Role of spermatogonia in the stage-synchronization of the seminiferous epithelium in vitamin-A-deficient rats

After 20-day-old rats are placed on a vitamin-A-deficient diet (VAD) for a period of 10 weeks, the seminiferous tubules are found to contain only Sertoli cells and a small number of spermatogonia and spermatocytes. Retinol administration of VAD rats reinitiates spermatogenesis, but a stage-synchronization of the seminiferous epithelium throughout the testis of these rats is observed. In order to determine which cell type is responsible for this synchronization, the germ cell population has been analyzed in whole mounts of seminiferous tubules dissected from the testes of rats submitted to the following treatments. Twenty-day-old rats received a VAD diet for 10 weeks and then were divided into three groups of six rats. In group 1, all animals were sacrificed immediately; in group 2, the rats were injected once with retinol and sacrificed 3 hr later; in group 3, the rats were injected once with retinol, placed on a retinol-containing diet for 7 days and 3 hr, and then sacrificed. Three rats from each group had one testis injected with ³H-thymidine 3 hr (groups 1 and 2) or 7 days and 3 hr (group 3) before sacrifice. Three normal adult rats (approximately 100 days old) served as controls. Labeled and unlabeled germinal cells were mapped and scored in isolated seminiferous tubules. In group 1, type A₁ and type A₀ spermatogonia as well as some preleptotene spermatocytes were present; type A₂ A₃ A₄ In, and B spermatogonia were completely eliminated from the testis. Neither type A₁ mitotic figures nor ³H-thymidine-labeled-type A₁ nuclei were seen. Three hr after retinol injection (group 2), type A₁ mitoses, but no labeled type A₁ nuclei were observed. At 7 days and 3 hr after retinol administration (group 3), type A₄ and In Spermatogonia as well as type A₁ spermatogonia were present. A few residual pachytene spermatocytes were found, and some type A₀ cells were labeled. These results indicate that VAD caused, in addition to an impairment of spermatogenesis at the preleptotene spermatocyte step, a selective momentary arrest of surviving type A₁ spermatogonia at the G₂ phase of their cell cycle. Following administration of vitamin A to VAD rats, these type A₁ cells reinitiated spermatogenesis synchronously and, after several cycless of proliferation and renewai, reconstituted the seminiferous epithelium in a stage-synchronized manner.     

Spermatogonial stem cells in the albino rat

The existence of two classes of spermatogonial stem cells in the rat testis, i.e., reserve type A₀ spermatogonia and renewing, types A₁-A₄ spermatogonia, postulated by Clermont and Bustos-Obregon (′68), was reexamined in a quantitative analysis of type A spermatogonia in both whole mounts of tubules and in radioautographed sections of testes from animals killed at various times, up to 26 days, after one or multiple injections of ³H-thymidine. The cell counts obtained from whole mounts of tubules revealed that the number of isolated type A₀ cells per unit area of limiting membrane remained constant throughout the cycle of the seminiferous epithelium. Paired type A₀ spermatogonia also remained unchanged in number per unit area of basement membrane from stage I to stage VIII of the cycle. The low mitotic index of type A₀ spermatogonia (0.1%) indicated that these cells were not actively involved in the production of spermatogonia or spermatocytes during each cycle of the seminiferous epithelium and thus were considered as reserve stem cells. The type A₁ spermatogonia, which are formed during stage I of the cycle, remained resting until stage IX, when they undertook a series of four successive divisions resulting in the production of new type A₁ and Intermediate-type spermatogonia. An analysis of the labeling indices of type A spermatogonia obtained from cell counts in radioautographed testicular sections after a single or multiple ³H-thymidine injections indicated that the percentages of labeled type A cells corresponded to the percentages of type A₁-A₄ at each stage, whereas the percentages of unlabeled type A cells corresponded to the percentages of type A₀ spermatogonia obtained from counts of cells in whole mounts. This confirmed that type A₀ cells were generally non-proliferative throughout the cycle of the seminiferous epithelium while the type A₁-A₄ spermatogonia underwent complete renewal during each cycle. The present results thus support the concept of the existence of two classes of spermatogonial stem cells in rats.     

