Spermatogenesis and ultrastructure of a peculiar acrosomal formation in the musk shrew, Suncus murinus

Spermatogenesis and acrosomal formation in the musk shrew, Suncus murinus, were studied by light and transmission electron microscopy. The cycle of the seminiferous epithelium was divided into 13 stages based on the characteristics of acrosomal change and nuclear shape, appearance of meiotic figures, location of spermatids, and period of spermiation. The relative frequencies of stages 1 to 13 were 5.1, 5.9, 10.1, 8.8, 12.5, 11.5, 10.6, 7.9, 6.0, 4.8, 8.9, 3.1 and 4.8, respectively. Additionally, spermatid development was subdivided into 13 steps. Acrosomal formation during spermiogenesis in the musk shrew was quite characteristic. However, in contrast to other mammalian species, the nucleus remained in the middle region of the seminiferous epithelium, and only the acrosome extended towards the basement membrane, beginning at step 7. The extension of the acrosome was conspicuous and reached maximum at step 9. At that time, the tip of the acrosome extended nearly to the basement. The acrosome of maturing spermatids was about 3-fold longer than that of spermatozoa. Thereafter, the acrosome gradually shortened and became flat. The enormous fan-shaped acrosome was completely formed at step 13. The prominent extension and subsequent shortening and flattening of the acrosome in the musk shrew appears to be a unique process to form the enormous fan-shaped acrosome.