n = 6) 10.20 fO.91 (n = 6) 8.52 & 1.11 (n = 6) 13.62 f 1.03 (n = 6) (x f S.D. mm) Controls Experimentals Left Right Left 14.55 f 0.84' (n = 6) 20.40 f 1.07 (n = 6) 16.32 f 0.85' (n = 5) 18.65 f 1.24' (n = 6) 19.45 f 1.12' (n = 6) 15.18 f 0.94' (n = 6) 34.40 f1.79' (n = 6) 6.27 f 0.50' (n = 6) 16.70 f 0.80' (n = 6) 11.35 f 0.49' (n = 6) 17.02 f 0.80' (n = 6) 8.30 f 0.36' (n = 6) 11.32 f 0.66' (n = 6) 11.88 f 0.88' (n = 6) 11.40 f 0.33' (n = 6) 9.68 f 0.70' (n = 6) 15.10 f 0.55' (n = 6) Right 14.55 k0.82' (n = 6) 20.47 f 1.07t (n = 6) 16.38 f 0.84' (n = 5) 18.60 f 1.24 (n = 6) 19.44 f 1.08 (n = 6) 15.15 f 0.94' (n = 6) 34.41 f 1.75' (n = 6) 6.28 f 0.52' (n = 6) 16.68 f 0.80' (n = 6) 11.33 f 0.52t (n = 6) 17.00 f 0.77' (n = 6) 8.25 f 0.32' (n = 6) 11.35 f 0.5gt (n = 6) 11.88 f 0.87' (n = 6) 11.43 f 0.38t (n = 6) 9.68 f 0.68' (n = 6) 15.08 f 0.52' (n = 6) *Left significantly different from right P < .01 +Controlssignificantly different from experimentals P 4 .01. above and 1,800 pm anterior to the FMNu lesion site used in this study (Paxinos and Watson, 1982). Because the mean FMNu lesion width (and assumed anteroposterior extent) was approximately 645 (k131) pm and because its anteriormost extent occurred at least 400 pm posterior to lesion-side TMNu, it does not appear that the initial lesion caused the secondary TMNu damage documented in this study. Histological sections of the two rats killed and perfused immediately after lesioning confirmed the accurate placement of the initial FMNu lesion and revealed no apparent damage to the TMNu (see the section on results; in addition, previously reported postlesion behavioral data in that section also support this interpretation). One possible explanation for the apparent secondary damage to the lesion-side TMNu might be delayed vascular effects. Secondary hemorrhage can cause severe damage to pontine nuclei within the brainstem and can create both facial palsies (Milland-Gublerand Foville's Syndromes) and jaw-muscle paralysis (alternating trigeminal hemiplegia) as described by Chusid (1979).The delayed TMNu sequelae observed in this study therefore could be due to neurovascular microhemorrhages caused by the initial FMNu electrolytic lesion. One way to determine if the delayed trigeminal effects are due to hemorrhagic events resulting from the FMNu electrolytic lesions would be to produce chemical lesions in the FMNu by microiontophoretic techniques using ibotenic acid (IBO).IBO has the distinct advantage of producing neuronal death without destroying adjacent structures (Markowska et al., 1985). Preliminary research involving microinjections of IBO into growing rat TMNu has just started (Byrd 98 K.E. BYRD unpublished data); no TMNu-FMNu interactions have yet been observed. Clearly, more discrete and motoneuron-specific lesioning techniques are needed to fully understand the data presented here. The replication of this experiment using identical and smaller size electrolytic lesions, combined with physiological data previously described, is in progress and should answer some of the specific questions raised here. A neurophysiological basis for TMNu damage by a small electrolytic lesion placed in the FMNu may be the presence of reticular formation projections between the facial and trigeminal motor nuclei in the rat (Travers and Norgren, 1983). These projections are probably involved with the masticatory a n d or facial CPGs. It is possible that the eledrolytic lesions produced in this study damaged not only the FMNu but also the reticular formation projections between the FMNu and TMNu; the subsequent damage to the TNC could then be accounted for by transneuronal degeneration (Brodal, 1981) even though no actual TMNe were stimulated during the actual lesioning procedure. Masticatory