Early stages of testicular differentiation in the rat

Summary The initial phases of the development of the seminiferous cords (future seminiferous tubules) were studied with histological techniques and with electron microscopy. On day 14 after fertilization, seminiferous cords are well differentiated in the anterior part of the testis near the mesonephric tubules. They comprise Sertoli cells which encompass the primordial germ cells. The Sertoli cells show an expanded clear cytoplasm and microfilaments beneath the outer surface; they differentiate complex contact zones. On day 13 a few cells localized near the mesonephric tubules display the characteristics of the Sertoli cells. These cells become more and more numerous. They aggregate and they form the seminiferous cords.The primordia of male gonads explanted in vitro on the mesonephros, realize testicular organogenesis in a synthetic medium. Adding 15% fetal calf serum to the medium prevents the morphogenesis of the testicular cords, although the Sertoli cells seem to differentiate morphologically and physiologically. In these gonads differentiation of the Sertoli cells was obtained but their aggregation and the morphogenesis of the seminiferous cords were prevented. This gives new insights into testicular morphogenesis and probably provides an experimental model for a new type of gonadal anomaly.     

鸡肾发生的组织学观察

采用组织学方法和电镜技术,对9个不同发育时期的鸡(Callus domestiaus)胚胎进行了观察.通过对鸡胚胎肾组织发生过程的观察,探讨鸡胚中肾的发生与退化,后肾的发生、分化规律和特点.结果表明,孵育到第16期在中肾前端附近出现一些中肾小泡.孵育到第18期形成中肾小管.孵育到第26期,中肾小管的盲端内陷,原始的肾小囊和肾血管球形成,中肾小管显著伸长并迂回曲折.孵育到第33～37期,体前后部中肾组织均已形成完整的肾单位.第37～46期体前部至后部的中肾组织依次退化.孵育到第26期从泄殖腔附近发出的输尿管芽向生后肾组织侵入生长,生后.肾组织产生许多生后肾小泡.第33期出现肾小囊和肾小管,肾小管伸长并发生折叠,出现集合小管、近端小管和远端小管的形态分化.第37～46期肾小体逐渐发育成熟,肾小管继续分化出现细段.鸡的中肾具有排泄功能.鸡后肾的发生与分化存在明显的时间差异.肾单位的分化中,同一胚龄肾组织内可存在不同发育阶段的肾小体,集合小管分化较早,诱导近端小管和远端小管分化,细段分化较迟.     

Differentiation and development of testis in the oviparous lizard, Calotes versicolor (Daud.)

The differentiation and development of the testis in the lizard Calotes versicolor was studied histologically and histoenzymatically from the day of oviposition (stage 27) to 2 months after hatching. The study reveals the appearance of the gonadal component as a genital ridge at stage 27. The first sign of testis differentiation is observed at stage 33, which displays a well-developed medulla consisting of seminiferous cords comprising Pre-Sertoli cells. The sex differentiation of the embryonic gonads occurs at stage 34. At this stage, seminiferous cords of the testis are prominent and extensive with many pre-Sertoli cells and few spermatogonia. The interstitial space consists of immature fibroblast-type Leydig cells. Pre-Sertoli cells of the seminiferous cords differentiate into Sertoli cells with a triangular nucleus becoming apparent around stages 36-37. The fibroblast-like Leydig cells differentiate into round matured Leydig cells at stage 40. Quantitative estimation of germ cells reveals that the number of germ cells increases in individual gonads, and in 5-day-old hatchling's, this number multiplies by manifold. Spermatogonia show reductional division in the testis of 1-day-old hatchlings.Histochemical localization of Delta5-3beta-HSDH and G-6-PDH activity appears in the seminiferous cords (medulla) of the testis after sexual differentiation (stage 36), indicating that the embryonic medulla is the site of steroidogenesis and not the cortex in C. versicolor. This study also suggests that morphological differentiation of the gonad precedes detectable steroidogenesis in this species. In 10-day-old hatchling's, Delta5-3beta-HSDH activity is seen in the interstitial cells of the testis, which, however, is not detected in the seminiferous tubules. The intensity of the enzyme activity remains more or less the same in the testis up to 10 days after hatching and begins to increase thereafter. The increase in steroidogenesis parallels the progressive post-hatching increase of the interstitial/Leydig cells.     

