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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 2nd 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.
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