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Myelin sheath formation in the peripheral nervous system and the ensuing saltatory conduction rely on differentiated Schwann cells. We have previously shown that transition of Schwann cells from an immature into a differentiated state requires Brg1 that serves as the central energy generating subunit in two related SWI/SNF-type chromatin remodelers, the BAF and the PBAF complex. Here we used conditional deletion of Pbrm1 to selectively interfere with the PBAF complex in Schwann cells. Despite efficient loss of Pbrm1 early during lineage progression, we failed to detect any substantial alterations in the number, proliferation or survival of immature Schwann cells as well as in their rate and timing of terminal differentiation. As a consequence, postnatal myelin formation in peripheral nerves appeared normal. There were no inflammatory alterations in the nerve or other signs of a peripheral neuropathy. We conclude from our study that Pbrm1 and very likely the PBAF complex are dispensable for proper Schwann cell development and that Schwann cell defects previously observed upon Brg1 deletion are mostly attributable to altered or absent function of the BAF complex. [ABSTRACT FROM AUTHOR]

The three SoxD proteins, Sox5, Sox6 and Sox13, represent closely related transcription factors with important roles during development. In the developing nervous system, SoxD proteins have so far been primarily studied in oligodendroglial cells and in interneurons of brain and spinal cord. In oligodendroglial cells, Sox5 and Sox6 jointly maintain the precursor state, interfere with terminal differentiation, and thereby ensure the proper timing of myelination in the central nervous system. Here we studied the role of SoxD proteins in Schwann cells, the functional counterpart of oligodendrocytes in the peripheral nervous system. We show that Schwann cells express Sox5 and Sox13 but not Sox6. Expression was transient and ceased with the onset of terminal differentiation. In mice with early Schwann cell-specific deletion of both Sox5 and Sox13, embryonic Schwann cell development was not substantially affected and progressed normally into the promyelinating stage. However, there was a mild and transient delay in the myelination of the peripheral nervous system of these mice. We therefore conclude that SoxD proteins—in stark contrast to their action in oligodendrocytes—promote differentiation and myelination in Schwann cells. [ABSTRACT FROM AUTHOR]

Early gonadal development was studied in the sterlet, Acipenser ruthenus, by means of histological and semithin serial sections and scanning electron microscopy. Special attention was given to the role of opisthonephric nephrostomial tubules and their coelomic funnels (nephrostomes, coelomostomes) in the origin of the gonad. Specimens of about 1 mm in length (about 7 days post hatching) have a continuous kidney complex (holonephros) that extends from the branchial region to the level of the cloaca and may be divided into a cranial pronephros and a caudally following opisthonephros, with no overlapping of either portion. In specimens of 10 to 25 mm in length the regression of pronephros and cranial opisthonephros can already been seen; as a consequence, these parts of the kidney complex are not involved in gonadogenesis. The initial gonadal anlage is seen in specimens of 30–40 mm in length. The somatic cells of the gonadal primordium develop from the medial lips of segmentally arranged opisthonephric nephrostomes situated in a line that extends from the level of the stomach through that of the spiral valve. The nephrostomes involved in this process belong to the first-order set of nephrons, since nephrons of higher order that arise continuously from blastema cells during further growth of the animals never send nephrostomial tubules to the coelomic surface. The cells of the medial nephrostomial lips proliferate by many mitoses. They grow over and surround the large germ cells that have accumulated on the medial side of the nephrostomes. The proliferating nephrostomial cells are elongated in shape, and their long axes are oriented in the cranio-caudal direction. By their size, shape and arrangement they replace not only the flat, polygonal, mesothelial cells of lateral plate origin on the medial side of the nephrostomes, but also those in the interstices in between. The result is the formation of a continuous gonadal crest situated medially from the nephrostomial line. At 100–130 mm in length, the gonadal crest has reached the stage of a gonadal fold that is attached to the dorsal body wall by a thin mesogonadium. Stroma cells and blood vessels start to invade the gonadal fold. At 240–290 mm in length, parts of the gonadal fold are converted into a fat body, a structure that is typically present in adult sturgeons of either sex. Nephrostomial tubules and their nephrostomes are visible only for a short period in Acipenser ruthenus. They are completely developed in specimens of 25–40 mm in length and start continuous retrogression immediately afterwards. Therefore, they cannot play an important role in excretion. Their only purpose apparently is to function as precursor tissue for other organs, such as the gonad, in establishing a route by which cells of the intermediate mesoderm can gain access to and spread out over the coelomic surface. In conclusion, the observations made in Acipenser prove that this species is a most suitable model to explain the origin of the gonad from opistho/mesonephric nephrostomial tubules in vertebrates with a less-developed or early regressing pronephros. Most mammals, including man, belong to this category. [ABSTRACT FROM AUTHOR]

