Материал: Kaplan USMLE-1 (2013) - Anatomy

Chapter 10 • Male Reproductive System

Spermatogenesis

The spermatogenic cells (germinal epithelium) are stacked in 4 to 8 layers that occupy the space between the basement membrane and the lumen of the semi­ niferous tubule. The stem cells (spermatogonia) are adjacent to the basement membrane. As the cells develop, they move from the basal to the luminal side ofthe tubule.

At puberty the stem cells resume mitosis, producing more stem cells as well as differentiated spermatogonia (type A and B) that are committed to meio­ sis. Type B spermatogonia differentiate into primary spermatocytes that enter meiosis. Primaryspermatocytes (4n, diploid) pass through a long prophase ( 10 days to 2 weeks) and after the first meiotic division form 2 secondary spermato­ cytes (2n, haploid). The secondary spermatocytes rapidly undergo the second meiotic division in a matter of minutes (and are rarely seen in histologic sec­ tions) to produce the spermatids (ln, haploid).

The progeny ofa single maturing spermatogonium remain connected to one an­ other by cytoplasmic bridges throughout their differentiation into mature sperm.

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Figure 1-10-3. Seminiferous tubule surrounded by a basement membrane (A) and myoepithelial cells

Spermatogonia (B) lie on the basement membrane, primary spermatocytes (C), and spermatozoa (D) are inside the blood testis barrier.

Sertoli cells (arrow) have elongate, pale-staining nuclei.

Spermiogenesis

Spermiogenesis transforms haploid spermatids into spermatozoa. This process of differentiation involves formation of the acrosome, condensation, and elon­ gation of the nucleus; development of the flagellum; and loss of much of the cytoplasm.

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Chapter 10 • Male Reproductive System

Sertoli Cells and the Blood-Testis Barrier

Sertoli cells are tall columnar epithelial cells. These multifunctional cells are the predominant cells in the seminiferous tubule prior to puberty and in elderly men but comprise only 10% of the cells during times of maximal spermatogenesis.

•Irregular in shape; the base adheres to the basal lamina and the apical end extends to the lumen. The nucleus tends to be oval with the long axis oriented perpendicular to the basement membrane.

•The cytoplasmic extensions make contact with neighboring Sertoli cells via tight junctions, forming the blood-testis barrier by separating the seminiferous tubule into a basal and an adlumenal compartment.

•Have abundant smooth endoplasmic reticulum, some rough endoplas­ mic reticulum, a well-developed Golgi complex, lysosomes, microtu­ bules, and microfilaments. Microtubules and microfilaments likely func­ tion to aid in the release of the spermatozoa.

•Secrete androgen-binding protein that binds testosterone and dihy­ drotestosterone. High concentrations of these hormones are essential for normal germ-cell maturation. The production of androgen-binding protein is stimulated by follicle-stimulatinghormone (FSH receptors are on Sertoli cells).

The blood-testis barrier is a network of Sertoli cells which divides the seminif­ erous tubule into a basal compartment (containing the spermatogonia and the earliest primary spermatocytes) and an adlumenal compartment (containing the remaining spermatocytes and spermatids). The basal compartment has free ac­ cess to material found in blood, while the more advanced stages of spermato­ genesis are protected from blood-borne products by the barrier formed by the tight junctions between the Sertoli cells. The primary spermatocytes traverse this barrier by a mechanism not yet understood.

Interstitial Tissues ofthe Testis

The interstitial tissue lying between the seminiferous tubules is a loose network of connective tissue composed of fibroblasts, collagen, blood and lymphatic ves­ sels, and Leydig cells (also called interstitial cells). The Leydig cells synthesize testosterone.

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