muscle weight data (Table 4) confirm the histological damage to the TMNu shown in Table 3. Previous studies lesioning the TMNu in experimental animals have documented muscular and skeletal alterations similar to those described here (Gardner et al., 1980; Phillips et al., 1982; Byrd, 1984). The essential lack of statistically significant weight differences for the posterior digastric of experimental animals (Table 4) suggests that despite massive damage to the FMNu (Table 2, Figs. 2, 3, and 6), the motoneuron pool directly supplying the posterior digastric was little affected by the initial lesion. Posterior digastric motoneurons in the rat are located within an accessory facial motor nucleus immediately dorsal to the FMNu proper (Matsuda et al., 1979; Szekely and Matesz, 1982; Shohara and Sakai, 1983). Because the initial FMNu lesion was placed an additional 0.5 mm below that position eliciting the lowest threshold voltage in the main FMNu (see the section on materials and methods), lesion effects were most pronounced in the main FMNu. This explains why the posterior digastric, with motoneurons in the accessory facial motor nucleus, was relatively unaffected by the initial FMNu lesion compared to other facial muscles with their FMNe located in the FMNu proper. It should be noted that many of the differences between experimental and control animals documented in Tables 2, 5, and 6 may be due to the relatively poor weight gain of the experimentals compared with that of their control counterparts (see the results section). This unavoidable sequela of producing electrolytic lesions suggests that differences between left (lesion) and right (nonlesion) sides of experimental animals should be weighted more than differences between experimental and control groups during any interpretations of unilateral FMNu lesion effects on craniofacial form. CNS lesion studies of the trigeminal system of the rat include those by Frank and Karli (1962), Pimenidis and Gianelly (1977), and Behrents and Johnston (1984). All these studies concentrated on the trigeminal sensory system, although it appears that morphological alterations resulting from V3 motoneuron axonal damage occurred in each investigation. It is a major problem to discern the role of the trigeminal neuromuscular complex (TNC)in craniofacial growth and to separate effects resulting from sensory neurons from those resulting from motoneuron activity. In this study, the sensory component of the TNC was largely unaffected by the lesion, as determined by the postoperative reflex testing and behavioral observations documented in the section on results. The absence of eyeblink reflex was apparently due to damaged FMNe innervating the orbicularis oculi muscle because touching lesion-side vibrissae elicited behavioral responses from experimental animals. A distinction should be made between “neurotrophic” and “neuromuscular” effects resulting from lesion studies. Following the definitions provided by Behrents and Johnston (1984), neurotrophic effects would be those resulting from a disturbance or interuption of a nutritive or sustaining substance within the nervous system, possibly biochemical in nature. Neurornuscular effects, on the other hand, would be due to a disturbance or interruption of mechanical forces between bone, muscle, and cartilage that are modulated by CNS sensorimotor systems. Data presented here and by previous researchers (Frank and Karli, 1962; Pimenidis and Gianelly, 1977; Sarnat et al., 1977; Gardner et al., 1980;Behrents and Johnston, 1984; Byrd, 1984) suggest that those studies that lesion/ resect sensory portions of the TNC would largely alter supposed neurotrophic effects; LESIONED FACIAL AND TRIGEMINAL MOTOR NUCLEI 99 TABLE 6. Mandibular dimensions of