Seminiferous tubules of testes

Seminiferous tubules of testes 

The testes have an unusually thick connective tissue capsule, the tunica albuginea. An unusually thick, dense connective tissue capsule, the tunica albuginea, covers each testis

 The inner part of this capsule, the tunica vasculosa, is a loose connective tissue layer that contains blood vessels. Each testis

is divided into approximately 250 lobules by incomplete connective tissue septa that project from the capsule. Along the posterior surface of the testis, the tunica albuginea thickens and projects inward as the mediastinum testis. Blood vessels, lymphatic vessels, and the genital excurrent ducts pass through the diastinum as they enter or leave the testis.

Each lobule consists of several highly convoluted seminiferous tubules.

Each lobule of the testis consists of one to four seminiferous tubules, in which sperm are produced, and a connective tissue stroma, in which Leydig (interstitial) cells are contained

The seminiferous tubules consist of a seminiferous epithelium surrounded by a tunica propria.

Each seminiferous tubule is approximately 50 cm long (range, 30 to 80 cm) and 150 to 250 _m in diameter. The seminiferous epithelium is an unusual and complex stratified epithelium composed of two basic cell populations:

• Sertoli cells, also known as supporting, or sustentacular,cells. These cells do not replicate after puberty. Sertoli cells are columnar cells with extensive apical and lateral processes that surround the adjacent spermatogenic cells and occupy the spaces between them.

Seminiferous Tubules

Each highly convoluted seminiferous tubule is composed of a fi bromuscular tunica propria, which is separated from the seminiferous epithelium by a basal membrane.

Seminiferous Epithelium

The seminiferous epithelium is composed of sustentacular Sertoli cells and a stratifi ed layer of developing malegametes. Sertoli cells establish a blood-testis barrier by forming occluding junctions with each other, thus subdividing the seminiferous tubule into adluminal and basal compartments. The basal compartment houses spermatogonia A (both light and dark), spermatogonia B, and the basal aspects of Sertoli cells. The adluminal compartment contains the apical portions of Sertoli cells, primary spermatocytes, secondary spermatocytes, spermatids,and spermatozoa. Spermatogenic cells, which regularly replicate and differentiate into mature sperm. These cells are derived from primordial germ cells originating in the yolk sac that colonize the gonadal ridges during early development of the testis.

2. Tunica Propria: The tunica propria consists of loose collagenous connective tissue, fi broblasts, and myoid cells. The tunica (lamina) propria, also called peritubular tissue, is a multilayered connective tissue that lacks typical fibroblasts. In man, it consists of three to five layers of myoid cells (peritubular contractile cells) and collagen fibrils, external to the basal lamina of the seminiferous epithelium . At the ultrastructural level, myoid cells demonstrate features associated with smooth muscle cells, including abasal lamina and large numbers of actin filaments. They also exhibit a significant amount of rough endoplasmic reticulum (rER), a feature indicating their role in collagen synthesis in the absence of typical fibroblasts. Rhythmic contractions of the myoid cells create peristaltic waves that help move spermatozoa and testicular fluid through the seminiferous tubules to the excurrent duct system. Blood vessels and extensive lymphatic vasculature as well as Leydig cells are present external to the myoid layer.

As a normal consequence of aging, the tunica propria increases in thickness. This thickening is accompanied by a decreased rate of sperm production and an overall reduction in the size of the seminiferous tubules. Excessive thickening of the tunica propria earlier in life is associated with infertility.

C. Stroma

The loose vascular connective tissue stroma surrounding seminiferous tubules houses small clusters of large, vacuolated-appearing endocrine cells, the interstitial cells (of Leydig). Leydig Cells

Leydig cells (interstitial cells) are large, polygonal, eosinophilic cells that typically contain lipid droplets Lipofuscin pigment is also frequently present in these cells as well as distinctive, rod-shaped cytoplasmic crystals, the crystals of Reinke.

Like other steroid-secreting cells, Leydig cells have an elaborate smooth endoplasmic reticulum (sER), a feature that accounts for their eosinophilia . The enzymes necessary for the synthesis of testosterone from cholesterol are associated with the sER. Mitochondria with tubulovesicular

cristae, another characteristic of steroid-secreting cells, are also present in Leydig cells.

Leydig cells differentiate and secrete testosterone during early fetal life. Secretion of testosterone is required during embryonic development, sexual maturation, and reproductive

function:

• In the embryo, secretion of testosterone and other androgens is essential for the normal development of the gonads in the male fetus.

At puberty, secretion of testosterone is responsible for the initiation of sperm production, accessory sex gland secretion, and development of secondary sex characteristics.

• In the adult, secretion of testosterone is essential for the maintenance of spermatogenesis and of secondary sex characteristics, genital excurrent ducts, and accessory sex glands.

The Leydig cells are active in the early differentiation of the male fetus and then undergo a period of inactivity beginning at about 5 months of fetal life. Inactive Leydig cells are difficult to

distinguish from fibroblasts. When Leydig cells are exposed to gonadotropic stimulation at puberty, they again become androgen-secreting cells and remain active throughout life.