#13361 - Spermatogenesis Essay - Organisation of the Body

Give an account of spermatogenesis and discuss how abnormalities of spermatozoa can give rise to infertility.

Functional spermatozoa, which are haploid cells are formed from diploid primordial germ cells in a process known as spermatogenesis that occurs in the seminiferous tubules of the testis. The process is initiated at puberty by the activation of Kisspeptin neurons which stimulate the puslatile release of gonadotrophin releasing hormone. These hormones enter the anterior pituitary via the pituitary stalk and stimulate the release of FSH and LH. It is LH that is essential for the formation of mature spermatozoa as KO of LH led to the complete absence of functional spermatozoa, whereas KO of the FSH results in a smaller testes but with still functional spermatozoa. LH acts on Leydig cells in the testes and stimulate the release of testosterone which causes primordial germ cells that lie on the basement membrane of seminiferous tubules to divide by mitosis, causing replenishment of the primordial germ cell pool and also causes some of the cells to differentiate into spermatogonium. It is essential to maintain the primordial germ cell pool because males mate opportunistically which means it is essential that the testis produces active spermatozoa continuously.

Spermatogenesis

The first stage of spermatogenesis is meiois. During meiosis 1, DNA replication, which marks the beginning of spermatogenesis, converts the diploid cell from 2N to 4N and the type B spermatogonia becomes the primary spermatocyte. Following this, is prophase where the double stranded chromosomes condense and form chiasmas through homologous pairing. The formation of chiasma is very important as it allows recombination to occur which allows exchange of genetic material between maternal and paternal chromosomes and this increases the genetic variability. Following this, in metaphase, the chiasma become lined at the equator of the spindle apparatus and then the double stranded chromosomes of each homologous pair is separated to the opposite poles of the cell in anaphase. The end product of meiosis 1 is the formation of two daughter nuclei contain 2N and after cytokinesis occurs these cells are called the secondary spermatocytes.

The secondary spermatocytes then undergo 2^nd meiotic division to form definitive spermatocytes or spermatids. In this phase there is no DNA replication and begins with prophase where 23 double stranded chromosomes condense and then lines up at the centre of cells and centromere replication takes place. Then in anaphase the double stranded chromosomes are pulled apart into 2 single chromatids. The spermatids that are formed are linked to each other by cytoplasm to form a syncitium and this is essential as it ensures differentiation occurs synchronously through transfer of mRNA and proteins between the haploid cells.

Spermiogenesis

The four haploid spermatids then differentiate in a processes known as spermiogenesis and as the cells mature they are displaced towards the lumen. They key cells that mediate this process are the Sertoli cells which are attached to the spermatocytes and spermatids by intercellular junctions and tubulobulbar complexes. Spermiogenesis consists of individual processes which partially proceed at the same time

a) Nuclear condensation

During this process, there is thickening and reduction of the nuclear size. This occurs due to the condensation of the DNA. Initially the DNA is packed with histones but during maturation these are replaced by protamines which result in the DNA being packaged into annulus like structures. The tightly packed chromosomes results in the DNA becoming transcriptionally inactive.

b) Acrosome formation

The acrosome contains enzymes that play a key role in the penetration of the zona pellicuda of the oocyte. It is synthesised by the golgi apparatus which secretes large number of small vesicles containing lysosomal enzymes such as hydraulinases which then fuse to form a single vesicle. The acrosome lies close to the nucleus and then invertsitself like a cap over the nucleus and it is the combination of the nucleus and the acrosome that forms the head of the spermatoxoa.

c) Flagella formation which forms the tail of the sperm

The flagella is formed by the outgrowth of a distal centriole which forms the primitive axenomal structure and consists of a bundle of 9 peripheral doubles of microtubules and 2 single ones in the centre. The axenome also consists of dyenin arms which are anchored to the doublet microtubule and walk along to the adjacent microtubule doublet. Synchronous movement of many dyenin arms results in the axenome bending and the flagellum beating. Mutations in the dyenin arms results in primary ciliary dyskinesia and results in infertility due to the abscence of motility.

While the flagellum is forming the nucleus and the acrosomal vesicle rotates so that the flagellum primordium lies on the opposite side of the acrosome. During this phase the spermatozoa also orientate themselves so that their tails point away from the lumen of the epithelium.

By the end of flagella formation the tail can be divided into four distinct segments

-Neck- contains two centrioles

-Midpiece: sheath of ring shaped mitochondria, grouped around the axenome to provide energy for flagellar movement

-Principle piece: Fibrous sheath around the axenome

-End piece: consists only of the 9 + 2 structure of the axenome

d) The last stage is cytoplasm reduction. Excess cytoplasm of the spermatids form residual bodies and are phagocytosed by the Sertoli cells through tubulobulbar complexes.

Spermiation

Once mature spermatozoa are formed they are released from the protective sertoli cells into the lumen of the seminiferous tubule. At this stage the spermatozoa are non motile and are surrounded by testicular fluid which is secreted by the sertoli cells. They are then transported into the epidiymis by peristalsic contractions.

The epididymis is a single coiled tube which takes spermatozoa 4-6 weeks to travel through and during this period it again undergoes further maturation. The epididymis when activated by testosterone...