FERTILISATION

From Big Medical Encyclopedia

FERTILISATION (synonym syngamy) — merge of a men's sex cell (a spermatozoon, a spermiya) with women's (egg, or an ovum), leading to formation of a zygote, edges gives rise to a new organism.

As a result of the association of paternal and maternal genes happening in the course of O. there is a set of new combinations of hereditary factors and the genetic variety of organisms is created.

In an animal kingdom there are 2 types of process of O. — fiziol, mono - and a polyspermia. At fiziol, monospermiya thanks to the special mechanism of blocking of a polyspermia only one spermatozoon gets into an ovum. This mechanism comes into effect after one of spermatozoa connects with ovum (see). The mechanism of blocking of a polyspermia operates on the level of a surface of an ovum and its cover. Fiziol, a monospermiya is eurysynusic among animals with outside insemination. It is inherent also a nek-eye to animals with internal insemination, in particular mammals.

At fiziol, polyspermias gets into an ovum from 2 — 3 to several tens spermatozoa, however the kernel only of one of them connects to a female pronucleus, and the others are excluded from development. The mechanism eliminating excess spermatozoa operates in the ooplazma. Such type of fertilization is inherent in several groups of the animals with internal insemination belonging to types of arthropods, mollusks and chordates and is, apparently, evolutionarily secondary.

Rapprochement and merge gametes (see) and their subsequent transformation at animals with different types O. proceed generally by a unified plan.

Preservation of diploid (double) set of chromosomes among generations is provided that in sex cells even before fertilization there is a meiotic division (see. Meiosis ), followed by a reduction (reduction twice) chromosome numbers. Merge haploid (i.e. the chromosomes containing unary set) gametes at O. leads to recovery in a zygote of diploidy. Meiotic divisions happen at all animals to a differentiation of spermatozoa (see. Spermatogenesis ). During the formation of female sex cells (see. Oogenesis ) meiosis usually does not come to the end (it is blocked at a certain stage, various for different types of animals) and the ovum remains at this stage, without changing, until the Lake. Penetration of a spermatozoon into egg (ovum), i.e. O., can come at a stage professional elements of meiosis (sponges, nek-ry worms, mollusks, from mammals — a dog, a fox, a horse), metaphases I of meiotic division (nek-ry worms, mollusks, insects, astsidiya), metaphases II of meiotic division (a lancelet, the vast majority of vertebrate animals) and after completion of meiosis (coelenterates, sea hedgehogs). For the further course of process of O. these distinctions have no essential value as when the spermatozoon prg nikat in an ovum at early stages of meiosis, he does not experience changes prior to the II meiotic division.

At the majority of the animals living and breeding in water (many invertebrates, the majority of fishes, etc.), insemination outside: eggs and spermatozoa are thrown out water where there is

O. U of the animals breeding on the land, insemination internal: the male usually through coital body enters sperm directly into a genital tract of a female. Transport of spermatozoa to O.'s place is carried out by hl. obr. due to muscular contractions of walls of a sexual path of a female and the directed movement of cilia of a mucous membrane of oviducts (uterine tubes). Progress of spermatozoa plays, apparently, an essential role only at their passing through the slime filling the channel of a neck of uterus upon transition from a uterus to oviducts (uterine tubes). A meeting of spermatozoa with the ovulating ova (see. Ovulation ) occurs in a venter of an upper third of a uterine tube, an ampoule (see. Matochnyetruby ).

At the person the ejaculate is entered into a vagina in number from 2 to 5 ml. The normal ejaculate contains from 30 million to 100 million spermatozoa in 1 ml, gets into a vagina from 60 million to 500 million spermatozoa, via the channel of a neck of uterus several millions from them get into her cavity, and an upper part of an oviduct is reached by only about 100 spermatozoa. Transport of spermatozoa lasts 5 — 30 hour.

The ovulating ova of mammals keep in an oviduct ability to fertilization rather short time: at a rabbit of 6 — 8 hours, a mouse of 15 hours, a Guinea pig of 20 hours, a horse of 2 — 3 days. The period, during to-rogo spermatozoa in a genital tract of a female keep ability to fertilization, the same order: at a mouse of 6 hours, at a Guinea pig 22 hours, at a rabbit to 36 hours, at the few types it is slightly longer, napr, at a dog more than 5 days. However there are also exceptions: at nek-ry bats pairing happens in the fall, and an ovulation of ova and their fertilization — only in the spring (spermatozoa at them keep ability to O. for several months). At the person O. it is carried out usually during the first 12 hours after an ovulation. Spermatozoa in a female genital tract keep ability to O. for 1 — 2 days.

The ovum is surrounded with covers, to-rye at nek-ry animals, napr, at insects and many fishes, micropyles — openings are supplied, through to-rye spermatozoa get into an ovum. However at the majority of animals, including and at mammals, micropyles are absent and to reach an ooplazma, spermatozoa shall pass through thickness of covers. It is carried out by means of an acrosome — the special organella of a spermatozoon which is located at top of its head.

At mammals the ovulating ovum is surrounded with own brilliant cover (zona pellucida), and also the follicular cells of an oviparous hillock shipped in zhelatinoobrazny weight.

