CellTiss. Res. 178, 183
188 (1977)
Cell and Tissue Research s by Springer-Verlag 1977
Fine Structure of Seminiferous Tubules in Antarctic Seals* Akhouri A. Sinha, Albert W. Erickson, and Ulysses S. Seal Medical Research Service, Veterans Administration Hospital and Department of Genetics and Cell Biology, University of Minnesota, Minneapolis, Minnesota; College of Fisheries, University of Washington, Seattle, Washington : and Department of Biochemistry, University of Minnesota and Veterans Administration Hospital, Minneapolis, Minnesota, USA
Summary. The fine structure of seminiferous tubules from 5 crabeater, 2 leopard and 2 Ross seals showed that during the nonbreeding season the tubules were essentially similar in possessing spermatogenic and Sertoli cells. However, the tubules of leopard and Ross seals had more primary and secondary spermatocytes and spermatids than the crabeater seals. In general, the tubules were devoid of spermatozoa. The spermatids showed stages of maturation such as Golgi phase of acrosome formation, acrosomal cap formation and condensation of nuclei. Some spermatids degenerated in tubules. Both maturing and degenerating spermatids were closely associated with Sertoli cells. Junctional complexes with plaques of filaments were observed between Sertoli cells and the spermatogenic cells. Sertoli cells, irregular and polygonal, contained highly convoluted nuclei, strands of rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi complexes, small mitochondria, variable amounts of lipid droplets, lysosomes, lipofuscin granules and highly plicated plasma membranes. In brief, the spermatogenic activity had practically ceased in the testes and the animals probably secreted low levels of testosterone during the nonbreeding season. Key words: Seminiferous tubules - Sertoli cells - Spermatogenic cells Antarctic seals - Ultrastructure.
Although antarctic seals exist in great abundance, only a limited study has been made of their population dynamics and their reproduction (Bertram, 1940; Harrison et al., 1952; Mansfield, 1958; Smith, 1966; Oritsland, 1970 and Stifling, 1971). These studies have suggested that the crabeater seal breeds in October and the leopard and Ross seals in November. All species apparently have very limited breeding seasons. Previous morphological analyses of pinniped testis have been limited to light microscopy except for a recent paper on the ultrastrucSendo{]'print requests to." A.A. Sinha, Building 49, Room 207, V.A. Hospital,Minneapolis, Minnesota
55417 USA * This research was supported in part by National Science Foundation Grants G.U. 30270 and G.U. 29829X from the Office of Polar Program and by NIH Grant 5 R01 AM11-376
184
A.A. Sinha et al.
ture of Leydig cells (Sinha et al., 1977). In fact, there is no published description of testes in the Ross seal. In this paper comparative fine structure of seminiferous tubules of crabeater, leopard and Ross seals during the nonbreeding season has been described and compared with those of other mammals. Unfortunately, materials from breading animals was unavailable.
Materials and Methods Portions of testes from 5 adult crabeater seals (Lobodon carcinophagus), 2 adult leopard seals (Hvdrurga leptonyx), and 2 adult Ross seals (Ommatophaca rossi) were collected in A m u n d s e n and Bellinghausen seas during January and February. Specimens were fixed for 2 to 3 h in 3% glutaraldehyde in 0.1 M phosphate buffer at pH 7.3, washed in 0.1 M phosphate buffer, post-fixed in buffered 2% osmium tetroxide for I to 2 h, dehydrated with graded ethyl alcohol, embedded in Epon 812 and sectioned with an LKB Ultratome III. Thin sections were stained with uranyl acetate and lead citrate and examined with an R C A - E M U - 3 or -4 electron microscope. Thick sections were stained with methylene blue. Small pieces were also fixed in 10% phosphate buffered formalin, embedded in paraffin wax, sectioned at 6 8 la and stained with hematoxylin and eosin (H and E), and periodic acid Schiff (PAS). From each testis, the greatest diameter of 10 seminiferous tubules, and the epithelium was measured by an ocular micrometer. In addition, blood samples were collected before killing the animals and serum testosterone levels were measured by radioimmunoassay (McMillin et al., 1974; Sinha et al., 1977).
