Your body’s sperm cells develop within a system of tubes called the seminiferous tubules, inside your testicles. These simple round cells must transform into sperm with a head and tail, like a tadpole, in a process called spermatogenesis.
It is a complex process regulated by sex hormones such as FSH, LH and testosterone. Mature sperm cells have a head full of genetic material, a middle section and a tail that propels the sperm cell to break through an egg.
Seminiferous tubules
The seminiferous tubules are a system of tubes that transport mature sperm cells to the epididymis. In the testis, these tubes are covered by a thick connective tissue coat called the tunica albuginea. Thin septa radiate from this layer to separate the testis into lobules, each of which contains one or more seminiferous tubules.
Each tubule is lined by a sheet of epithelial support cells known as Sertoli cells. These cells are tall, simple columnar structures that span from the basement membrane to the lumen of the tube. These cells provide nutrients and raw materials to proliferating germ cell stems, which transform into spermatocytes and then spermatids. They also phagocytose excess spermatid cytoplasm that is not required in the formation of sperm cells.
As the first stage of [spermatogenesis] progresses, [Sertoli cells] multiply and change their shape, as shown in Figure 1. They lose their sheath-like appearance and become narrower and of a different shape. Eventually, they create niches in the walls of the tubule tract that are occupied by second-stage [spermatogonia] and [spermatids].
Sertoli cells
Originally described by Enrico Sertoli in 1865, somatic Sertoli cells play an essential nursing function during spermatogenesis. They form the lining of the seminiferous tubules, which are filled with developing germ cells (spermatogonia). The tight junctions between adjacent Sertoli cells form the blood-testis barrier (BTB), which protects the diploid germ cells from foreign agents and from the host’s own immune system.
During spermatogenesis, a number of growth factors and cytokines are secreted by the Sertoli cells. These growth factors regulate the self-renewal of SSCs and the differentiation of spermatocytes into spermatozoa. They also control the meiosis of spermatocytes and the transformation of round spermatids into sperm cells.
In addition, Sertoli cells produce B-cells and a protein that binds testosterone. They also secrete a fluid that nourishes the sperm cells and provides them with a medium through which they can swim. These secretions are also necessary for sperm maturation. The sperm cells consist of a head, body and tail. The tail is a bundle of proteins that enables them to propel themselves through the seminal fluid.
Primary sperm cells
The spermatogonia in the testes produce germ cells that eventually mature into sperm. The cells that are destined to become sperm first multiply by the process of mitosis, which results in two new cells with the same number of chromosomes as the original parent cell. One of these new cells is destined to become a sperm cell and is called a primary spermatocyte.
The primary spermatocytes undergo a special metamorphosis to become mature sperm cells (spermatids). Each sperm cell has 23 chromosomes and is round with a head, midpiece, and tail. The head contains a nucleus and is surrounded by a thick protein layer, the midpiece contains mitochondria to produce energy, and the tail forms a structure that aids in locomotion.
As the spermatids continue maturing they are grouped together and bound to each other by thin cytoplasmic bridges. The spermatids then form a cyst, which contains an ideal liquid medium with the necessary nutrients for the cellular process of meiosis to occur. The spermatocytes within the cyst then go through meiosis, which is a series of specialized cell division that reduces the number of chromosomes by producing four haploid spermatids from each primary spermatocyte.
Secondary sperm cells
Sperm cells mature in the testes, a pair of coiled structures that contain seminiferous tubules. The process by which these precursor cells transform into fully-formed sperm is called spermatogenesis, and it takes around 74 days to complete. Sperm cell development begins with spermatogonia, which are germ cells that grow in the epithelial lining of the seminiferous tubules. They duplicate themselves mitotically to renew their population and then differentiate into primary spermatocytes. The primary spermatocytes go through meiosis I, resulting in two diploid secondary spermatocytes. These then undergo meiosis II, resulting in four haploid spermatids.
Spermatids are round and non-motile in shape, with mitochondria present throughout their cytoplasm. They also have tightly packaged chromatin, which is transcriptionally inactive. The acrosome, a structure at the head of the spermatid, is surrounded by the nucleus and the centrioles. The acrosome allows the sperm to enter the body of a female.
Sperm cells then mature into spermioctes, which are spherical and have tails. They are transported from the testes to the epididymis, a long and coiled tube that sits atop each of the testes. They become motile and fertile in the epididymis and are then expelled from the body through a duct called the ejaculatory duct and the urethra.
Maturation
Sperm cells are produced inside a system of coiled tubes called seminiferous tubules within the testicles. The seminiferous tubules connect the testicles to the rest of the male reproductive organs, including the seminal vesicles and prostate gland. The testes are also connected to the urethra through a long, coiled tube known as the epididymis. This is where sperm cells mature and acquire the ability to swim.
The production of sperm is a complex and highly regulated process that takes an average of 74 days to complete. It begins with diploid, sperm-producing cells called spermatogonia. Then these cells undergo a series of metamorphoses to become fully mature sperm cells called spermatozoa. This process is known as spermatogenesis and it’s controlled by hormones such as FSH, LH, and testosterone.
Once sperm cells have matured, they’re transported out of the testes into the epididymis, where they remain until they’re ejaculated. During ejaculation, the sperm cells mix with seminal fluid (a whitish liquid produced by the seminal vesicles, the prostate gland, and the epididymis) to form semen. Then it’s released through the urethra.
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