Gonadal dysgenesis induced by prenatal exposure to ethinyl estradiol in mice

Daily oral administration of ethinyl estradiol (0.02, 0.2, or 2.0 mg/kg of body weight) to pregnant Jc1:ICR mice resulted in ovotestis and intra-abdominal testis with persistent Müllerian duct and Wolffian duct in male fetuses and ovarian hypoplasia in female fetuses when it was given from day 11 through day 17 of gestation (before gonadal differentiation in the fetus). The ovotestis consisted of testicular and ovarian portions. In the testicular portion, a few solid seminiferous tubules containing spermatogonia, some with pachytene nuclei with Sertoli cells and compact interstitial tissue including Leydig cells, were seen. In the ovarian portion, pachytene nuclei were seen. The intra-abdominal testis was smaller and contained more spermatogonia per tubule in cross section than the control testis. These findings suggest that in male fetuses ethinyl estradiol affects Sertoli cell differentiation resulting in suppression of Müllerian inhibiting factor. On the other hand, in the ovarian hypoplasia, the primordial follicles and follicular cells in a primordial follicle were significantly decreased in number, and the number of the degenerated primordial follicles was significantly increased. It seems likely that ethinyl estradiol affects the intimate contact between follicular cells and oocytes to cause degeneration of primordial follicles.     

Mechanisms of gonadal morphogenesis are not conserved between chick and mouse

To understand mechanisms of sex determination, it is important to know the lineage relationships of cells comprising the gonads. For example, in mice, the Y-linked gene Sry triggers differentiation of Sertoli cells from a cell population originating in the coelomic epithelium overlying the nascent gonad that also gives rise to uncharacterised interstitial cells. In contrast, little is known about origins of somatic cell types in the chick testis, where there is no Sry gene and sex determination depends on a ZZ male/ZW female mechanism. To investigate this, we performed fate mapping experiments in ovo, labelling at indifferent stages the coelomic epithelium by electroporation with a lacZ reporter gene and the underlying nephrogenous (or mesonephric) mesenchyme with chemical dyes. After sex differentiation, LacZ-positive cells were exclusively outside testis cords and were 3betaHSD-negative, indicating that the coelomic epithelium contributes only to non-steroidogenic interstitial cells. However, we detected dye-labelled cells both inside and outside the cords. The former were AMH-positive while some of the latter were 3betaHSD-positive, showing that nephrogenous mesenchyme contributes to both Sertoli cells and steroidogenic cells. This is the first demonstration via lineage analysis that steroidogenic cells originate from nephrogenous mesenchyme, but the revelation that Sertoli cells have different origins between chick and mouse suggests that, during evolution, mechanisms of gonad morphogenesis may diverge alongside those of sex determination.     

Characterization of Mesonephric Cells That Migrate into the XY Gonad during Testis Differentiation

In mouse fetal gonads, sex differentiation begins at 10.5-11.5 days postcoitum (dpc). With XY gonads of 12.5 dpc, cord-like structures are visible and stromal cells migrate from adjacent mesonephros, unlike in XX gonads. However, the migrated mesonephric cells, except for the endothelial cells, have not been specifically identified because they have not expressed differentiation markers over the course of organ coculture in previous experiments. In this study, we have for the first time succeeded in isolating only the mesonephric cells that migrate into the XY gonad from the mesonephros with alive and then cultured these cells in vitro through the use of an organ coculture system using EGFP-transgenic mice and a FACS Vantage. The migrated and isolated cells were used for morphological and molecular characterization. The migrated mesonephric cells contained three cell forms; a sharp cell form, a round cell form, and a cluster-forming cell. The sharp cells have the characters of peritubular myoid cells. The round cells and cluster-forming cells have the potential to differentiate into Leydig cells, as some of them are 3beta-HSD-positive. In in vitro culture of migrated mesonephric cells, the cluster-forming cells proliferated well and then differentiated into round cells, suggesting that the cluster-forming cells may be stem or precursor cells for the round cells. Thus, our findings provide important information related to the migration and differentiation of migrated mesonephric cells in the XY gonad.     

