Golgi acrosomal vesicle acrosome manchette flagellum centriole sheath FIGURE 22.11 • Schematic diagram of spermiogenesis in the human. The basic changes in the structure of the key organelles of the spermatid are illustrated (see text for detailed explanation). (Modified from Dym M. In: Weiss L, ed. Cell and Tissue Biology: A Textbook of Histology, 6th ed. Baltimore: Urban & Schwarzenberg, 1988.) proacrosomal granules, rich in glycoproteins, coalesce into a membrane-bounded vesicle, the acrosomal vesicle, adjacent to the nuclear envelope. The vesicle enlarges and its contents increase during this phase. The position of the acrosomal vesicle determines the anterior pole of the developing sperm. Also during this phase, the centrioles migrate from the juxtanuclear region to the posterior pole of the spermatid, where the mature centriole aligns at right angles to the plasma membrane. The centriole initiates the assembly of the nine peripheral microtubule doublets and two central micro-tubules that constitute the axoneme of the sperm tail.
 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.
The centrioles, which had earlier initiated the development of the flagellum, now move back to the posterior surface of the nucleus where the immature centriole becomes attached to a shallow groove in the nucleus. They are then modified to form the connecting piece, or neck region, of the developing sperm. Nine coarse fibers develop from the centrioles attached to the nucleus and extend into the tail as the outer dense fibers peripheral to the microtubules of the axoneme. These fibers unite the nucleus with the flagellum; hence, the name connecting piece.
As the plasma membrane moves posteriorly to cover the growing fagellum, the manchette disappears, and the mitochondria migrate from the rest of the cytoplasm to form a tight, helically wrapped sheath around the coarse fibers in the neck region and its immediate posterior extension (Fig. 22.12). This region is the middle piece of the tail of the sperm. Distal to the middle piece, a fbrous sheath consisting of two longitudinal columns and numerous connecting ribs surrounds the nine longitudinal fibers of the principal piece and extends nearly to the end of the flagellum. This short segment of the tail distal to the fibrous sheath is called the end piece.  Maturation phase. This last phase of spermatid remodel ing reduces excess cytoplasm from around the flagella to form mature spermatozoon. The Sertoli cells then phagocytose this excess cytoplasm, also termed the resid ual body. The intercellular bridges that have characterized the developing gametes since the prespermatocyte stages remain with the residual bodies. Spermatids are no longer attached to each other and are released from the Sertoli cells.
Spermatids are released into the lumen of the seminiferous tubules during the process called spermiation.