Reaggregates of cells from rat testis resemble developing gonads

Effects of growth factors on testicular morphogenesis

Since the discovery of the sex-determining gene Sry in 1990, research effort has focused on the events downstream of its expression. A range of different experimental approaches including gene expression, knocking-out and knocking-in genes of interest, and cell and tissue culture techniques have been applied, highlighting the importance of growth factors at all stages of testicular morphogenesis. Migration of primordial germ cells and the mesonephric precursors of peritubular myoid cells and endothelial cells to the gonad is under growth factor control. Proliferation of both germ cells and somatic cells within the gonadal primordium is also controlled by cytokines as is the interaction of Sertoli cells (with each other and with the extracellular matrix) to form testicular cords. Several growth factors/growth factor families (e.g., platelet-derived growth factor, fibroblast growth factor family, TGFbeta family, and neurotrophins) have emerged as key players, exerting an influence at different time points and steps in organogenesis. Although most evidence has emerged in the mouse, comparative studies are important in elucidating the variety, potential, and evolution of control mechanisms.

Gonadal development in mammals at the cellular and molecular levels

In mammals, although sex is determined chromosomally, gonads in both sexes begin development as similar structures. Until recently it was widely held that female development constituted a "default" pathway of development, which would occur in the absence of a testis-determining gene. This master gene on the Y chromosome, SRY in the human and Sry in the mouse, is thought to act in a cell-autonomous fashion to determine that cells in the gonadal somatic population develop as pre-Sertoli cells. Triggering of somatic cell differentiation along the Sertoli cell pathway is therefore a key event; it was thought that further steps in gonadal differentiation would follow in a developmental cascade. In the absence of Sertoli cells, the lack of anti-Mullerian hormone would allow development of the female Mullerian duct and absence of Leydig cells would prevent maintenance of the Wolffian duct. Recent findings that female signals not only maintain the Mullerian duct and repress the Wolffian duct but also suppress the development of Leydig cells and maintain meiotic germ cells, together with the finding that an X-linked gene is required for ovarian development and must be silenced in the male, have shown that the female default pathway model is an oversimplification. Morphological steps in gonadal differentiation can be correlated with emerging evidence of molecular mechanisms; growth factors, cell adhesion, and signaling molecules interact together, often acting within short time windows via reciprocal control relationships.

Transforming growth factor-beta2 mediates mesenchymal-epithelial interactions of testicular somatic cells

Transforming growth factor-beta2 (TGFbeta2) is an important mediator of growth and differentiation. We here describe for the first time the complete sequence of the TGFbeta2 complementary DNA derived from peritubular myoid cells of the rat testis. The size of the rat TGFbeta2 complementary DNA was 1245 bp, and the deduced protein sequence contained 414 amino acids. Sequence comparison with the human and mouse amino acid sequences demonstrated 96.4% and 97.9% sequence identities, respectively. To elucidate the functional role of TGFbeta2 in testicular somatic cells, we studied its secretion in vitro in monocultures and cocultures of mesenchymal peritubular and epithelial Sertoli cells. The highest amounts of TGFbeta2 protein were secreted in the cocultures and by peritubular cells, whereas Sertoli cells secreted only minor amounts. Stimulation experiments with FSH revealed a reduced secretion of TGFbeta2 in cocultures, probably mediated by a paracrine interaction of the FSH-responsive Sertoli cells. In contrast, TGFbeta2 secretion by peritubular cells was increased after stimulation with glucocorticoids and after addition of recombinant TGFbeta2, indicating an autoregulation of TGFbeta2. Furthermore, application of recombinant TGFbeta2 to cocultures resulted in an enhanced aggregation and cell clustering of Sertoli cells, pointing to an important role of TGFbeta2 in the paracrine interaction of peritubular and Sertoli cells of the developing rat testis.