Results
The gross morphology of reproductive organs from all three species showed that the testes were inguinal, compressed dorso-laterally and positioned lateral to the femoral veins. The epididymis was closely attached to the testis and led into a slender ductus deferens which progressively passed anteriorly, medially, and then caudally before entering into the ejaculatory duct, much as in other pinnipeds (Harrison et al., 1952; Laws, 1956; Smith, 1966). Microscopically, testes of all three species were similarly lobulated. Each lobule contained 8-10 highly tortuous seminiferous tubules surrounded by a PAS positive basal lamina. Diameter of tubules was 100 155 lam and the epithelium height 45-75 gm. The tubules contained spermatogonia, primary spermatocytes and Sertoli cells, but in crabeater seals, unlike the leopard and Ross seals, secondary spermatocytes, spermatids and spermatozoa were usually absent (Fig. 1). Each spermatogonium had a large, round nucleus, having abundant granular, homogeneous chromatin, a large nucleolus with nucleolonema and pars amorpha, a few round mitochondria, occasional strands of rough endoplasmic reticulum (ER), abundant free ribosomes and a small Golgi complex (Figs. 2, 4). The primary spermatocytes had aggregates of chromosomes, occasional mitochondria, strands of rough and smooth ER, and some free ribosomes. Junctional complexes and associated plaques of filaments were observed between the plasma membranes of spermatocytes and Sertoli cells and between the spermatogonia and Sertoli cells, much as in other mammals (Fawcett, 1975) (Fig. 3). Occasional spermatids were attached to Sertoli cells, but the lumina of tubules were essentially azoospermic as evidenced by smears of the testis and epididymis. Sertoli
Fig. 1. The micrograph illustrates seminiferous tubules, spermatogenic cells and absence of spermatozoa in the tubule lumen. Ross seal 25. H and E. x 270 Fig. 2. A primary spermatocyte showing aggregates of chromosomes and nucleolus. Crabeater seal 20. x 5,700 Fig. 3. Detail of tight junction (arrow) between Sertoli cells, electron opaque plaques of filaments (F), and subjacent smooth endoplasmic reticulum (ER). x 97,350 Fig. 4. Micrograph showing spermatogonium (S), portions of Sertoli cells (SE), junctional complexes (JC), lipid droplet (LP), lipofuscin granules (LG), Golgi complex (G), and basal lamina (B). Crabeater seal 36. x 5,750
Fig. 5. A spermatid in the Golgi phase showing acrosomic granule (arrow), Golgi complex (G), thickened nuclear membranes, and tubule lumen (L). Figures 5 8 are from leopard seal 18. x 4~660 Fig. 6. A degenerating spermatid closely associated with Sertoli cell. Note an aggregate of lipofuscin granules in the Sertoli cell, condensed nuclear membrane (arrow) of the spermatid, its nucleus, and acrosomal cap (AC). x 8,025 Fig. 7. A portion of a late spermatid illustrating acrosomal cap (AC) and condensed nucleus. Note close association of the plasma membranes (arrow) of spermatid and the Sertoli cells, x 14,620 Fig. 8. Micrograph illustrates relationship of early (ST) and late spermatids with Sertoli cells. x 5,780
Testis of Antarctic Seals
187
cells, irregular and polygonal, possessed large, highly convoluted nuclei, many strands of rough and smooth ER, some free ribosomes, Golgi complexes, small rod-to round mitochondria, strands of microfilaments, variable amounts of lipid droplets, lysosomes, lipofuscin granules or residual bodies and highly plicated plasma membranes (Fig. 4). However, some differences in the stages of spermatogenesis were also observed. For example, the testis of leopard and Ross seals had more primary and secondary spermatocytes than the crabeater seals. The tubules showed spermatids in different stages of maturation such as the Golgi phase of acrosome formation, differentiation of the acrosome, acrosomal cap formation and condensation of the nuclei (Figs. 5 8). But occasional degenerating spermatids were associated with the Sertoli cells which possessed large lipid droplets, lysosomes, and residual bodies (Fig. 6). Serum testosterone levels were between 112 and 116 ng/100 ml in the crabeater seals, 80 ng/100 ml in the leopard seals, and 80 and 125 ng/100 ml in the Ross seals. Following analysis of the fine structure of the Leydig cells Sinha et al. (1977) concluded that the secretion of testosterone was low in the antarctic seals during the nonbreeding season.