Prespermatogenesis and spermatogoniogenesis in the bovine testis

The bovine male germ cell population was studied over the entire period from testicular differentiation in the embryo through onset of spermatogenesis in the pubertal calf. Germ cells were identified by protein gene product 9.5 immunohistochemistry and characterized by their ultrastructure. The proliferation pattern of germ cells was studied with immunohistochemical anti-Ki 67 and anti-proliferating cell nuclear antigen reactions. Germ cells with a high proliferation rate are observed from day 50 p.c. to day 80 p.c. These cells are in transition from primordial germ cells to prespermatogonia. From day 80 p.c. until approximately the 15th postnatal week the germ cells present are identified as prespermatogonia. From day 80 p.c. to day 200 p.c. germ cell multiplication decreases continuously; then the prespermatogonia enter a phase of relative mitotical quiescence that lasts until the 4th postnatal week. Between the 4th and the 15th postnatal week, testicular tubular diameters grow from 40 to 80 μm and the prespermatogonia resume their proliferation. In seminiferous tubules with diameters between 80 and 120 μm, found in animals between 18 and 27 weeks of age, a central lumen is normally still absent. During this period germ cell proliferation reaches a second maximum. The cells involved represent the members of the spermatogonia stem and precursor cell line kinetically interpolated between the prespermatogonia and the first differentiating A-spermatogonia. This second phase of prepubertal germ cell multiplication coincides with the period when the pre-Sertoli cells transform into adult-type Sertoli cells and enter the G₀-phase for the rest of life. Accepted: 8 May 2000     

Re-examination of spermatogonial renewal in the rat by means of seminiferous tubules mounted “in toto”

Observations on dissected tubules, fixed in Carnoy, stained with hematoxylin and mounted “in toto” revealed that there were five distinct classes of type A spermatogonia. The type A₁ found in stages II–VIII of the cycle of the seminiferous epithelium had round, pale-stained nuclei, typically arranged in linear clusters of four or eight along the tubular wall. They all divided at stage IX to produce type A₂ cells. These in turn divided at stage XII to produce type A₃ spermatogonia. The type A₂ and A₃ cells had large ovoid nuclei containing globular masses of deeply stained chromatin and were randomly distributed in the space between Sertoli nuclei. The type A₃ spermatogonia divided at stage XIV to produce type A₄ cells. These had smaller nuclei, sometimes lobulated, containing more deeply stained chromatin granulation, free in the nucleus or adhering to the nuclear membrane. They divided in stage I of the cycle to yield two classes of spermatogonia: intermediate type and new type A₁. Hence, type A₁–type A₄ spermatogonia were considered as “renewing stem cells.” The fifth class of type A spermatogonia (A₀) was found at all stages of the cycle. Rare, isolated or in pairs, they had oval nuclei with deeply stained chromatin granulations. Seldom seen to divide, they did not appear to be actively involved in cell renewal and were tentatively considered as “reserve stem cells”.     

Proliferation and apoptosis of spermatogonia in postpuberal boar (Sus domesticus) testes with spontaneous unilateral and bilateral abdominal cryptorchidism

Cryptorchidism is a frequent male sexual disorder in mammals, which affects the histology of the tunica propria, interstitial tissue, blood vessels, seminiferous epithelium and testis functioning. In this paper, proliferation and apoptosis were examined in the seminiferous epithelium of both testes from unaffected boars and from boars suffering unilateral and bilateral cryptorchidism. In germ cells, proliferation was studied using the immunohistochemical PCNA technique, and apoptosis was analysed by in situ TUNEL labelling. An index was obtained for the proliferation and apoptosis observed in seminiferous tubules. In abdominal testes the epithelium contained few spermatogonia and Sertoli cells. In the testes of unaffected boars, numerous spermatogonia proliferated, whereas in cryptorchid testes such proliferation was lower and the proliferation/apoptosis ratio diminished. In the unaffected group, the TUNEL-positive germ cells were spermatogonia and spermatocytes in different phases of meiosis. In abdominal testes, the TUNEL-positive germ cells were spermatogonia alone. The apoptosis index of both abdominal and scrotal testes was similar. In conclusion, spontaneous cryptorchid testes showed a lower rate of spermatogonia proliferation in the seminiferous epithelium.     

