Structure of the Gametes

The Sperm. The sperm of the rabbits is being described as an example of a typical sperm. The sperm is 60-70 μm long, and is divided into three main parts, the head, the middle piece and the tail. The head is almost entirely filled with the nucleus, which appears homogeneous. In front of the nucleus is a double walled sac, the acrosome. The acrosome contains the egg-lytic enzyme hyaluronidase. Behind the nucleus is a post nuclear cap.  At the anterior end of the middle piece is a proximal centirole, from which arises the axial filament of the tail. A ring-like distal centriole surrounds the axial filaments at the posterior end of the middle piece. A tightly coiled spiral of elongated mitochondria, arranged the source of energy for the movement of the tail. A narrow neck region separates the head from the middle piece. The tail consists of a main piece and an end piece. A sperm plasma membrane surrounds the sperm. The axial filament shows the typical 9+2 structure. It consists of two central tubules and nine peripheral fibers. The peripheral fibers are in the form of doublets of microtubules.



The egg. The egg differs from the sperm in having a much larger amount of cytoplasm. The nucleus is usually vesicular and does not have the compact appearance of the sperm nucleus. The egg is bounded by a vitelline membrane, which apparently consists of two closely applied unit membranes. In mammalian eggs a thick zona pellucida and a radially arranged layer of follicle cells forming the corona radiata surround the egg and serve as protective layers. The outer part of the egg cytoplasm contains cortical granules. These granules are enclosed in the single unit membrane, which arises by invagination of the inner vitelline membrane. After the entry of the sperm the cortical granules explode, and their contents become applied to the outer membrane. This membrane now becomes very thick and is called the fertilization membrane.

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