the distal centriole disappears after axoneme assembly. The proximal centriole enters into the egg during fertilisation and starts the first cleavage division of the egg, which has no centriole. The distal centriole gives rise to the axial filament which forms the tail and has a (9+2) arrangement. A transitory membrane called the Manchette lies in the middle piece. Middle piece: It has 10–14 spirals of mitochondria surrounding the axial filament in the cytoplasm. It provides motility, and hence is called the powerhouse of the sperm. It also has a ring centriole (annulus) that form a diffusion barrier between the middle piece and the principal piece and serve as a stabilizing structure for tail rigidity. Tail: The flagellum is the longest part (50µm), having an axial filament surrounded by cytoplasm and plasma membrane, but at the posterior end the axial filament is naked. The flagellum gives movement to the cell. Semen has an alkaline nature and the spermatozoa do not reach full motility (hypermotility) until they reach the vagina, where the alkaline pH is neutralized by acidic vaginal fluids. This gradual process takes 20–30 minutes. During this period, fibrinogen from the seminal vesicles forms a clot, securing and protecting the sperm. Just as they become hypermotile, fibrinolysin from the prostate gland dissolves the clot, allowing the sperm to progress optimally. The spermatozoon is characterized by a minimum of cytoplasm and the most densely packed DNA known in eukaryotes. Compared to mitotic chromosomes in somatic cells, sperm DNA is at least sixfold more highly condensed. The specimen contributes with DNA/chromatin, a centriole, and perhaps also an oocyte-activating factor (OAF). It may also contribute with paternal messenger RNA (mRNA), also contributing to embryonic development. The human spermatozoon contains at least 7500 different proteins. Human sperm genetics has been associated with human evolution, per a 2020 study. DNA damage and repair DNA damages present in spermatozoa in the period after meiosis but before fertilization may be repaired in the fertilized egg, but if not repaired, can have serious deleterious effects on fertility and the developing embryo. Human spermatozoa are particularly vulnerable to free radical attack and the generation of oxidative DNA damage. (see e.g. 8-Oxo-2'-deoxyguanosine) Exposure of males to certain lifestyle, environmental or occupational hazards may increase the risk of aneuploid spermatozoa. In particular, risk of aneuploidy is increased by tobacco smoking, and occupational exposure to benzene, insecticides, and perfluorinated compounds. Increased aneuploidy of