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Proceedings of the First Symposium on Marsupials in New Zealand
Structure of the Possum Epididymis
Structure of the Possum Epididymis
The epididymis of the possum is a long, highly convoluted duct which, in gross appearance, is very similar to that of eutherian mammals (Fig. 1). The testes and epididymides lie within a pendulous scrotum and the vasa deferentia and the vascular pedicles containing the testicular arteries and veins lie close together when passing through the inguinal canal. In analysing epididymal structure and sperm maturation patterns, I (Cummins 1976) divided the duct into five regions (Fig. 1): (1) The Ductuli Efferentes and Proximal Caput; (2) The Distal Caput; (3) The Proximal Corpus; (4) The Distal Corpus; (5) The Cauda and commencement of the Vas Deferens.
The cytology of the tract also bears remarkable similarity to that of eutherian mammals. Thus, regions 1 and 2 have high columnar epithelium with prominent stereocilia, enclosing an irregularly shaped lumen with relatively few spermatozoa (Fig. 2). Regions 3 and 4 have lower epithelium, with a heterogeneous cell population and active secretion of PAS-positive material (Fig. 3). Region 5 has low, almost cuboidal epithelium enclosing a wide lumen densely packed with spermatozoa; in the terminal region approaching the caudal flexure and merger into the vas deferens, the lumen becomes large enough to be seen with the naked eye (Fig. 1). Sperm concentration increases progressively in descending the tract, reaching a maximum in Region 4.
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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).