25.1:
Spermatogenesis
During human spermatogenesis, stem cells give within the testes give rise to functional sperm cells.
This process occurs within seminiferous tubules, muscle-wrapped structures that contain a peripheral layer of cells surrounding a hollow center. Importantly, the inner layer contains stem cells, each known as a spermatogonium, and supporting sertoli cells.
Factors produced during puberty, stimulate a spermatogonium to divide via mitosis. Of the two ensuing cells, one continues to serve as a stem cell, while the other is a precursor, that will ultimately go on to form sperm.
Multiple, subsequent mitotic divisions of this precursor cell, eventually result in a new type of cell, called a spermatocyte, located closer towards the interior of the tubule.
At this point, additional factors modulated by sertoli cells, direct spermatocytes to enter meiosis, causing them to divide twice, thus every spermatocyte produces four haploid cells, each containing a single chromosome set.
These haploid spermatids are situated at the boundary of the cell layer, and hollow core of the seminiferous tubule. As spermatogenesis proceeds, the spermatids lose cytoplasm, thin, and develop characteristic tails called flagella.
The resulting, differentiated sperm are then released into the tubules hollow center, and eventually make their way through the male genital tract.
Thus, the final outcome of spermatogenesis is numerous haploid, motile sperm cells, equipped to fertilize eggs.
25.1:
Spermatogenesis
Spermatogenesis is the process by which haploid sperm cells are produced in the male testes. It starts with stem cells located close to the outer rim of seminiferous tubules. These spermatogonial stem cells divide asymmetrically to give rise to additional stem cells (meaning that these structures “self-renew”), as well as sperm progenitors, called spermatocytes. Importantly, this method of asymmetric mitotic division maintains a population of spermatogonial stem cells in the male reproductive tract, ensuring that sperm will continue to be produced throughout a man’s lifespan. As spermatogenesis proceeds, spermatocytes embark on meiosis, and each ultimately divides to form four sperm—each with only 23 chromosomes— that are expelled into the male reproductive tract. Interestingly, this is in contrast to oogenesis in women, during which only a single egg is generated for every progenitor cell.
Spermatogenesis and Capacitation
At the end of spermatogenesis, sperm demonstrate their characteristic shape: a “head” harboring minimal cytoplasm and a highly condensed nucleus, as well as a motile tail (flagellum). They are small cells, with no organelles such as ribosomes, ER or Golgi, but do have many mitochondria around the flagellum for power. Just below the head is the acrosomal vesicle which contains hydrolytic enzymes to penetrate the egg outer coat—the contents are released by exocytosis.
Sperm cells endure an additional maturation process after spermatogenesis termed capacitation. Interestingly, capacitation only occurs in sperm once these cells have been introduced into the female genital tract. Here, components of the female environment—like mucus—elicit critical changes in these gametes, among them the removal of cholesterol from their membranes. This enables different molecules to enter into sperm, such as bicarbonate ions, which can activate new proteins and increase tail movements. Such rigorous motions help sperm to navigate towards, and ultimately infiltrate, the thick protein-based zona pellucida surrounding an egg cell. In addition, cell membrane alterations lead to an increase in the concentration of calcium ions in sperm, which is also necessary for zona pellucida penetration. Thus mature sperm, demonstrating rapid tail activity and the ability to locate, bind and penetrate an egg, require the unique conditions of the female reproductive organs.
Factors that Affect Spermatogenesis
Several factors can affect sperm production. One well-documented influence is exposure to heat in the scrotum: men shown to sit for long periods of time (like cab drivers) or metal welders demonstrate increased risk of infertility or sperm with decreased motility. In addition to temperature, certain drugs have also been determined to have adverse effects on spermatogenesis. These include hormone antagonists used to address prostate conditions, cancer treatments, and even specific antibiotics. As a result, some professionals are emphasizing the need to warn men of the effects of these drugs on their reproductive health, rather than only focusing on what medications a woman planning a pregnancy should avoid.
Suggested Reading
Bailey, Janice L. “Factors Regulating Sperm Capacitation.” Systems Biology in Reproductive Medicine 56, no. 5 (October 1, 2010): 334–48. [Source]
De Jonge, Christopher. “Biological Basis for Human Capacitation—Revisited.” Human Reproduction Update 23, no. 3 (May 1, 2017): 289–99. [Source]
Ding, Jiayi, Xuejun Shang, Zhanhu Zhang, Hua Jing, Jun Shao, Qianqian Fei, Elizabeth R. Rayburn, and Haibo Li. “FDA-Approved Medications That Impair Human Spermatogenesis.” Oncotarget 8, no. 6 (October 27, 2016): 10714–25. [Source]