Among the spermatogonia (all in all, over 1 billion in both testicles) that form the basal layer of the germinal epithelium, several types can be distinguished: certain type A cells are seen as spermatogonia that divide mitotically and reproduce themselves (homonymous division), whereby the spermatogonia population is maintained.
The beginning of spermatogenesis is introduced through the so-called heteronymous division, in which the daughter cells (second group of type A cells) remain bound together by thin bridges of cytoplasm. Through the preservation of these cytoplasmic connections, spermatogonia are inducted into the spermatogenesis process.
After a further mitotic division type B spermatogonia are engendered that also divide themselves mitotically into primary spermatocytes (I).
The freshly created primary spermatocytes (I) now enter into the first meiosis. They then go immediately into the S phase (that is, into the preleptotene meiosis), double their internal DNA, leave the basal compartment and reach the special milieu of the luminal compartment. Following the S phase, these cells attain the complex stage of the prophase of the meiosis and become thereby noticeably visible with a light microscope.
This prophase, which lasts 24 days, can be divided into five sections:
- Leptotene
- Zygotene
- Pachytene
- Diplotene
- Diakinesis
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Commentary
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In the heteronymous division the cytoplasmic division is not completed; the daughter cells stay bound together through thin cytoplasmic bridges.
Also in the subsequent meiosis the cytoplasmic division is incomplete, so that from one spermatogonium a network of daughter cells arises that doubles in size in each generation. The forming of such networks assures that all of the processes in each generation occur in step with each other.
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