experimental and control animals (52 f S.D.mm) Measurement S-T s-w s-z T-W T-Y T-Z u-Y u-z w-z MMH MCW MCL Controls ExDerimentals Right Left 23.98 f 1.93 (n = 5) 9.98 1.62 (n = 6) 7.73 f 0.97 (n = 6) 14.44 f 0.43 (n = 5) 19.54 f 1.45 (n = 5) 22.22 0.90 (n = 5) 18.72 1.18 (n = 5) 20.54 f 1.12 (n = 5) 8.87 k 0.47 (n = 6) 11.90 & 0.99 (n = 6) 1.08 0.41 (n = 6) 2.42 f 0.44 + + + + (n = f 3 25.07 k 1.46 (n = 6) 10.57 f 1.00 (n = 6) 7.83 f 0.74 (n = 6) 14.60 0.36 (n = 6) 19.83 k 0.96 (n = 3) 21.93 f 1.36 (n = 6) 18.50 k 0.90 (n = 3) 19.92 f 1.44 (n = 6) 8.25 0.93* (n = 6) 11.80 f 0.53 (n = 3) 1.50 f 0.21* (n = 6) 3.15 f 0.19* in = 6) + Left Right 26.70 f 1.26tt (n = 6) 12.13 5 0.73'' (n = 6) 9.42 k 0.35" (n = 6) 15.08 k 0.59++ (n = 6) 21.98 k 0.91tt (n = 5) 22.92 k 1.12 (n = 6) 21.24 f 0.84tt (n = 5) 21.60 k 0.65tt (n = 6) 9.21 0.58' (n = 6) 13.58 f 0.61'' (n = 5) 1.73 f 0.05'+ (n = 6) 3.85 f 0.24++ 26.67 1.24'+ (n = 6) 12.10 f 0.75'1 (n = 6) 9.40 k 0.2gtt (n = 6) 15.13 0.67'' (n = 6) 21.63 k 0.81t' (n = 4) 22.83 f 1.06 (n = 6) 20.93 k 0.54'? (n = 4) 21.65 0.68" (n = 6) 9.23 k 0.56+' (n = 6) 13.57 f 0.65'+ (n = 5) 1.75 k 0.05tt (n = 6) 3.85 0.18" in = 6 ) + + + + + "Left significantly different from right P < .01. +Controls significantly different from experimentals P < .05. ttControls significantly different from experimentals P < .01. likewise, those studies that lesionhesect motor portions of the TNC would alter those neuromuscular effects. The problem, it seems, is largely a methodological one: better discrimination of sensory and motor components of the TNC in lesionhesection studies should allow a better understanding of the respective roles played by both neurotrophic and neuromuscular factors in craniofacial form. Although neurotrophic substances are defined as "nonnerve impulse," maintainancel homeostatic agents found in both motor and sensory nerves (Gutmann, 19761, these debates do not detract from the value of such studies; rather, they indicate that there is much interaction between sensory and motor components not only of the TNC, but of all motor systems in the craniofacial complex. A pressing question is, What are the relative contributions of the respective sensory and motor systems to craniofacial growth and development? Neurophysiological factors Byrd (1984) documented that electrolytic lesions of the TMNu do not completely sup- press masticatory muscle activity in growing guinea pigs; in addition, profound morphological alterations of the craniofacial complex were produced by such lesions. It was therefore suggested that muscular paresis and altered neuromuscular activity patterns can produce skeletal changes in the craniofacial region: total paralysis is not required for significant changes (Byrd, 1984). Data presented here also indicated that significant musculoskeletal alterations occurred without complete destruction of facial and trigeminal motor nuclei in young rats (Tables 2 and 3). Data presented here also suggest the presence of neurophysiological interactions between the FMNu and TMNu that have direct and indirect effects on craniofacial growth in the laboratory rat. The observed secondary damage to the TNC resulting from primary lesions in the FMNu is of interest because similar-size electrolytic lesions (2 mA, 15 sec.) in guinea pigs (Gardner et al., 1980; Byrd, 1984) do not produce secondary damage to the FMNu. This difference between guinea pigs and rats may be due to their physical size differences; there is an interval of ap- 100 K.E. BYRD proximately 1,600 pm between caudalmost TMNu and rostralmost FMNu in guinea pigs (Byrd, unpublished data), whereas the same distance in rats is approximately 1,100 pm (Paxinos and Watson, 1982). In addition, significantly different neurophysiological and neuroanatomical