MICROSCOPY RESEARCH AND TECHNIQUE 42:226–233 (1998) Testicular Microvascularization in the Common Tree Shrew (Tupaia glis) as Revealed by Vascular Corrosion Cast/SEM and by TEM W. PRADIDARCHEEP,1 S. KONGSTAPONKIT,1 P. WARAKLANG,1 P. CHUNHABUNDIT,2 AND R. SOMANA1* 1Department 2Department of Anatomy, Faculty of Science, Mahidol University, Bangkok, Thailand of Anatomy, Faculty of Dentistry, Mahidol University, Bangkok, Thailand KEY WORDS testis; microcirculation; scanning electron microscope; transmission electron microscope; vascular cast ABSTRACT Testicular angioarchitecture in lower primates has not been established and the route of androgens from Leydig cells entering the systemic circulation is still a matter of controversy. In the present study, the common tree shrew (Tupaia glis) was used as the model for vascular corrosion cast/SEM and conventional TEM studies. With vascular corrosion cast/SEM, it was revealed that while coursing in the spermatic cord, the testicular artery convoluted and gave off branches to supply the epididymis, the coverings of the spermatic cord and the pampiniform plexus. Upon approaching the testis, it encircled the organ, then penetrated into the testicular parenchyma near the rostro-medial pole before further dividing into arterioles that gave rise to capillary plexuses looping around the seminiferous tubules. These capillaries converged into the intratesticular venules, then into larger venules on ventral and dorsal surfaces of the testis and finally into the collecting veins on medial and lateral borders of the testis. In addition, the capillaries in the central or medullary portion of the gland collected the blood into the medullary venules and central (medullary) vein, respectively. The collecting veins as well as central vein joined together before dividing into pampiniform plexus. With transmission electron microscopy, the capillaries in the testis were shown to be of the thick basement membrane and continuous type. The Leydig cells were found adjacent to lymphatic vessels among the seminiferous tubules. This structure is compatible with the idea that most of the androgens drain into the lymphatic vessels rather than into the capillaries. Microsc. Res. Tech. 42:226–233, 1998. r 1998 Wiley-Liss, Inc. INTRODUCTION The testes of most higher mammals descend into the scrotum. Some exceptions are the elephant (Short et al., 1967) and rock hyrax (Glover, 1973). The species having testes permanently lodged in the abdominal cavity have a straight testicular artery without pampiniform plexus (Glover, 1973; Short et al., 1967). Those with scrotal testes have a coiled testicular artery with pampiniform plexus (Christensen, 1964; Chubb and Desjardins, 1982; Dhingra, 1979; Ohtsuka, 1984; Osman et al., 1979; Noordhuizen-Stassen et al., 1985; Sisson, 1969). In most animals with scrotal testis, the testicular artery encircles the gland on the capsular surface before arteriolization into the testicular tissue (Chubb and Desjardins, 1982; Suzuki, 1982) but, in man, the artery does not surround the gland and it sends off many branches before or upon reaching the surface of the testis (Kormano and Suoranta, 1971). In addition, the intratesticular capillary organization varies in different mammals. In mouse, such capillary arrangement can be visualized in a rope-ladder-like pattern (Suzuki, 1982) while in man, the pattern does not follow any discernible pattern (Kormano and Suoranta, 1971; Suzuki and Nagano, 1986; Takayama and Tomoyoshi, 1981). Three-dimensional study of the testicular vascular pattern with the corrosion cast technique combined r 1998 WILEY-LISS, INC. with scanning electron microscopy has previously been performed in only a restricted number of species, notably, the mouse (Suzuki, 1982), rat (Ohtsuka, 1984), and man (Suzuki and Nagano, 1986). Such studies have not been conducted in the common tree shrew, an animal regarded as a lower primate (DeVore and Eimerl, 1970; Palley et al., 1984). The present study, therefore, aimed to carry out three-dimensional analysis of the vascular pattern in this animal. Additional study with transmission electron microscopy was undertaken to determine