A microarray analysis of the XX Wnt4 mutant gonad targeted at the identification of genes involved in testis vascular differentiation

One of the earliest morphological changes during testicular differentiation is the establishment of an XY specific vasculature. The testis vascular system is derived from mesonephric endothelial cells that migrate into the gonad. In the XX gonad, mesonephric cell migration and testis vascular development are inhibited by WNT4 signaling. In Wnt4 mutant XX gonads, endothelial cells migrate from the mesonephros and form a male-like coelomic vessel. Interestingly, this process occurs in the absence of other obvious features of testis differentiation, suggesting that Wnt4 specifically inhibits XY vascular development. Consequently, the XX Wnt4 mutant mice presented an opportunity to focus a gene expression screen on the processes of mesonephric cell migration and testicular vascular development. We compared differences in gene expression between XY Wnt4+/+ and XX Wnt4+/+ gonads and between XX Wnt4-/- and XX Wnt4+/+ gonads to identify sets of genes similarly upregulated in wildtype XY gonads and XX mutant gonads or upregulated in XX gonads as compared to XY gonads and XX mutant gonads. We show that several genes identified in the first set are expressed in vascular domains, and have predicted functions related to cell migration or vascular development. However, the expression patterns and known functions of other genes are not consistent with roles in these processes. This screen has identified candidates for regulation of sex specific vascular development, and has implicated a role for WNT4 signaling in the development of Sertoli and germ cell lineages not immediately obvious from previous phenotypic analyses.     

A microarray analysis of the XX Wnt4 mutant gonad targeted at the identification of genes involved in testis vascular differentiation

One of the earliest morphological changes during testicular differentiation is the establishment of an XY specific vasculature. The testis vascular system is derived from mesonephric endothelial cells that migrate into the gonad. In the XX gonad, mesonephric cell migration and testis vascular development are inhibited by WNT4 signaling. In Wnt4 mutant XX gonads, endothelial cells migrate from the mesonephros and form a male-like coelomic vessel. Interestingly, this process occurs in the absence of other obvious features of testis differentiation, suggesting that Wnt4 specifically inhibits XY vascular development. Consequently, the XX Wnt4 mutant mice presented an opportunity to focus a gene expression screen on the processes of mesonephric cell migration and testicular vascular development. We compared differences in gene expression between XY Wnt4+/+ and XX Wnt4+/+ gonads and between XX Wnt4+/+ and XX Wnt4+/+ gonads to identify sets of genes similarly upregulated in wildtype XY gonads and XX mutant gonads or upregulated in XX gonads as compared to XY gonads and XX mutant gonads. We show that several genes identified in the first set are expressed in vascular domains, and have predicted functions related to cell migration or vascular development. However, the expression patterns and known functions of other genes are not consistent with roles in these processes. This screen has identified candidates for regulation of sex specific vascular development, and has implicated a role for WNT4 signaling in the development of Sertoli and germ cell lineages not immediately obvious from previous phenotypic analyses.     

Gonadal development and sex differentiation in the cichlid fish Cichlasoma dimerus (Teleostei, Perciformes): a light- and electron-microscopic study