The ultrastructure of the developing extratesticular rete testis, the efferent ductules and the establishment of the urogenital junction were studied in bovine embryos and fetuses of 41 through 95 days post conceptionem. The efferent ductules originate as a new set of secondary mesonephric tubules from the dorsal aspect of the nephric giant corpuscle and grow in the direction of the Wolffian duct. Cytological differentiation of the efferent ductules proceeds in a proximo – distal direction. At about 50–60 days, the simple columnar epithelium of the proximal portions of the efferent ductules already consists of the two typical cell types, i.e. reabsorptive principal cells with an endocytotic apparatus and a brush-border and ciliated cells. The lumen of the proximal portion is temporarily filled with intraductular blood vessels and perivascular tissue which may represent vestigial rudiments of glomeruli associated with the efferent ductules. At 50 to 60 days, the extratesticular rete still has a blastema – like appearance and consists of irregular cells with abundant glycogen. Extensions of the extratesticular rete come into contact with the efferent ductules and create the first end-to-side anastomoses with the latter. Somewhat later, the separating basal laminas vanish and invading rete cells intermingle with the epithelium of the efferent ductules, thus establishing the urogenital junction. [ABSTRACT FROM AUTHOR]

Nerve growth factor receptor (low-affinity form) was demonstrated immunohistochemically in bovine testis by using a monoclonal mouse anti-human antibody. In the 7-month-old fetus and in the early postnatal testis, the peritubular and intertubular fibroblast-like mesenchymal cells showed a strong reaction. Following differentiation of these cells into Leydig and myoid peritubular cells, the nerve growth factor receptor was no longer expressed. However, peritubular and intertubular testicular fibroblasts/fibrocytes, which are also derived from mesenchymal precursors, remained positive. Additionally, the nerve growth factor receptor was demonstrated in postnatal prespermatogonia, A-spermatogonia, I-spermatogonia and members of the spermatogonia precursor cell line; B-spermatogonia remained negative. In A-spermatogonia and I-spermatogonia, the expression of the nerve growth factor receptor was cell-cycle-dependent and was mostly observed during G-phase. Pre-embedding ultrahistochemistry with gold-conjugated antibody followed by silver-enhancement revealed that the nerve growth factor receptor was localized at the outer cell surface. The metal granules showed a regular distribution in positive spermatogonia. In testicular fibroblasts/fibrocytes the long narrow processes were preferentially decorated. [ABSTRACT FROM AUTHOR]

The spermatogonial stem cell line in prepubertal and adult bovine testis was studied by electron microscopy and protein gene product 9.5 immunohistochemistry. Three successive spermatogonia precursor cell configurations were observed. Small basal stem cells were found to possess a spherical shape and nuclei with two to three nucleoli. They were observed in prepubertal testes (25 and 30 weeks) and in low numbers during all the stages of the seminiferous epithelial cycle in the adult. Aggregated spermatogonia precursor cells are the dominating germ cell type in the 25-week-old and 30-week-old calf. In the adult seminiferous epithelium, they cause expansion of the basal tubular compartment as they form dense groups containing up to 15 cells. These groups are observed concomitantly with cycling A-spermatogonia and preleptotenes at the beginning of spermatocytogenesis. At the end of A-spermatogonia propagation, the aggregated spermatogonia precursor cells separate and intermingle with cycling A-spermatogonia. The spermatogonia precursor cells can later be found together with I-spermatogonia as members of an interconnected cellular network of medium-sized cells. When the I-spermatogonia divide to form the smaller B-spermatogonia, the precursor cells, which stay connected with the cycling spermatogonial population, pass through a growth phase. They can now be considered as committed spermatogonia precursor cells and are continuously being transformed into A-spermatogonia to start a new round of spermatocytogenesis. Ultrastructurally, all members of the precursor cell line are similar. However, a number of features have been found to show a quantitative increase (endoplasmic reticulum, mitochondria) or to exhibit a rising degree of complexity (nucleolus) during the progression from basal stem cells to committed spermatogonia precursor cells. [ABSTRACT FROM AUTHOR]