the characteristics of the intratesticular capillaries in relation to the Leydig cells and seminiferous tubules and of the lymphatic vessels in the tree shrew testis. MATERIALS AND METHODS Eight adult male common tree shrews (Tupaia glis), weighing between 120–180 g, were used. The preparation of animals and the injection of Batson’s no. 17 plastic mixture (casting medium) and the preparation Contract grant sponsor: National Science and Technology Development Agency (NSTDA). *Correspondence to: Prof. Reon Somana, M.D., Ph.D., Department of Anatomy, Faculty of Science, Mahidol University, Rama VI Road, Phayathai, Bangkok 10400, Thailand. Received 23 January 1998; accepted in revised form 28 May 1998 TESTICULAR MICROVASCULARIZATION IN THE COMMON TREE SHREW TESTIS of testicular vascular casts were processed according to the method previously described by Chunhabundit and Somana (1988, 1991). Each vascular cast of the testis was air-dried, stuck onto a brass stub with silver paint, and coated with gold/palladium prior to being examined and photographed under a scanning electron microscope at an accelerating voltage of 30 kV. For transmission electron microscopy, testes from three animals were fixed by vascular perfusion with 2.5% glutaraldehyde in phosphate buffer, pH 7.4, and left overnight in the same fixative at 4°C. The samples were postfixed in 1% osmium tetroxide, dehydrated in a graded series of ethanol, infiltrated and embedded in Araldite 502 resin. Sections 70–100 nm thick were stained with 1% uranyl acetate, then lead citrate, and examined under a transmission electron microscope at an accelerating voltage of 75 kV. RESULTS The testis of the common tree shrew is an ellipsoidal organ with the approximate dimensions of 5 3 10 3 4 mm. The average combined weight of both testes is 1.2 g. Each testis receives arterial blood supply from the testicular artery, which arises from the abdominal aorta just caudal to the renal artery. Each testicular artery runs along the side of vertebral column on the ventral surface of the psoas major muscle, passes over the external iliac vessels and then joins the spermatic cord. The average diameter of the artery is 0.25 mm. The diameter of the seminiferous tubule in this animal is 100–200 µm. The vascular corrosion cast technique in conjunction with SEM at low magnification revealed the tree shrew testis to be highly vascularized. The morphology of the cast conforms to the appearance of the testis (Fig. 1). The proximal portion of the testicular artery is somewhat straight, accompanied by a single testicular vein. The artery becomes convoluted upon approaching the pelvis. The degree of convolution gradually increases as the artery runs caudally to the testis and is prominent in the middle and distal portions of the spermatic cord. The convoluting segments are surrounded by the pampiniform plexus of numerous anastomosing veins (Fig. 2). The convoluted part of the artery also gives off small branches to supply the epididymis and the coverings of the spermatic cord (Fig. 3). In addition, some of these small branches give rise to vasa vasorum, supplying the LIST OF ABBREVIATIONS a CoV CV E EA L LY LS Pe PP S ST T TA v VV arteriole collecting vein central vein epididymis epididymal artery lymphocyte interstitial cell of Leydig lymphatic space pericyte pampiniform plexus seminiferous epithelium space for seminiferous tubule testis testicular artery venule vasa vasorum 227 wall of pampiniform plexus, especially in its distal portion (Fig. 4). When the artery emerges from the pampiniform plexus and approaches the testis at the rostro-medial pole, it courses caudally along the medial border, then rostrally along the lateral border, and finally penetrates into the testicular parenchyma near the rostral pole (Figs. 1,5). While curving on the lateral border of the gland, it gives off 4–5 parenchymal branches penetrating perpendicularly into the testicular tissue (Fig. 5). When viewing the cut surface of the testicular vascular cast, it is clearly revealed that each parenchymal branch courses tortuously in parallel to the seminiferous tubule before further dividing into large and then