Discussion
Based on the light microscopy of testis of Weddell seal (Leptonychotes weddelli Lesson), Smith (1966) showed that the male reproductive cycle was essentially similar to that of the common seal (Phoca vitulina) and grey seal (Halichoerus grypus) (Harrison et al., 1952), ringed (Phoca hispida) and bearded seals (Erignathus barbatus) (McLaren, 1958a, b), northern fur seal (Callorhinus ursinus) (Kenyon et al., 1954), and elephant seals (Mirounga leonina Linn) (Laws, 1956). He recognized four phases in the spermatogenic cycle; these were (1) initiation of spermatogenesis (August September), (2) active spermatogenesis (OctoberDecember), (3) epithelial degeneration (January March), and (4) resting phase (April-July). Seminiferous tubules studied here were comparable to the phase three of Weddell seal. However, unlike the Weddell seal, extensive degeneration, shedding of spermatogenic cells and formation of spermatocyte giant cells were not observed in the crabeater, leopard and Ross seals. The spermatogenic activity and testosterone levels in antarctic seals showed a decline in spermatogenic activity during the postbreeding season, much as in other mammals (Erickson et al., 1968; Joffre and Joffre, 1973; Short and Mann, 1966). Our analysis of testis suggested that spermatogonia, primary spermatocytes, spermatids and their maturation were essentially like other mammals (Roosen-Runge, 1969; Steinberger and Steinberger, 1975; Bryan and Wolosewick, 1973; Fawcett, 1975).
References Bertram, G.C.L. : The biology of the Weddell and crabeater seals, with a study of the comparative behavior of the Pinnipedia. Sci. Rep. G r a h a m Land Exped. 1934 1937. Br. Mus. Nat. Hist. Lond. 1. 1 139 (1940)
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A.A. Sinha et al.
Bryan, J.H.D., Wolosewick, J.J. : Spermatogenesis revisited. II. Ultrastructural studies of spermiogenesis in multinucleate spermatids of the mouse. Z. Zellforsch. 138, 155 169 (1973) Erickson, A.W., Mossman, H.W., Hensel, R.J., Troyer, W.A.: The breeding biology of the male brown bear (Ursus arctos). Zoologica Sci. Contri., N.Y. Zool. Soc. 53, 85 105 (1968) Fawcett, D.W. : UItrastructure and function of the Sertoli cell. In: Handbook of physiology, male reproductive system, Vol. 5, Sect. 7, Chap. 2, pp. 21 55 (eds. D.W. Hamilton and R.O. Greep). Washington, D.C.: American Physiological Society 1975 Harrison, R.J., Matthews, L.H., Roberts, J.M.: Reproduction in some Pinnipedia. Trans. Zool. Soc. Lond. 27, 437 540 (1952) Joffre, J., Joffre, M. : Seasonal changes in the testicular blood flow of seasonally breeding mammals: Dormouse, Glis glis, ferret, Mustella furo, and fox, Vulpes vulpes. J. Reprod. Fertil. 34, 227 233 (1973) Kenyon, K.W., Scheffer, U.B., Chapman, D.G.: A population study of the Alaska fur seal herd. Spec. Sci. Rep. U.S. Fish Wildl. Serv. 12, 1 77 (1954) Laws, R.M. : The elephant seal (Mirounga leonina Linn). 