12.6Specialization of Gametes
A particular characteristic of male gametes is that they exclude most organelles (other than the nucleus) and the cytoplasm. The same applies to spermatids in the pollen tube and mammalian sperms. This may be because the main task of male gametes is to pass nuclear DNA on to ovum, and other intracellular components may adversely affect embryogenesis in the female body. Indeed, it has been reported that male mitochondrial DNA is actively destroyed in egg after fertilization.
In the sperm nucleus, nuclear proteins are replaced by different kinds of nuclear proteins and DNA is condensed during the maturation process, which inactivates the nucleus. In addition to the nucleus, sperm have a flagellum, mitochondria and an acrosomal vesicle. All of these are devices to introduce the male nucleus into an ovum; the flagellum is a strong propellant device enabling the sperm to swim to the ovum, mitochondria are devices that supply the energy for the swim, and the acrosomal vesicle is a sac containing hydrolases that allow the sperm to penetrate the oolemma (Fig. 12-13). However, these devices become unnecessary once fertilization takes place.
Ova are specialized in a number of ways. First, they are large (Table 12-1), since they contain the nutrients, organelles, protein synthesis devices, etc. needed after fertilization for embryogenesis. Second, animal ova have a zona pellucida on the surface; this layer physically supports the bulk of the ovum from the outside, protects it from physical damage, and functions as a species-specific barrier that only particular sperms (i.e., those of the same species) can penetrate.