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Cover: From "Equatorial segment protein defines a discrete acrosomal subcompartment persisting throughout acrosomal biogenesis," by Michael J. Wolkowicz, J. Shetty, A. Westbrook, K. Klotz, F. Jayes, A. Mandal, C.J. Flickinger, and J.C. Herr (Biol Reprod 2003; 69:735-745). Equatorial segment protein (ESP) is the earliest marker yet recognized for specification of the equatorial segment, and it allows this region of acrosomal matrix to be traced through all phases of acrosomal biogenesis. Human spermatids were stained with anti-ESP polyclonal sera (red), and nuclear DNA was counter stained with DAPI (blue) to demonstrate the location of the ESP protein during spermiogenesis. A series of staged developing spermatids is presented. During acrosome biogenesis, ESP first appeared in the nascent acrosomal vesicle in early Golgi phase round spermatids (top row, left panel) and subsequently segregated to the periphery of the expanding acrosomal vesicle. In early cap phase (top row, right panel; center row, left and center panels), mid to late cap phases (center row, right panel; bottom row, left panel), elongating spermatids (bottom row, center panel), and testicular sperm (bottom row, right panel), an ESP-containing domain can be traced at the leading edge of the expanding acrosomal vesicle. At the fine structural level the ESP protein is concentrated within electron-lucent regions of the acrosomal matrix in Golgi and early cap phase spermatids (see Fig. 8 in Wolkowicz et al., this issue). ESP thus allows the definition of an equatorial segment sub-compartment in the matrix of acrosomal vesicles as early as the Golgi phase of acrosome biogenesis.
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