The blood–testis barrier in the toad (Bufo arenarum hensel): A freeze-fracture and lanthanum tracer study

Intercellular junctions between Sertoli cells in the toad testis were studied by freeze-fracture and electron-opaque intercellular markers. These junctional specializations are characterized in thin sections by a series of focal fusions on the outer leaflets of both adjacent cell plasmalemmas, associated with bundles of fine filaments in the subjacent Sertoli cell cytoplasms. However, the wide subsurface cisterna of the endoplasmic reticulum, a component constantly associated with Sertoli cell junctions in mammals, is absent in the toad. The intravascularly injected lanthanum hydroxide, used as a tracer compound, gains access to the seminiferous tubules and surrounds spermatogonia and leptotene spermatocytes, but is persistently excluded from germ cells in later stages of development. This indicates that, as is the case in the mammalian testis, a permeability barrier to lanthanum is established which isolates all germ cells beyond leptotene spermatocytes. Freeze-fracture reveals the characteristic occluding junctions between Sertoli cells, but a variation in their geometric patterns was clearly observed in different regions of the toad seminiferous epithelium. The membrane-fractured faces of Sertoli cells embracing differentiating spermatids exhibit a deep junctional complex: up to 50 rows of particles between adjacent Sertoli cells separate these late germ cells from the periphery of the seminiferous tubules. Sertoli cells surrounding early germ cells generally exhibit, instead, a discontinuous, poorly developed network of interconnected rows of particles with few widely spaced strands. This seems to permit the percolation of the intercellular marker in areas of the seminiferous epithelium containing spermatogonia and leptotene spermatocytes.     

The blood-testis barrier in the toad (Bufo arenarum Hensel): a freeze-fracture and lanthanum tracer study

Intercellular junctions between Sertoli cells in the toad testis were studied by freeze-fracture and electron-opaque intercellular markers. These junctional specializations are characterized in thin sections by a series of focal fusions on the outer leaflets of both adjacent cell plasmalemmas, associated with bundles of fine filaments in the subjacent Sertoli cell cytoplasms. However, the wide subsurface cisterna of the endoplasmic reticulum, a component constantly associated with Sertoli cell junctions in mammals, is absent in the toad. The intravascularly injected lanthanum hydroxide, used as a tracer compound, gains access to the seminiferous tubules and surrounds spermatogonia and leptotene spermatocytes, but is persistently excluded from germ cells in later stages of development. This indicates that, as is the case in the mammalian testis, a permeability barrier to lanthanum is established which isolates all germ cells beyond leptotene spermatocytes. Freeze-fracture reveals the characteristic occluding junctions between Sertoli cells, but a variation in their geometric patterns was clearly observed in different regions of the toad seminiferous epithelium. The membrane-fractured faces of Sertoli cells embracing differentiating spermatids exhibit a deep junctional complex: up to 50 rows of particles between adjacent Sertoli cells separate these late germ cells from the periphery of the seminiferous tubules. Sertoli cells surrounding early germ cells generally exhibit, instead, a discontinuous, poorly developed network of interconnected rows of particles with few widely spaced strands. This seems to permit the percolation of the intercellular marker in areas of the seminiferous epithelium containing spermatogonia and leptotene spermatocytes.     

Seminiferous epithelium damage after short period of busulphan treatment in adult rats and vitamin B12 efficacy in the recovery of spermatogonial germ cells

Several different strategies have been adopted in attempt to recover from chemotherapy‐damaged spermatogenesis that is often seen in oncologic patients. In this study, we have evaluated the impact of short period of exposure to busulphan on the haemogram and seminiferous epithelium of adult rats, focusing on spermatogonial depletion and Sertoli cell (SC) integrity. We then examined whether vitamin B₁₂ supplementation improves the haematological parameters and spermatogonia number. The animals received 10 mg/kg of busulphan (BuG) or busulfan+vitamin B₁₂ (Bu/B₁₂G) on the first and fourth days of treatment. In H.E.‐stained testicular sections, the areas of the seminiferous tubule (ST) and seminiferous epithelium were measured. The number of spermatogonia in H.E‐stained and PCNA‐immunolabelled testicular sections was quantified. The frequency of tubules with abnormal SC nuclei or TUNEL‐positive SC was evaluated. Vimentin immunofluorescence in ST was also evaluated. In BuG and Bu/B₁₂G, the animals showed leukopenia and thrombocytopenia, but the body weight reduced only in BuG. The areas of ST and seminiferous epithelium decreased in Bu/B₁₂G and BuG. In BuG, the number of H.E.‐stained and PCNA‐immunolabelled spermatogonia reduced significantly. The frequency of tubules containing abnormal SC nuclei and TUNEL‐positive SC increased and the vimentin immunoexpression pattern changed. In Bu/B₁₂G, the number of H.E.‐stained or PCNA‐immunolabelled spermatogonia increased fourfold in comparison with BuG. The structural changes in ST after 6 days of busulphan exposure may be associated with the potential effect of this anti‐neoplastic agent on SC. The increased number of spermatogonia in the busulphan‐treated animals receiving vitamin B₁₂ indicates that this vitamin can be an adjuvant therapy to improve the fertility in male cancer patients.     

