Cell to cell relationships in the seminiferous epithelium in the mouse embryo

The cell-cell junctions of mammalian testes: I. The adhering junctions of the seminiferous epithelium represent special differentiation structures

The seminiferous tubules and the excurrent ducts of the mammalian testis are physiologically separated from the mesenchymal tissues and the blood and lymph system by a special structural barrier to paracellular translocations of molecules and particles: the "blood-testis barrier", formed by junctions connecting Sertoli cells with each other and with spermatogonial cells. In combined biochemical as well as light and electron microscopical studies we systematically determine the molecules located in the adhering junctions of adult mammalian (human, bovine, porcine, murine, i.e., rat and mouse) testis. We show that the seminiferous epithelium does not contain desmosomes, or "desmosome-like" junctions, nor any of the desmosome-specific marker molecules and that the adhering junctions of tubules and ductules are fundamentally different. While the ductules contain classical epithelial cell layers with E-cadherin-based adherens junctions (AJs) and typical desmosomes, the Sertoli cells of the tubules lack desmosomes and "desmosome-like" junctions but are connected by morphologically different forms of AJs. These junctions are based on N-cadherin anchored in cytoplasmic plaques, which in some subforms appear thick and dense but in other subforms contain only scarce and loosely arranged plaque structures formed by α- and β-catenin, proteins p120, p0071 and plakoglobin, together with a member of the striatin family and also, in rodents, the proteins ZO-1 and myozap. These N-cadherin-based AJs also include two novel types of junctions: the "areae adhaerentes", i.e., variously-sized, often very large cell-cell contacts and small sieve-plate-like AJs perforated by cytoplasm-to-cytoplasm channels of 5-7 nm internal diameter ("cribelliform junctions"). We emphasize the unique character of this epithelium that totally lacks major epithelial marker molecules and structures such as keratin filaments and desmosomal elements as well as EpCAM- and PERP-containing junctions. We also discuss the nature, development and possible functions of these junctions.

Of mice and men (and rats): phthalate-induced fetal testis endocrine disruption is species-dependent

For over 15 years, reproductive toxicologists have explored the physiological outcomes and mechanism of fetal phthalate exposure to determine the risk posed to human male reproductive health. This review examines the fetal male reproductive system response to phthalate exposure across species including rat, mouse, and human, with emphasis on the testis. In the rat, in utero phthalate exposure causes male reproductive tract malformations, in large part, by targeting the testis and inhibiting fetal Leydig cell hormone production. Despite mouse phthalate pharmacokinetics being similar to the rat, inhibition of fetal Leydig cell hormone synthesis is not observed in the mouse. The species-specific differences in testicular response following in utero phthalate exposure and the discordant reaction of the rodent fetal testis when exposed to phthalates ex vivo versus in vivo have made determining risk to humans difficult, yet critically important. The recent use of fetal testis xenotransplants to study phthalate toxicity suggests that the human fetal testis responds like the mouse fetal testis; it appears refractory to phthalate-induced inhibition of testosterone production. Although this result is unfulfilling from the perspective of identifying environmental contributions to human reproductive maldevelopment, it has important implications for phthalate risk assessment.

The blood-testis barrier: the junctional permeability, the proteins and the lipids

The elucidation of how individual components of the Sertoli cell junctional complexes form and are dismantled to allow not only individual cells but whole syncytia of germinal cells to migrate from the basal to the lumenal compartment of the seminiferous epithelium without causing a permeability leak in the blood-testis barrier is amongst the most enigmatic yet, challenging and timely questions in testicular physiology. The intriguing key event in this process is how the barrier modulates its permeability during the periods of formation and dismantling of individual Sertoli cell junctions. The purpose of this review is therefore to first provide a reliable account on the normal formation, maintenance and dismantling process of the Sertoli cells junctions, then to assess the influence of the expression of their individual proteins, of the cytoskeleton associated with the junctions, and of the lipid content in the seminiferous tubules on the regulation of the their permeability barrier function. To help focus on the formation and dismantling of the Sertoli cell junctions, several considerations are based on data gleaned not only from rodents but from seasonal breeders as well because these animal models are characterized by exhaustive periods of junction assembly during development and the onset of the seasonal re-initiation of spermatogenesis as well as by an extensive junction dismantling period at the beginning of testicular regression, something unavailable in normal physiological conditions in continual breeders. Thus, the modulation of the permeability barrier function of the Sertoli cell junctions is analyzed in the physiological context of the blood-epidydimis barrier and in particular of the blood-testis barrier rather than in the context of a detailed account of the molecular composition and signalisation pathways of cell junctions. Moreover, the considerations discussed in this review are based on measurements performed on seminiferous tubule-enriched fractions gleaned at regular time intervals during development and the annual reproductive cycle.