Intercellular invasion and the organizational stability of tissues: a role for fibronectin

Intracellular invasion is the movement of cells of one type into the fabric of other, contiguous tissues. Invasion is a signature behavior of the malignant tumor and also is found as part of the normal behavior of inflammatory blood cells and tissues engaged in the morphogenetic movements of normal embryogenesis and in a number of instances of normal and pathological tissue remodeling in the adult. Informed by the view that the underlying mechanisms of invasion will be similar for tumor cells and invasive blood and embryonic cells, this review adopts a comparative approach to the analysis of invasion. Invasion results in the development of a diffuse interface between contiguous tissues. Its alternative is the maintenance of stable, planar tissue boundaries. This is the more usual condition for contiguous tissues in the animal. This review will focus on the processes that, on the one hand, stabilize planar contact interfaces between tissues, and, on the other, promote the destabilization of tissue integrity by fostering intercellular invasion. Particular attention is devoted to a role for adhesive interactions mediated by the matrix adhesion molecule, fibronectin. In certain instances, fibronectin in the matrix promotes invasion whereas in others, the presence of fibronectin prevents invasion. The distinction appears to depend on whether the invasive tissue is migrating into an acellular extracellular matrix or whether invasion involves densely cellular tissues. In the first instance, fibronectin promotes invasion, whereas in the second, it stabilizes the interface of the contacting tissues and prevents invasion.

Mesenchymal entactin-1 (nidogen-1) is required for adhesion of peritubular cells of the rat testis in vitro

Epithelial-like Sertoli cells isolated from immature rat testis aggregate to form tubule-like structures when cultured on a monolayer of mesenchyme-derived peritubular cells. At the end of this morphogenetic process both cell types are separated by a basement membrane. In this study the gene expression of monocultures and direct cocultures of peritubular cells and Sertoli cells was examined using DD-RT-PCR. One of the isolated cDNA clones showed high homology to the cDNA encoding the basement membrane component entactin-1 (nidogen-1). Even though the entactin-1 (nidogen-1) gene is transcribed in peritubular cells, Sertoli cells, and in direct cocultures, the mRNA is translated only by the peritubular cells. No entactin-1 (nidogen-1) was detected in the Sertoli cells by Western blotting. Moreover, peritubular cell monocultures and cocultures showed the presence of one single band at 152 kDa in the supernatant, whereas in cell lysates two bands were detectable at 152 kDa and 150 kDa. Perturbation experiments using monoclonal antibodies directed against entactin-1 (nidogen-1) were performed with peritubular cells and Sertoli cells, respectively, and demonstrated loss of cell adhesion of the peritubular cells, while the Sertoli cells remained adherent. From these data we conclude that entactin-1 is exclusively produced and secreted by mesenchymal peritubular cells, and affects adhesion of peritubular cells in an autocrine manner.

Mesonephric contribution to testis differentiation in the fetal mouse

Testes from 11.5-day-old mouse embryos, with and without attached mesonephroi, were cultured for 7 days. Isolated testes failed to develop well-differentiated testis cords: however, when cultured attached to a mesonephros from either a male or a female donor embryo, testes developed cords that were normal in appearance. Testes cultured next to a mesonephric region but separated from it by a permeable filter, did not develop normal cords, nor did testes grafted to fragments of embryonic limb or heart. When testes were grafted to mesonephric regions from mice carrying a transgenic marker, the marker was found in some of the peritubular myoid cells and other interstitial cells of the testis, but not in the Sertoli cells or the germ cells. We conclude that after 11.5 days post coitum, cells can migrate from the mesonephric region into the differentiating testis and can contribute to the interstitial cell population, and that this contribution is necessary for the establishment of normal cord structure. The germ cells in all cultured testes, whether or not differentiated cords were present, were T1 prospermatogonia: no meiotic germ cells were seen.