Proliferative activity of the developing seminiferous epithelium during prespermatogenesis in the golden hamster testis measured by bromodeoxyuridine labeling

Bromodeoxyuridine (BrdU)-labeling was used to study the cell kinetics of the developing seminiferous epithelium in the testes of golden hamsters aged 10.5 to 27.5 days post conception (dpc), i.e., during a period beginning one developmental day before testicular differentiation (11.5 dpc) and extending to the appearance of the first mature spermatogonia. Supporting (Sertoli) cells continuously proliferate throughout the period studied. Labeling indices amount to about 30% between the 10.5th and 16.5th dpc, and subsequently decrease to levels below 10% on the 26.5th and 27.5th dpc. Germ cells (prespermatogonia) proliferate between the 10.5th and 15.5th dpc and again, after a period of mitotic quiescence, from the 24.5th dpc onwards. This pattern of prespermatogonial proliferation substantiates and further specifies the successive appearance of M-prespermatogonia (10.5th to 15.5th dpc: proliferating), T ₁-prespermatogonia (16.5th to 23.5th dpc: quiescent), and T ₂-prespermatogonia (24.5th to 27.5th dpc: proliferating). Thus, the M-prespermatogonial phase of germ cell proliferation is shown to commence at least 24 h before testicular differentiation. Transitions from M- to T ₁-phase and from T ₁-to T ₂-phase are rather abrupt. Both the latter observation and the comparison with oogonial development in the female at the corresponding time (onset of meiosis) indicate the presence of an underlying control mechanism operative during prespermatogenic development. Due to different nuclear staining patterns, the BrdU-labeling method allows temporal subdivision of the S-phase, thus opening up prospects of more detailed cell-kinetic analyses of the seminiferous epithelium.     

Role of spermatogonia in the repair of the seminiferous epithelium following X-irradiation of the rat testis

In the normal adult rat testis, type A₀ spermatogonia do not appear to participate to a significant extent in the production of spermatocytes, while type A₁ spermatogonia periodically initiate a series of divisions resulting in the production of spermatocytes and new type A₁ spermatogonia. The behavior of type A₀ and A₁ spermatogonia was investigated following administration of a single dose of x-rays (300 r) to the testis. Using whole mounts of seminiferous tubules, the type A₀ and A₁ cells were counted at various intervals after irradiation. At 8 and 13 days after irradiation, type A₁ spermatogonia reached lowest values, i.e., 6% and 3% of non-irradiated control, while type A₀ reached the lowest value, i.e., 62% of control at eight days. Thereafter the numbers of type A₀ and A₁ progressively increased to return to normal at 39 days. It was thus concluded that the type A₀ were comparatively more resistant to x-irradiation than type A₁ spermatogonia. To verify if the surviving type A₀ proliferated in the irradiated testis, animals were injected with ³H-thymidine three hours before they were sacrificed at various times after x-irradiation. In irradiated testes the labeling indices of the surviving type A (A₀, A₁–A₄) were the same as in the non-irradiated testes except in stages V-VI of the cycle of the seminiferous epithelium. While in the controls only 2% of type A cells were labeled at these two stages of the cycle, after irradiation the labeling index of type A reached a maximum of 31% at 13 days to return to control values by 39 days. Since at 13 days after irradiation type A₀ spermatogonia were the predominant component of the spermatogonial population, it was concluded that these cells must have incorporated ³H-thymidine and thereby contributed to the reconstruction of the spermatogonial population partially destroyed by irradiation.     

The distribution pattern of cytokeratin and vimentin immunoreactivity in testicular biopsies of infertile men

Testicular biopsies of infertile patients are often characterized by a mixed atrophy, in which different types of spermatogenic lesions are found in adjacent tubules. In order to evaluate a possible involvement of the state of differentiation of the Sertoli cells, the distribution pattern of cytokeratin and vimentin intermediate filaments within the seminiferous epithelium of 228 biopsy specimens with normal spermatogenesis (n=10), mixed atrophy (n= 206) or Sertoli Cell Only Syndrome (n=12) were investigated by means of immunohistochemical techniques. Sertoli cells were regularly found to show vimentin expression in tubules with normal spermatogenesis as well as in tubules with any kind of spermatogenic impairment including SCO. Cytokeratin expression as a marker showing lack of differentiation was common in Sertoli cells of tubules with arrest of spermatogenesis at the level of spermatogonia, and was occasionally associated with arrest at the level of primary spermatocytes or with SCO. Ultrastructural examination of tubules with spermatogonial arrest revealed Sertoli cells with features of typical fetal or prepubertal Sertoli cells, such as round to ovoid nuclei without indentations, stacks of rough ER and spot desmosomes. These data suggest that spermatogenic arrest at the level of spermatogonia might be due to functional impairment of the associated Sertoli cells, which have maintained or regained an undifferentiated state and are not able to initiate or trigger the process of spermatogonial differentiation.     