Characterization of membrane rafts isolated from rat sertoli cell cultures: caveolin and flotillin-1 content

Membrane rafts from Sertoli cell cultures were isolated as detergent-insoluble glycosphingolipid-enriched (DIG) fractions on the basis of their enriched content of glycosphingolipids and cholesterol and the resulting insolubility in 1% Triton X-100 and their low buoyant density. Because lipid rafts have been implicated in numerous cell functions, including cell signaling and sites for actin/membrane attachment, studies were initiated to characterize Sertoli cell rafts. This study reports the distribution of the raft structural proteins, caveolin and flotillin-1, implicated in raft microdomain organization. Methods employed included the immunoblotting of cell lysates and detergent-insoluble glycosphingolipid-enriched (DIG) fractions, the immunofluorescent microscopy of peritubular myoid cell (PMC) cultures and cryostat-sectioned testis, and the immunohistochemical staining of paraffin-embedded sections following microwave antigen retrieval techniques. Sertoli cells and Sertoli DIG fractions were found to lack the common raft-associated protein, caveolin, a marker protein for caveolae, but they are enriched in the 48-kd protein, flotillin-1, a protein also implicated in raft formation, cell signaling, and cell motility. Since the primary cell contaminant of Sertoli cell cultures is the PMC, these cells, along with spermatogenic cell fraction (SPGC), were also examined for caveolin and flotillin-1 content. The PMCs contained significant concentrations of both caveolin and flotillin-1. PMCs in culture exhibited a punctate caveolin staining pattern at the cell surface characteristic of a caveolar location. These data support the idea that the pinocytotic vesicles observed in PMCs are caveolae. PMCs also show a perinuclear location for caveolin characteristic of a Golgi location. Cryostat sections of rat testis showed a marked concentration of caveolin in the PMCs. The PMC location of caveolin was also confirmed by the immunohistochemical staining of sections from paraffin-embedded rat testis following microwave antigen retrieval techniques. Similar experiments showed a more ubiquitous, stage-specific distribution of flotillin-1 among testicular cell types.

Developmental regulation of connexin 43 expression in fetal mouse testicular cells

Multiple connexins have been identified in testicular cells. Several lines of evidences indicate that, among them, connexin 43 (Cx43) may be unique for control of gonad development and spermatogenesis. To date, however, it is not known whether Cx43 is expressed in the fetal testis and what possible types of cellular interactions mediated by this connexin are critical to male fertility. In the present work, expression of Cx43 was investigated at various developmental ages in cryosections from mouse testis by using specific antibodies against Cx43. In serial or double-labeled sections, Cx43 localization was compared with immunocytochemical distribution of steroidogenic enzyme, 3beta-hydroxysteroid dehydrogenase (3betaHSD), Mullerian inhibitory hormone (MIH), and germinal nuclear cell antigen (GCNA1), which are specific markers, respectively, of interstitial Leydig, Sertoli, and germinal cells. Sections were analyzed by fluorescence microscopy. We found that Cx43 immunofluorescence (IF) was uniformly distributed in the undifferentiated gonad at 11.5 days post coitus (dpc) and in cells of the mesonephric tubules. In the undifferentiated gonad, Cx43 was localized between primordial germ cells and somatic cells. At 12.5 dpc, when the gonad has undergone sexual differentiation, in the interstitium Cx43 was localized in Leydig cells and in the seminiferous cord it was localized between adjacent Sertoli cells. In Leydig and Sertoli cells, Cx43 labeling increased at 14.5, 16.5, and 18.5 dpc. From day 12.5 up to 18.5 dpc, Cx43 was also localized in cell borders between germinal and Sertoli cells. In conclusion, this study demonstrates that from the earliest stages of gonadal development, Cx43 is expressed in the principal cell types that participate in the control of male fertility. It also shows that Cx43 expression in Leydig and Sertoli cells increase during fetal life. Finally, it provides evidence that, throughout embryonic life, Cx43 forms gap junctions between Sertoli and germinal cells.