mechanisms exist between different mammalian taxa (Byrd, 1985). If craniofacial growth is indeed correlated with muscle attachment sites but not necessarily with growth of individual muscles (Nakata, 1981), it would seem that muscle activity (EMG) patterns are of prime importance in the determination of skull form. Both tonic and phasic EMG activity patterns have been studied in this regard (Miller and Chierici, 1982; Miller et al., 1984; Byrd, 1984). These activity patterns are currently thought to be initiated by at least one, and possibly by several, central pattern generators (CPG) within the reticular formation of the pontine brainstem (Luschei and Goldberg, 1981). In this study, then, CPGs involved with both facial and trigeminal motor nuclei and their respective muscular terminations would have been affected by the produced lesion. Physiological data are currently being collected to test this hypothesis (Byrd, in preparation). Role of the facial neuromuscular complex The facial neuromuscular complex (FNC) is defined here as (1)the muscles innervated by cranial nerve VII; (2) nerve VII and its branches; (3) afferent and efferent nerve fibers to those muscles supplied by VII; and (4) nuclei within the CNS giving rise to those fibers. Because both facial and trigeminal neuromuscular complexes were altered in this study, the relative contributions of each to the observed morphological alterations must be determined by their respective muscle attachment sites and by research reported by previous investigators. Both Washburn (1946) and Jarabak et al. (1949) resected the facial nerve at the stylomastoid foramen in growing rats. They both observed deviation of the premaxillary and nasal bones to the nonlesion side. This agrees with the results of this study in which nonlesion-side snout deviation and bone loss in the nonlesion-side infraorbital region were detected (Fig. 8). Washburn (1946) further asserted that the mechanism involved seemed to be “purely mechanical.” Assuming that mechanical forces generated by facial muscles are capable of producing both rostral deviation and infraorbital region bone loss (as shown in this study), which muscles could have caused those morphological changes? Meinertz (1942) describes in great detail the facial musculature of the rat and provides the answer to this question. The region of nonlesion-side infraorbital bone loss shown in Figure 8 may be due to the unbalanced activity of the nonlesion-side nasolabialis, dilator nasi, maxillonasialis, and bucconasolabialis muscles that attach to that area (Meinertz, 1942). In this sense, the nonlesionside infraorbital bone loss shown in Figure 8 may be a n example of “flexure-drift” bone modeling (Frost, 1963, 1983) caused by unchecked activity of these nonlesion-side rostral muscles. All these muscles are supplied by FMNe and are continuously active in the “whisker-moving” activity of rats and other rodents (Wineski, 1983). It would appear, then, that the major effect of the FMNu lesion on the experimental rats’ facial skeleton was primarily rostral; this is confirmed by statistical analyses of osteometric data (dimensions C-I, D-J, and D-E in Fig. 1, Table 5). If control animals are also considered, facial dimension K-M was also affected by the altered FNC (see Table 5). Based on masticatory-muscle attachment sites, as outlined by Turnbull (19701, it appears that all remaining cranial and mandibular skeletal alterations in this study were due to alteration of the TNC. This is consistent with skeletal sequelae following TMNu lesions in previous studies (Gardner et al., 1980; Phillips et al., 1982; Byrd, 1984). Neuromuscular effects As described earlier, it appears that effects of the FNC on the skull are secondary to those of the TNC; this