The structural changes during spermiogenesis were studied on developing spermatids in seminal vesicles and receptacles of Lumbricus terrestris fixed in glutaraldehyde-osmium tetroxide and embedded in Epon-Araldite. The centriole plays a prominent role in the morphogenesis and organization of the microtubules of the manchette and flagellum. Microtubules arising from the centriole extend anteriorly to encase the developing middle piece, the nucleus, and the acrosome. The manchette not only provides a supporting framework for the cell during elongation, but also may provide the motive force for the elimination of both nucleoplasm and cytoplasm. The manchette participates in segregation and elimination of the nuclear vesicle that contains the nonchromatin nucleoplasm. Compartmentalization and conservation may also be a function of the manchette since those elements which remain within the framework of microtubules are retained, while all the cytoplasm outside the manchette is discarded. At maturation, the endoplasmic reticulum plays a key role in dismantling the manchette and reducing the cytoplasm external to it. During the early stages of middle-piece formation, six ovoid mitochondria aggregate at the posterior pole of the spermatid nucleus. Concurrent with manchette formation, the mitochondria are compressed laterally into elongate wedge-shaped components, and their outer limiting membranes fuse to form an hexagonal framework that surrounds the dense intramitochondrial matrices. Dense glycogen granules are arranged linearly between the peripheral flagellar tubules and the outer membrane of the mature sperm tail.

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