Differential distribution of the alpha 6 subunit of integrins in the development and sexual differentiation of the mouse testis

The distribution of the alpha 6 subunit of integrins in the development and sexual differentiation of mouse testis was analyzed by light and electron microscopy during the embryonic, fetal and early postnatal periods. At the pregonadal phase only the epithelial cells of the mesonephric duct and of the distal mesonephric tubules showed a reaction to alpha 6, whereas the surface epithelium and the mesenchyme of the mesonephros were negative or contained only a rudimentary amount of the alpha 6 subunit. With the formation of the gonadal ridge and the testicular blastema, the gonadal cells became positive for the alpha 6 subunit. This expression remained in embryonic cord cells and in the vascular endothelial cells, whereas the differentiating cells of the surface epithelium, tunica albuginea, the Leydig cells, and the interstitial mesenchymal cells were negative. With the fetal and postnatal differentiation, the expression of the alpha 6 subunit gradually diminished in the cord cells, and by the prepubertal phase, alpha 6 was found only at adhesion sites between some Sertoli cells. Similar changes were seen in the mesonephric duct and tubules, and in the rete cords. The presence of alpha 6 in regions undergoing developmental cell aggregation processes and their disappearance during tissue maturation, suggest that alpha 6 plays a specific but transient role in gonadal cell adhesion necessary for the histogenetic organization of the testis. In addition to its role in developing and organizing cells, alpha 6 integrin was also a prominent component in degenerating cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The blood-testis barrier and Sertoli cell junctions: structural considerations

In this review, a few well-established axioms have been challenged while others were viewed from a new perspective. The extensive literature on the blood-testis barrier has been scrutinized to help probe its mechanics and hopefully to promote understanding of the constant adaptation of the barrier function to germ cell development. Our principal conclusions are as follows: (1) Although the barrier zonule is topographically located at the base of the seminiferous epithelium it actually encircles the apex of the Sertoli cell. Consequently the long irregular processes specialized in holding and shaping the developing germ cells should be considered as apical appendages analogous to microvilli. (2) The development of the barrier zonule does not coincide with the appearance of a particular class of germ cells. (3) The barrier compartmentalizes the epithelium into only two cellular compartments: basal and lumenal. (4) Although the blood-testis barrier does sequester germ cells usually considered antigenic, immunoregulator factors other than the physical barrier seem to be involved in preventing autoimmune orchitis. (5) Structurally, a Sertoli cell junctional complex is composed of occluding, gap, close, and adhering junctions. The Sertoli cell membrane segments facing germ cells are presumably included in the continuum of the Sertoli cell junctional complex that extends all over the lateral and apical Sertoli cell membranes. (6) The modulation (i.e., formation and dismantling) of the junctions in a baso-apical direction is characteristic of the seminiferous epithelium and may be dictated by germ cell differentiation. The formation of tubulobulbar complexes and the following internalization of junction vesicles conceivably represent sequential steps of a single intricate junction elimination process that involves junction membrane segments from different cell types as part of a continual cell membrane recycling system. (7) The preferential association of junctional particles with one or the other fracture-face reflect a response to various stimuli including seasonal breeding. Changes in the affinity of the particles are generally coincidental with cytoskeletal changes. However, changes in the cytoskeleton are not necessarily accompanied by permeability changes. The number of strands seems to reflect neither the junctional permeability nor the transepithelial resistance. The diverse orientation of the strands seems to be related to the plasticity of the Sertoli cell occluding zonule. (8) Cooperation between all constituents (Sertoli cells, myoid cells, cell substratum, and germ cells) of the epithelium seems essential for the barrier zonule to function in synchrony with the germ cell differentiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Topography of the rat blood-testis barrier after intratubular administration of intercellular tracers

