From Nature:
“What’s in sperm?” demands Tim Karr. Because sperm have to swim far and fast, biologists have come to view them like racing cars: streamlined and stripped down of all unnecessary bits and pieces. Generally speaking, the DNA in animal sperm is tightly packed inside a sleek head structure that contains little of the cytoplasm that fills most other cells. Behind the head is the midpiece, containing more than 50 power units called mitochondria that drive the lashing motion of the attached tail. But what does a sperm deliver? One popular misconception is that only the head enters the egg, while the tail is discarded. But in most species, the entire cell enters the egg — midpiece, tail and all. And in many mammals, midpiece and tail structures persist in the embryo for several cell divisions. This results in a large number of proteins and other molecules being delivered to the egg.
More here.
