Spermatogenesis is the process through which immature sperm cells called Spermatogonia (which are actually stem cells) are transformed through a series of different types of cell divisions in order to produce mature sperm cells called Spermatozoa. Spermatogonia (or Spermatogonium singular) are undifferentiated spermatogenic cells that need to be differentiated through the processes of mitosis and meiosis before they become mature sperm cells. Spermatogenesis occurs in the seminiferous tubules of the testis. Both Spermatogenesis and Oogenesis are forms of Gametogenesis (formation of male and female gametes from primordial germ cells).
Explanation of the Process of Spermatogenesis
At birth, germ cells (or stem cells) in the male are found in structures called sex cords which are in the testis. These stem cells appear as large and pale cells surrounded by supporting cells. The supporting cells are derived from the epithelial surface of the seminiferous tubules these supporting cells are called Sertoli cells or Sustentacular cells and are involved in the support and regulation of spermatogenesis.
Before the onset of puberty, the sex cords acquire a lumen to become the seminiferous tubules and the primordial germ cells or stem cells give rise to spermatogonial stem cells. This stem cell then continues to produced type A spermatogonia at regular intervals, and their production marks the beginning of spermatogenesis. These Type A cells undergo a limited number of mitotic divisions called Spermatocytogenesis to form a clone of cells with the last cell division producing type B spermatogonia, which then divides to form primary spermatocytes. Primary spermatocytes then enter prolonged prophase with a duration of about 22 days; this is followed by rapid completion of meiosis I and the formation of secondary spermatocytes.
During meiosis II, these cells immediately begin to form haploid spermatids. From the time type-A cells leave the stem cell population to the formation of spermatids, the division of the cytoplasm of these cells (cytokinesis) is incomplete such that successive cell generations are joined by cytoplasmic bridges making the progeny of a single type A spermatogonium to form a clone of germ cells that maintain contact throughout differentiation.
The Spermatids then undergo a series of changes resulting in their transformation into Spermatozoa through the process of Spermiogenesis.
Sperm Production Cycle
- The spermatogonia first divide by mitosis to form dark type A spermatogonia, which act as stem cells. Each dark type A spermatogonium undergoes mitosis to form one dark A spermatogonium and another light type-A spermatogonium. The dark type A spermatogonia are kept in reserve for repetition of the next cycle. The light type A undergoes mitotic division to form two dark type B spermatogonia.
- The type B spermatogonium undergoes mitotic division to form two primary spermatocytes (largest germ cells). The process of mitosis which produces more spermatogonia is called Spermatocytogenesis this process involves a series of mitotic divisions to form a large number of spermatogonia. Depending upon their appearance, three types of spermatogonia are distinguished which are: dark type A spermatogonia, light type A spermatogonia, and type B spermatogonia.
- The primary spermatocytes undergo first meiotic division (reductional division) to form two secondary spermatocytes. The secondary spermatocytes thus have a haploid number of chromosomes.
- Each secondary spermatocyte immediately undergoes second meiotic division (i.e., mitotic division) to form two spermatids, each with a haploid number of chromosomes. Thus, four haploid spermatids are produced from the meiotic division of one primary spermatocyte. The spermatids are small cells of about half the size of the secondary spermatocyte and have round and darkly stained nuclei. The spermatids lie close to the lumen of the seminiferous tubule.
- Each circular spermatid gradually changes through a process known as Spermiogenesis to become an elongated spermatozoon or sperm.
Thus from one primary spermatocyte, four spermatozoa are formed two with 22 autosomes and one X chromosome (22 + X, 22 + X) and two with 22 autosomes and one Y chromosomes (22 + Y, 22 + Y)
Changes that occur during Spermiogenesis to form a Spermatozoa from Spermatids
- Formation of the acrosome – which covers half of the nuclear surface and contains enzymes to assist in penetration of the egg and its surrounding layers during fertilization
- Condensation of the nucleus
- Formation of the neck, middle piece, and tail of the spermatozoa
- Shedding of most of the cytoplasm.
Duration of Spermatogenesis
In humans, the time required for complete spermatogenesis that is, for a spermatogonium to develop into a mature spermatozoon is approximately 64 days. When fully formed, the spermatozoa enter the lumen of seminiferous tubules and are pushed toward the epididymis by contractile elements in the wall of the seminiferous tubules.
Hormonal regulation of Spermatogenesis
Spermatogenesis is regulated by the effect of luteinizing hormone (LH) which is produced by the pituitary gland. Luteinizing hormone does this by binding to receptors on Leydig cells of the testes and stimulates testosterone production; Testosterone then binds to Sertoli cells ( the testicular target of follicle-stimulating hormone ) to promote Spermatogenesis. Follicle-stimulating hormone (FSH) is also essential because of its binding to Sertoli cells stimulates testicular fluid production and synthesis of intracellular androgen receptor proteins.