2. General, social and reproductive behavior. Sci. Rep. Falkld. Isl. Depend. Surv. 13, l 88 (1956) Mansfield, A.W. : The breeding behaviour and reproductive cycle of the Weddell seal: (Leptonychotes weddelli Lesson). Sci. Rep. Falkld. Isl. Depend. Surv. 18, 1 41 (1958) McLaren, J.A.: Some aspects of growth and reproduction of the bearded seal, Erignathus barbatus Erxleben. J. Fish. Res. Bd. Canada 15, 219 227 (1958a) McLaren, J.A. : The biology of ringed seal, Phoca hispida Schreber, in the eastern Canadian Arctic. Bull. Fish. Res. Bd. Canada 118, 1 97 (1958b) McMillin, J.M., Seal, U.S., Keenlyne, K.D., Erickson, A.W., Jones, J.E.: Annual testosterone rhythm in the adult white-tailed deer (Odocoileus virginianus borealis). Endocrinology 94, 1034-1040 (1974) Oritsland, T.: Sealing and seals research in the Southwest Antarctic pack ice, September-October 1964. In: Antarctic Ecology 1, 367 376 (ed. M.W. Holdgate). New York: Academic Press 1970. Roosen-Runge, E.C.: Comparative aspects of spermatogenesis. Biol. Reprod. 1, 24-39 (1969) Short, R.V., Mann, T. : The sexual cycle of a seasonally breeding mammal, the roebuck (Capreolus capreolus). J. Reprod. Fertil. 12, 337 351 (1966) Sinha, A.A., Erickson, A.W., Seal, U.S. : Fine structure of Leydig cells in crabeater, leopard and Ross seals. J. Reprod. Fertil. 49 (1977) Smith, M.S.R.: Studies on the Weddell seal in McMurdo Sound, Antarctica. Ph.D. Thesis, pp. 1 161, University of Canterbury (1966) Steinberger, E., Steinberger, A. : Spermatogenic function of the testis. In: Handbook of physiology, male reproductive system, Vol. 5, Sect. 7, Chap. 1, pp. 1-19 (eds. D.W. Hamilton and R.O. Greep). Washington, D.C. : American physiological Society 1975 Stirling, I. : Population dynamics of the Weddell seal (Leptonychotes weddelli) in McMurdo Sound, Antarctica, 1966 1968. Antarctic Res. 18, 141 161 (1971)
Received December 1, 1976
188 (1977)
Cell and Tissue Research s by Springer-Verlag 1977
Fine Structure of Seminiferous Tubules in Antarctic Seals* Akhouri A. Sinha, Albert W. Erickson, and Ulysses S. Seal Medical Research Service, Veterans Administration Hospital and Department of Genetics and Cell Biology, University of Minnesota, Minneapolis, Minnesota; College of Fisheries, University of Washington, Seattle, Washington : and Department of Biochemistry, University of Minnesota and Veterans Administration Hospital, Minneapolis, Minnesota, USA
Summary. The fine structure of seminiferous tubules from 5 crabeater, 2 leopard and 2 Ross seals showed that during the nonbreeding season the tubules were essentially similar in possessing spermatogenic and Sertoli cells. However, the tubules of leopard and Ross seals had more primary and secondary spermatocytes and spermatids than the crabeater seals. In general, the tubules were devoid of spermatozoa. The spermatids showed stages of maturation such as Golgi phase of acrosome formation, acrosomal cap formation and condensation of nuclei. Some spermatids degenerated in tubules. Both maturing and degenerating spermatids were closely associated with Sertoli cells. Junctional complexes with plaques of filaments were observed between Sertoli cells and the spermatogenic cells. Sertoli cells, irregular and polygonal, contained highly convoluted nuclei, strands of rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi complexes, small mitochondria, variable amounts of lipid droplets, lysosomes, lipofuscin granules and highly plicated plasma membranes. In brief, the spermatogenic activity had practically ceased in the testes and the animals probably secreted low levels of testosterone during the nonbreeding season. Key words: Seminiferous tubules - Sertoli cells - Spermatogenic cells Antarctic seals - Ultrastructure.