Atypical development of Sertoli cells and impairment of spermatogenesis in the hypogonadal (hpg) mouse

Testes of hypogonadal (hpg) mice show arrested postnatal development due to congenital deficiencies of gonadotrophin-releasing hormone (GnRH) and gonadotrophin synthesis and secretion. Follicle-stimulating hormone (FSH), androgen or oestrogen treatment restore qualitatively normal spermatogenesis in hpg testes. Understanding the cellular and molecular changes accompanying hormone-induced spermatogenesis in hpg mice requires detailed morphological analyses of the germ cells and Sertoli cells in the untreated hpg testis. We compared seminiferous epithelial cytology in adult hpg, immature and adult wild-type mice using unbiased optical disector-based stereology, immunolocalization of Sertoli cell microtubules (MT), espin (a component of the blood-testis barrier), markers of Sertoli cell maturity (p27(kip1) and WT-1), and electron microscopy. Hpg testes had marked reductions in weight, seminiferous cord volume and length, and severe spermatogenic impairment with germ cells per testis < 1% of adult wild-type testes. Sertoli cell nuclei expressed WT-1 in hpg testes, but often were centrally located, similar to 9-14-day-old wild-type testes, and they expressed p27(kip1), indicating that hpg Sertoli cells were post-mitotic. Hpg testes had significantly (P < 0.05) reduced Sertoli cells per testis (0.56 million) compared with 10-day wild-type (1.15 million) and adult wild-type testes (2.06 million). Immunofluorescence labelling of normal adult Sertoli cells showed supranuclear MT columns and basally located espin, but these features were absent in 10-day-old and hpg Sertoli cells. Hpg Sertoli cells showed pleomorphic nuclear ultrastructure with mature-type nucleoli, similar to normal adult-type Sertoli cells, but hpg Sertoli cells exhibited incomplete tight junctions that lacked ectoplasmic specializations. We conclude that in hpg mice, chronic gonadotrophin insufficiency restrains Sertoli cell proliferation and maturation, forming pseudo-adult-type Sertoli cells that are incapable of supporting germ cell proliferation and maturation.     

Relative volume of sertoli cells in monkey seminiferous epithelium: A stereological analysis

Techniques of quantitative stereology have been utilized to determine the relative volume occupied by the Sertoli cells and germ cells in two particular stages (I and VII) of the cycle of the seminiferous epithelium. Sertoli cell volume ranged from 24% in stage I of the cycle to 32% in stage VII. Early germ cells occupied 3.4% in stage I (spermatogonia) and 8.7% in stage VII (spermatogonia and preleptotene spermatocytes). Pachytene spermatocytes occupied 15% (stage I) and 24% (stage VII) of the total volume of the seminiferous epithelium. In stage I the two generations of spermatids comprised 58% of the total epithelium by volume, whereas in stage VII, after spermiation, the acrosome phase spermatids occupied 35% of the total seminiferous epithelial volume.     

β_1整合素在生后不同年龄阶段小鼠睾丸生精细胞分布的研究

目的　观察 β1 整合素在出生后不同年龄阶段小鼠睾丸生精细胞中的免疫组化定位。方法　用免疫组织化学观察 β1 整合素在生后 1d、4d、7d、1 1d、2周、3周、4周、5周、6周和成年昆明小鼠睾丸生精细胞中的分布。结果　出生后 1～ 1 0d的小鼠睾丸生精小管上皮细胞未见 β1 整合素的表达 ;出生后 1 1d到成年小鼠的不同年龄阶段均可检测到 β1 整合素在睾丸生精小管上皮细胞中的表达。在生后 1 1d到 4周小鼠 ,β1整合素主要表达于增殖能力旺盛的精原细胞的胞膜 ,5周后 β1整合素主要表达于精原细胞和拉长期精子细胞头部细胞膜。结论　β1 整合素是生后 1 1d～ 4周小鼠精原细胞的表面标志     

