Biology · Ch 2 — Human Reproduction
Microscopic Anatomy of the Testis
Microscopic Anatomy of the Testis
A single seminiferous tubule, seen in transverse section under the microscope, has a wall composed of a specialised, thick, multi-layered lining called the germinal (or spermatogenic) epithelium, surrounding a central hollow lumen. This epithelium contains two fundamentally different populations of cells living side by side: the germ cells, which are actually dividing and differentiating into sperm, and the Sertoli cells, which are permanent, non-dividing supporting cells.
The germ cells are arranged in distinct concentric layers corresponding to successive stages of spermatogenesis, from the outermost layer (nearest the tubule's basement membrane) toward the innermost layer (nearest the lumen). Spermatogonia, the diploid stem-cell population from which all sperm ultimately derive, lie against the basement membrane and continue dividing mitotically throughout adult life, some daughter cells remaining as spermatogonia (to keep the stem-cell population going) and others committing to differentiate further. These committed cells enlarge into primary spermatocytes, which undergo the first meiotic division to give two secondary spermatocytes, which in turn complete the second meiotic division to give four haploid spermatids. The spermatids, initially rounded and non-motile, lie closest to the lumen and undergo spermiogenesis (described in section 2.6) to become slender, motile spermatozoa, which are finally released into the lumen by spermiation.
Interspersed among these germ cells, and running the full height of the epithelium from the basement membrane right up to the lumen, are the tall, columnar Sertoli cells (also called nurse cells or sustentacular cells). Unlike the germ cells, Sertoli cells do not themselves divide to form sperm; instead they perform a support role, providing nutrition to the developing germ cells, secreting the fluid that fills the tubule lumen and carries the released sperm forward, and secreting androgen-binding protein (ABP), which concentrates testosterone locally within the tubule to sustain spermatogenesis. Sertoli cells are themselves the direct target of FSH from the anterior pituitary and are also regulated by testosterone diffusing in from the surrounding interstitial tissue. …
What this figure shows. A transverse-section diagram of a single seminiferous tubule cut across its lumen, showing a wall composed of a stratified germinal (spermatogenic) epithelium. From the outer basal margin of the tubule inward toward the central lumen, the diagram shows successive concentric layers of germ cells at progressively more advanced stages of spermatogenesis — spermatogonia lying against the basement membrane, primary spermatocytes, secondary spermatocytes, spermatids, and finally maturing spermatozoa with their tails projecting into the lumen. Interspersed among these germ cells, running the full height of the epithelium from base to lumen, tall columnar Sertoli cells (also called nurse cells or sustentacular cells) are shown, each providing physical support and nourishment to the adjacent developing germ cells. Outside the tubule's own boundary wall, in the loose interstitial connective tissue that fills the spaces between adjacent tubules, clusters of rounded Leydig cells (interstitial cells) are shown alongside small blood vessels — these are the cells responsible for secreting the ma …