Intratubular injection of electron-opaque tracers (lanthanum hydroxide, peroxidase) by micromanipulation showed that the intercellular spaces of the adluminal compartment are in continuity with the lumen of the rat seminiferous tubule at all stages of the spermatogenic cycle. This continuity involves the intercellular spaces which surround zygotene spermatocytes and late leptotene spermatocytes in stages X-XI. The present observations would seem to cast doubt on the real existence of a third compartment, or intermediate compartment, in the seminiferous epithelium. In the material which, in addition to intratubular administration, had been treated with intravascular tracer perfusion, we found the presence of a parabasal region of interSertoli junctions where the tracer stopped whatever its direction. Freeze-fracture replicas exhibit interSertoli tight junctions arranged, from basal lamina to lumen, in three well-defined patterns: in the most basal area, which is the zone of free penetration of tracers from the peritubular environment, junctions are very sparse or absent; a belt of closely apposed continuous and parallel junctions, also parallel to the basal lamina, is the exclusion zone which prevents tracer penetration from either direction; the most apical membrane areas display irregularly arranged, loosely spaced and frequently interrupted rows of particles, most tending to be perpendicular to the basal lamina. This area corresponds to the territory where the intraluminally injected tracer permeates the interSertoli space.

A study of intercellular bridges during spermatogenesis in the rat

A morphological evaluation of intercellular bridges was undertaken during rat spermatogenesis. The dimensions and relationships of the bridges were shown to vary during different phases of spermatogenesis. Cellular divisions of spermatogonia and spermatocytes resulted in the partitioning of pre-existing bridges by complex structures termed bridge partitioning complexes, which are described in detail, as is the process whereby new bridges are formed. The structure of premeiotic bridges was generally consistent; however, during spermiogenesis, the structure of bridges and bridge contents were modified at specific phases of their development. The plasma membrane density associated with the cytoplasmic aspect of early step 1 spermatids separated into multiple dense bands that encircled the peripheral aspect of late step 1 spermatid bridges. By step 2 of spermiogenesis, these dense bands became associated with several cisternae of endoplasmic reticulum, which later coalesced into a single saccule that completely encircled the bridge structure by step 4. At steps 10-13 of spermiogenesis, the single saccule of endoplasmic reticulum vesiculated into many smaller cisternae. Also, filament-bounded densities (measuring 10-12 nm in diameter) appeared within the bridge channel. At step 17 of spermiogenesis, the filament-bounded densities were no longer apparent, but an anastomosing network of endoplasmic reticulum, often in the configuration of a sphere, occupied the entire central region of the bridge. In step 19 spermatids, the smooth endoplasmic reticulum within the bridge channel and the multiple cisternae lining the bridge density were gradually displaced. The subsurface density of bridges gradually lost its prominence. Some cytoplasmic lobes were connected by extremely narrow (approximately 22 nm) cytoplasmic channels. Similar-appearing channels were seen on the surface zone of cytoplasmic lobes or residual bodies, this observation suggesting that channels were sites of severence of bridges. Just prior to the separation or disengagement of the spermatid from the cytoplasmic lobe, selected bridges appeared to open to form large masses. After spermiation, residual bodies were not found joined by bridges; but from the size of some of the residual bodies, it was suspected that they were formed by coalescence of more than one cytoplasmic lobe. Freeze-fracture demonstrated few intramembranous particles on either the P or E face of the plasma membrane forming the bridge; this finding suggested bridge structures restricted free lateral movement of membrane constituents across the bridge.(ABSTRACT TRUNCATED AT 400 WORDS)

