Spermatogenesis is the production of sperm in the testes of animals. It begins at puberty and occurs in the epithelial lining of seminiferous tubules. It begins from spermatogonia which are germ cells found around the outer edge of the seminiferous tubules next to the basal lamina. They constantly reproduce by mitosis. Some cells then differentiate into primary spermatocytes which then divide by meiosis to form two secondary spermatocytes. These cells then divide by meiosis themselves to form 4 haploid cells called spermatids. Differentiation occurs producing mature sperm cells which migrate from the tubules to the epididymis where they are stored .
Developmental Stages of Spermatogenesis:
- Primary spermatocyte
- Secondary spermatocyte
- Sperm cell
During the course of spermatogenesis, germ cells move towards the lumen as they mature.
Spermatogenesis can be subdivided into two successive sections:
- The first includes the cells from the spermatogonium up to the secondary spermatocyte, this is called spermatocytogenesis.
- The second one includes the differentiation of the sperm cell, starting with the spermatid phase and is called spermiogenesis (or spermiohistogenesis).
Among the spermatogonia 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 heteronymous division, in which the daughter cells remain bound together by thin bridges of cytoplasm.
After a further mitotic division type B spermatogonia are engendered that also divide themselves mitotically into primary spermatocytes.
The freshly created primary spermatocytes now enter into the first meiosis. They then go immediately into the S phase (preleptotene meiosis), double their DNA, leave the basal compartment and reach 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 5 sections:
In the prophase in every germ cell a new combination of maternal and paternal genetic material occurs. After the long prophase follows the metaphase, anaphase and telophase that take much less time. One primary spermatocyte yields two secondary spermatocytes.
The secondary spermatocytes go directly into the second meiosis, where the spermatids emerge. Since in the secondary spermatocytes neither DNA reduplication nor a recombination of the genetic material occurs, the second meiosis occurs quickly. It lasts only around 5 hours. Through the division of the chromatids of a secondary spermatocyte, 2 haploid spermatids emerge that contain only half the original DNA content.
Besides the sperm cells the spermatids are the smallest cells of the germinal epithelium. In a process lasting several weeks, they are transformed into sperm cells with the assistance of the Sertoli's cells.
- ↑ Alberts et al. (2002)
2. Human embryology. Gametogenesis: Spermatogenesis. Available from: http://www.embryology.ch/anglais/cgametogen/spermato04.html