Spermatogenesis is the biological process whereby spermatogonia (the germ cell) develop into spermatozoa (the mature sperm cells). This process takes place in the seminiferous tubules of the testes; this is the starting point for spermatogenesis. Stem cells adjacent to the inner tubule wall divide, beginning at the walls and proceeding into the innermost part, or lumen producing immature sperm. Maturation occurs in the epididymis where sperm develop a tail and become motile.
Anatomy of the Testes
Seminiferous tubules comprise the majority of the structure of the testes. The remaining spaces between the tubules are occupied by interstitial tissue.
The interstitial tissue comprises mainly of blood vessels, lymph gaps, connective tissue and Leydig cells (specialised cells found in interstitial tissue adjacent to seminiferous tubules). Mast cells and macrophages are also present in small numbers.
Leydig Cells – These cells have an abundance of smooth endoplasmic reticulum and no rough endoplasmic reticulum, along with large amounts of mitochondria, lipid droplets and centrioles. They also have a prominent Golgi complex. Another cell specific feature is the receptors they have on their cell membrane that are highly specific to luteinising hormone (LH) – This differentiates them from other testicular cells, as they are the only cells to have these receptors.
Follicle-stimulating hormone (FSH) increases the response of Leydig cells by increasing the number of LH receptors expressed on their surface. The LH receptors when stimulated secrete steroidal hormones such as testosterone. Testosterone has a key role in the development of spermatozoa (spermatogenesis).
Typical seminiferous tubules include the Sertoli cells and germ cells. The epithelium of these tubules is known as the germinal epithelium. The seminiferous tubules have a fluid filled lumen; mature spermatids are released into this lumen as fully mature spermatozoa. Myoid cells surround the basement membrane of seminiferous tubules; they are contractile in nature and their contractions move sperm along the seminiferous tubules.
Sertoli cells have a main function in the nurturing of spermatozoa through their early stages of development from germ cells right up to their mature spermatid form (before being released into the seminiferous tubules to become spermatozoa). Sertoli cells therefore have a prime role in the co-ordinating of spermatogenesis – without Sertoli cells, spermatogenesis cannot take place.
Near the base of the cells in their lateral walls, tight junctions join Sertoli cells to one another. The formation of tight junctions means that inter-cellular diffusion of material is prevented. The tight junctions form a complete barrier that divides the tubule into a basal compartment and an adluminal compartment. Different types of germ cells occupy the different compartments. This barrier forms a blood-testes barrier, which isolates spermatocytes from the rest of the body, allowing for the environmental conditions required for spermatogenesis.
An important feature of the blood-testes barrier is that it prevents immune cells from reaching the haploid cells produced during spermatogenesis. This is important because the haploid cells are not recognisable as ‘self’ cells, meaning if the immune cells could reach them, they would destroy them. If damage were to occur to the tight junctions forming the blood-testes barrier, immune cells would be able to come into contact with the germ cells triggering an immune response and the production of antibodies causing the sperm cells to become non-functional – resulting in infertility of the male. Sertoli cells are also unable to proliferate; the body is unable to replace any lost Sertoli cells.
Sertoli cells have cell-membrane receptors specific for FSH, which when stimulated increases production of cyclic AMP
They are also able to convert cholesterol to pregnenolone, which is then converted to testosterone. They also produce specific proteins such as androgen binding protein under the influence of FSH and testosterone.
A diagram of a Sertoli cell:
- Basal lamina
- Primary Spermatocyte
- Secondary Spermatocyte
- Mature Spermatid
- Sertoli cell
- Blood-testes barrier – the tight junction
The process of spermatogenesis begins early in embryonic development ad continues after birth and mainly after puberty. The three main recognisable cell types that occur during spermatogenesis are:
- Spermatids – Which go on to be spermatozoa when released into the seminiferous tubules
Spermatogonia lie in the basal compartment of the seminiferous tubule, whilst the rest lie in the adluminal compartment. The spermatogonia divide mitotically. Spermatogonia can be split into three types:
- A-spermatogonia – These lie closest to the basement membrane and are the stem cells
- Intermediate Spermatogonia
- B-spermatogonia – These go on to form the primary spermatocytes as they are lifted off the basement membrane by changes within the adjacent Sertoli cells.
The initial stage of transformation from Spermatogonia (A-spermatogonia) into spermatocytes (primary spermatocytes) is known as spermatocytogenesis.
Spermatocytes divide meiotically. They can be split into primary and secondary spermatocytes, with the primary split even further:
- Primary Spermatocytes
- Pre-leptotene spermatocytes – These closely resemble the final stage of spermatogonia (the B-spermatogonia) but are inside of the Sertoli tight junction.
- Leptotene spermatocytes – Cells no longer diploid (2n), now teatraploid (4n)
- Zygotene spermatocytes
- Pachytene spermatocytes
- Diplotene spermatocytes
- Secondary Spermatocytes – Consist of diploid DNA again (2n), but each divides to form 2 spermatids – each containing haploid (1n) DNA
Spermatids are haploid (1n) and undergo a process known as spermiogenesis; this is the process where the cells change from round spermatids to the elongated spermatids with less cytoplasm and a tail. Spermatids are not called spermatozoa (sperm) until they undergo spermiation – the process of their release into the seminiferous tubules.