Global periodicity of development
The process of male germ cell development and maturation can be viewed in a broad biological sense (as ontogenesis) and in a narrow sense (as spermatogenesis proper). There are three global stages in total:
- Gonocyte stage. This period takes place during embryogenesis before the cells populate the developing gonads, i.e., before their determination by the surrounding somatic environment.
- Prespermatogenesis. Takes place within the gonads, starting from the embryonic period up to puberty. The main process at this stage is the proliferation of prespermatogonia.
- Spermatogenesis proper. Initiated with the onset of reproductive age. From this moment, a portion of spermatogonia enters the path of irreversible differentiation, which culminates in the formation of mature spermatozoa.
Stages of spermatogenesis proper
In the testis of an adult man, the cell maturation process is divided into four sequential stages. Each of them is characterized by the presence of a specific cell type and takes a strictly defined duration:
- Proliferation (mitotic division). Cells at this stage are called spermatogonia. The process of their mitotic proliferation lasts approximately 2 weeks.
- Maturation (meiotic division). At this stage, cells are classified as spermatocytes. The maturation stage lasts about 1 month, with the major part of this time occupied by the complex prophase of meiosis I. In older literature, this stage was often called the "growth period," but the modern approach considers this term unfortunate. It overly simplifies the essence of the stage, reducing the complex genetic events of meiosis prophase to trivial growth.
- Formation (spermiogenesis). The essence of this stage is the morphological maturation of cells without division. Spermatids gradually transform into spermatozoa. The duration of the stage is about 1 month.
- Extraepithelial maturation. Occurs outside the tubular epithelium — in the epididymis. This final stage takes from 1 to 3 weeks.
Mitotic proliferation: syncytium and mathematics
The proliferation process is initiated by divisions of isolated type A spermatogonia. A crucial feature of spermatogenesis is that after mitosis, daughter cells do not separate completely. Cytoplasmic bridges are preserved between them. The first stage of such union is paired spermatogonia.
In all subsequent divisions (both in mitosis and meiosis), the progeny of a single initial cell remain physically connected, forming a clonal syncytium. It is this structure that ensures the absolute synchrony of their further development.
During the 2 weeks of the proliferation stage, cells manage to undergo 9 mitotic divisions. Consequently, a single cell cycle lasts on average 1.5 days, of which mitosis itself takes only about an hour, while the rest of the time is allotted to interphase (volume restoration).
Theoretically, a single initial cell gives rise to about 500 spermatogonia as a result of 9 divisions. Taking into account the subsequent meiosis, which increases the cell count fourfold, the theoretical yield should be around 2,000 spermatozoa. However, in reality, the yield is significantly lower due to massive apoptosis (cell death) at intermediate stages.
Classification of spermatogonia and preparation for meiosis
With each new cell cycle, spermatogonia change their morphological characteristics. The chain of their differentiation looks as follows: Type A spermatogonia (with indices from 1 to 4) → Intermediate spermatogonia → Type B spermatogonia.
The key marker of differentiation is the change in chromatin structure:
- Early cells (type A): Chromatin is diffuse, nuclei are pale. This indicates high genome activity.
- Late cells: The proportion of heterochromatin gradually increases. Nuclei become darker, indicating a gradual decrease in the functional activity of the genome during interphase.
This stage concludes with type B spermatogonia undergoing their final mitotic division. As a result, primary spermatocytes are formed — cells destined to enter meiosis. Just before the onset of meiosis, a genetic rearrangement is observed: the number of active regulatory genes decreases, which limits the overall "guiding" role of the genome in subsequent processes.
Briefly: answers to topical questions
Which stages of meiosis I prophase do spermatocytes undergo during the maturation stage?
Primary spermatocytes sequentially pass through five stages of meiosis I prophase during the maturation stage. The following phases and key events take place during this period:
- Leptotene — the initial stage of prophase;
- Zygotene — pairwise convergence (conjugation) of homologous chromosomes begins;
- Pachytene — the formation of the retaining ribbon-like structure (synaptonemal complex) is completed;
- Diplotene — the subsequent prophase stage;
- Diakinesis — the final stage.
During prophase I, crossing over (exchange of segments) and "genome correction" also take place.
What specific morphological changes occur in spermatids during the formation stage (spermiogenesis)?
During the spermiogenesis stage, morphological maturation of spermatids occurs without division, during which they transform into spermatozoa. The following specific changes take place:
- Chromatin condensation;
- Flagellum formation — from centrioles;
- Acrosome formation — from the Golgi apparatus;
- Mitochondrial arrangement — mitochondria cluster at the base of the tail forming a sheath;
- Removal of excess cytoplasm — excess cytoplasm is phagocytosed by cells.
PMFE: Proliferation, Maturation, Formation, Extraepithelial maturation — the 4 stages of spermatogenesis proper.
Class and exam questions
Why is the term "growth period" considered obsolete?
This term greatly simplifies the complex molecular genetic processes of the prolonged prophase of the first meiotic division, reducing them simply to an increase in cell volume.
What is the role of cytoplasmic bridges?
They connect cells originating from a single spermatogonium into a single clonal syncytium. This guarantees strict developmental synchrony of the entire clone during the stages of mitosis and meiosis.
Why is the actual sperm count lower than the theoretical calculation?
Although 9 mitoses and subsequent meiosis should yield about 2,000 cells from a single initial one, a significant portion of the population undergoes apoptosis at various maturation stages.
From which cells do primary spermatocytes form?
They are formed as a result of the final mitotic division of type B spermatogonia and then directly enter meiosis.
What else to study on this topic
- Structure and functions of the clonal syncytium
- Genetic regulation prior to entering meiosis
- Dynamics of chromatin changes in spermatogonia
- Morphological changes during spermiogenesis (formation)
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