Although the overall pattern and timing of gonadal sex differentiation have been established in a considerable number of teleosts, the ultrastructure of early stages of gonadal development is not well documented. In this study, gonads from larval and juvenile stages of laboratory-reared Cichlasoma dimerus were examined at the light-microscopic and ultrastructural levels. This freshwater species adapts easily to captivity and spawns with high frequency during 8 months of the year, providing an appropriate model for developmental studies. Larvae and juveniles were kept at a water temperature of 26.5 +/- 1 degrees C and a 12:12 hour photoperiod. Gonadal development was documented from 14-100 days postfertilization, covering the period of histologically discernible sex differentiation. Gonadal tissue was processed according to standard techniques for light and electron microscopy. C. dimerus, a perciform teleost, is classified as a differentiated gonochorist, in which an indifferent gonad develops directly into a testis or ovary. On day 14, the gonadal primordium consists of a few germ cells surrounded by enveloping somatic cells. Ovarian differentiation precedes testicular differentiation, as usual in teleost fishes. The earliest signs of differentiation, detected from day 42 onward, include the onset of meiotic activity in newly formed oocytes, which is soon accompanied by increased oogonial mitotic proliferation and the somatic reorganization of the presumptive ovary. The ovarian cavity is completely formed by day 65. Numerous follicles containing perinucleolar oocytes are observed by day 100. In contrast, signs of morphological differentiation in the presumptive testis are not observed until day 72. By day 100, the unrestricted lobular organization of the testis is evident. The latest stage of spermatogenesis observed by this time of testicular development is spermatocyte II.     

Early Morphogenesis of Chick Gonad in the Absence of Mesonephros

The development of the gonads in male and female chick embryos with induced unilateral mesonephric agenesis was studied using grafting, histoenzymology, and electron microscopy. As in embryos with a mesonephros, proliferation of the coelomic epithelium and its interaction with mesenchymal cells to form the medullary cords take place in the amesonephric gonads. In a similar manner, gonadal sexual differentiation and the differentiation of steroidogenic tissue, detectable by the presence of Δ5-3β-hydroxysteroid dehydrogenase, do not appear to be affected by the absence of an organized mesonephros. However, the initiation of gonadal development, further growth, and the onset of meiosis observable in developing ovaries are retarded. This delay appears to be reversible, as was demonstrated by experiments in which ovaries from chicks with complete mesonephric agenesis were transplanted into the coelomic cavity of male and female 3 1/2-day-old embryos. Meiosis finally occurred in the oocytes of all ovaries, regardless of the sex of the host. Therefore, the presence of a differentiated mesonephros in chick embryos is not required for the establishment of an undifferentiated gonad and sexual differentiation, or for initiation of meiosis.     

Matrix metalloproteinases regulate mesonephric cell migration in developing XY gonads which correlates with the inhibition of tissue inhibitor of metalloproteinase-3 by Sry

In the mouse, the sex determining gene Sry, on the Y chromosome, controls testis differentiation during embryogenesis. Following Sry expression, indifferent XY gonads increase their size relative to XX gonads and form cord-like structures with the adjacent mesonephros, providing XY gonad somatic cells. This mesonephric cell migration is known to depend on Sry, but the molecular mechanism of mesonephric cell migration remains unknown. In this study, it was shown that cells expressing Sry induced proliferation of mesonephric cells migrating into male gonads, and inhibited expression of the tissue inhibitor of metalloproteinases (TIMP)-3 gene, which is the endogenous inhibitor of matrix metalloproteinases (MMP). In addition, the mesonephric cell migration was blocked by a chemically synthesized inhibitor of MMP in a gonad/mesonephros organ co-culture system with enhanced green fluorescent protein transgenic embryos. The findings indicate that MMP may play a critical role in mesonephric cell migration, and the function of MMP may be regulated by a Sry-TIMP-3 cascade. These findings are an important clue for the elucidation of testicular formation in developing gonads.     

