Morphology and cell interactions
Sertoli cells feature pronounced polar differentiation, dividing into two functional zones. The basal part is located on the basement membrane between spermatogonia. It houses the 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 part faces the lumen of the seminiferous tubule and is equipped with numerous cytoplasmic processes. In histological preparations, the apical zone is usually masked by tightly adjacent cells, so primarily the basal fragment with the nucleus is visualized.
The lateral surfaces of sustentacular cells feature special pouch-like invaginations into which developing germ cells sink. As they mature, they slide along the surface of the Sertoli cell following the «conveyor belt principle». An important rule: each germ cell contacts only one Sertoli cell, while a single sustentacular cell simultaneously services several germ cells at different stages.
During the development stages of middle and late spermatids, specialized tubulobulbar complexes are formed. They consist of 4 to 24 cytoplasmic projections of the spermatid that invaginate into the Sertoli cell. The name reflects their shape: a narrow tubular part (tubulo-) ends in a spherical bulbous expansion (-bulbar).
Blood-testis barrier (BTB)
Sertoli cells are connected to each other by specialized tight junctions (zonula occludens) located at a height of approximately one cell layer from the basement membrane. These junctions represent a "sewing together" of the plasmalemmas using at least 50 rows of protein molecules, ensuring the highest level of tightness.
By closing off, these processes divide the tubular space into two isolated compartments:
- Basal compartment: contains spermatogonia and early (preleptotene) spermatocytes.
- Adluminal compartment: contains cells at subsequent stages of meiosis and spermiogenesis.
For spermatocytes to cross into the adluminal compartment without disrupting isolation, the «lock mechanism» operates. Junctions between sustentacular cells close below the migrating cell and open above it, maintaining the continuity of the barrier.
The BTB itself is a multilayered structure (from vessel to lumen): the fenestrated capillary endothelium and its basement membrane, narrow interstitial layers, the wall of lymphatic sinusoidal capillaries, the proper sheath of the tubules, the basement membrane of the spermatogenic epithelium, and the key component—tight junctions of Sertoli cells.
The biological role of the BTB consists of immune isolation (preventing antibodies from contacting antigenic spermatozoa that appear only at puberty) and maintaining a specific chemical microenvironment for meiosis.
Functional spectrum of sustentacular cells
In addition to their barrier role, Sertoli cells perform numerous tasks critical for spermatogenesis:
- Supporting function: formation of a "skeleton" on whose processes spermatogenic cells are anchored.
- Trophic function: nourishment of cells in the adluminal compartment. Due to barrier isolation, all nutrients reach them exclusively in transit through the cytoplasm of sustentacular cells.
- Phagocytic function: utilization of germ cells that died via apoptosis and digestion of residual bodies (residual drops of cytoplasm from late spermatids).
- Exosecretory function: secretion of fluid into the tubule lumen. This fluid is necessary for the transport of still immotile spermatozoa.
- Coordinating function: regulation of the rate of cell development and control of the "wave of spermatogenesis initiation".
- Production of transport proteins: synthesis of androgen-binding protein (ABP) and testicular albumin for testosterone transport, as well as transferrin for iron transport.
- Endocrine regulation: sustentacular cells produce inhibin (providing negative feedback by suppressing FSH) and various growth factors that stimulate spermatogonia division.
Briefly: answers to topical questions
What hormones do Sertoli cells produce?
Sertoli cells produce/synthesize:
- Anti-Müllerian hormone (AMH) — secreted from the 7th week until puberty; causes suppression (regression) of the Müllerian ducts.
- Inhibin — a protein providing negative feedback: suppresses FSH production by the pituitary gland; sources also indicate that inhibins inhibit gonadotropin-releasing hormone and follitropin.
- Estrogens — formed in Sertoli cells via aromatization of testosterone; participate in the completion of sperm formation.
In addition, Sertoli cells produce growth factors that stimulate the division of spermatogonia.
What hormones regulate the activity of Sertoli cells?
The activity of Sertoli cells is regulated by pituitary follicle-stimulating hormone (FSH). Sertoli cells are the main target for FSH and possess specific receptors for it.
The "lock" principle during cell migration is easy to remember: junctions between Sertoli cells close BELOW the cell and open ABOVE it — like lock gates on a river canal, letting the vessel (cell) pass without mixing the water (the chemical environment of the compartments).
Class and exam questions
Why is it difficult to see an entire Sertoli cell on histological slides?
The apical part of the cell with its numerous processes is masked by closely adjacent spermatogenic cells, so usually only the basal part and the nucleus are clearly visualized.
How do spermatids receive nutrition if they are isolated by the blood-testis barrier?
Cells of the adluminal compartment lack direct access to blood vessels, so all nutrients reach them strictly in transit through the cytoplasm of Sertoli cells.
What is a tubulobulbar complex?
This is a specialized junction at the mid and late spermatid stages, when 4 to 24 cytoplasmic extensions of the spermatid (narrow with a bulbous expansion) invaginate into the Sertoli cell.
What else to study on this topic
- The mechanism of the "spermatogenic conveyor belt"
- Molecular organization of tight junctions (zonula occludens)
- Interaction of androgen-binding protein (ABP) with testosterone
- The role of inhibin in the pituitary-testicular endocrine axis
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