The configuration and distribution of bovine spermatogonia, preleptotene primary spermatocytes and Sertoli cells in the basal seminiferous tubular compartment have been studied by means of whole-mount preparations, immunohistochemistry and quantitative morphology. Three types of spermatogonia (Sg) can be identified. Large A-spermatogonia are irregularly distributed in the tubular periphery. Following the period of propagation of the A-spermatogonia, an interconnected meshwork of medium-sized spermatogonia with different cytogenetic potency is observed. Although the majority of the medium-sized spermatogonia are kinetically of the I type and divide to produce small B-spermatogonia, some members of the medium-sized population are seen in a growth phase and differentiate into large A-spermatogonia. These mark the beginning of a new round of spermatocytogenesis. Only one generation of B-spermatogonia divides into preleptotene primary spermatocytes. The architectural arrangement of multiplying spermatogonia in circles or rows is primarily the result of the distribution of the Sertoli cells. Spermatogonial multiplication is not strictly coordinated with the stages of the seminiferous epithelial cycle. Spermatogonial degeneration amounts on average to 3.6% and has therefore no decisive impact on the yield of primary spermatocytes. [ABSTRACT FROM AUTHOR]

The morphology of the bovine Sertoli cell was studied during 6 different phases of the spermatogenetic cycle. Tubular dimensions do not vary significantly during the phases. Sertoli cells occupy 27.0% (phase 4) to 38.4% (phase 8) of the tubular epithelium. Sertoli cells of phase 1 are approximately 20% larger than during the other phases. 30-35% of Sertoli cell volume consists of organelles. Mitochondrial (about 5.0%) and nuclear (about 5.7%) volume densities remain remarkably stable during the cycle, irrespective of changes in Sertoli cell size. Phagocytic capacity of bovine Sertoli cells is only moderate. Elimination of excess spermatid cytoplasm occurs to a large extent prior to spermiation. The majority of spermatid residual bodies undergoes autolytic decay while attached to the Sertoli cell apical surface. Aggregates of densely packed cisternae of the smooth endoplasmic reticulum (ER) located in a basal position and associated with the acrosome-phase and maturation-phase spermatids contribute between 14 and 17% to Sertoli cell volume. During phase 3 the ER pinches off a large number of small, smooth-walled vesicles filled with flocculent content. The contact area between Sertoli cells and other tubular constituents changes considerably during the different phases. It is concluded that the blood-testis barrier is particularly impassable during phases 1 and 8. A lipid cycle does not exist in the bovine testicular tubular epithelium. [ABSTRACT FROM AUTHOR]

The postnatal development of intertubular cells and vessels and of the tubular lamina propria was studied in three locations of perfusion-fixed bovine testes from 31 animals ranging from 4 to 78 weeks. The postnatal morphological differentiation of the testis is not uniform, regional differences have to be considered. The intertubular cell population is composed of mesenchyme-like cells, fibrocytes, Leydig cells, peritubular cells and mononuclear cells. In 4 and 8-week-old testes mesenchyme-like cells are the dominating element. These pluripotent cells proliferate by frequent mitoses and are the precursors of Leydig cells, contractile peritubular cells and fibrocytes. Morphologically differentiated Leydig cells are encountered throughout the entire period of postnatal development. In 4-week-old testes degenerating fetal and newly formed postnatal Leydig cells are seen in juxtaposition to each other. From the 8th week on, only postnatal Leydig cells are present. Between 16 and 30 weeks large-scale degeneration of prepuberal Leydig cells is observed. The Leydig cells that survive this degenerative phase constitute the long-lasting adult population. 20-30% (numerically) of all intertubular cells at all ages are free mononuclear cells. These are found as lymphocytes, plasma cells, monocytes, macrophages and light intercalated cells (LIC). The latter are monocyte-derived, Leydig cell-associated typical cells of the bovine testis. The differentiation of the two main components of the tubular lamina propria, (i) basal lamina and (ii) peritubular cell sheath, seems to be effected rather independent from each other and also from hormonal signals important for the development of the germinal cells. The laminated basal lamina reaches nearly 3 μm at 16 weeks and is later on continuously reduced. At 25 weeks the peritubular cells have transformed into contractile myofibroblasts. At this period the germinal epithelium is still in a prepuberal state. [ABSTRACT FROM AUTHOR]