Fertilization

According to the cell theory, the development of the multi-cellular organism begins from one cell, called zygote. The zygote appears as a result of the fusion of the mature gametes and this process is called fertilization. Thats why, first of all we should study the structures and particularities of male and female gametes – spermatozoon and ovum. These cells are developed and located in the reproductive organs – gonads. The male gonad is the testis and the female gonad is the ovary.

Male and female gametes differ in morphological and physiological features. It means these cells have a sex differentiation. Moreover they differ from somatic, that is the rest cells of the human body.

Ovum (egg, oocyte) is a specialized big round cell about 130 micrometers in diameter, formed in the ovary, where there is deposited around it a noncellular covering (called oolemma or zona pellucida) and where ovum is surrounded by a corona radiata of adhering granulosa cells. They form several layers around the ovum and supply nutrition and protection of the ovum.

The ovum properly consists of cytoplasm, which contains all organelles, but only one centriole. There is a large nucleus in it, within which is a nucleolus. Correlation of nucleus to cytoplasm is 1 to 10. That is to say, nucleus has one tenth part of a cell, while each somatic cell, as a rule, has this correlation as one to three.

The cytoplasm of ovum contains a little amount of yolk, which is distributed throughout the cytoplasm. Yolk or lecithin is the nutritive inclusion and the ovum contains it for the subsequent nourishment of a germ at the early embryonic stages.

According to the different conditions of the embryo development there are different types of the ova. Type of the human ovum with the small amount of yolk inclusions distributed throughout the cytoplasm is called secondary isolecithal and oligolecithal type of an ovum.

And finally, ovum contains only 23 chromosomes – haploid amount of chromosomes.

Spermatozoon (sperm). The mature human sperm is a highly specialized cell modified for the movement – it must reach the ovum during fertilization. So, the sperm is the mobile male cell which (like the ovum) contains the genetic information to be transmitted to the zygote by the male. It has remarkable shape and consists of a head, a neck, a middle piece and a tail. This is a very small cell. It measures about 60 micrometers long. There is a big dense nucleus and acrosome in a head. The head is capped by the acrosomal cap. Acrosome is a dense Golgi apparatus with many vesicles, containing the enzymes necessary for the sperm penetration of the ovum covering.

The neck contains a proximal centriole. The middle piece consists of two central microtubules and nine outer doublet microtubules, surrounded by a lot of mitochondria as a sheath. The tail begins with a distal centriole, contains the microtubules and is enclosed by a thin layer of a cytoplasm.

There are no more organelles in the sperm. The correlation of nucleus to cytoplasm is 1 to 1. The nucleus of the sperm has a single set of chromosomes, the haploid number.

Note the following: this cell has only necessary for the fertilization organelles. Golgi apparatus for the penetration the ovum cover, mitochondria and mobile tail for the movement, proximal centriole penetrates in the ovum and is necessary for the subsequent division. While the ovum has all organelles (except one centriole – the zygote gets another centriole from the sperm during fertilization).

Comparative description of the gametes structure

	Ovum	Sperm
1. Measurement	130 μm in diameter	60 μm long
2. Shape	round,	head, neck, middle piece, tail
3. Organelles	all, except one centriole	Golgi apparatus, centrioles, mitochondria, microtubules
4. Nucleus/ cytoplasm	1:10	1:1
5. Chromosomes		23
6. Particularities	yolk inclusions, zona pellucida, corona radiata,	motile

The first stage, calling fertilization happens in the uterine tube, where the ovum enters from the ovary, and consists of two phases – distant phase and contact phase.

In time of distant phase sperms and ovum produce physiological active substances, which are necessary for distant interaction, for stimulation of the sperms moving and gametes meeting.

Note the following: the sperms get the ability to fertilize the ovum only after they have been in the female genital tract. This final step in their maturation is called capacitation.

The second phase of fertilization is contact phase, when in the result of the fusion of the sperm with the ovum the zygote makes up. How does it happen? A few hundreds of the sperms from millions reach the ovum, but only one, which has maximum activity, goes across adhering granulosa cells, surrounding the ovum. Then sperm’s acrosomal cap is shed and the enzymes are released from the acrosome. This process is called the acrosomal reaction. The enzymes make a local lysis in the zona pellucida, after that the sperm’s head penetrates the ovum membrane and sinks into the cytoplasm. Only the head with nucleus and proximal centriole sink. The middle piece and the tail don’t penetrate into ovum.

The membrane of the fertilized ovum overlying the region of the sperm head fuses and is named the cover of fertilization. After that the sperm cannot penetrate the ovum.

The condition of the fertilized ovum, when it has two pronuclei (nucleus of a sperm and nucleus of an ovum) is called syncaryon. Pronuclei draw together and fuse.

So, in summary, fertilization properly consists of the entry of the sperm’s head into the ovum. This is followed by the fusion of the male and female pronuclei to restore the diploid number of chromosomes.

The process of fertilization leads to formation of the zygote and ends with the initiation of its cleavage.