The insemination. The fertilization

Insemination is a condition providing sperm and ovum meeting. There are external (in fishes and amphibia) and internal (in reptilia, birds and mammalia) insemination. During external insemination, sperms and ova are ejected to external environment. During internal insemination, sperms are ejected directly to female sexual ways that provides gametes meeting in approximately stable conditions. Such insemination is provided by system of reflexes and is performed by special copulation organs.

It is believed that gametes secret special substances - hamons, which provide their interaction on a distance. Ovicell produce ginohamon I and II, sperm produce androhamon I and II. Ginohamon I is non-protein structure with low molecular weight, stimulating sperm movement and increasing probability of sperm and ovicell contact. An antagonist of ginohamon I is androhamon I with similar chemical structure. It suppresses sperm movement and preserves them from preliminary energy waste. Ginohamons II (fertilysins) are proteins or glycoproteins.

They totally block sperm movement facilitating sperm attachment to ovicell membrane. Androhamone II helps to dissolve ovicell coats. The methods of artificial insemination of fishes and farm animals were suggested by scientists V. P. Vrasskiy and I. I. Ivanov. The artificial insemination of human was permitted in USSR in 1987.

Fertilization is a process of two gametes fusion resulting in zygota formation. Fertilization consists of three stages: penetration, activation and fusion.

The sperm and ovicell meeting is provided by unspecific factors facilitating their merging such as excessive sperm production, large sizes of ovum, secretion of hamons.

The acrosome reaction occurs at the moment of sperm and ovicell touching. The liberated acrosome enzymes help to dissolve ovum coats. It allows fusing sperm and ovum cell membranes. Then cytoplasms of sperm and ovum fuse. The sperm nucleus and centriol come to ovum cytoplasm.

The ovicell activation is a series of events initiated by sperm penetration. The region of membrane, which is made of sperm membrane, is permeable for sodium ions. They come in ovum and change membrane charge. Then cortical reaction occurs. The contents of cortical granules assist dense coat exfoliation. It is become more solid and impermeable for sperms. It is called fertilization coat. The amphibians and bony fishes have cytoplasm changes called cytoplasm segregation. Activation is finished by protein synthesis start.

Many mammalians have ovicell at the time of sperm meeting in the diakine- sis stage. After fertilization, a meiosis block is removed. At the moment of meiosis termination in ovicell, a sperm nucleus appearance changes firstly to an interphase nucleus appearance, and than to prophase nucleus appearance. Such nucleus with doubled DNA concentration and haploid chromosome set has a name “male pronucleus”. The nucleus of ovicell after meiosis has a name “female prcnucleus”. It also has a DNA concentration 2C. Both promucleuses merge and fuse. This is a moment of full gametes fusion resulting in zygote formation.

The hermaphroditism. The formation of sex dimorphism. When organisms had become multicellular, they received a possibility to form gametes of two types in one organism at the same time, i.d. they were hermaphrodits. It is less possible that multicellular organisms at first had different sexes. Indeed, there are many hermaphrodits between plants, flatworms, annelids, mollusks. In spite of producing both types of sex cells, the self-fertilization is untypical for them. Usually it happens because of asynchronic maturation of male and female gametes.

Even human may have true hermaphroditism. It appeares as a disturbance of embryogenesis when all body cell have same chromosome set - XX or XY. However, some human hermaphrodits have mosaic distribution of chromosomes in somatic cells. Some cells have XX, some XY.

A human and mammalian gonad is developed from a bisexual germ. It cortex (outlayer) has a properties to form ovarium, but medulla (inner layer) has a properties to form testis. If organism has genotype XX, the cortex is developed better than medulla, forming ovarium. If organism has genotype XY, the medulla is developed better than cortex, forming testis. Accordinary to those facts M. Chartman (1936) assumed a concept about organism’s bisexuality.

The formation of sex dimorphism was closely connected with eukaryotes appearance. This process was related with increasing gametes sizes and with formation of big and small gametes, with appearance of anizogamy as copulation type, with sperm production in large amount. The next step was a specialization of organisms on that which produce mostly spermatozoa and that which produce mostly ovicells.