Seminiferous tubules: convoluted portion

A tunica propria surrounds the convoluted portion of each seminiferous tubule. The tunica propria is composed of the basal lamina of the seminiferous 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. Lateral extensions from adjacent Sertoli cells form occluding, tight junctions that form the blood-testis barrier.

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 can be used for comparison in identifying the other later 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. They contain prominent, condensed chromosomes and many primary spermatocytes are seen because they remain in prophase of the first meiotic division for an extended period of time.

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. The image on the left is at a different developmental stage and lacks secondary spermatocytes.

Spermatids are haploid cells formed when secondary spermatocytes 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. The image on the right is at a different developmental stage and lacks spermatids.

Spermatids are haploid cells formed when secondary spermatocytes 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. The image on the right is at a different developmental stage and lacks spermatids.

At the completion of spermiogenesis, late spermatids are shed into the lumen of the tubule by a process termed spermiation. This process results in free spermatozoa in the lumen, though they are non-motile at this point.

During spermiogenesis, spermatocytes shed much of their cytoplasm which is visible as residual bodies. They are eventually phagocytosed by Sertoli cells or shed into the lumen of the tubule.

Images taken of a slide from the University of New England College of Osteopathic Medicine slide collection.