statement is substantiated by physiological data from hemifacial microsomia patients (Vargervik and Miller, 1984). It was found that EMG signals from facial muscles (FNC) were predominantly normal in all manifestations of hemifacial microsomia, whereas masticatory muscles (TNC) could demonstrate abnormal EMG patterns in any malformation type. Furthermore, neuromuscular effects on the craniofacia1 skeleton by the TNC have recently been identified for plagiocephaly (Kreiborg et al., 1985) and skull shape in general (Weijs and Hillen, 1984). Perhaps the most important finding of this study is that damage to the CNS can effect LESIONED FACIAL AND TRIGEMINAL MOTOR NUCLEI massive changes in the TMJ (Figs. 8 and 9, Table 6). The lesion-side condylar changes shown in Figures 8 and 9 may be an example of regressive modeling of the TMJ (Moffett, 1966) if there was actual movement of the condylar articular surface away from the articular cavity; future histological studies should resolve the question. It is assumed that the neuromuscular effects on the lesionside condyles were those modulated by an altered (postlesion)TNC, because the lesionside lateral pterygoid muscles (although not weighed) appeared smaller and paler than nonlesion-side ones. This finding agrees with Petrovic’s view that the lateral pterygoid muscle plays a major role in normal condylar and subsequent mandibular growth (Petrovic et al., 1975; Oudet and Petrovic, 1978). Paired t-tests between lesion- and nonlesion-side condyles of experimental animals displayed significant differences for both maximum condylar length and maximum condylar width (Table 6). Actual values o f t also demonstrated that the greatest difference between condylar dimensions of experimental and control animals was for condylar length (x f s.d. t value of 15.14 0.69 at 10 d.f.) and not width (2 + s.d. t value of 7.48 1.61 at 10 d.f.). However, Herring and Lakars (1981) observed that dorsoventral width of the condylar cartilage was more affected than anteroposterior length by the absence of i n utero muscular contractions of mutant mice with muscular dysgenesis. This observation suggests that condylar length may be under less genetic control than are other condylar dimensions. The presence of sexual dimorphism in human condylar width, but not length (Wedel et al., 1978; Hinton and Carlson, 1979), also suggests that condylar width is under more genetic control than length. * + Evolutionary considerations This study demonstrated that significant morphological alterations of the craniofacial complex can be manifested by lesions to motoneurons within the CNS. The secular modifications of the craniofacial complex during the course of human and primate evolution have been and continue to be well documented by numerous investigators (see Moore and Lavelle, 1974 for a review). Hypotheses for explaining these morphological changes have often entailed some change@ in masticatory function (HrdliEka, 1910; Carlson and Van Gerven, 1977; Hylander, 1977, 1979; Cachel, 1979; Hinton and Carl- 101 son, 1979; Cartmill, 1980; Smith et al., 1983). Data presented here and previously (Byrd, 1984) demonstrate that muscle activity patterns, masticatory function, CNS motoneuron systems, and craniofacial morphology are all interrelated. All muscle-activity patterns and functions influencing craniofacial morphology have a neurophysiological and neuroanatomical basis. In addition, different taxa possess different neurophysiological andor neuroanatomical mechanisms that effect their respective patterns of mastication (Byrd, 1985).This concept can also be applied to the muscles of facial expression. For example, Washburn (1946) repeated the facialnerve avulsion experiments he had performed on rats a second time on