Topography and general morphology
Convoluted seminiferous tubules are located inside the lobules of the testis. Each lobule contains from one to four such tubules. The length of a single uncoiled tubule is 30 to 70 centimeters. Since there are about 300–450 tubules in the organ, their total length is enormous and varies between 400 and 500 meters.
The tubules are folded into complex loops multiple times. The ends of these loops merge with each other and with neighboring loops toward the mediastinum testis.
Due to their high tortuosity, histological sections show a characteristic pattern: the same tubule is cut by the plane of sectioning many times. Therefore, the numerous rounded or oval profiles visible under the microscope within a single lobule are most often cross-sections of the same long tubule.
Microscopic structure of the wall
The wall of the convoluted seminiferous tubule has a complex organization and is divided into two main components: the outer lamina propria and the inner epithelial-spermatogenic layer facing the lumen.
Lamina propria (tunica propria)
This is a thin outer structure in which cellular elements strictly alternate with acellular layers. From outside to inside, four layers are distinguished:
- Outer cellular layer. Formed by fibrocytes-like cells. This is the least specific part of the layer, directly bordering the surrounding interstitial tissue.
- Outer acellular layer. Consists of an amorphous matrix penetrated by collagen fibers.
- Myoid cell layer. Represented by spindle-shaped cells tightly connected to each other by tight junctions, forming continuous chains around the tubule.
- Inner acellular layer. Acts as the basement membrane to which the epithelium attaches. It has a bilayer structure (including homogeneous and fibrous layers).
Functions of the wall components
The leading functional load in the tunica propria is carried by myoid cells and the inner basement membrane. They ensure the normal course of spermatogenesis and the protection of maturing cells.
- Barrier formation: The capsule is an important part of the blood-testis barrier, reliably preventing the penetration of immunocompetent cells from the bloodstream to the spermatogenic epithelium.
- Mechanical role: Through the contraction of myoid cells, a constant movement of tubular fluid and maturing spermatozoa toward the rete testis is ensured.
- Trophic and transport functions: Myoid cells possess transport vesicles through which active two-way metabolism is carried out between blood, lymph, and the tubule epithelium.
- Regulation: The capsule is able to secrete specific paracrine factors. These substances stimulate Sertoli cells to synthesize androgen-binding protein.
Organization of the spermatogenic epithelium
The epithelial-spermatogenic layer forms the bulk of the tubule wall. It lies on the inner acellular layer of the capsule and is represented by two distinct cell populations:
- Somatic cells (sustentocytes / Sertoli cells). These are supporting elements that perform auxiliary and supportive functions. They are large and extend through the entire thickness of the epithelial sheet — from the basement membrane up to the tubule lumen.
- Spermatogenic cells. These are developing male germ cells at various stages of maturation. During spermatogenesis, they gradually migrate from the basement membrane toward the central lumen.
Interstitium and hormone transport pathways
The space between the convoluted tubules is filled with loose fibrous connective tissue, the interstitium. Blood and lymph vessels run here, along with typical connective tissue fibers and cells.
Of particular importance are specific glandulocytes — interstitial Leydig cells. Their main task is the production of male sex hormones, primarily testosterone and its derivatives.
According to modern data, microcirculation in this zone has a unique feature. Each seminiferous tubule is surrounded by a peculiar lymphatic "sheath" (sinusoidal capillary). Unlike most other endocrine glands of the body, the hormonal secretions of Leydig cells are initially transported not into the bloodstream, but into the lymphatic system.
Briefly: answers to topical questions
Which structures make up the blood-testis barrier?
The blood-testis barrier is a multi-layered structure located in the direction from the vessel to the tubule lumen. It includes the following components:
- Testicular capillary endothelium — fenestrated, along with its basement membrane.
- Interstitial tissue — narrow layers.
- Wall of lymphatic vessels — sinusoidal capillaries that form the "sheath".
- Lamina propria (tunica propria) — the capsule of the seminiferous tubules.
- Basement membrane of the spermatogenic epithelium.
- Tight junctions (zonula occludens) — specialized connections between Sertoli cells that are the key link closing the barrier.
What stages does a male germ cell go through during spermatogenesis?
During spermatogenesis, a male germ cell sequentially goes through four main phases.
- Proliferation phase — initial cells divide by mitosis, resulting in diploid spermatogonia.
- Growth phase — DNA replication occurs, cells increase in size and develop into primary spermatocytes.
- Maturation phase — cells divide by meiosis; after the first division, secondary spermatocytes are formed, and after the second, spermatids.
- Differentiation phase (spermiogenesis) — cell differentiation occurs (morphological maturation without division), during which mature spermatozoa are formed.
Additionally, there is a stage of extra-epithelial maturation that occurs in the epididymis.
The layers of the tubule lamina propria (from outside to inside) are easy to remember by the alternation rhythm: Cellular (fibrocytes) — Acellular (collagen) — Cellular (myoid) — Acellular (basement membrane). Formula: C—A—C—A.
Class and exam questions
Why are so many tubule cross-sections visible in a single lobule on a histological specimen?
This is an optical illusion associated with the morphology of the tubules. Since each tubule is up to 70 cm long and highly convoluted (folded into loops multiple times), the plane of the microtome section crosses the same tube dozens of times, creating multiple rounded and oval profiles.
Where do the hormones synthesized by interstitial Leydig cells go first?
Unlike most endocrine secretions, testosterone and its derivatives enter the lymphatic system first rather than the bloodstream. This is because each seminiferous tubule is surrounded by a special lymphatic sinusoidal capillary (lymphatic "sheath").
What is the role of myoid cells in the wall of the convoluted seminiferous tubule?
Myoid cells perform a contractile function (move fluid and spermatozoa), participate in metabolism via transport vesicles, are part of the blood-testis barrier, and secrete paracrine factors to stimulate Sertoli cells.
Where exactly are spermatozoa formed?
The process of sperm formation from stem cells (spermatogenesis) occurs within the epithelial-spermatogenic layer of the convoluted seminiferous tubules. As they mature, cells migrate from the basement membrane to the lumen of the tubule.
What else to study on this topic
- Detailed structure of the epithelial-spermatogenic layer
- Morphology and functions of Sertoli cells (sustentocytes)
- Stages of spermatogenesis and cell migration to the lumen
- Blood-testis barrier: structural components
- Synthesis and role of androgen-binding protein
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Convoluted seminiferous tubules" and 3,485 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the "Male Reproductive System" section
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.