Testis: convoluted tubules

The convoluted portions of the seminiferous tubules form the main mass of the testis and are the site of spermatogenesis. Stratified seminiferous epithelium lining these tubules contains Sertoli cells and several layers of germ cells. A basal lamina and a thin layer of connective tissue surrounds each tubule. 600x

A tunica propria surrounds the convoluted portion of each seminiferous tubule. The tunica propria is composed of the basal lamina of the seminiferus epithelium, three to five layers of myoid cells and collagen fibrils. The myoid cells secrete the fibrils and contract in peristaltic waves to propel spermatozoa through the seminiferous tubules.

Sertoli cells extend the entire thickness of the epithelium. Their cell outline is difficult to discern because of numerous lateral processes extending around developing germ cells. Therefore, Sertoli cells must be identified primarily by their nuclei, which are oval, euchromatic, infolded and display a prominent nucleolus. Sertoli cells form the blood-testis barrier and do not divide.

Spermatogonia are diploid, somatic cells that always lie on the basal lamina of the epithelium; they are the earliest of the germ cell line. Spermatogonia divide by mitosis to perpetuate themselves or to form primary spermatocytes. The diameter of these nuclei will be used as a measure for identifying the other germ cells.

Primary spermatocytes are diploid cells that form in the basal compartment but quickly move through the blood-testis barrier to the adluminal compartment. Many primary spermatocytes are seen because they remain in prophase of the first meiotic division for an extended period. Their nuclei are always seen in cell division and are about 150% the size of the spermatogonia nuclei.

Secondary spermatocytes are haploid cells formed when primary spermatocytes complete the first meiotic division. Secondary spermatocyte nuclei are spherical and equal in diameter to spermatogonia nuclei. Secondary spermatocytes complete the second meiotic division very quickly, so these cells are not frequently seen in sections.

Spermatids are haploid cells formed when secondaries complete the second meiotic division. Early spermatid nuclei are spherical and smaller than spermatogonia nuclei. Spermatids do not divide but differentiate by condensing and elongating their nuclei, developing a flagellum and losing excess cytoplasm. Cytoplasmic continuity between adjacent spermatids is lost during this process.

Spermatids are haploid cells formed when secondaries complete the second meiotic division. Early spermatid nuclei are spherical and smaller than spermatogonia nuclei. Spermatids do not divide but differentiate by condensing and elongating their nuclei, developing a flagellum and losing excess cytoplasm. Cytoplasmic continuity between adjacent spermatids is lost during this process.

Spermatids are haploid cells formed when secondaries complete the second meiotic division. Early spermatid nuclei are spherical and smaller than spermatogonia nuclei. Spermatids do not divide but differentiate by condensing and elongating their nuclei, developing a flagellum and losing excess cytoplasm. Cytoplasmic continuity between adjacent spermatids is lost during this process.