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In the mid and apical regions of the seminiferous epithelium of wild-type mice, the heads of elongating spermatids were deeply embedded in niches of Sertoli cell processes (Fig. 7a). The cytoplasm of the latter contained mitochondria, lysosomes, and ER cisternae, all embedded in a finely flocculent ground substance. In addition, bundles of filaments overlaid by ER cisternae, and forming the so-called ectoplasmic specializations, were closely applied to the spermatid heads (Fig. 7a). In the semilunar affected areas of the seminiferous epithelium of FORKO mice, a gross enlargement and dilation of the apical Sertoli cell cytoplasm was evident (Fig. 7b). In some planes of section, the dilated apical Sertoli cell processes, of enormous sizes, contained few organelles other than membranous profiles of various sizes (Fig. 8a). However, in other cases, such processes contained numerous organelles such as mitochondria, ER cisternae, lysosomes, and small vesicular profiles (Fig. 8b). The ectoplasmic specializations of the Sertoli cell processes also appeared to be affected. They showed fewer filaments and occasional swellings of their ER cis-ternae (Fig. 8, a and b). The dilated processes did not appear to be continuous with the lumen of the tubule, as they were consistently delimited by a plasma membrane (Fig. 8, a and b). FIG. 8. Apical Sertoli cell processes enveloping the heads of elongating spermatids near the tubular lumen of 6-mo-old FORKO mice. In a, an extremely dilated space (solid stars) envelops the heads of elongating spermatids and contains no apparent ground substance or organelles of identifiable nature, aside from several membranous profiles (curved arrows). Such spaces are not territories of the tubular lumen, as they are consistently delimited by a plasma membrane (short arrows). Note that the latter is closely applied to the plasma membrane delimiting the neighboring elongating germ cell cytoplasm (Gc). The ectoplasmic specialization enveloping the spermatid head shows few bundles of filaments (small arrows), while the ER cisternae overlying them (arrowheads) are at times dilated (open stars). In b, the grossly dilated Sertoli cell process (solid stars) enveloping the elongating spermatid head contains organelles such as mitochondria (M), lysosomes (Ly), and ER cisternae, embedded in what appears to be a highly diluted ground substance. The plasma membrane of the dilated Sertoli process (short arrows) closely approximates that of the intact germ cell cytoplasm. A, Acrosome. a) Original magnification X10 750; (b) original magnification X8600.