rabbits; whereas the rats showed rostral deviation away from the operated side (identical to data presented here for FMNu lesioned rats), rabbits demonstrated rostral deviation toward the operated side. In recent years, many paleoneurology studies have concentrated on encephalization and cerebral cortex changes during the course of primate evolution (Falk, 1980). Data presented here, however, suggest that an appreciation of the roles played by pontine motor nuclei (FMNu and TMNu) and by their manifestations as faciallmasticatory musculature acting upon craniofacial form are also important for more fully understanding the evolution of primate craniofacial complexes. ACKNOWLEDGMENTS I would like to thank the following people for their support, comments, advice, and assistance during the course of this study: Drs. Cedric Minkin, Michael Melnick, Tina Jaskoll, Messrs. Pablo Bringas and Val Santos, and Martin Fong for his photographic expertise. This research was supported by NIH grants 2 SO7 RRO5303-24, and 1R23 DE07380. LITERATURE CITED Alherti, PW, and Biagioni, E (1972) Facial paralysis in children. A review of 150 cases. Laryngoscope 82t10131020. Avis, V (1959) The relation of the temporal muscle to the form of the coronoid process in the cat. Am. J. Phys. Anthropol. 17t99-108. Avis, V (1961) The significance of the angle of the mandible: An experimental and comparative study. Am. J. Phys. Anthropol. 19:55-61. Baer, MJ, Bosma, JF, and Ackerman, JL (1983) The Postnatal Development of the Rat Skull. Ann Arbor: University of Michigan Press. Behrents, RG, and Johnston, LE, Jr. (1984)The influence 102 K.E. BYRD of the trigeminal nerve on facial growth and development. Am. J. Orthod. 85t199-206. Blumenbach, JF (1795) On The Natural Variety of Mankind. Gottingen. Brodal, A (1981) Neurological Anatomy in Relation to Clinical Medicine. New York Oxford University Press. Byrd, KE (1984) Masticatory movements and EMG activity following electrolytic lesions of the trigeminal motor nucleus in growing guinea pigs. Am. J. Orthod. 86r146-161. Byrd, KE (1985) Research in mammalian mastication. Am. Zool.25t365-374. Cachel, SM (1979) A functional analysis of the primate masticatory system and the origin of the anthropoid postorbital septum. Am. J. Phys. Anthropol. 50tl-18. Cammermeyer, J (1963)Differential response of two neuron types to facial nerve transection in young and old rabbits. J. Neuropath. Exp. Neurol. 22.594-616. Carlson, DS, and Van Gerven, DP (1977) Masticatory function and post-Pleistoceneevolution in Nubia. Am. J. Phys. Anthropol. 46:495-506. Cartmill, M (1980) Morphology, function, and evolution of the anthropoid postorbital septum. In RL Ciochon and AB Chiarelli (eds.) Evolutionary Biology of the New World Monkeys and Continental Drift. New York: Plenum, pp. 243-274. Chusid, J G (1979) Correlative Neuroanatomy and Functional Neurology. Los Altos: Lange Medical Publications. Delaire, J (1978) The potential role of facial muscles in monitoring maxillary growth and morphogenesis. In DS Carlson and JA McNamara, Jr. (eds.) Muscle Adaptation In The Craniofacial Region. Ann Arbor: Monog. No. 8, Craniofacial Growth Series, Ctr. for Human Growth and Development, pp. 157-180. Delaire, J and Chateau, JP (1977) Comment le septum nasal influence-t-illa croissance premaxillaire et maxillaire. Deductions en chirurgie des fentes labiomaxillaires. Rev. Stom. 78r241-254. Falk, D (1980) Hominid brain evolution: the approach from paleoneurology. Yearb. Phys. Anthropol. 23:93107. Frank, RM, and Karli, P (1962) Destruction electrolytique des noyaux centraux du trijumeau chez le rat et ses repercussions a u niveau de la region faciale. Arch. Oral Biol. 7t381-389. Frost, HM (1963) Bone Remodeling Dynamics. Springfield CC Thomas. Frost. HM (1983) A determinant of bone architecture. The minimum effective strain. Clin. Orthop. Rel. Res. 175286-292. Gardner, DE, Luschei, ES, and Joondeph, DR (1980)Alterations in the facial skeleton of the euinea Die following a lesion of the trigeminal motor nucleus. Am. J. Orthod. 