Spermatogenesis is a sperm differentiating process which develops inside the testicles and starts with spermatogonia’ mitotic proliferation (Spermatocytogenesis), going through spermatocyte development (meiosis) and culminating with spermatid alteration –the round haploid cell becomes elongated sperm cell through the process of spermiogenesis. This complex process of transformation takes place inside the testicles’ seminiferous epithelium. Testicular epithelium is formed of two populations of cells: germinal cells, in different stages of differentiation, and Sertoli somatic support cells.
Primordial germinal cells have been detected for the first time at the end of the third gestational week. These cells are initially placed right next to the Yolk bag and migrate through amoeboid movements towards the genital ridge. After an intense process of proliferation, two primitive germinative cell populations called gonocytes. Up to the 6th gestational week, the gonocytes have a large nucleus with one nucleolus, and its mitochondria and cytoplasm present many microfilaments formations. The 2nd type of gonocytes contains large deposits of glycogen and mitochondria inside their cytoplasm, which vanish after the 10th week of gestation.
During the 3rd month of gestation, fetal spermatogonia can be found, but their origin isn’t very well known. It is believed that gonocytes differentiate into spermatogonias, through mitotic division. Spermatogonia which passes mitosis, generates certain cell types which evolve during male’s entire reproductive life. Most common types of spermatogonia are: Ap, Ad, B and AL.
Type B spermatogonium completes a final mitotic division and then, differentiates itself in two primary spermatocyte initiating meiosis. Meiosis involves two successive nuclear divisions in order to reach to the stage of haploid cell. The first meiotic division, called prophase, is a complex process which evolves chromosome’s condensation, pairing, exchange and separation.
The preleptotene spermatocyte is a result of mitotic division of type-A spermatogonia. Type-A cells have smaller nucleus than type-B and they are located at the periphery of seminiferous tubules. They can replicate their DNA, then they spermatocyte enters in meiosis leptotene stage where chromosomes appear as ultrathin filaments. Leptotene stage if followed up by a zygotene stage, where genetic material synapses. The chromosomes thicken; they pair up, and continually interchange, then, pachytene stage of prophase follows. After the completion of pachytene stage, a spermatocyte enters diplotene stage; the paired chromosomes separate, and chiasma (chromosomal bridges) can be observed. Finally, during diakinesis, the chromosomes continue their separation and the prophase of the first meiosis is complete.
After diakinesis the nuclear envelope disintegrates initiating metaphase I. Afterwards, anaphase I and telophase I are completed by the migration of the chromatid towards the opposite pole of the new cell, called secondary spermatocyte, it contains a haploid number of chromosomes, it has a shorter life and it is smaller then the primary spermatocyte, it has spherical nuclei which contain granular chromatid.
DNA synthesis takes place between the two meiotic cellular divisions. Meiosis’ II interphase is shorter and the cells rapidly pass through metaphases II, anaphases II, and telophase II. The completion of these phenomena leads to the origination of the young spermatid, which after functional and morphologic changes will become a spermatozoid.