small arterioles in the interstitial spaces. Each small arteriole leads into capillaries within the interstitium. The capillary then runs either semi- or total circumferentially around the seminiferous tubules (Fig. 6). The distal ends of these capillaries converge into intratesticular small venules (Fig. 7), which tend to run at right angles to the testicular surfaces (Fig. 8). These small venules drain the blood into cortical large venules on both ventral and dorsal surfaces of the testis (Fig. 8). The cortical venules on the ventral and dorsal surfaces empty the blood into the collecting veins on the lateral and medial borders of the testis, respectively (Fig. 9). In addition, the capillaries from the medullary region of the testis drain the blood into the intratesticular venules and finally into the medullary vein or central vein that courses rostro-medailly (Fig. 9). The two collecting veins as well as the central vein join together to become the pampiniform plexus just before leaving the rostral testicular pole (Fig. 9). Occasionally, the cortical large venules at the upper rostral pole of ventral and dorsal surfaces of the testis drain the blood directly into the pampiniform plexus (Fig. 10). With TEM, it is shown that the capillaries in the testicular tissue are without fenestrations and have a thick basement membrane (Fig. 11). The capillaries are not in close proximity to the Leydig cell clusters or to the wall of seminiferous tubules. Moreover, the lymphatic vessels are frequently observed adjacent to the clusters of Leydig cells and to the walls of the seminiferous tubules (Fig. 12). DISCUSSION The testicular arterial blood supply in the tree shrew is similar to that in man (Kormano and Suoranta, 1971), rat (Chubb and Desjardins, 1982), and most mammals in that it derives from a direct branch of the abdominal aorta. However, the testicular artery of the tree shrew almost completely encircles the testis and sends off parenchymal branches while curving along the lateral border of the gland. This differs from what has been reported in rabbit (Chubb and Desjardins, 1982) and man (Kormano and Suoranta, 1971). In rabbit, the artery encircles the testis more than once before giving off branches into testicular tissue, while in man, the artery does not surround the gland and it gives off many branches immediately either before reaching or when it is on the surface of the testis. It seems that the rabbit testis rotates more than one turn during the process of descending while in the tree shrew and man testes make only one turn and less than one turn, respectively. Fig. 1. SEM micrograph at low magnification of testicular vascular cast and of related organs in the tree shrew. Arrowhead, collecting vein; E, epididymis; PP, pampinifotm plexus; T, testis; arrow, testicular artery. Bar 5 250 µm. Fig. 2. SEM micrograph, at low magnification, of the testicular artery in the spermatic cord. Note the coiled artery (TA) is surrounded by pampiniform plexus (PP). Bar 5 250 µm. Fig. 3. SEM micrograph, at low magnification, of the coiling testicular artery (TA) in spermatic cord. Note the TA gives off branches to supply various components of the cord. EA, epididymal artery. Bar 5 250 µm. Fig. 4. SEM micrograph of the spermatic cord vascular cast showing the vasa vasorum (VV) deriving from branches of testicular artery to supply the wall of pampiniform plexus (PP). Note the small venule (arrow) draining the blood from VV into the vein of the pampiniform plexus. Bar 5 100 µm. TESTICULAR MICROVASCULARIZATION IN THE COMMON TREE SHREW TESTIS Fig. 5. SEM micrograph of the casts of major blood vessels supplying the tree shrew testis. Note the testicular artery (TA) encircling the testis after emerging from the pampiniform plexus (PP). The TA gives off branches (arrowheads) while curving along the lateral border of the gonad. Bar 5 500 µm. Fig. 6. SEM micrograph, at high magnification, of the testis vascular cast illustrating a small arteriole (a) branching to form 229 capillary loops supplying each seminiferous tubule. ST, space for seminiferous tubule. Bar 5 50 µm. Fig. 7. SEM micrograph at high magnification of the testicular