Somatic and germ cell interactions during histogenetic aggregation of mouse fetal testes

In the present study we examined the capacity of somatic and germ cells dissociated from fetal mouse testes at various stages to reform seminiferous cords in culture. We found that after 12 h in culture, seminiferous cords became segregated from stromal cells. Although Sertoli cells were incorporated into seminiferous cords at all stages studied, the germ cells dramatically changed their histogenetic behavior with age. Most germ cells which had been dissociated at 12.5 days postcoitum (dpc) were incorporated into the seminiferous cords, whereas at 14.5 dpc or later the majority remained among the stromal cells or as clusters on the surface of the aggregates. We considered three possible causes for this change in behavior of germ cells: (i) Failure to deposit some extracellular matrix components in the aggregates. (ii) Decrease in adhesiveness of prospermatogonia to either extracellular matrix components or Sertoli cells. (iii) A change in adhesiveness of Sertoli cells to germ cells with age. We found that laminin and fibronectin were similarly deposited in aggregates at 12.5 and 15.5 dpc. When prospermatogonia at 15.5 dpc labeled with colloidal gold were reaggregated with somatic cells at 12.5 dpc, 50% were incorporated into seminiferous cords. Moreover, [3H]thymidine-labeled Sertoli cells at 15.5 dpc formed heterochronic seminiferous cords with Sertoli cells at 12.5 dpc. These results suggest that mouse Sertoli cells change their surface property which is essential for binding to germ cells when they enter the mitotic resting stage (T-prospermatogonia).

Mouse gonadal differentiation in vitro in the presence of fetal calf serum

Other workers have shown that fetal calf serum (F.C.S.) inhibits the differentiation of the rat testis, and disrupts established testicular cords, in vitro. To investigate the possibility of a serum effect in the mouse, indifferent urogenital complexes and differentiated gonads were removed from sexed fetuses and cultured for 7 days in medium with or without the addition of fetal calf serum. Cultures were assessed by light and electron microscopy. Testicular and ovarian differentiation occurred in the presence of fetal calf serum. Serum did not prevent basal lamina development in testicular cords of explants cultured to the equivalent of day-17 control testes.

Cell sorting out: the self-assembly of tissues in vitro

The question posed by the science of analytical histology is how the properties and interactions of the components of the tissues determine their organization in the organs. The relevant components of the tissues are the cells and the extracellular matrix. The ability of cohering populations of cells to self-assemble structured tissues by cell sorting out offers an important opportunity for the experimental study of the mechanisms by which the cells and extracellular matrix interact to determine structure. The investigator can manipulate the initial organization and the cellular composition of the system and, in favorable situations, the composition of the extracellular matrix and the activities of candidate adhesive molecules. It can reasonably be expected that the recent progress in the characterization of the molecular species involved in cell-cell and cell-extracellular matrix interaction will allow the analysis of the molecular basis of tissue organization, with study of the self-assembly of tissue structure during sorting out playing an important role in this analysis. The importance of the differential adhesion hypothesis is its success in describing the rules by which macroscopic tissue structure is governed by the adhesive interactions of cell with cell and cell with extracellular matrix. The DAH describes how the physical forces of cell-cell and cell-matrix adhesion determine structure. Elucidation of the particular adhesive molecules involved in these interactions (e.g., the CAMs, junctional proteins, and matrix adhesion molecules) will yield an explanation at the biochemical level. A complete understanding of structure requires both levels of explanation.

An in vitro model of gonad differentiation in the chick embryo. Roller cultures in gas permeable biofoil bags

Embryonic gonads of 6 1/2 to 12 days old chick embryos were enzymatically dissociated. The cell suspensions were cultured in small gas permeable bags of foil (Biofolie Heraeus) in a roller culture apparatus. The cells formed multiple small aggregates, in which sex specific differences developed within two days. In cell suspensions of embryonic testes smooth spheric aggregates formed with well delineated testicular cords in the center and a tunica albuginea-like mesenchymal layer at the outside. Most of the male germ cells were incorporated in the central cords. A number of germ cells were barred from entering the cords by the tunica albuginea-like mesenchymal layer and populated the outer surface of the aggregates. The aggregates of left ovary were irregular in shape and characterized by clusters of germ cells residing in an outer cortical zone. The aggregates of the right ovary, which regresses in vivo, showed poor growth and did not differentiate, thus, indicating that the suppression of right ovary was not removed in culture. In the roller cultures of dissociated embryonic gonads male and female morphogenesis was mimicked in a reproducible manner, so that the system can be used for further experimental studies of gonadal development.