Meiosis in Spermatogenesis

General Process Overview

The sperm maturation process involves two successive meiotic divisions. First, the primary spermatocyte (Sp I) divides, followed by the secondary spermatocyte (Sp II). Note the specific notation used for genetic material: the symbol n indicates the haploid set of DNA, whereas the symbol c denotes the haploid set of chromosomes.

The first division accounts for the major part of the duration of the entire process. This is precisely why primary spermatocytes are most frequently visualized during microscopy of the seminiferous tubules.

First meiotic division (reductional)

The first division begins with a preparatory phase (interphase I) during which DNA replication occurs. Prior to synthesis, the cell has a 2n, 2c set. Following the duplication of genetic material, the ploidy of preleptotene Sp I changes to 4n, 2c—forming a tetraploid amount of DNA with a diploid set of chromosomes. Each chromosome becomes two-chromatid.

The longest stage is prophase I, which takes about one month and includes a series of sequential phases:

1. Preleptotene: the preparatory stage when DNA is duplicated immediately before prophase. A distinct G2 period is absent.
2. Leptotene (thin thread stage): chromosomes begin to spiralize and become visible.
3. Zygotene (paired thread stage): active conjugation occurs, during which homologous chromosomes lie closely apposed to each other.
4. Pachytene (thick thread stage): the longest part of prophase. The paired chromosomes thicken and shorten, and protein synaptonemal complexes form. Against the background of partial decondensation, active crossing-over (exchange of segments), radical DNA repair, and mRNA synthesis take place, leading to an increase in nuclear and cytoplasmic volume. It concludes with the breakdown of the synaptonemal complexes.
5. Diplotene (double thread stage): homologues begin to repel each other, chiasmata (crossover crossovers) and tetrads become visible (each pair consists of 4 chromatids).
6. Diakinesis: chiasmata disappear, homologues completely separate, the nuclear envelope breaks down, and the spindle apparatus forms.

In metaphase I, 46 chromosomes align along the cell equator as bivalents. The key difference from mitosis appears in anaphase I: intact homologous two-chromatid chromosomes diverge to the poles rather than sister chromatids. In telophase I, nuclear envelopes reform and cytokinesis occurs, though complete separation does not happen—cells remain connected by a cytoplasmic bridge.

The outcome of the first division is the formation of secondary spermatocytes (Secondary Spermatocytes) with a 2n, 1c set (23 two-chromatid chromosomes). These cells are genetically heterogeneous and divide into pools carrying either an X or a Y chromosome.

Second meiotic division (equational)

Preparation for the second division (interkinesis) occurs very rapidly. The main feature of this stage is the complete absence of DNA replication and chromosomal protein synthesis, since the chromosomes already consist of two chromatids.

Division itself proceeds according to the classical mitotic type, passing through prophase II, metaphase II, anaphase II, and telophase II. During anaphase II, the 23 chromosomes separate, and individual sister chromatids migrate to the poles.

As a result of the second division, the number of cells doubles compared to Sp II. Early spermatids with a truly haploid set of 1n, 1c are formed. Like their precursors, spermatids possess a unique genome and are distributed into two populations depending on the presence of sex chromosomes (X or Y).

Class and exam questions

What else to study on this topic

• Mechanisms of formation and function of synaptonemal complexes
• The process of radical DNA repair at the pachytene stage
• Role and fate of cytoplasmic bridges between daughter cells
• Genetic diversity of spermatids: distribution of X and Y chromosomes

Other topics in the "Male Reproductive System" section

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