Two distinct origins for Leydig cell progenitors in the fetal testis

During the differentiation of the mammalian embryonic testis, two compartments are defined: the testis cords and the interstitium. The testis cords give rise to the adult seminiferous tubules, whereas steroidogenic Leydig cells and other less well characterized cell types differentiate in the interstitium (the space between testis cords). Although the process of testis cord formation is essential for male development, it is not entirely understood. It has been viewed as a Sertoli-cell driven process, but growing evidence suggests that interstitial cells play an essential role during testis formation. However, little is known about the origin of the interstitium or the molecular and cellular diversity within this early stromal compartment. To better understand the process of mammalian gonad differentiation, we have undertaken an analysis of developing interstitial/stromal cells in the early mouse testis and ovary. We have discovered molecular heterogeneity in the interstitium and have characterized new markers of distinct cell types in the gonad: MAFB, C-MAF, and VCAM1. Our results show that at least two distinct progenitor lineages give rise to the interstitial/stromal compartment of the gonad: the coelomic epithelium and specialized cells along the gonad–mesonephros border. We demonstrate that both these populations give rise to interstitial precursors that can differentiate into fetal Leydig cells. Our analysis also reveals that perivascular cells migrate into the gonad from the mesonephric border along with endothelial cells and that these vessel-associated cells likely represent an interstitial precursor lineage. This study highlights the cellular diversity of the interstitial cell population and suggests that complex cell–cell interactions among cells in the interstitium are involved in testis morphogenesis.     

In vitro studies of gonadal organogenesis in the presence and absence of H-Y antigen

Summary In a very strict sense, the primary (gonadal) sex of mammals is determined not so much by the presence or absence of the Y but the expression or nonexpression of the evolutionary extremely conserved plasma membrane H-Y antigen. The central somatic blastema of embryonic indifferent gonads contains one cell lineage characterized by the possession of S−F differentiation antigen that differentiates into testicular Sertoli cells in the presence of H-Y and into ovarian follicular (granulosa) cells in its absence. This cell lineage appears to play the most critical role in gonadal differentiation. Whether or not testicular Leydig cells and ovarian theca cells are similarly derived from the common cell lineage has not been determined. Nevertheless, if given H-Y antigen, presumptive theca-cell precursors of the fetal ovary acquire hCG (LH?)-receptors—the characteristic of fetal Leydig cells. Presented in the formal symposium on Sexual Differentiation in Vitro and in Vivo at the 29th Annual Meeting of the Tissue Culture Association, Denver, Colorado, June 4–8, 1978. This work was supported by Contract NO1-CB-33907, and Grants No. 1 RO1 AG00042 and No. 5 RO1 CA16952 from the National Institutes of Health.     

Male-specific cell migration into the developing gonad is a conserved process involving PDGF signalling

Male-specific migration of cells from the mesonephric kidney into the embryonic gonad is required for testis formation in the mouse. It is unknown, however, whether this process is specific to the mouse embryo or whether it is a fundamental characteristic of testis formation in other vertebrates. The signalling molecule/s underlying the process are also unclear. It has previously been speculated that male-specific cell migration might be limited to mammals. Here, we report that male-specific cell migration is conserved between mammals (mouse) and birds (quail-chicken) and that it involves proper PDGF signalling in both groups. Interspecific co-cultures of embryonic quail mesonephric kidneys together with embryonic chicken gonads showed that quail cells migrated specifically into male chicken gonads at the time of sexual differentiation. The migration process is therefore conserved in birds. Furthermore, this migration involves a conserved signalling pathway/s. When GFP-labelled embryonic mouse mesonephric kidneys were cultured together with embryonic chicken gonads, GFP+ mouse cells migrated specifically into male chicken gonads and not female gonads. The immigrating mouse cells contributed to the interstitial cell population of the developing chicken testis, with most cells expressing the endothelial cell marker, PECAM. The signalling molecule/s released from the embryonic male chicken gonad is therefore recognised by both embryonic quail and mouse mesonephric cells. A candidate signalling molecule mediating the male-specific cell migration is PDGF. We found that PDGF-A and PDGF receptor-alpha are both up-regulated male-specifically in embryonic chicken and mouse gonads. PDGF signalling involves the phosphotidylinositol 3-kinase (PIK3) pathway, an intracellular pathway proposed to be important for mesonephric cell migration in the mammalian gonad. We found that a component of this pathway, PI3KC2alpha, is expressed male-specifically in developing embryonic chicken gonads at the time of sexual differentiation. Treatment of organ cultures with the selective PDGF receptor signalling inhibitor, AG1296 (tyrphostin), blocked or impaired mesonephric cell migration in both the mammalian and avian systems. Taken together, these studies indicate that a key cellular event in gonadal sex differentiation is conserved among higher vertebrates, that it involves PDGF signalling, and that in mammals is an indirect effect of Sry expression.     

