Spermatozoa nuclei

The first cell cycle of the mouse embryo differs in many respects from the second and the following cell cycles. It is characterized by a long Gl phase that starts after the penetration of the spermatozoon or artificial activation of the oocyte. During this period the chromatin of the oocyte completes the second meiotic division and forms the female pronucleus. At the same time, in the fertilized egg, the highly condensed chromatin of the sperm nucleus decondenses and sperm-specific proteins, protamines, are replaced by histones. After the initial sperm chromatin... 

The spermatozoon consists of a head, a midpiece and a tail or flagellum. The head contains the nucleus and a vesicle known as the acrosome (Figure 19.4). It contains hydrolytic enzymes that are required during fertilisation of the ovum. 

Fertilization is the fusion of the sperm and ovum. The sperm head binds to the plasma membrane of the egg (oolemma), and the entire spermatozoon enters the cytoplasm of the ovum. Only capacitated spermatozoa with intact acrosomes can enter and pass through the cumulus oophorus. The acrosome is a membrane-bound, cap-like structure covering the anterior portion of the sperm nucleus. The acrosomal reaction is the release of materials that lyse the glycoprotein coat (zona pellucida) surrounding the ovum. This is necessary for fertilization to take place. Before undergoing the acrosomal reaction, sperm go through a type of hypermotility called hyperactivation. The... 

Figure 3. (Upper Panel). Scanning electron micrograph of an H. rufescens spermatozoon. The sperm head, from mitochondrion (M) to tip of the acrosome vesicle (granule AV) is 7 pm. The width of the nucleus (N) is 1 pm. (Lower Panel). Transmission electron micrograph of the acrosomal vesicle showing it attached to the nucleus (NF) by the rod of actin filaments (AF). The darker material labelled 1 shows the location of the 18K protein and 2 shows the location of lysin (from Lewis et al., 1980).

The quantity of DNA contained in the ordinary nucleus usually exceeds these calculated values considerably. The spermatozoon of the trout contains 6.4 x lO mg DNA with a mean molecular weight of 6 X 10 (Sadron et al., 1957), equivalent to 640,000 DNA molecules. Because of the possible existence of surplus DNA, there is no direct relationship between the mass of DNA in the nucleus and the animaPs position in the scale of evolution. The DNA content in the human spermatozoon, for example, is less (about 2.8 X 10" mg) than in the spermatozoon of the trout. Du Praw (1965) has calculated that if all DNA molecules of one diploid nucleus of a human cell were to be joined together into a single chain its length would be about 180 cm. 

It has been proved impossible to extract basic protein from mammalian spermatozoa by the usual methods. This has led to the sponge hypothesis advanced by Bril-Petersen and Westenbrink (1963), according to which the whole nucleus of the mammalian spermatozoon is regarded as a sponge consisting of a keratinoid network, held together by numerous disulfide bonds and associated with DNA to make the protein resistant against extraction or destruction. 

The nuclear basic protein of bull spermatozoa is thus considered to be a new class of histone (Coelingh, 1971), rich in arginine (resembling protamines in this respect) and cystine. The nucleus of the bull spermatozoon, therefore, may consist of a network of identical histone molecules linked by disulfide bridges and associated with DNA. 

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