vascular cast, intraglandular view, showing the capillaries (*) joining each other to form small venules (v) that surround the seminiferous tubule. Note the ovoid endothelial nuclear imprints (arrows) of the venule. ST, space for seminiferous tubule. Bar 5 50 µm. 230 W. PRADIDARCHEEP ET AL. Fig. 8. SEM micrograph of the upper portion of the testicular vascular cast, dorsal view, demonstrating the intratesticular small venules (arrowheads) draining the blood into larger venules (v) on the testicular surface. Bar 5 500 µm. Fig. 9. SEM micrograph, at low magnification, dorsal view, of the testicular vascular cast showing the venules on the testicular surface (arrowheads) joining each other to form the collecting veins (CoV) near the borders of the gland. Note the central vein (CV) emptying the blood from the medullary part of the testis. TA, testicular artery; PP, pampiniform plexus; **, vascular cast of caput epididymis. Bar 5 250 µm. Fig. 10. SEM micrograph, dorsal view, of upper medial portion of testis vascular cast demonstrating medial collecting vein (arrow), lateral collecting vein (arrowhead), and central vein (CV) joining to form pampiniform plexus (PP). Note some venules (v) drain the blood directly into the pampiniform plexus. Bar 5 250 µm. TESTICULAR MICROVASCULARIZATION IN THE COMMON TREE SHREW TESTIS Fig. 11. TEM micrograph of the common tree shrew testis illustrating the continuous type of capillary with thick basement membrane in the interstitial space. Pe, pericyte. Bar 5 1 µm. 231 Fig. 12. TEM micrograph of the interstitial space of the tree shrew testis containing blood vessels, Leydig cell (LY), and lymphatic vessel. S, seminiferous epithelium. Note the lymphocyte (L) in the lymphatic space (LS). Bar 5 5 µm. 232 W. PRADIDARCHEEP ET AL. With the corrosion cast technique combined with SEM, we clearly demonstrate that the tree shrew testicular artery becomes coiled and is surrounded by a pampiniform plexus while it is in the spermatic cord. Similar findings have been reported in the camel (Osman et al., 1979), the buffalo (Dhingra, 1979), the mouse and rabbit (Chubb and Desjardins, 1982), as well as in the rat (Ohtsuka, 1984). This special organization of the artery and the veins in the spermatic cord is generally considered to be related to thermoregulation, yielding the suitable environment for spermatogenesis (Dahl et al., 1959; Fawcett, 1986; Young, 1957). Furthermore, it may also contribute to the mechanism for maintaining high concentrations of testosterone intratesticularly, for the hormone is transferred from the pampiniform plexus to the testicular artery (Amann and Ganjam, 1976; Bayard et al., 1975; Dierschke et al., 1975; Free and Jaffe, 1975, 1978; Free et al., 1973; Ginther et al., 1974; Jacks and Setchell, 1973) by diffusion (Free et al., 1973; Free, 1977) or by passing through the arteriovenous connection (A-V shunt) in the cord (Godinho and Setchell, 1975; NoorhuizenStassen et al., 1985). On the basis of heat exchange and hormone transportation, the particular organization between the testicular artery and the pampiniform plexus is sometimes referred to as a ‘‘functional portal system’’ (Henderson and Daneil, 1978; Ohtani, 1981). It should be emphasized that this study is the first to elucidate that the coiling part of the testicular artery gives off small branches that divide into vasa vasorum supplying the wall of the pampiniform plexus. This finding is of interest because the vasa vasorum are usually found to supply the thick wall of the large vessels (Brook, 1977) whereas the wall of the pampiniform plexus in this species is somewhat thin. It is possible that these vasa vasorum play a role in controlling the blood flow rate and consequently the blood volume to the testis. Alternatively, they may be involved with another mechanism other than the known countercurrent mechanism in order to increase the blood temperature in the pampiniform plexus before it is drained back into the body. On the other hand, the pampiniform plexus could reduce the blood temperature before reaching the testis by absorbing the radiating heat from their encompassed artery to increase blood temperature, especially, in