Although antarctic seals exist in great abundance, only a limited study has been made of their population dynamics and their reproduction (Bertram, 1940; Harrison et al., 1952; Mansfield, 1958; Smith, 1966; Oritsland, 1970 and Stifling, 1971). These studies have suggested that the crabeater seal breeds in October and the leopard and Ross seals in November. All species apparently have very limited breeding seasons. Previous morphological analyses of pinniped testis have been limited to light microscopy except for a recent paper on the ultrastrucSendo{]'print requests to." A.A. Sinha, Building 49, Room 207, V.A. Hospital,Minneapolis, Minnesota
55417 USA * This research was supported in part by National Science Foundation Grants G.U. 30270 and G.U. 29829X from the Office of Polar Program and by NIH Grant 5 R01 AM11-376
184
A.A. Sinha et al.
ture of Leydig cells (Sinha et al., 1977). In fact, there is no published description of testes in the Ross seal. In this paper comparative fine structure of seminiferous tubules of crabeater, leopard and Ross seals during the nonbreeding season has been described and compared with those of other mammals. Unfortunately, materials from breading animals was unavailable.
Materials and Methods Portions of testes from 5 adult crabeater seals (Lobodon carcinophagus), 2 adult leopard seals (Hvdrurga leptonyx), and 2 adult Ross seals (Ommatophaca rossi) were collected in A m u n d s e n and Bellinghausen seas during January and February. Specimens were fixed for 2 to 3 h in 3% glutaraldehyde in 0.1 M phosphate buffer at pH 7.3, washed in 0.1 M phosphate buffer, post-fixed in buffered 2% osmium tetroxide for I to 2 h, dehydrated with graded ethyl alcohol, embedded in Epon 812 and sectioned with an LKB Ultratome III. Thin sections were stained with uranyl acetate and lead citrate and examined with an R C A - E M U - 3 or -4 electron microscope. Thick sections were stained with methylene blue. Small pieces were also fixed in 10% phosphate buffered formalin, embedded in paraffin wax, sectioned at 6 8 la and stained with hematoxylin and eosin (H and E), and periodic acid Schiff (PAS). From each testis, the greatest diameter of 10 seminiferous tubules, and the epithelium was measured by an ocular micrometer. In addition, blood samples were collected before killing the animals and serum testosterone levels were measured by radioimmunoassay (McMillin et al., 1974; Sinha et al., 1977).
Results
The gross morphology of reproductive organs from all three species showed that the testes were inguinal, compressed dorso-laterally and positioned lateral to the femoral veins. The epididymis was closely attached to the testis and led into a slender ductus deferens which progressively passed anteriorly, medially, and then caudally before entering into the ejaculatory duct, much as in other pinnipeds (Harrison et al., 1952; Laws, 1956; Smith, 1966). Microscopically, testes of all three species were similarly lobulated. Each lobule contained 8-10 highly tortuous seminiferous tubules surrounded by a PAS positive basal lamina. Diameter of tubules was 100 155 lam and the epithelium height 45-75 gm. The tubules contained spermatogonia, primary spermatocytes and Sertoli cells, but in crabeater seals, unlike the leopard and Ross seals, secondary spermatocytes, spermatids and spermatozoa were usually absent (Fig. 1). Each spermatogonium had a large, round nucleus, having abundant granular, homogeneous chromatin, a large nucleolus with nucleolonema and pars amorpha, a few round mitochondria, occasional strands of rough endoplasmic reticulum (ER), abundant free ribosomes and a small Golgi complex (Figs. 2, 4). The primary spermatocytes had aggregates of chromosomes, occasional mitochondria, strands of rough and smooth ER, and some free ribosomes. Junctional complexes and associated plaques of filaments were observed between the plasma membranes of spermatocytes and Sertoli cells and between the spermatogonia and Sertoli cells, much as in other mammals (Fawcett, 1975) (Fig. 3). Occasional spermatids were attached to Sertoli cells, but the lumina of tubules were essentially azoospermic as evidenced by smears of the testis and epididymis. Sertoli