78t66-80. Gutmann, E (1976) Neurotrophic relations. In E Knobil, RR Sonnenschein, and IS Edelman (eds.) Annual Review of Physiology. Palo Alto: Annual Reviews Inc., Vol. 38, pp. 177-216. Herring, SW, and Lakars, TC (1981) Craniofacial development in the absence of muscle contraction. J. Craniofac. Gen. Dev. Biol. 1t341-357. Hinton, RJ, and Carlson, DS (1979)Temporal changes in human temporomandibular joint size and shape. Am. J. Phys. Anthropol. 50:325-334. Horowitz, SL, and Shapiro, HH (1951) Modifications of mandibular architecture following removal of temporalis muscle in the rat. J. Dent. Res. 30r276-280. Horowitz, SL, and Shapiro, HH (1955) Modifications of - .- skull and jaw architecture following removal of the masseter muscle in the rat. Am. J. Phys. Anthropol. 13t301-308. HrdliEka, A (1910) Contribution to the anthropology of Central and Smith Sound Eskimos. Anthropol. Papers Am. Mus. Nat. Hist. 5: part 2. Huber, E, and Hughson, W (1926) Experimental studies on the voluntary motor innervation of the facial musculature. J. Comp. Neurol. 42113-163. Hylander, WM (1977) The adaptive significance of eskimo craniofacial morphology. In AA Dahlberg and TM Graber (eds.) Orofacial Growth and Development. The Hague: Mouton, pp. 129-169. Hylander, WM (1979) The functional significance of primate mandibular form. J. Morph. 16Ot223-240. Jarabak, JR, Kamins, M, and Vehe, K (1949) Effect of unilateral resection of facial nerve on facial growth of the rat. J. Dent. Res. 28t654-655. Jenkins, HA, Herzog, JA, and Coker, N J (1985) Bell’s palsy in children. Cases of progressive facial nerve degeneration. Ann. Otol. Rhin. Laryng. 94:331-336. Johnston, LE, Jr. (1976)The functional matrix hypothesis: Reflections in a jaundiced eye. In JA McNamara, Jr. (ed.) Factors Affecting the Growth of the Midface. Ann Arbor: Monog. No. 6, Craniofacial Growth Series, Center for Human Growth and Development, pp. 131168. Kreiborg, S, MBller, E, and Bjork, A (1985) Skeletal and functional craniofacial adaptations in plagiocephaly. J. Craniofac. Genet. Develop. Biol. Supp. 1t199-210. LaBossiere, E, and Glickstein, M (1976)Histological Processing for the Neural Sciences. Springfield Thomas. Luschei, ES, and Goldberg, L J (1981) Neural mechanisms of mandibular control: Mastication and voluntary biting. In VB Brooks (ed.) Handbook of Physiology-the Nervous System 11. Bethesda: American Physiological Society, pp. 1237-1274. Manning, J J , and Adour, KK (1972) Facial paralysis in children. Pediatrics 49:102-109. Markowska, A, Bakke, HK, Walther, B, and Ursin, H (1985) Comparison of electrolytic and ibotenic acid lesions in the lateral hypothalamus. Brain Res. 328:313323. Matsuda, K, Uemura, M, Takeuchi, Y, Kume, M, Matsushima, R, and Mizuno, N (1979) Localization of motoneurons innervating the posterior belly of the digastric muscle: A comparative anatomical study by the HRP method. Neurosci. Lett. 1247-52. Meinertz, T (1942) Das Superfizielle Fazialisgebiet der Nager. IV Die Muriden. 1.Epimys noruegicus (Erxleben) und Cricetus cricetus Gegenbaurs. Morph. Jahrb. 87t254-324. Miller, AJ, and Chierici, G (1982) Concepts related to adaptation of neuromuscular function and craniofacial morphology. Birth Def. I8r21-43. Miller, AJ, Vargervik, K, and Chierici, G (1982) Sequential neuromuscular changes in rhesus monkeys during the initial adaptation to oral respiration. Am. J. Orthod. 81t99-107. Miller, AJ, Vargervik, K, and Chierici, G (1984) Experimentally induced neuromuscular changes during and after nasal airway obstruction. Am. J. Orthod. 