Gonadal development and fertility of mice treated prenatally with cadmium during the early organogenesis stages

Gonadal development was studied in mouse embryos that were exposed to cadmium during the early organogenesis stages. At 13.5 days, both the male and the female embryos had small genital ridges. Fewer primordial germ cells were found in the male embryos. In both sexes, many primordial germ cells were left outside the genital ridges, presumably as a result of retarded cell migration. In 16.5-day embryos, the size of the testes and ovaries and the number of differentiating germ cells were reduced. Many germ cells degenerated during the differentiation to spermatogonia and meiotic oocytes. The perturbed gonadal development was less likely to be caused primarily by a defective hypothalamopituitary axis but was more a part of the general cadmium-induced damage. The fertility of the male offspring was impaired by the prenatal cadmium insult, but the females were apparently fertile. The epididymal spermatozoa of the cadmium-affected offspring showed a lower fertilizing capacity in vitro. The impaired fertility of the cadmium-affected mice was the result of poor gonadal growth, paucity of germ cells, and defective maturation of the gametes.

Desmoplakins of epithelial and myocardial desmosomes are immunologically and biochemically related

Guinea pig antibodies against desmoplakins from bovine muzzle epidermis showed specific reaction in several epithelial tissues with desmoplakin I (Mr 250,000) and desmoplakin II (Mr 215,000). By immunofluorescence microscopy, prominent punctate staining was observed in various lines of cultured epithelial cells, revealing desmosomal junctions at sites of established cell-to-cell contacts as well as hemidesmosomes and internalized desmosome-derived membrane domains. On frozen tissue sections punctate staining was observed along plasma membranes of epithelial cells, and electron microscopy using the immunoperoxidase technique revealed that the antibodies were specifically localized at the plaques associated with desmosomes and hemidesmosomes. Of a large number of non-epithelial cells examined positive staining was only observed on desmosome-like junctions of myocardial cells and Purkinje fiber cells. In both epithelial and myocardial tissues the antibodies showed a broad range of cross-reactivity between diverse vertebrate species such as man, cow, rodent, and chicken, indicating that desmoplakins contain determinants strongly conserved during evolution. When binding of these antibodies to cytoskeletal polypeptides separated by gel electrophoresis and blotted on nitrocellulose paper sheets was examined, specific reaction was noted with desmoplakin I and, to a variable degree, also desmoplakin II from various epithelial cells. Reaction was also observed with a myocardial polypeptide from bovine and human hearts which had a similar Mr value (250,000) and isoelectric pH range as desmoplakin I. We conclude that desmoplakins are the major proteins present in the desmosomal plaques of both epithelial and myocardial cells and that the desmoplakin polypeptides present in these two different cell types are very similar, if not identical.

Spatial distribution of proteins specific for desmosomes and adhaerens junctions in epithelial cells demonstrated by double immunofluorescence microscopy

The spatial relationships between the protein constituents to two junctional structures, adhaerens junctions and desmosomes, were determined by double immunofluorescence microscopy using marker proteins specific for these structures. Adhaerens junctions were visualized by immunofluorescent labeling for the membrane-associated protein vinculin and by their association with actin filaments. Desmosomal components were identified by labeling with antibodies to a group of minor desmosomal plaque proteins (DP1 antigens) and their association with filaments stained by cytokeratin antibodies. Double immunofluorescence microscopy of these components was performed in several tissues and cultured cells, including intact intestine, dissociated intestinal cells, and two morphologically different types of epithelial cells, cultured bovine kidney (MDBK), and mammary gland (BMGE) epithelial cells. This allowed the direct demonstration that each filament system is associated exclusively with its specific membrane-bound junctional protein. Vinculin and DP1-protein were found in distinct sites in the subapical intercellular junctional complex of intestinal epithelium and MDBK cells. Cell-substrate focal contacts contained vinculin and actin and showed no apparent relationships to the tonofilament system whereas intercellular contacts of BMGE cells were characterized by positive staining for DP1-protein and associated cytokeratin filaments. Immunolabeling of the cultured cells at different intervals after plating for the cytoskeletal elements and their membrane anchorage proteins was used to determine the temporal sequence of their organization. We propose that this approach may be used for the molecular definition and identification of cellular contacts and junctions as well as for studies of junction topology, dynamics and junction-cytoskeleton interactions, and junction biogenesis.