the distal portion of the pampiniform plexus. Temperature exchange may not only be by direct heat transfer between the testicular artery and the pampiniform plexus, but also between the vasa vasorum and the pampiniform plexus, given the abundance of the vasa vasorum in the distal portion of the pampiniform plexus. The organization of the capillaries in the testis has been described as a ‘‘rope-ladder-like’’ system by Muller (1957) and this kind of organization has been reported for the testes of several other species (Kormano, 1967; Setchell and Brooks, 1988; Suzuki, 1982; Weerasooiya and Yamamoto, 1985). The present study shows that the capillary architecture in the tree shrew testicular parenchyma is rather similar to that in man (Suzuki and Nagano, 1986; Takayama and Tomoyoshi, 1981) in that there is no rope-ladder-like organization enclosing the seminiferous tubule. Ultrastructurally, testicular capillaries are of the continuous nonfenestrated type. This finding is quite unique since the capillaries in other endocrine glands of the same animals including pineal (Chunhabundit and Somana, 1991), thyroid (Rattanachaikunsopon et al., 1991), pituitary (Sudwan et al., 1991), endocrine pancreas (Bamroongwong et al., 1992), and adrenal (Thongpila et al., 1997) are with fenestrations. The presence of continuous capillaries in this endocrine gland leads to the speculation that the androgens produced by the Leydig cells may pass directly through the capillary wall since the steroid hormones could easily propagate through the cell membrane (Daryl, 1993). By means of electron microscopy, Davidoff et al. (1990) and Ergun et al. (1994) have shown that the capillaries in the human testis are also found within the lamina propria of the seminiferous tubules. Such capillaries, called intramural capillaries, are not present in the tree shrew testis, as in rat and mouse. The seminiferous tubules in tree shrews themselves seem to be avascular structures. As in humans, the intramural capillaries make close contact to cells of the seminiferous tubules, and they have been assumed to associate with the paracrine regulation for spermatogenesis. In tree shrew testis, the lymphatic vessels are frequently observed in close relation to the walls of the seminiferous tubules and the clusters of the Leydig cells as has been reported in the guinea pig and chinchilla (Fawcett et al., 1969). Such lymphatics might play a role in the distribution of androgens within the testis as, although the lymph flow rate is very slow (Fawcett et al., 1969), the wall of the lymph vessels is very thin. The route of androgens from Leydig cells entering the systemic circulation has been reported as being via blood (Amann and Ganjam, 1976; Bayard et al., 1975; Free and Jaffe, 1975; Ginther et al., 1974). The concept of blood testis barrier (BTB) is as old as that of the blood brain barrier (BBB) and had been derived by the scientists who observed that when dyes were injected into animals, most tissues were stained but not the brain or testis (Bouffard, 1906; Goldmann, 1909). The BTB is believed to be constituted by tight junctions between Sertoli cells in the seminiferous tubules and possibly by myoid cells that encircle these tubules. Recently, Holash et al. (1993) using immunocytochemical and histochemical techniques, showed that the endothelium of testicular microvessels also contributes to the BTB. The present study also demonstrates that capillaries in the interstitium of the tree shrew testis are with a quite thick basement membrane and without fenestrations. Such a finding could lead to the speculation that the thick basement membrane might be another component of the BTB. In conclusion, the endothelium, the thick basement membrane of the capillaries, and the epithelial (Sertoli) components of BTB are ‘‘in series’’ and complement each other in achieving a stable milieu for spermatogenesis in this animal. SEM of microvascular corrosion casts has enabled us to study novel aspects of arterial and venous patterns of the tree shrew testis. It demonstrates that the main route of the testicular artery in this animal is similar to that of nonprimates (rat, mouse, rabbit), as it surrounds the gland before arteriolization, yet the architecture of intratesticular capillaries is similar to those of primates (man), as the rope-ladder-like pattern is not clearly apparent. This implies that the vascular organi- TESTICULAR MICROVASCULARIZATION IN THE COMMON TREE SHREW TESTIS zation of the tree shrew testis shares some characters of both nonprimates and primates. ACKNOWLEDGMENTS This study was supported by Senior Fellowships, National Science and Technology Development Agency (NSTDA) of Thailand. REFERENCES Amann, R.P., and Ganjam, V.K. (1976) Steroid production by the bovine testis and steroid transfer across the pampiniform plexus. Biol. Reprod., 15:695–703. Bamroongwong, S., Chunhabundit, P., Rattanachaikunsopon, P., and Somana, R. (1992) Pancreatic microcirculation in the common tree shrew (Tupaia glis) as revealed by electron microscopy of vascular corrosion cast. Acta Anat., 143:188–194. Bayard, F., Boulard, P.Y., Huc, A., and Pontonnier, F. (1975) Arteriovenous transfer testosterone in the spermatic cord of man. J. Clin. Endocrinol. Metabol., 40:345–346. Bouffard, G. (1906) Injection des couleurs de benzidine aux animau normaux. Ann. Immunol. (Paris), 20:539–546. Brook, W.H. (1977) Vasa vasorum of veins in dog and man. Angiology, 28:351–360. Christensen, G.C. (1964) The urogenital system and mammary glands. In: Anatomy of the Dog. Miller, Christensen and Evans, eds. Saunders, Philadelphia, pp. 741–806. Chubb, C., and Desjardins, C. (1982) Vasculature of the mouse, rat, and rabbit testis-epididymis. Am. J. Anat., 165:357–372. Chunhabundit, P., and Somana, R. (1988) Modification of plastic mixture for vascular cast to withstand electron beam at high magnification under SEM. In: Proceedings of the 4th Asia-Pacific Conference and Workshop on Electron Microscopy. V. Mangclaviraj, W. Bachorndhevakul, and P. Ingkaninun, eds. Chulalongkorn University, Printing House, Bangkok, pp. 411–412. Chunhabundit, P., and Somana, R. (1991) Scanning electron microscopy on microvascularization of the common tree shrew (Tupaia glis). J. Pineal Res., 10:59–64. Dahl, E.V., and Herrick, J.F. (1959) A vascular mechanism for maintaining testicular temperature by counter current exchange. Surg. Gynecol. Obstet., 108:697–704. Daryl, K.G. (1993) Hormones of the gonad. In: Harper’s Biochemistry. K.M. Robert, K.G. Daryl, A.M. Peter, and W.R. Victor, eds. Connecticut: Appleton and Lange, pp. 542–546. Davidoff, M.S., Breucker, H., Holstein, A.F., and Seidl, K. (1990) Cellular architecture of the lamina propria of human seminiferous tubules. Cell Tissue Res., 262:253–261. DeVore, I., and Eimerl, S. (1970) The Primates. New York, Time-Life Books. Dhingra, L.D. (1979) Angioarchitecture of the buffalo testis. Anat. Anz., 146:60–68. Dierschke, D.J., Walsh, S.W., Mapletoft, R.J., Robinson, J.A., and Ginther, O.J. (1975) Functional anatomy of the testicular vascular pedicle in the rhesus monkey: Evidence for a local testosterone concentrating mechanism. Proc. Soc. Exp. Biol. Med., 148:236–242. Ergun, S., Stingl, J., and Holstein, A.F. (1994) Segmental angioarchitecture of the testicular lobule in man. Andrologia, 26:143–150. Fawcett, D.W. (1986) Male reproductive system. In: A Textbook of Histology. Saunders, Philadelphia, pp. 796–848. Fawcett, D.W., Heidger, P.M., and Leak, L.V. (1969) Lymph vascular system of the interstitial tissue of the testis as revealed by electron microscopy. J. Reprod. Fertil., 19:109–119. Free, M.J. (1977) Blood supply to the testis and its role in local exchange and transport of hormones in the male reproductive tract. In: The Testis. A.D. Johnson, and W.R. Gomes, eds. Academic Press, New York, pp. 39–84. Free, M.J., and Jaffe, R.A. (1975) Dynamics of venous-arterial testosterone transfer in the pampiniform plexus of the rat. Endocrinology, 97:169–177. Free, M.J. and Jaffe, R.A. (1978) Target organs for testosterone transferred from vein to artery in the pampiniform plexus: the epididymis. Biol. Reprod., 18:639–642. Free, M.J., Jaffe, R.A., Jain, S.K. and Gomes, W.R. (1973) Testosterone concentrating mechanism in the reproductive organs of the male rat. Nature (New Biol)., 244:24–26. 