At penetration of a spermatozoon through this barrier and a brilliant cover of an ovum an important role is played by acrosome reaction. After the meeting of a spermatozoon with an ovum plasmatic and akrosomny membranes stick together among themselves and in many places there are small openings, through to-rye there are enzymes (hyaluronidase, akrosomny proteases) concluded in an akrosomny bubble. Then bound together membranes break up to a set of bubbles (vesiculation), the spermatozoon gets through the zhelatinoobrazny intercellular substance of an oviparous hillock loosened with enzymes and, having approached the surface of an ovum, merges with a cortical layer of an ooplazma a side surface of a head.

The spermatozoa of mammals which just got into a genital tract of a female are not capable even to implementation of acrosome reaction and consequently, and to connection with egg. They gain this ability, undergoing the specific changes called by a kapatsitation. Kapatsitation is carried out under the influence of substances, serfetiruyemy by a mucous membrane of uterine tubes and a uterus.

Merge of plasma membranes of a spermatozoon and egg leads to activation of the last. Activation is a difficult complex of the phenomena including cortical reaction, formation of the perceiving hillock, stimulation of an egg kernel etc. Cortical reaction begins with the impulse of activation (excitation wave) extending in a surface layer of an ooplazma from the place of an attachment of a spermatozoon that leads to removal of the nuclear block. After this there is an allocation of contents of special organellas — cortical little bodies. Contents of these little bodies rich with mucopolysaccharides ovodnyatsya, and as a result between covers and an ooplazmy ovum the perivitellinovy space filled with liquid is formed. Activation of an ovum is followed by increase in permeability of a plasma membrane of a zygote for nek-ry ions, activation of protein synthesis and other processes.

In the place of an attachment of a spermatozoon of an ooplazm of a cortical layer of an ovum it is pulled together, and the perceiving hillock is quite often formed.

Ooplazma of the perceiving hillock gets under a plasma membrane of a spermatozoon, flows round its kernel and organellas then they begin to move to the basis of the perceiving hillock whereas the plasma membrane of a spermatozoon remains on a surface of a zygote, built in a plasma membrane.

A head of a spermatozoon, having plunged into an ooplazma, is guided a middle part, adjacent to it, in depth of an ovum. At the same time around a microcentrum of a spermatozoon there is a radiance and there is «a seed star», than edge is later, apparently, is divided into two and participates in formation of a spindle of the I division of crushing (see. Crushing of egg ). Moving ahead to the meeting place with a female kernel, the kernel of a spermatozoon will gradually be transformed to a men's pronucleus. Its chromatin which earlier is densely packed is loosened and dissipates, the nuclear envelope breaks up to separate bubbles, and the membrane of a pronucleus forms again.

These transformations happen along with end of maturation divisions and formation of a female pronucleus. Both pronucleus approach in the center of a yaytsegeletka (at hypodispersion of vitelline inclusions as at mammals) or in the center of area of the ooplazma containing less than a yolk and come into contact. At nek-ry animals, napr, at sea hedgehogs, a pronucleus merges, forming a uniform kernel of a zygote, however much more often they remain in close contact, without merging, before the I division of crushing when there is an association of fatherly and maternal chromosomal complements. This association is completion of process of fertilization.

Disturbances of process of fertilization

When several spermatozoa get into an ovum of physiologically monospermny animal, the germ is impractical. This phenomenon carries the name of a pathological polyspermia. At the majority of animals of disturbance of development begin with the earliest stages. At mammals in polispermny ova (usually dispermny) the normal bipolar spindle forms and crushing proceeds normally; nevertheless germs are impractical and perish, as a rule, in the middle of pregnancy. In a similar way development of germs of mammals at other disturbance of O. — polygenies proceeds, at a cut there is no department of the II polar little body. In this case two female pronucleus form and arises, as at a dyspermia, a triploid germ. One more type of disturbances is observed in case of O. of ova at their «aging», i.e. later considerable time after an ovulation. Aging is sometimes shown in the shift of a spindle of the II maturation division in depth of an ooplazma and spontaneous department of very big polar little body (the phenomenon of «immediate crushing»). Such polar little body can be impregnated; assume that it in nek-ry cases merges with an oospore and gives rise to a himerny organism, in structure to-rogo there are cells of a different genotype. The phenomenon of a himerizm was observed also at the person.

In certain cases spermatozoa pass all length of an oviduct and impregnate ova right after an ovulation, before their hit in a funnel of an oviduct; it can lead to an attachment of a germ to an ovary, a wall of an abdominal cavity, etc. (see. Extrauterine pregnancy ).

See also Reproduction .



Bibliography: Ginzburg A. S. Fertilization at fishes and a problem of a polyspermia, M., 1968, bibliogr.; it, Patterns of fertilization at animals, M., 1977; Dorfman V. A. Physical and chemical bases of fertilization, M., 1963 * G w a t k i n R. B. L. Fertilization mechanisms in man and mammals, NY _ L., 1977; Reproduction in mammals, ed. by C. R. Austin a. R. V. Short, v. 1, Cambridge, 1972.


A.S. Ginzburg.

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