The formation of sperm in humans, also called spermatogenesis is a process that spans 65-75 days and permits the production of approximately 300 million mature sperm per day in a normal male individual (Tortora and Derrickson, 2006, p. 1062). Sperm develop from the specialization of stem cells located close to the basement membrane of seminiferous tubules in the testis. These stem cells are called spermatogonia which form from the yolk sac or the extraembryonic membrane and migrate into the testis during the fifth week of embryonic development (Tortora and Derrickson, 2006, p. 1059, G37).
Spermatogonia are inactive during childhood as spermatogenesis begins at the onset of puberty – around 10 years of age when hormones effect the maturation of the reproductive system. During this period, some spermatogonia still remain dormant because they function as reserve cells for sperm formation in the future. The rest undergo mitosis and are subsequently released from the basement membrane. They pass through the tight junctions of the blood-testis barrier and further differentiate into primary spermatocytes until they complete their development.
Spermatogenesis follows stages wherein spermatogonia develop into primary and secondary spermatocytes, then further specialize into early and late spermatids and finally into spermatozoon or sperm cells (Tortora and Derrickson, 2006, p. 1061). These cells follow an arrangement of least mature to most mature starting from the basement membrane to the lumen of the seminiferous tubules. Primary spermatocytes as well as their stem cells are diploid – containing just 46 chromosomes but after they are formed, they replicate their DNA and undergo meiosis.
During meiosis I, pairs of chromosomes which are homologous line up at the metaphase plate and cross over where the meiotic spindle pulls a duplicated chromosome of each chromosome pair to the opposite pole of the cell undergoing division (Seeley, Stephens and Tate, 2007, p. 545). The two cells produced by a single primary spermatocyte during meiosis I are the secondary spermatocytes. These cells are haploid as they contain just 23 chromosomes – half the number of chromosomes found in primary spermatocytes.
However, each chromosome has two chromatids or copies of DNA present in its centromere and serve in the subsequent cell division process of spermatogenesis as DNA replication occurs only in primary spermatocytes (Tortora and Derrickson, 2006, p. 1062). Meiosis II follows meiosis I where the chromosomes once again line up at the metaphase plate and the two chromatids of each chromosome of the two secondary spermatocytes detach from each other.
The four haploid cells produced are the spermatids. Thus, one primary spermatocyte results in four cells by undergoing meiosis I and meiosis II (Seeley, Stephens and Tate, 2007, p. 546). Spermatids are not completely separated from each other as what happens in cytokinesis. Rather they are connected to each other by cytoplasmic bridges. Finally, each of the spherical spermatids develops an acrosome on top of their nuclei to form a head which compresses and elongates.
Flagella also develop in each spermatid, the number of mitochondria significantly increases while excess cytoplasm is removed by Sertoli cells (Seeley, Stephens and Tate, 2007, p. 545). This end process wherein a spermatid changes into an elongated and slender sperm is also known as spermiogenesis. Aside from the head and mitochondria, a sperm also has a neck, a centriole and a tail divided into the middle, principal and end pieces.
Sperm cells then detach from the Sertoli cells during the process of spermiation and go into the lumen of the seminiferous tubules (Tortora and Derrickson, 2006, p. 1063). The Sertoli cells also perform an added function in secreting fluids that push the sperm cells toward the ducts of the testes.
List of References Seeley, R. R. , Trent, D. S. and Tate, P. (2007). Essentials of Anatomy and Physiology (6th ed). New York: McGraw-Hill Companies, Inc. Tortora, G. J. and Derrickson, B. (2006). Principles of Anatomy and Physiology (11th ed). New Jersey: John Wiley and Sons Inc.