Fig. 1. The micrograph illustrates seminiferous tubules, spermatogenic cells and absence of spermatozoa in the tubule lumen. Ross seal 25. H and E. x 270 Fig. 2. A primary spermatocyte showing aggregates of chromosomes and nucleolus. Crabeater seal 20. x 5,700 Fig. 3. Detail of tight junction (arrow) between Sertoli cells, electron opaque plaques of filaments (F), and subjacent smooth endoplasmic reticulum (ER). x 97,350 Fig. 4. Micrograph showing spermatogonium (S), portions of Sertoli cells (SE), junctional complexes (JC), lipid droplet (LP), lipofuscin granules (LG), Golgi complex (G), and basal lamina (B). Crabeater seal 36. x 5,750
Fig. 5. A spermatid in the Golgi phase showing acrosomic granule (arrow), Golgi complex (G), thickened nuclear membranes, and tubule lumen (L). Figures 5 8 are from leopard seal 18. x 4~660 Fig. 6. A degenerating spermatid closely associated with Sertoli cell. Note an aggregate of lipofuscin granules in the Sertoli cell, condensed nuclear membrane (arrow) of the spermatid, its nucleus, and acrosomal cap (AC). x 8,025 Fig. 7. A portion of a late spermatid illustrating acrosomal cap (AC) and condensed nucleus. Note close association of the plasma membranes (arrow) of spermatid and the Sertoli cells, x 14,620 Fig. 8. Micrograph illustrates relationship of early (ST) and late spermatids with Sertoli cells. x 5,780
Testis of Antarctic Seals
187
cells, irregular and polygonal, possessed large, highly convoluted nuclei, many strands of rough and smooth ER, some free ribosomes, Golgi complexes, small rod-to round mitochondria, strands of microfilaments, variable amounts of lipid droplets, lysosomes, lipofuscin granules or residual bodies and highly plicated plasma membranes (Fig. 4). However, some differences in the stages of spermatogenesis were also observed. For example, the testis of leopard and Ross seals had more primary and secondary spermatocytes than the crabeater seals. The tubules showed spermatids in different stages of maturation such as the Golgi phase of acrosome formation, differentiation of the acrosome, acrosomal cap formation and condensation of the nuclei (Figs. 5 8). But occasional degenerating spermatids were associated with the Sertoli cells which possessed large lipid droplets, lysosomes, and residual bodies (Fig. 6). Serum testosterone levels were between 112 and 116 ng/100 ml in the crabeater seals, 80 ng/100 ml in the leopard seals, and 80 and 125 ng/100 ml in the Ross seals. Following analysis of the fine structure of the Leydig cells Sinha et al. (1977) concluded that the secretion of testosterone was low in the antarctic seals during the nonbreeding season.
Discussion
Based on the light microscopy of testis of Weddell seal (Leptonychotes weddelli Lesson), Smith (1966) showed that the male reproductive cycle was essentially similar to that of the common seal (Phoca vitulina) and grey seal (Halichoerus grypus) (Harrison et al., 1952), ringed (Phoca hispida) and bearded seals (Erignathus barbatus) (McLaren, 1958a, b), northern fur seal (Callorhinus ursinus) (Kenyon et al., 1954), and elephant seals (Mirounga leonina Linn) (Laws, 1956). He recognized four phases in the spermatogenic cycle; these were (1) initiation of spermatogenesis (August September), (2) active spermatogenesis (OctoberDecember), (3) epithelial degeneration (January March), and (4) resting phase (April-July). Seminiferous tubules studied here were comparable to the phase three of Weddell seal. However, unlike the Weddell seal, extensive degeneration, shedding of spermatogenic cells and formation of spermatocyte giant cells were not observed in the crabeater, leopard and Ross seals. The spermatogenic activity and testosterone levels in antarctic seals showed a decline in spermatogenic activity during the postbreeding season, much as in other mammals (Erickson et al., 1968; Joffre and Joffre, 1973; Short and Mann, 1966). Our analysis of testis suggested that spermatogonia, primary spermatocytes, spermatids and their maturation were essentially like other mammals (Roosen-Runge, 1969; Steinberger and Steinberger, 1975; Bryan and Wolosewick, 1973; Fawcett, 1975).
References Bertram, G.C.L. : The biology of the Weddell and crabeater seals, with a study of the comparative behavior of the Pinnipedia. Sci. Rep. G r a h a m Land Exped. 1934 1937. Br. Mus. Nat. Hist. Lond. 1. 1 139 (1940)
188
A.A. Sinha et al.