Chemotherapy does not affect the development of inter-Sertoli junctions in childhood leukaemia

The inter-Sertoli junctions of children aged between 5 and 12 years, affected by acute lymphoblastic leukaemia, were analyzed in sections and freeze-fracture replicas. The testicular biopsies were performed at the end of therapy, when patients were in continuous remission for over 30 months. Chemotherapy does not seem to affect the development of junctions that were studied in sections and freeze fracture. Two age groups were considered (I, 5 to 8 years; II, 9 to 12 years). In age group I, oval Sertoli cells were connected by occasionally focal points of fusion, which in replicas appeared as scattered, interrupted ridges on the P face and grooves on the corresponding E face. In age group II Sertoli cells presented cytoplasmic extensions and interdigitations. Tight junctions appeared close to one another in conventional sections. Freeze fracture evidenced extensive although isolated areas formed by intervining strands. Lanthanum penetrated freely the intercellular spaces and gap junctions were observed in both age groups. The results suggest that tight junctions formation is initiated long before puberty; a progression in the complexity of the strand organization is present as the tubules mature; the strands reorganize in parallel and continuous rows only at puberty.

Antibodies to high molecular weight polypeptides of desmosomes: specific localization of a class of junctional proteins in cells and tissue

Desmosome-enriched fractions were isolated from bovine muzzle epidermis either as desmosome-tonofilament complexes using a procedure involving treatment at pH9 or in the form of desmosomal residue fractions using a modification of the citric acid buffer (pH 2.3) method of Skerrow and Matoltsy [1]. Major polypeptides of high molecular weights (mol. wt.) were separated by gel electrophoresis, individual polypeptide bands were excised, and protein was eluted and used for immunization. Guinea pig antibodies raised against two prominent polypeptides of high mol. wt. (250,000 and 215,000) showed, on nitrocellulose paper blots of desmosome-tonofilament polypeptides separated by gel electrophoresis, extensive cross-reaction between a group or large polypeptides characteristic of desmosome-containing fractions, most notably polypeptides of 250 K, 215 K, 200 k, 175 K, and 164 K. These antibodies allowed, when used in immunofluorescence microscopy, the specific localization of desmosomal junctions (i) in sections through epithelia-containing tissue (e.g., epidermis, mucosae of tongue and esophagus, cornea, mammary gland, small intestine, liver, thymus, urothelium of bladder) and myocardium; (ii) on dissociated cells from these tissues; (iii) on various epithelial cells grown in culture; an (iv) in tumor-like proliferations of cultured epithelial cells injected into nude mice. Individual desmosomes could be visualized and resolved at the light microscopic level. No reaction was found in cells devoid of desmosomes and on other classes of intercellular junctions. Electron microscopic localization using immunoperoxidase techniques indicated that these proteins are located in, or close to, the desmosomal plague structure. It is proposed to use such antibodies against desmosomal proteins as markers specific to this so far only morphologically define class of junctions. Use of these markers will (i) improve identification and classification of intercellular junctions; (ii) facilitate determinations of the specific patterns of distributions of desmosomes and desmosomal protein in various cells and tissue; (iii) allow studies of formation and disintegration of desmosomes, and of the biosynthesis and possible recycling of their constituents; and (iv) provide tissue group-specific markers valuable in histology and diagnosis, especially for identification of epithelial and carcinoma cells.

Permeability to lanthanum of blood testis barrier in human germinal aplasia

The permeability of Sertoli tight junctions to lanthanum administrated during fixation is demonstrated in biopsies of patients with partial germinal aplasia. In freeze-fracture replicas the number of fibrils is not significantly different from the data obtained in normal testis. Thus, in these pathological conditions junctional permeability is not related solely to the complexity of the network revealed by freeze-fracture.