Updated by: Arabinda Naik
Spermatogenesis is the process by which sperm cells (spermatozoa) are produced in the male reproductive system. It occurs in several phases, including the multiplication phase, also known as the mitotic phase or spermatogonial phase.
Multiplication Phase (Spermatogonial Phase): This phase represents the initial step of spermatogenesis. It involves the multiplication or proliferation of spermatogonial cells, which are the stem cells of the process. These spermatogonial cells are located in the seminiferous tubules of the testes multiply by mititic cell division and increase in number.
There are two main types of spermatogonia:
Type A Spermatogonia: These are the stem cells that give rise to new spermatogonia through mitosis. Type A spermatogonia can either undergo self-renewal to maintain the population of stem cells or differentiate into type B spermatogonia.
Type B Spermatogonia: Type B spermatogonia are the immediate descendants of type A spermatogonia. They undergo further divisions through mitosis and eventually differentiate into primary spermatocytes, which are the first cells in the spermatogenesis process to undergo meiosis.
Growth phase:Spermatogonia grow and increase in size and form primary spermatocytes. Each spermatogonium is diploid and contains 46 chromosomes.
Multiplication phase:Each primary spermatocyte undergoes a specialized type of cell division called meiosis to produce haploid (n) secondary spermatocytes.
Meiosis I: Meiosis is a two-step cell division process that reduces the chromosome number by half. During the first meiotic division (meiosis I), each primary spermatocyte undergoes homologous chromosome pairing, recombination, and segregation, resulting in two haploid cells known as secondary spermatocytes.
Secondary Spermatocytes: Secondary spermatocytes are haploid (n) cells. They are the product of meiosis I and contain half the number of chromosomes as the original primary spermatocytes. Secondary spermatocytes do not undergo DNA replication between meiosis I and meiosis II.
Meiosis II: Each secondary spermatocyte then undergoes a second meiotic division (meiosis II), without DNA replication to produce four equal haploid spermatids.
Spermatids: Spermatids are immature sperm cells. They have the same genetic content as the secondary spermatocytes but differ in their cellular morphology. Spermatids undergo a process called spermiogenesis, during which they undergo extensive structural and functional changes to develop into mature, motile spermatozoa.
which is the process by which male germ cells (spermatogonia) develop into mature sperm cells (spermatozoa or simply sperm). Spermiogenesis occurs in the seminiferous tubules of the testes and involves a series of dramatic changes in the structure and function of developing sperm cells. These changes are necessary to ensure that the sperm cells are fully equipped to fertilize an egg during sexual reproduction.
During spermiogenesis, several key events take place:
Reduction in Cell Size: The developing sperm cell, called a spermatid, undergoes a reduction in cell size as it matures.
Condensation of Chromatin: The chromatin (genetic material) in the nucleus of the spermatid becomes highly condensed, resulting in a compact and streamlined structure. This helps protect the genetic material during its journey to the egg.
Formation of Acrosome: The acrosome is a specialized organelle that develops at the anterior (head) of the sperm cell. It contains enzymes that are essential for penetrating the protective layers of the egg during fertilization.
Elongation of the Tail: The spermatid's tail (flagellum) elongates and becomes a long, whip-like structure that provides the motility needed for the sperm to swim toward the egg.
Shedding of Excess Cytoplasm: Excess cytoplasm is shed from the developing sperm cell to reduce its size and make it more streamlined. This process is known as "residual body extrusion."
Maturation of Mitochondria: Mitochondria, which provide the energy for sperm motility, become concentrated in the midpiece of the sperm cell.
Once spermiogenesis is complete, the spermatid has transformed into a fully mature sperm cell capable of fertilizing an egg. It is released into the lumen of the seminiferous tubule and then transported to the epididymis for further maturation and storage until ejaculation.
Head: The head of a sperm cell is oval or ellipsoid in shape and makes up the majority of the cell. It contains the genetic material necessary for fertilization in the form of a condensed nucleus. The head is covered by a cap-like structure called the acrosome, which contains enzymes that help the sperm penetrate the protective layers surrounding the egg during fertilization.
Midpiece: The midpiece is a slender, cylindrical region located behind the head. It is densely packed with mitochondria, which provide the energy (in the form of ATP) needed for the sperm's motility. This region is essential for the sperm's ability to swim toward the egg.
Tail (Flagellum): The tail is a long, whip-like structure extending from the midpiece. It is responsible for the sperm's locomotion and propelling it through the female reproductive tract to reach the egg. The tail contains microtubules, which form the axoneme and provide the structural support for flagellar movement.
Neck: The neck is a short region that connects the head and midpiece. It contains structures called centrioles, which are important for organizing the microtubules in the sperm tail and are involved in the formation of the mitotic spindle during fertilization.
Plasma Membrane: The sperm cell is enclosed by a plasma membrane, which separates the internal components of the cell from the external environment. The plasma membrane contains receptors that allow the sperm to interact with molecules on the surface of the egg during fertilization.
A human male ejeculate about 200-300 million sperms during a coitus. Seminal plasma along with the sperms constitute the semen.