85:385392. Moffett, B (1966)The morphogenesis of the temporomandibular joint. Am. J. Orthod. 521401-415. Moore, WJ, and Lavelle, CLB (1974) Growth of the Facial Skeleton in the Hominoidea. New York Academic Press. Nakata, S (1981)Relationship between the development a,). LESIONED FACIAL AND TRIGEMINAL MOTOR NUCLEI and growth of cranial bones and masticatory muscles in postnatal mice. J. Dent. Res. 60:1440-1450. Oudet, C, and Petrovic, AG (1978)Variations in the numbers of sarcomeres in series in the lateral pterygoid muscle as a function of the longitudinal deviation of the mandibular position produced by the postural hyperpropulsor. In DS Carlson and JA McNamara, Jr. (eds.) Muscle Adaptation in the Craniofacial Region. Ann Arbor: Monog. No. 8, Craniofacial Growth Series, Center for Human Growth and Development, pp. 233246. Paxinos, G, and Watson, C (1982) The Rat Brain In Stereotaxic Coordinates. New York Academic Press. Petrovic, AG, Stutzmann, JJ, and Oudet, CL (1975) Control processes in the postnatal growth of the condylar cartilage of the mandible. In JA McNamara, Jr. (ed.) Determinants of Mandibular Form and Growth. Ann Arbor: Monog. No. 4, Craniofacial Growth Series, Center for Human Growth and Development, pp. 101-153. Phillips, C, Shapiro, PA, and Luschei, ES (1982)Morphologic alterations in Macaca rnulatta following destruction of the motor nucleus of the trigeminal nerve. Am. J. Orthod. 81:292-298. Pimenidis, M Z , and Gianelly, AA (1977)Class I11 malocclusion produced by oral facial sensory deprivation in the rat. Am. J. Orthod. 71:94-102. Sarnat, BG, Feigenbaum, JA, and Krogman, WM (1977) Adult monkey coronoid process after resection of trigeminal nerve motor root. Am. J. Anat. 15Ot129-137. Shohara, E, and Sakai, A (1983)Localization of motoneurons innervating deep and superficial facial muscles in the rat: A horseradish peroxidase and eIectrophysiologic study. Exp. Neurol. 81:14-33. Smith RJ, Petersen, CE, and Gipe, DP (1983) Size and shape of the mandibular condyle in primates. J. Morph. 177:59-68. Sokal, RR, and Rohlf, FJ (1969)Biometry. The Principles and Practice of Statistics in Biological Research. San 103 Francisco: W.H. Freeman and Co. SzBkeley, G, and Matesz, C (1982) The accessory motor nuclei of the trigeminal, facial, and abducens nerves in the rat. J. Comp. Neurol. 210:258-264. Travers, JB,and Norgren, R (1983) Afferent projections to the oral motor nuclei in the rat. J. Comp. Neurol. 220:280-298. Turnbull, WD (1970) The mammalian masticatory apparatus. Field. Geol. 18:149-356. Vargervik, K, and Miller, AJ (1984) Neuromuscular patterns in hemifacial microsomia. Am. J. Orthod. 86:3342. Vargervik, K, Miller, AJ, Chierici, G, Harvold, E, and Tomer, BS (1984) Morphologic response to changes in neuromuscular patterns experimentally induced by altered modes of respiration. Am. J. Orthod. 85115-124. Walker, DG (1961) Malformations of the Face. London: Livingstone. Washburn, SL (1946)The effect of facial paralysis on the growth of the skull of rat and rabbit. Anat. Rec. 94:163168. Washburn, SL (1947)The relation of the temporal muscle to the form of the skull. Anat. Rec. 99:239-248. Watson, CRR, Sakai, S, and Armstrong, W (1982) Organization of the facial nucleus in the rat. Brain Behav. Evol. 20:19-28. Wedel, A, Carlsson, GE, and Sagne, S (1978) Temporomandibular joint morphology in medieval skull material. Swed. Dent. J.2177-187. Weijs, WA, and Hillen, B (1984) Relationships between masticatory muscle cross-section and skull shape. J. Dent. Res. 63:1154-1157. Wineski, LE (1983) Movements of the cranial vibrissae in the golden hamster (rnesocricetus auratus). J. Zool., Land. 200:261-280. Wolff, J (1892) Das Gesetz der Transformation der Knochen. Berlin: Hirschwald.