Morphology and Cell Interactions
Sertoli cells exhibit pronounced polar differentiation, dividing into two functional zones. The basal region rests on the basement membrane between spermatogonia. It contains an irregularly shaped nucleus with indented contours and concentrated organelles. Smooth endoplasmic reticulum is present here, though it is not as dominant a feature as in Leydig cells. The apical region faces the lumen of the seminiferous tubule and features numerous cytoplasmic extensions. In histological sections, the apical zone is typically obscured by tightly packed adjacent cells, making the basal fragment and nucleus the most readily visualized structures.
Specialized crypt-like invaginations on the lateral surfaces of sustentocytes accommodate developing germ cells. As they mature, germ cells glide along the Sertoli cell surface via a "conveyor-belt principle". An important rule: each germ cell contacts only one Sertoli cell, whereas a single sustentocyte simultaneously supports several germ cells at different stages of development.
During the development of mid and late spermatids, specialized tubulobulbar complexes are formed. These consist of 4 to 24 cytoplasmic extensions of the spermatid that invaginate into the Sertoli cell. The name reflects their morphology: a narrow tubular portion (tubulo-) terminating in a spherical bulbous expansion (-bulbar).
Blood-Testis Barrier (BTB)
Sertoli cells are linked to one another by specialized tight junctions (zonula occludens) located approximately one cell layer above the basement membrane. These junctions consist of apposed plasma membranes fused by a minimum of 50 rows of protein molecules, ensuring exceptionally high tightness and sealing.
By closing off the intercellular space, these junctions divide the tubule into two isolated compartments:
- Basal compartment: contains spermatogonia and early (preleptotene) spermatocytes.
- Adluminal compartment: contains cells undergoing subsequent stages of meiosis and spermiogenesis.
To allow spermatocytes to cross into the adluminal compartment without breaking the barrier, a "lock-and-key / sluice-gate principle" operates. Junctions between sustentocytes form below the migrating cell while breaking down above it, maintaining continuous barrier integrity.
The BTB itself is a multilayered structure (from blood vessel to lumen): fenestrated capillary endothelium and its basement membrane, narrow interstitial connective tissue layers, lymphatic sinusoid walls, the tunica propria of the tubules, the basement membrane of the seminiferous epithelium, and the key component—the Sertoli cell tight junctions.
The biological role of the BTB includes immune isolation (preventing antibodies from contacting autoantigenic spermatozoa, which appear only at puberty) and maintaining a specific chemical microenvironment necessary for meiosis.
Functional Spectrum of Sustentocytes
In addition to their barrier role, Sertoli cells perform multiple tasks critical for spermatogenesis:
- Supportive function: formation of a structural scaffold whose processes anchor spermatogenic cells.
- Trophic function: nourishment of adluminal compartment cells. Due to barrier isolation, all nutrients reach them exclusively via transit through the Sertoli cell cytoplasm.
- Phagocytic function: clearance of germ cells undergoing apoptosis and digestion of residual bodies (cytoplasmic droplets discarded by late spermatids).
- Exosecretory function: fluid secretion into the tubular lumen, which is necessary for the transport of immotile spermatozoa.
- Coordinating function: regulation of the rate of cell development and control over the "wave of the cycle of the seminiferous epithelium."
- Transport protein production: synthesis of androgen-binding protein (ABP) and testicular transferrin for testosterone and iron transport, respectively.
- Endocrine regulation: sustentocytes produce inhibin (providing negative feedback to suppress FSH) and various growth factors that stimulate spermatogonial division.