Histological organization of roe deer testis throughout the seasonal cycle: variable and constant components of tubular and interstitial compartment

Seasonally regulated breeding in roe deer, Capreolus capreolus, is associated with significant changes in testis mass, structure and function. This study has quantified seasonal changes of morphometric parameters and cellular composition in roe deer testis parenchyma. Tissue samples were collected bimonthly during a complete annual cycle. Morphometric parameters of seminiferous tubules were measured and the number of different cell types was counted using a computer-aided image-analyzing system. A scheme of eight tubular epithelium stages for active spermatogenesis was devised according to the spermatid development. Stage I is characterized by the occurrence of new round spermatids, stage IV by spermiation and stage VIII by the meiotic division of spermatocytes. The average diameter of seminiferous tubules varied between 88.4±3.6 µm (February) and 216.8±9.2 µm (June). Also numbers of spermatogonia, spermatocytes and spermatids per tubule cross-section showed considerable seasonal changes. In December and February the germinative epithelium mainly consists of Sertoli cells and spermatogonia. In February, the first differentiated spermatogonia enter meiosis, and in April even spermatids occasionally occur, which reach their highest numbers during the rut in August. Both the expansion and the proportion of tubular and interstitial compartment change seasonally and result in differing cell densities. Assuming numerically constant populations of Sertoli cells and interstitial cells during the entire year, the hypothetical cell numbers per mm² of the tubular and interstitial areas were calculated for the seasonally variable total areas of tissue cross-sections. The concordance of these theoretical values with measured cell densities provided evidence that the total numbers of Sertoli cells, as well as interstitial cells, remain really constant throughout the seasonal cycle. The exact quantification of variable and constant components provides basic data for characterization of cell type and stage-specific processes of spermatogenesis.     

Stability of the intra-epithelial component of the blood-testis barrier in epinephrine-induced testicular degeneration in Syrian hamsters

Adult male Syrian hamsters were given daily intraperitoneal injections of epinephrine (1.0 mg/kg) and papaverine, a vasodilator, (60 mg/kg) for a period of ten days. After the treatment period, lanthanum and horseradish peroxidase tracer studies were used to examine the intra-epithelial component of the blood-testis barrier. Degenerating tubules often exhibited only Sertoli cells and spermatogonia, or Sertoli cells alone. Sertoli cell processes in the degenerating tubules often arched out from the main cell body to make contact with other Sertoli cell processes, forming a series of vacuole-like spaces in the germinal epithelium, adluminal to the Sertoli-Sertoli junctions. At the site of contact between these arching Sertoli cell processes one to eight tight junctions had formed, with hexagonal arrays of Sertoli cell cytoplasmic filaments located immediately adjacent to these junctions. Cisternae of the Sertoli cell endoplasmic reticulum lay deep to the layer of cytoplasmic filaments. It appeared that these junctions had originated after the expulsion of the germinal elements of the seminiferous epithelium. Penetration of the tracers in the degenerating seminiferous tubules was prevented by what appeared to be normal Sertoli-Sertoli junctions located between apposed Sertoli cells, adluminal to the remaining spermatogonia when these resisted degeneration, or just adluminal to the basal lamina in those tubules in which spermatogonia were absent.     

Ultrastructure of germ cells and sertoli cells in the postnatal rabbit testis

Spermatogenesis begins in the rabbit at seven to eight weeks of age. During the preceding postnatal period, seminiferous tubules contain round to oval prespermatogonia arranged in pairs at the tubular periphery and elongated Sertoli cells lying perpendicular to the basal lamina. Prespermatogonia are distinguished from fetal gonocytes by their abundant cytoplasm and the presence of intercellular bridges. Preceding the onset of spermatogenesis, prespermatogonia undergo diminution in cell size and cytoplasmic condensation. At the same time there is marked nucleolar enlargement, with elaborate proliferation of the nucleolonema. By the end of the seventh week, characteristic type A and B spermatogonia are present. Spermatocytes are seen at the end of the eighth week. Spermatids appear in the twelfth week, coincident with tubular lumen formation. The duration of the initial spermatogenic cycle is similar to that in the adult. Sertoli cells are closely associated with adjacent germinal elements throughout the postnatal period. The Sertoli cell cytoplasm includes elongated mitochondria, abundant smooth endoplasmic reticulum and prominent Golgi complexes. Many of the cells contain phagocytized material, presumably derived from degenerating germ cells. The observations suggest possible mechanisms by which Sertoli cells support early germ cell maturation.