THE ANATOMICAL RECORD 2 4 4 5 7 2 4 7 8 (1996) Posterior “Septum” of Human Spinal Cord: Normal Developmental Variations, Composition, and Terminology D. PARKINSON AND M.R. DEL BIG10 Departments of Anatomy and Pathology, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada ABSTRACT Background: The boundary separating the posterior columns of the spinal cord is formed by the lateral margins of the neural groove approximating to form the neural canal. In anatomy texts this line is usually drawn as continuous, uniform, centered, and straight. It is universally termed posterior or dorsal, median “septum.” Methods: Sections from the cervical and lumbar enlargements and the mid-thoracic region were examined from 35 human autopsy specimens from 20 weeks gestation to 70 years with no history of spinal cord disease or trauma. They were stained with Masson’s trichrome, and by immunohistochemistry for collagen types 1 and 4, and for glial fibrillary acidic protein (GFAP). Results: One or more variations were found in the position character, shape, or extent of the line at one or more levels in every case. There was no midline staining for collagen other than that associated with blood vessels. There is intense immunoreactivity for GFAP from 20 weeks gestation to 35 weeks diminishing thereafter. When the posterior columns are separated the “septum” divides. Conclusion: In the absence of any collagen this line of separation is more akin to a raphe than a septum. Inasmuch as there is an immediately adjacent subarachnoid posterior median septum it would be advantageous to re-name this intraspinal structure “dorsal” or “posterior” median raphe. 0 1996 Wiley-Liss, Inc. Key words: Human, Spinal cord developmental variations, Septum, “Raphe” Credit for the first observation and naming of the posterior spinal cord sulcus and “septum” goes wanting. There is no evidence that Leonard0 observed these features (Todd, 1983) and neither Vesalius (1543) nor Willis (1680) drew the sulci or fissures or septa in their depictions of the upper cord. According to Spillane (1981),in 1774 Charles Bell noted “the shallow posterior and deep anterior median Sulci.” These were probably noted by many of his contemporary anatomists. By 1889 standard anatomy texts (Macallister 1889) described andlor drew in cross-sections of the spinal cord, a straight midline separation between the posterior columns, extending from the cord surface to approximate the posterior grey commissure. The terms “posterior septum” and “posterior fissure” were applied to this structure. [We suspect branching clefts seen in some of our specimens and in some text illustrations are drying artifacts.] Subsequent standard texts (Table 1)use the word “septum” with posterior or dorsal, several with addition of “median,” for this midline separation of the posterior columns. “Sulcus” or “furrow” is used for the surface midline indentation or those at the dorsal root entrance. These are without bibliographical credit references and do not change their original de0 1996 WILEY-LISS, INC. scription in subsequent editions. Many of these same anatomy texts describing the pia-arachnoid septum in the posterior midline subarachnoid space also use the term “septum posticus” or “posterior septum.” This term is credited to Schwalbe by Morris and McMurrich (1907) and to Magendie by Schafer et al. (19081, without reference in either case. Medical dictionaries (Becker, 1989; Dorland, 1985; Wiley, 1986) under the term “posterior [or “posticum”1 septum” state, “a glial-pia mater septum which dips into the posterior median sulcus of the spinal cord and separates the posterior funiculi.” The other they term “subarachnoidal septum.” One dictionary, Stedman’s (Hensyl, 19901, has no reference to either spinal septum. Valentino et al. (1983) reported intense glial fibrillary acidic protein (GFAP) immunoreactivity in the dorsal median septum of the rat spinal cord on day one Received May 8, 1995; accepted October 2, 1995. Address reprint requests to Dwight Parkinson, M.D., #128 Basic Sciences Building, 730 William Avenue, Winnipeg, Manitoba, Canada R3E OW3. POSTERIOR MEDIAN “SEPTUM OR “RAPHE”? TABLE 1. Representative texts that use the term posterior (median) septum Austin, G.W. 1984 The Spinal Cord, 3rd ed. Igaku-Shoin, New York. Barr, M.L, and J.A. Kiernan 1988 The Human Nervous System. An Anatomical Viewpoint. J. Lippincott Co., Philadelphia, pp. 64. Brash, J.C. 1951 Cunningham’s Text Book of Anatomy, 9th ed. Oxford University