Explanation of Figures 1-11

An idea of the magnifications in Figures 4–11 will be given by the fact that the sperm nucleus is approximately 5 micrometers (.005 mm) long.

Fig. 1. The testis (T) and epididymis removed from the scrotum and displayed. Epididymal regions 1–5 are numbered (see text). The vas deferens (V) runs along the side of the 'corpus' (3 and 4) and leaves the scrotal sac in close association with the testicular artery and vein.

Fig. 2. Light micrograph of a section of epididymal region 2. Note the high columnar epithelium rich in lipid droplets, and the sparse luminal contents. The spermatozoa are recognizably immature, with large cytoplasmic droplets.

Fig. 3. Epididymal region 4. The spermatozoa are now nearly all morphologically mature, and have become densely packed. The epithelium shows several different cell types.

Fig. 4. Scanning electron micrograph (SEM) of a naked nucleus, showing the ventral groove. The background is a Millipore filter.

Fig. 5. Transmission electron micrograph (TEM) of the possum testis, showing a spermatozoon at the point of liberation from the Sertoli cell (S). The bowl-shaped acrosome sits on the anterior third of the nucleus (n), and is still filled with the Sertoli cell process. Note the large cytoplasmic droplet, with reticular channels running the length of the midpiece.

Fig. 6. Phase-contrast light micrograph of immature sperm from epididymal region 1. Note the perpendicular head-neck angle, the large cytoplasmic droplet around the neck, and the protruding acrosome (A).

Fig. 7. SEM of immature sperm from epididymal region 1. The position of the nucleus within the cytoplasmic droplet is indicated by a dotted line. Note the bowl-shaped acrosome (A).

Fig. 8. SEM of condensing sperm from epididymal region 2. The acrosome (A) is collapsing, with the loss of membranous vesicles from its surface, and the cytoplasmic droplet has a pitted appearance (arrowed).

Fig. 9. TEM of sperm section. The cell is at approximately the same stage in maturation as that shown in Figure 8: the nucleus (N) is starting to straighten out on the neck, the cytoplasmic droplet is losing much of its internal organization, and the fibrous sheath interspersed with invaginations has appeared in the distal midpiece (arrowed).

Fig. 10. Phase contrast micrograph of mature spermatozoon from epididymal region 5. The acrosome and cytoplasmic droplet have fully contracted, and the head and tail are in line, presenting a streamlined appearance.

Fig. 11. TEM of mature sperm, showing the fully contracted acrosome (A). This specimen was air-dried, which caused some contraction of membranes permitting observation of the mode of insertion of the neck into the nuclear groove (here viewed side-on).