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This process occurs in the seminiferous tubules of the testis. It begins at puberty and continues throughout life.

Type A spermatogonia, which are derived from the primordial germ cells that migrated from the wall of the yolk sac, undergo mitosis. Some of the daughter cells remain as type A spermatogonia to serve as a reservoir of stem cells while others differentiate into type B spermatogonia, which contain the diploid number of chromosomes (46), but which will undergo meiosis to produce sperm cells which contain the haploid number of chromosomes (23).

Type B spermatogonium will produce 2 primary spermatocytes. Note that the primary spermatocytes are the result of a mitotic division and are therefore diploid.

At the end of the first meiotic division, each
primary spermatocyte will produce 2 secondary spermatocytes. During meiosis I, the chromosomal DNA is replicated and each chromosome pair consists of four strands of DNA called chromatids. At the end of the first meiotic division, each secondary spermatocyte contains 23 chromosomes. Each of these chromosomes consists of paired chromatids.

Each secondary spermatocyte completes the second meiotic division without the replication of DNA and produces 2 spermatids each containing 23 chromosomes.
Spermatids undergo morphologic alteration (spermiogenesis) to become mature spermatozoa.

These changes include the formation of the
acrosome, condensation of the nucleus, formation of a neck, middle piece and tail and shedding of most of the cytoplasm.
Read about the relationship between the developing spermatocytes and Sertoli cell in the textbook