Press, London, pp. 859-866. Carpenter, M.B. 1984 Core Text of Neuroanatomy, 3rd ed. William and Wilkins, Baltimore, pp. 59. Daube, J.A., and B.A. Sandok 1978 Medical Neurosciences. Little, Brown and Company, Boston, pp. 302. Everett, N.B. 1972 Functional Neuroanatomy. Lea and Febiger, Philadelphia, pp. 31-33. Haines, D.E. 1983 Neuroanatomy: An atlas of structures, sections and systems. Urban and Schwarzenberg, Baltimore, pp. 65, 67, 137. Martin, J.H. 1989 Neuroanatomy Text and Atlas. Elsevier, New York, pp. 116. Macallister, A. 1989 Text Book of Human Anatomy. Philadelphia, P. Blakiston & Son and Co., Philadelphia, pp. 743, 745. Marinekovic, S.S., A. Ilic, M. Milisavjevic, and V. Kostic 1989 Funkcionalna I Topografska Neuroanatomija. Savremena Administrackja Beograd, pp. 265. Montemurro, D.G., and J.E. Bruni 1981 The Human Brain in Dissection. W.B. Saunders Co., Philadelphia, pp. 115. Moore K.L. 1992 Clinically Oriented Anatomy, 3rd ed. William and Wilkins, Baltimore, pp. 361. Romero-Scierra, R. 1986 Neuroanatomy. A Conceptual Approach. Churchill Livingstone Inc., New York, pp. 79. Wilson, D.B., and W.J. Wilson 1983 Human Anatomy. Oxford University Press, New York, pp. 256. post partum. Bohme (19881, using cats, postulated that the increasing bulk of the dorsal funiculi causes the elongation of the ependymal cells, the basal process of which remain to form the dorsal glial septum. Thus until 2 or 3 months after birth (in cats) there is a small wedge-shaped area in the dorsal wall of the central canal consisting of fetal matrix cells with long tapering basal processes extending into the glial septum. After this date the matrix is exhausted and the ependyma forms the complete lining of the canal (see Figs. 1-3). Joosten and Gribnau (1989) reported a prominent glial septum in the midline raphe of the medulla oblongata and in the spinal cord whereas it was absent in the decussation area of the corticospinal tract in rats. After the first postnatal week, the major vimentin-immunoreactive glial barrier completely disappears in the medullary levels and reduces to a minor GFAP immunoreactive line in the spinal cord for the rat. Sarnat (1992)found that cells in the dorsal roof plate of spinal cords in humans were immunoreactive for GFAP as early as 8 weeks gestation and that the expression of GFAP diminished by 34 weeks. He believed, as did Snow et al. (1990) working with rats, that this midline collection of parallel fibers was important in fetal development through its potential for repelling growing axons and thus preventing aberrant decussation of neuronal pathways from the posterior columns. During our earlier study of the midline subarachnoid posterior septum o f the human spinal cord [Septum pos- 573 ticum, posterior septum] (Parkinson, 19911, it was noted that the configuration and structure of the midline separation of the posterior columns within the spinal cord exhibited considerable variation. The present study enlarges on these earlier observations, and considers the structure of, and terminology for, what has classically been called the posterior (median) septum. MATERIALS AND METHODS Thirty-five human fresh autopsy spinal cords and coverings that ranged from 20 weeks gestation to 70 years were examined. None had a history of cord disease or injury. Ten 6 pm cross-sections were taken from paraffin blocks at the mid-cervical enlargement, midthoracic region, and the mid-lumbar enlargement and mounted; a total of thirty for each of the thirty-five cords. These were stained with Masson’s trichrome (Masson, 1929; Lillie, 19481, and, in addition, rabbit See figures on following pages Figs. 1-6. Chronologically left to right. Bottom, ventral; top, dorsal. Fig. 1. Twenty week female. Lumbar level. Early phase of approximation with incomplete closure of canal consisting of ependymal and matrix cells. x 40. Fig. 2. Twenty-two week male. Thoracic level. Early tenting of fibers dwindling into one to two fibers forming wavy line. The two circular areas of loose tissue just below the canal are associated with blood vessels. x 80. Fig. 3. Twenty-two week male. Thoracic level. Completion of canal lining with ependyma. Note “wavy” nature of Zine in Figures 2 and 3. Fig. 4. Twenty-five week male. Thoracic level. Early interposition of transverse fibers in region of posterior commissure. Note “reverse flow” connection of tributary vessel. x 40. Fig. 5. Thirty-seven week female. Cervical level. Tenting fibers into one or two strands forming separating Zine. Note