<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> Spermatogenesis is the process in which an animal produces spermatozoa from spermatogonial stem cells by way of mitosis and meiosis . The initial cells in this pathway are called spermatogonia, which yield primary spermatocytes by mitosis . The primary spermatocyte divides meiotically (Meiosis I) into two secondary spermatocytes; each secondary spermatocyte divides into two spermatids by Meiosis II . These develop into mature spermatozoa, also known as sperm cells . Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa . </P> <P> Spermatozoa are the mature male gametes in many sexually reproducing organisms . Thus, spermatogenesis is the male version of gametogenesis, of which the female equivalent is oogenesis . In mammals it occurs in the seminiferous tubules of the male testes in a stepwise fashion . Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for sexual reproduction . DNA methylation and histone modification have been implicated in the regulation of this process . It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age (see Male infertility). </P> <P> Spermatogenesis produces mature male gametes, commonly called sperm but more specifically known as spermatozoa, which are able to fertilize the counterpart female gamete, the oocyte, during conception to produce a single - celled individual known as a zygote . This is the cornerstone of sexual reproduction and involves the two gametes both contributing half the normal set of chromosomes (haploid) to result in a chromosomally normal (diploid) zygote . </P>

When does spermatogenesis begin what causes it to begin