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Ultrastructural features and morphometric values of ovine Sertoli and spermatogenic cells are reported with special reference to the 6 stages of the seminiferous epithelial cycle. Seminiferous tubules occupy about 83% of the testicular parenchyma. Average tubular diameter (about 275 μm) and epithelial height (about 95 μm) do not vary significantly during the cycle. From preleptotene to late diplotene the cellular volume of primary spermatocytes increases nearly five-fold; the nuclear volume increases threefold in the same period. Secondary spermatocytes are observed exclusively during stage 4 of the seminiferous epithelial cycle. Due to partial cell necrosis and autolytic events, ovine spermatids lose a considerable amount of their cytoplasm during acrosome and maturation phases prior to spermiation. Sertoli cells occupy between 27.6% (stage 3) and 36.6% (stage 1) of the tubular epithelium. The average volume of a Sertoli cell varies between 6 380 μm3(stage 2) and 7 195 μm3(stage 4), the absolute surface area between 10 550 μm2and 12 305 μm2. The irregularly contoured Sertoli cell nucleus contains a vesicular nucleolus and occupies about 7.5% of the cell. Mitochondria (about 5%) and smooth endoplasmic reticulum are other prominent organelles, but three-quarters of the Sertoli cell are taken up by cytoplasmic matrix with a well-developed cytoskeleton. Ovine Sertoli cells contain large basal lipid droplets, but no typical lipid cycle can be observed.

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.

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.

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.

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 A1-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.

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 G₁-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.