nest of closely packed nuclei within tent. x 40. Fig. 6. Two month post partum male. Lumbar level. Tenting into line of two or three fibers. Again note nest of closely packed nuclei with sparse cytoplasm and cell processes. Wider separation of line and canal. x40. Figs. 7-12. Chronologically left to right. Bottom, ventral; top, dor- sal. Fig. 7. Four month male. Thoracic level. Very narrow posterior commissure. Incomplete alar plate approximation with pia covering smooth shoulders to bottom of cleft. Note arachnoid septum. Collagen staining ends a t base of valley. Normal tributary branching. X 40. Fig. 8. Eighteen month male. Thoracic level. Forked or split line at arrows. x40. Fig. 9. Two and one-half year female. Thoracic level. Gently curved line from off center tented takes off becoming straight thereafter. Usual vessel branching. x 20. Fig. 10. Two and one-half year female. Thoracic level. Line absent or indistinct from commissure to lower arrow. Single fibre from there on. x 40. Fig. 11. Four year male. Cervical level. Off center tenting origin (arrow) quickly tapering to single fiber line to vessel and beyond. x 40. Fig. 12. Five year male. Lumbar level. Tenting origin dwindles to 2-3 fibers. Nest of 14 nuclei within tent with sparse cytoplasm and processes. Posterior column deeper into commissure region on left than right. x 60. 574 D. PARKINSON AND M.R. DEL BIG10 Figs. 1-6. POSTERIOR MEDIAN “SEPTUM” OR “RAPHE”? Figs. 7-1 2. 575 576 D. PARKINSON AND M.R. DEL BIG10 polyclonal anti-collagen type I (Monosan: M O O dilu- presumably venous as Herren and Alexander (1939) tion), mouse monoclonal anti-collagen type IV (Dako, state that there are only veins posteriorly in the midCarpinteria, CA: 1/100 dilution), and rabbit polyclonal line. Craigie (1944a,b) and Gray (Williams and Waranti GFAP (BioGenix, San Ramon, CA: 1/100 dilution). rick, 1980) draw both arteries and veins in the postePrimary antibodies were detected using biotinylated rior midline. The usual branching configuration would secondary antibodies followed by avidin-biotin peroxi- fit with either an artery running centripetally from the dase method and diaminobenzidine reaction. The ex- surface or a vein running centrifugally from the inteaminations were done under an Olympus BH2 micro- rior, i.e., the branches are at an acute angle towards scope and the photographs taken on Pantomatic-X film the center. However, several of our specimens clearly depict a “reverse” branching, i.e., acutely away from 100 with an Olympus camera. the center of the cord (Fig. 4). RESULTS Collagens type I and IV were detected in arachnoid, From twenty to twenty-five weeks gestation ependy- pia, and the connective tissue associated with the ma1 cells are seen (Figs. 1-4) approximating dorsal to larger blood vessels. In addition, small blood vessels the central canal. These cells give off processes extend- exhibited collagen type IV immunoreactivity. There ing centrifugally along the dorsal midline. The central was no immunoreactivity in the posterior midline nor canal origin of the line is evident up to about 25 weeks, collagen staining with Masson’s unless a blood vessel after which there are transversely arranged strands was present. The line did, however, exhibit strong iminterspersed in the region of the posterior grey com- munoreactivity for GFAP at 20 weeks gradually diminmissure (Fig. 4). The posterior grey commissure attains ishing to 35 weeks and persisting faintly into adultadult proportions by about 2 months post partum (Fig. hood, corresponding t o Sarnat’s (1992) finding in the 6). Beyond this period no age related differences were human roof plate ependymal cells. If the surface pia-arachnoid is opened, the posterior found (Figs. 6-18). The mature line (2 months or older) usually arises from a “tenting” of fibers at the posterior columns can be separated evenly with a relatively grey commissure (Figs. 2-6, 9, 11, 12, 14, 16). The smooth plane of cleavage. Fine filaments and occasionnumber of these “tenting” centrifugal fibers diminishes ally narrow ribbons or a vessel can be picked up from with increasing distance from the central canal. The the exposed surfaces, but on neither side is a septum fibers are replaced at varying intervals by additional demonstrable. The “septum” itself divides when the cells, rarely more than two at any one level (Figs. 2-14, two sides are pulled apart. 