Bryan, J.H.D., Wolosewick, J.J. : Spermatogenesis revisited. II. Ultrastructural studies of spermiogenesis in multinucleate spermatids of the mouse. Z. Zellforsch. 138, 155 169 (1973) Erickson, A.W., Mossman, H.W., Hensel, R.J., Troyer, W.A.: The breeding biology of the male brown bear (Ursus arctos). Zoologica Sci. Contri., N.Y. Zool. Soc. 53, 85 105 (1968) Fawcett, D.W. : UItrastructure and function of the Sertoli cell. In: Handbook of physiology, male reproductive system, Vol. 5, Sect. 7, Chap. 2, pp. 21 55 (eds. D.W. Hamilton and R.O. Greep). Washington, D.C.: American Physiological Society 1975 Harrison, R.J., Matthews, L.H., Roberts, J.M.: Reproduction in some Pinnipedia. Trans. Zool. Soc. Lond. 27, 437 540 (1952) Joffre, J., Joffre, M. : Seasonal changes in the testicular blood flow of seasonally breeding mammals: Dormouse, Glis glis, ferret, Mustella furo, and fox, Vulpes vulpes. J. Reprod. Fertil. 34, 227 233 (1973) Kenyon, K.W., Scheffer, U.B., Chapman, D.G.: A population study of the Alaska fur seal herd. Spec. Sci. Rep. U.S. Fish Wildl. Serv. 12, 1 77 (1954) Laws, R.M. : The elephant seal (Mirounga leonina Linn). 2. General, social and reproductive behavior. Sci. Rep. Falkld. Isl. Depend. Surv. 13, l 88 (1956) Mansfield, A.W. : The breeding behaviour and reproductive cycle of the Weddell seal: (Leptonychotes weddelli Lesson). Sci. Rep. Falkld. Isl. Depend. Surv. 18, 1 41 (1958) McLaren, J.A.: Some aspects of growth and reproduction of the bearded seal, Erignathus barbatus Erxleben. J. Fish. Res. Bd. Canada 15, 219 227 (1958a) McLaren, J.A. : The biology of ringed seal, Phoca hispida Schreber, in the eastern Canadian Arctic. Bull. Fish. Res. Bd. Canada 118, 1 97 (1958b) McMillin, J.M., Seal, U.S., Keenlyne, K.D., Erickson, A.W., Jones, J.E.: Annual testosterone rhythm in the adult white-tailed deer (Odocoileus virginianus borealis). Endocrinology 94, 1034-1040 (1974) Oritsland, T.: Sealing and seals research in the Southwest Antarctic pack ice, September-October 1964. In: Antarctic Ecology 1, 367 376 (ed. M.W. Holdgate). New York: Academic Press 1970. Roosen-Runge, E.C.: Comparative aspects of spermatogenesis. Biol. Reprod. 1, 24-39 (1969) Short, R.V., Mann, T. : The sexual cycle of a seasonally breeding mammal, the roebuck (Capreolus capreolus). J. Reprod. Fertil. 12, 337 351 (1966) Sinha, A.A., Erickson, A.W., Seal, U.S. : Fine structure of Leydig cells in crabeater, leopard and Ross seals. J. Reprod. Fertil. 49 (1977) Smith, M.S.R.: Studies on the Weddell seal in McMurdo Sound, Antarctica. Ph.D. Thesis, pp. 1 161, University of Canterbury (1966) Steinberger, E., Steinberger, A. : Spermatogenic function of the testis. In: Handbook of physiology, male reproductive system, Vol. 5, Sect. 7, Chap. 1, pp. 1-19 (eds. D.W. Hamilton and R.O. Greep). Washington, D.C. : American physiological Society 1975 Stirling, I. : Population dynamics of the Weddell seal (Leptonychotes weddelli) in McMurdo Sound, Antarctica, 1966 1968. Antarctic Res. 18, 141 161 (1971)
Received December 1, 1976
