Centrosome reduction during mammalian spermiogenesis

Spermiogenesis is the terminal differentiation of postmeiotic germ cells into spermatozoa. The cellular and molecular pathways of centrosome reduction during mouse, rhesus, and human spermiogenesis are described. This chapter discusses the fundamental events and stages of centrosome reduction during mammalian spermiogenesis. The developmental stages of isolated mouse spermatids show remarkable events of centrosomal changes: round spermatid stage, elongating spermatid stage, and late elongating stage. The process of centrosome reduction extends throughout the entire testicular and epididymal stages of spermiogenesis and comprises four stages: (a) loss of microtubule nucleating function, (b) formation of transitional microtubule organizing centers (MTOCs), (c) loss of centrosomal proteins, and (d) centriole degeneration. The observation of a systematic degeneration of distal centriole and γ-tubulin in nonrodent mammalian sperm implies that centrosome reduction could be a ubiquitous phenomenon taking place in all mammalian sperm to varying degrees, the mouse sperm representing the highest state and exhibiting complete loss of both centrioles and centrosomal proteins. The complete absence of centrosomes in mouse sperm consolidates the hypothesis that murine oocytes can compensate for the lack of paternal centrosomal contribution. The lack of a standard distal centriole and γ-tubulin in rhesus and human sperm does not interfere with the formation of the zygotic centrosome during fertilization and bipolar spindles during cleavages.