233 Ginther, O.J., Mapletoft, R.J., Zimmerman, N., Meckley, P.E. and Nuti, L. (1974) Local increase in testosterone concentration in the testicular artery in rams. J. Animal Sci., 38:835–837. Glover, T.D. (1973) Aspects of sperma production in some East African mammals. J. Reprod. Fertil., 35:45–53. Goldmann, E.E. (1909) Die aussere und innere sekretion des gesunden and kranken Organismus im Lichte der ‘‘vitalen Farbung’’. Beitr. Klin. Chirug., 64:192–265. Godinho, H.P., and Setchell, B.P. (1975) Total and capillary blood flow through the testes of anaesthetized rams. J. Physiol., 251:19–20. Henderson, J.R., and Daniel, P.M. (1978) Portal circulations and their relation to countercurrent systems. Q.J. Exp. Physiol., 63:355–369. Holash, J.A., Harik, S.I., Perry, G., and Stewart, P.A. (1993) Barrier properties of testis microvessels. Proc. Natl. Acad. Sci. U.S.A., 90:11069–11073. Jacks, F., and Setchell, B.P. (1973) A technique for studying the transfer of substances from venous to arterial blood in the spermatic cord of wallabies and rams. J. Physiol. (Lond.), 233:17–18. Kormano, M. (1967) An angiographic study of the testicular vasculature in the postnatal rat. Z. Anat. Entwickl. Gesch., 126:138–153. Kormano, M., and Suoranta, H. (1971) Microvascularization of the adult human testis. Anat. Rec., 170:31–40. Muller, I. (1957) Kanalchen and Capillar architektonik des Rattenhodens. Z. Zellforsch., 45:522–537. Noordhuizen-Stassen, E.N., Charbon, G.A., de Jong, F.H., and Wensing, C.J.G. (1985) Functional arterio-venous anastomoses between the testicular artery and the pampiniform plexus in the spermatic cord of rams. J. Reprod. Fertil., 75:193–201. Ohtani, O. (1981) Microcirculation studies by the injection replica method with special reference to the portal circulation. In: Three Dimensional Microanatomy of Cells and Tissue Surfaces. D.J. Allen, P.M. Motta, L.J.A. Didio, eds. Elsevier North Holland, New York, pp. 51–70. Ohtsuka, A. (1984) Microvascular architecture of the pampiniform plexus-testicular system in the rat. Am. J. Anat., 169:285–293. Osman, D.I., Tingari, M.D., and Moniem, K.A. (1979) Vascular supply of the testis of the camel (Camelus dromedarius). Acta Anat., 104:16–22. Palley, L.S., Schlossman, S.F., and Letvin, N.L. (1984) Common tree shrew and primates share leukocyte membrane antigens. J. Med. Primatol., 13:67–71. Rattanachaikunsopon, P., Chunhabundit, P., Bamroongwong, S., and Somana, R. (1991) Microvasculture of the thyroid gland in the common tree shrew (Tupaia glis): Microvascular corrosion cast/ SEM. Acta Anat., 142:208–214. Setchell, B.P., and Brooks, D.E. (1988) Anatomy, vasculature, innervation, and fluids of the male reproductive tract. In: The Physiology of Reproduction, vol. 1. E. Knobil, and J. Neill, eds. New York, Raven Press, pp. 753–835. Short, R.V., Mann, I., and Hay, M.F. (1967) Male reproductive organs African elephant. J. Reprod. Fertil., 13:517–536. Sisson, S. (1969) The Anatomy of the Domestic Animals. Saunders, Philadelphia. Sudwan, P., Chunhabundit, P., Bamroongwong, S., Rattanachaikunsopon, P., and Somana, R. (1991) Hypophyseal angioarchitecture of common tree shrew (Tupaia glis) revealed by scanning electron microscopy study of vascular corrosion casts. Am. J. Anat., 192:263– 273. Suzuki, F. (1982) Microvasculature of the mouse testis and excurrent duct system. Am. J. Anat., 163:309–325. Suzuki, F., and Nagano, T. (1986) Microvasculature of the human testis and excurrent duct system. Cell Tissue Res., 243:79–89. Takayama, H., and Tomoyoshi, T. (1981) Microvascular architecture of rat and human testis. Invest. Urol., 18:341–344. Thongpila, S., Rojananeungnit, S., Chunhabundit, P., Cherdchu, C., Samritthong, A., and Somana, R. (1998) Adrenal microvascularization in the common tree shrew (Tupaia glis) as revealed by scanning electron microscopy of vascular casts. Acta Anat. (in press). Weerasooiya T.R., and Yamamoto, T. (1985) Three-dimensional organisation of the vasculature of the rat spermatic cord and testis. Cell Tissue Res., 241:317–323. Young, J.Z. (1957) The reproductive tract of mammals. In: The Life of Mammals. J.Z. Young, ed. Oxford: Pergamon Press, pp. 663–675.