16-18). These “tenting” fibers are not compacted as DISCUSSION though being pushed dorsally but rather are progressively more separated as though pulled from the cenThe term “septum” denotes a free-standing structure tral canal (Figs. 6, 12, 14). These linear arranged cen- (Hensyl, 1990) which could be removed intact without trifugal fibers originating near the canal and the disturbing the separated structures as for instance the additional cell processes along the way do not look to be atrial septum, septum pellucidurn, or the nasal septum. separable from the posterior columns; in fact some fi- Even muscle septa with muscle attachments are freebers appear to be entering or leaving this linear bundle standing separable structures. Our sections of the hufrom or into the adjacent posterior columns. These fi- man spinal cord from 20 weeks gestation to 70 years do bers do not stain for collagen with Masson’s stain. not show such a structure except for an occasional short Within the tented or wedge-shaped origin of the line extension of the arachnoid into the spinal cord, prethere are frequently clusters of small dense nuclei. sumably indicating incomplete approximation of the (Figs. 5 , 6, 12, 16). neural plate dorsally (Fig. 7). At any section there may be one or more of eight The constituents of the line separating the posterior major variations. Rarely is the line classic: i.e., cen- columns begin as the neural plate folds on either side of tered, straight, and complete from commissure to pial surface (Type I) (Figs. 5-7). More often it is missing in part (Fig. 10) (Type 11),or missing completely (Fig. 15) Figs. 13-18. Chronologically left to right. Bottom, ventral; top, (Type 111). It may have a single curve (Figs. 9, 16, 17) dorsal. (Type IV), or multiple curves (Fig. 13)with long (Type Fig. 13. Seven year female. Thoracic level. Gently curved line from Va) or short (Type Vb) (Figs. 2,3) wavelengths. It may arise off center (Figs. 9,11,16,17) (Type VI). It may be blood vessel to pial surface. x 40. forked and/or divided with one or more components Fig. 14. Nine year male. Lumbar level. Open tent containing vessel (Fig. 8) (Type VII). The duplicated or dividing lines do and loose collagen staining reticulum leading to multiple fiber line. not suggest enclosure of the septo-marginal bundle. x 60. There may, or may not, be a pial surface indentation Fig. 15. Nine year male. Cervical level. Absent line. x 40. (Figs. 9, 18) with any of these variations. When the approximation is incomplete (Type VIII), a valley or Fig. 16. Nine year female. Thoracic level. Tenting origin off center sulcus of varying depth remains with smooth rounded to left tapers to single cell curving line. Arrows: Cluster of 5 nuclei pial shoulders and usually a pia-arachnoid extension of with sparse cytoplasm in tent. x 40. the subarachnoid dorsal median septum into the defect Fig. 17. Eleven year male. Lumbar level. Off center curving origin. (Fig. 7). Any collagen staining ends at the bottom of x 40. such a sulcus (Fig. 7). The line separating the posterior Fig. 18. Thirty year female. Thoracic level. Multiple parallel similar columns may be occupied by a blood vessel (Suh and lines all blood vessels which stained for collagen below arrows. No Carpenter, 1939)for part or all of the distance from the staining for collagen above arrows including pale wedge of cells at the surface to the commissure (Figs. 4, 9, 13, 18). This is pial margin. x 40. POSTERIOR MEDIAN “SEPTUM” OR “RAPHE”? Figs. 13-18. 577 578 D. PARKINSON AND M.R. DEL BIG10 the neural groove approximate to form and enclose the central canal. At this time in development there is no collagen included on, or between, the approaching surfaces (Rutka et al., 1988; Sarnat, 1992; Joosten and Gribnau, 1989). In the absence of any collagen or any semblance of a separable structure, this line of separation, which apparently arises from the ependyma of the central canal early in development and is subsequently modified during maturation, is more akin to a ruphd (Parkinson, 1995) than a septum. One can only speculate as to whether the occasional asymmetry andlor curvature of the line is due t o chance during embryonic approximation or differential development of various portions of the posterior columns. Examination of 35 normal cords from 20 weeks gestation to age 70 reveals a great variation in the line separating the posterior columns of the cord. There is no evidence of a true septum, or even of any midline condensation of collagen other than that seen in blood vessel walls. 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