Figure 2110

Comparison of mitosis and meiosis in a spermatogonia! cell. The two pairs of chromosomes (2n) of maternal and paternal origin are depicted in red and blue, respectively. The mitotic division produces daughter cells that are genetically identical to the parental (2n) cell. The meiotic division, which has two components, a reductional division and an equatorial division, produces a cell that has only half the number of chomosomes (n). In addition, during the chromosome pairing in prophase I of meiosis, chromosome segments are exchanged, crossing-over, creating genetic diversity. In humans, the first polar body does not divide, but it does so in other species. *Note that prophase II, anaphase II, and telophase 11 are not shown.

nine peripheral microtubule doublets and two central microtubules that constitute the axoneme of the sperm tail (see page 92).

• Cap phase. In this phase, the acrosomal vesicle spreads over the anterior half of the nucleus. This reshaped structure is called the acrosomal cap. The portion of the nuclear envelope beneath the acrosomal cap loses its pores and becomes thicker. The nuclear contents also condense.

• Acrosome phase. In this phase, the spermatid reorients itself so that the head becomes deeply embedded in the

Sertoli cell and points toward the basal lamina. The developing flagellum extends into the lumen of the seminiferous tubule. The condensed nucleus of the spermatid flattens and elongates, the nucleus and its overlying acrosome also move to a position immediately adjacent to the anterior plasma membrane, and the cytoplasm is displaced posteriorly. The cytoplasmic microtubules become organized into a cylindrical sheath, the manchette, which extends from the posterior rim of the acrosome toward the posterior pole of the spermatid.

Golgi

mitochondrial sheath acrosome manchette manchette

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