Gonadal morphogenesis and sex differentiation in cultured Ussuri catfish Tachysurus ussuriensis

The objective of this study was to investigate the optimal developmental time to perform sex reversal in Ussuri catfish Tachysurus ussuriensis, to develop monosex breeding in aquaculture. Systematic observations of gonadal sex differentiation of P. ussiriensis were conducted. The genital ridge formed at 9 days post fertilization (dpf) and germ cells begin to proliferate at 17 dpf. The ovarian cavity began forming on 21 dpf and completed by 25 dpf while presumptive testis remained quiescent. The primary oocytes were at the chromatin nucleolus stage by 30 dpf, the peri‐nucleolus stage by 44 dpf and the cortical alveoli stage by 64 dpf. The germinal vesicle migrated towards the animal pole (polarization) at 120 dpf. In presumptive testis, germ cells entered into mitosis and blood vessels appeared in the proximal gonad on 30 dpf. The efferent duct anlage appeared on 36 dpf and formation of seminal lobules with spermatogonia and lobules interstitium occurred at 120 dpf. Therefore, gonadal sex differentiation occurred earlier in females than in males, with the histological differentiation preceding cytologic differentiation in T. ussuriensis. This indicates that undifferentiated gonads directly differentiate into ovary or testis between 17 and 21 dpf and artificial induction of sexual reversal by oral steroid administration must be conducted before 17 dpf.     

Pathology of 46, XY pure gonadal dysgenesis: absence of testis differentiation associated with mutations in the testis-determining factor

Abstract. Individuals with 46, XY pure gonadal dysgenesis present with a completely female phenotype. These individuals develop bilateral streak gonads and have normal Müllerian structures. The apparent absence of testicular tissue in these individuals suggests a mutation in the initial steps of the male sex-determining pathway. A candidate gene for the primary signal in this pathway was recently cloned ( SRY ) which encodes a protein with a DNA-binding capacity. In a study of 14 XY females with pure gonadal dysgenesis harbouring SRY , we analysed the histology of the gonads and compared it to the presence or absence of mutations in the SRY open reading frame ( SRY -orf). The histological analysis revealed two distinct groups of streak gonads. In the first group, the gonad was composed of exclusively ovarian-like stroma, with sclero-hyaline nodules in some areas. No tubules were observed. The gonads in the second group were composed of undifferentiated stroma harbouring either tubules or a rete structure. This suggests that in the latter group some differentiation (towards testis formation) has occurred, whereas in the first group ovarian differentiation has been interrupted. Individuals with mutations in the SRY -orf were found to have streak gonads of the first group, whereas most of the remaining XY females without detectable mutation in the SRY -orf had streak gonads belonging to the second group.
On the basis of histology, it may be possible to distinguish between mutations in the sex-determining or sex-differentiation pathways. We suggest that SRY may play a role in rete testis formation. We also present arguments favouring the mesonephros as the origin of testicular somatic cells in humans.     

In vitro studies of gonadal organogenesis in the presence and absence of H-Y antigen.

In a very strict sense, the primary (gonadal) sex of mammals is determined not so much by the presence or absence of the Y but the expression or nonexpression of the evolutionary extremely conserved plasma membrane H-Y antigen. The central somatic blastema of embryonic indifferent gonads contains one cell lineage characterized by the possession of S-F differentiation antigen that differentiates into testicular Sertoli cells in the presence of H-Y and into ovarian follicular (granulosa) cells in its absence. This cell lineage appears to play the most critical role in gonadal differentiation. Whether or not testicular Leydig cells and ovarian theca cells are similarly derived from the common cell lineage has not been determined. Nevertheless, if given H-Y antigen, presumptive theca-cell precursors of the fetal ovary acquire hCG (LH?)-receptors-the characteristic of fetal Leydig cells.     

