General Characteristics of the Process
The process of sperm maturation includes two consecutive meiotic divisions. First, the primary spermatocyte divides, followed by the secondary spermatocyte. Note the specific notation used for the genetic material: the symbol n indicates the haploid amount of DNA, whereas the symbol c denotes the haploid chromosome set.
The first division contributes most to the overall duration of the process. For this reason, primary spermatocytes are the most frequently visualized germ cells during microscopy of the seminiferous tubules.
First Meiotic Division (Reductional Division)
The first division begins with a preparatory phase (interphase I) during which DNA replication occurs. Before synthesis begins, the cell has a 2n, 2c complement. Following the duplication of genetic material, the ploidy of preleptotene primary spermatocytes changes to 4n, 2c—yielding a tetraploid amount of DNA with a diploid chromosome set. Each chromosome becomes a two-chromatid structure.
The longest stage is prophase I, which lasts about one month and includes several sequential substages:
- Preleptotene: A preparatory stage where DNA is duplicated immediately before prophase. A distinct G2 period is absent.
- Leptotene (thin thread stage): Chromosomes begin to condense and become visible.
- Zygotene (paired thread stage): Active synapsis occurs, during which homologous chromosomes align closely together.
- Pachytene (thick thread stage): The longest substage of prophase. Paired chromosomes thicken and shorten, and protein synaptonemal complexes form. Amidst partial decondensation, crossing over (genetic exchange), radical DNA repair, and mRNA synthesis take place actively, leading to an increase in nuclear and cytoplasmic volume. It ends with the breakdown of the synaptonemal complexes.
- Diplotene (double thread stage): Homologs begin to repel each other; chiasmata (sites of crossing over) and tetrads (each pair consisting of 4 chromatids) become visible.
- Diakinesis: Chiasmata terminalize and disappear, homologs separate completely, the nuclear envelope breaks down, and the meiotic spindle forms.
In metaphase I, 46 chromosomes align along the cell equator as bivalents. The key difference from mitosis is seen in anaphase I: whole homologous two-chromatid chromosomes segregate to opposite poles, rather than sister chromatids. In telophase I, nuclear envelopes re-form and cytokinesis occurs, though separation is incomplete—the 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 complement (23 duplicated chromosomes). These cells are genetically diverse and segregate into pools carrying either an X or a Y chromosome.
Second Meiotic Division (Equational Division)
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, as the chromosomes already consist of two chromatids.
The division itself proceeds in a classic mitotic fashion through prophase II, metaphase II, anaphase II, and telophase II. During anaphase II, the 23 chromosomes split, and individual sister chromatids migrate to opposite poles.
As a result of the second division, the cell count doubles compared to secondary spermatocytes. Early spermatids are formed with a true haploid complement of 1n, 1c. Like their precursors, spermatids possess a unique genome and are distributed into two populations depending on the presence of sex chromosomes (X or Y).