Interaction with Sertoli cells
Throughout spermiogenesis, developing cells (from early to late spermatids) are partially embedded in the cytoplasm of Sertoli cells (sustentocytes).
- Nutrition and regulation: Sustentocytes supply the maturing cells with essential substances via specialized tubulobulbar complexes.
- Spermiation (completion of the process): Late spermatids break their connection with Sertoli cells and enter the lumen of the seminiferous tubule. At this point, the cytoplasmic bridges connecting the cell clone disappear, the cells become individual, and are henceforth called spermatozoa.
Morphological cellular remodeling
The transformation of a spermatid involves a radical change in all its structures to fulfill a single task: the transport of genetic material to the oocyte.
- Nucleus: In early stages, the haploid nucleus remains active. Then histones are replaced by more basic proteins, protamines. Chromatin is super-densely packed, the nucleus decreases in volume 30-fold, loses functional activity, and shifts to one of the poles (the future head).
- Acrosome: Formed from the Golgi apparatus. It is a lysosome analogue that evolves from a membranous granule into a flattened "double cap" over the anterior part of the nucleus. It contains enzymes (acrosin, hyaluronidase, collagenase) necessary for dissolving the egg coats.
- Flagellum (locomotor apparatus): Begins to grow from a single centriole located at the posterior pole of the nucleus. The core of the flagellum is the axoneme with a classic microtubule arrangement (9 peripheral doublets and 1 central pair).
- Membrane: The plasmalemma over the acrosome changes its chemical composition, which will subsequently ensure chemotaxis, binding to the oocyte, and facilitate membrane rupture during fertilization.
Energy supply and cytoplasm reduction
For autonomous existence, the spermatozoon develops robust metabolic pathways while shedding excess ballast.
- Enzyme system: Enzymes are synthesized in the cytoplasm to utilize exogenous substrates. Epididymal secretion provides acetylcarnitine (imported into mitochondria via acetylcarnitine transferase), and seminal vesicle secretion provides fructose (converted into sorbitol and then into glucose by sorbitol dehydrogenase).
- Mitochondria: Align in a helix around the base of the axoneme, forming the mitochondrial sheath in the midpiece of the tail. Upon entering the female reproductive tract, they switch to oxidizing lactate and pyruvate.
- Cytoplasm reduction: Organelles (ER, Golgi apparatus, ribosomes) and excess cytoplasm shift along the tail into a droplet—the residual body. It is then pinched off, leaving only a ultra-thin residual layer of cytoplasm around the axoneme and acrosome.
Post-testicular maturation and capacitation
Upon leaving the seminiferous tubules, spermatozoa are not yet capable of fertilizing an egg. Their maturation continues in several stages.
In the epididymis (duration 1–3 weeks):
- In the caput epididymidis, morphological formation of the acrosome is completed.
- In the cauda epididymidis and vas deferens, cells acquire motility.
- The surface acquires a negative charge (to prevent agglutination) and binds specific glycoproteins.
In the female reproductive tract, capacitation takes place:
- Secretions of the female tract, prostate, and seminal vesicles act on the spermatozoon.
- Cell membrane lability increases (preparing for the acrosome reaction).
- Metabolism and motility reach their maximum.
Organization of the spermatogenic epithelium
Unlike typical epidermis, the seminiferous tubules simultaneously contain cells of different generations. Stem cells enter differentiation strictly periodically.
- Temporal ordering (Cycle): The total duration of spermatogenesis is 75 days. New cells enter the process every 19 days. As a result, at any point in the tubule, 4 layers of cells develop simultaneously (with a developmental difference of exactly 19 days). The set of cells in the same location changes cyclically.
- Spatial ordering (Wave): The process starts in the middle of a tubular loop and spreads in both directions (in rodents at a rate of about 2.2 mm/day). Moving along the tubule "against the wave," one can observe the same sequence of stage changes as observing a single point over time.
Briefly: answers to topical questions
What phases (stages) are distinguished in spermiogenesis?
Spermiogenesis involves the development of spermatids from early to late forms, passing through five processes of morphological remodeling.
- Early spermatids — round in shape with a pale nucleus, retaining functional activity.
- Intermediate spermatids — acquire an oval shape and a dark nucleus; flagella are still absent.
- Late spermatids — characterized by the formation of the tail.
The following remodeling processes are distinguished: morphological remodeling of the nucleus, formation of the acrosome, flagellum, energy supply systems, and reduction of cytoplasm.
Which hormones regulate spermiogenesis and Sertoli cell functions?
Sertoli cell activity and spermatogenesis are controlled by pituitary hormones and local steroids.
- FSH — stimulates Sertoli cells and the synthesis of specific proteins within them.
- Testosterone — activates sustentocytes and directly stimulates spermatogenic cells.
- Estradiol — produced in sustentocytes from testosterone, locally suppresses testosterone synthesis.
- Inhibin — a protein that suppresses FSH production via negative feedback.
- Activin — stimulates spermatogonial division.
What structural elements (besides the axoneme and mitochondria) are part of the sperm flagellum?
In addition to the axoneme and the mitochondrial sheath, the sperm flagellar structure includes cytoskeletal elements and specific sheaths.
- Centrioles — proximal and distal, located in the neck of the flagellum; the axoneme originates from the distal centriole.
- Outer dense fibers — 9 cytoskeletal elements composed of keratin-like proteins surrounding the axoneme in the midpiece and principal piece of the tail.
- Fibrous sheath — consists of circularly arranged fibrils, present in the principal piece of the tail instead of mitochondria.
- Plasmalemma — covers the midpiece and principal piece of the tail, remaining the sole covering in the endpiece without additional sheaths.
Acrosome contents: "ABC" rule — Acrosin, Hyaluronidase, Collagenase. These enzymes "Attack Boundaries of the Cell" (the oocyte).
Class and exam questions
Why are the testes located in the scrotum, outside the abdominal cavity?
The process of protein synthesis in spermatids is extremely temperature-sensitive. The optimal temperature for spermiogenesis is about 34 °C, which is lower than normal body temperature.
What is the essence of capacitation?
This is the final stage of sperm maturation in the female reproductive tract, during which membrane lability increases (to enable membrane rupture upon encountering the oocyte) and motility sharply surges.
What is spermiation?
This is the process by which late spermatids release their contact with Sertoli cells and are released into the lumen of the seminiferous tubule. At this point, cytoplasmic bridges disappear and the cells become spermatozoa.
What else to study on this topic
- Mathematical model of the spermatogenic epithelium cycle (75 days and 19-day intervals)
- The phenomenon of the «wave» of the spermatogenic epithelium and its propagation speed
- Histological picture of spermatogenesis: differences in cell composition in sections I, II, and III
- Detailed structure and functions of tubulobulbar complexes
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete methodical guide on the topic «Spermiogenesis» 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.