Morphometric studies on hamster testes in gonadally active and inactive states: light microscope findings

This study provides quantitative information on the testes of seasonally breeding golden hamsters during active and regressed states of gonadal activity. Seminiferous tubules occupied 92.5% of testis volume in adult gonadally active animals. Leydig cells constituted 1.4% of the testicular volume. The mean volume of an individual Leydig cell was 1092 microns 3, and each testis contained about 25.4 million Leydig cells. The volume of an average Sertoli cell nucleus during stage VII-VIII of the cycle was 502 microns 3. A gram of hamster testis during the active state of gonadal activity contained 44.5 million Sertoli cells, and the entire testis contained approximately 73.8 million Sertoli cells. Testes of the hamsters exposed to short photoperiods for 12-13 wk displayed a 90% reduction in testis volume that was associated with a decrease in the volume of seminiferous tubules (90.8% reduction), tubular lumena (98.8%), interstitium (72.7%), Leydig cell compartment (79.3%), individual Leydig cells (69.7%), Leydig cell nuclei (50.0%), blood vessels (85.5%), macrophages (68.9%), and Sertoli cell nuclei (34.1%). The diameter (61.1%) and the length (36.8%) of the seminiferous tubules were also decreased. Although the number of Leydig cells per testis was significantly lower (p less than 0.02) after short-photoperiod exposure, the number of Sertoli cells per testis remained unchanged. The individual Sertoli cell in gonadally active hamsters accommodated, on the average, 2.27 pre-leptotene spermatocytes, 2.46 pachytene spermatocytes, and 8.17 round spermatids; the corresponding numbers in the regressed testes were 0.96, 0.20, and 0.04, respectively. The striking differences in the testicular structure between the active and regressed states of gonadal activity follow photoperiod-induced changes in endocrine function and suggest that the golden hamster may be used as a model to study structure-function relationships in the testis.     

Ultrastructure of the early ovary and testis in pig embryos.

Pig embryos aged 26-27 days were used for an ultrastructural study of the early ovary and testis. Sex was identified by both chromosomal analysis and gonadal histology, with consistent results. The gonads occupied their original site in the medial coelomic angles in both sexes. The female gonad was composed of three tissues: the surface epithelium, the gonadal blastema and the mesenchyme. The gonadal structure was similar to that seen earlier at the age of 24 days. At 26 days the testis had distinctly differentiated into four tissues. The new components were the testicular cords and the interstitium, both derived from the gonadal blastema. The testicular cords resembled anastomosing sheets more than cords. The ultrastructure of the tissues and their cell types are described and compared to the previous indifferent stage at the age of 24 days. The cells of the surface epithelium, of the primitive cords, of the mesenchyme, and the primordial germ cells had an ultrastructure that was similar in both sexes. The sustentacular cells of the testicular cords resembled the primitive cord cells and the spermatogonia were similar to the primordial germ cells. No Leydig cells were present yet. The process of testicular differentiation is described on the basis of the present and a previous study, and a new hypothesis, based on the vascular organization, is presented.     

An in vitro model of gonad differentiation in the chicken. Estradiol-induced sex-inversion results in the occurrence of serological H-Y antigen

Dissociated cells from the gonads and mesonephros of 8-day-old chicken embryos were reorganized in rotation culture. The aggregates obtained from gonadal cells exhibited specific morphologic and histologic sex differences. In the presence of estradiol, aggregates from testicular cells showed characteristics similar to control ovarian aggregates, while in ovarian aggregates under estradiol treatment the female organization became more pronounced. Determination of serological H-Y antigen revealed that male aggregates of gonads and mesonephros were negative for H-Y and those of female embryos were positive for H-Y. Administration of estradiol did not change the H-Y findings in female aggregates. In contrast, in the male, gonadal cultures became H-Y positive while mesonephros cultures remained negative. It is assumed that estradiol induces the occurrence of H-Y antigen in the gonads.