Developing into an individual (or a group of individuals) with hereditary characteristics of both parental organisms that produced gametes.
In some species, the development of a single gamete (unfertilized egg) in the body is also possible - parthenogenesis.
Gamete morphology and types of gametogamy
Isogamy, heterogamy and oogamy
The morphology of gametes of different species is quite diverse, while the produced gametes can differ both in the chromosomal set (with the heterogamety of the species) and in the size and mobility (the ability to move independently), while the gamete dimorphism in different species varies widely - from the absence dimorphism in the form of isogamy to its extreme manifestation in the form of oogamy.
Isogamy
If merging gametes do not differ morphologically from each other in size, structure and chromosome set, then they are called isogametes, or asexual gametes. Such gametes are mobile, can carry flagella or be amoeba-like. Isogamy is typical of many algae.
Anisogamy (heterogamy)
Gametes capable of fusion differ in size, mobile microgametes carry flagella, macrogametes can be both mobile (many algae) and immobile (macrogametes of many protists lacking flagella).
Oogamy
Gametes of one biological species capable of fusion differ sharply in size and mobility into two types: small mobile male gametes - spermatozoa - and large immobile female gametes - eggs. The difference in the size of gametes is due to the fact that the eggs contain a supply of nutrients sufficient to ensure the first few divisions of the zygote during its development into the embryo.
Parker's disruptive selection theory... If the size of the zygote is important enough for its survival (in organisms with external fertilization), then anisogamy would be an evolutionarily stable strategy. In such cases, a population of males (breeders of small gametes) and females (breeders of large gametes) will be stable. The theory of disruptive selection makes it possible to explain the emergence and maintenance of dioeciousness in many plants and some animals with external fertilization.
Gamete type and gender
The concept of sex is associated with differentiation by the size of gametes, that is, we refer to the male sex as individuals that produce small mobile gametes, and to the female sex - those that produce large ones. At the same time, differentiation by the type of gamety (homo - XX or heterogametic constitution - XY) in some species may not coincide with the differentiation in size.
In the process of evolution, in most species, small gametes and a heterogametic constitution XY were found in males, and large gametes and a homogametic constitution XX were found in females. These are species with gamete type Drosophila... On the contrary, in species with gametes of the type Abraxas (English)Russian(birds, butterflies, moths, some fish species, etc.) the directions of these differentiations did not coincide. The ova in the female are heterogametic, and the spermatozoa in the male are homogametic.
Conception occurs at the moment of the union of the mother's egg and the father's sperm, or gametes - from that time on, a new organism develops in the mother for nine months before birth and the beginning of independent life.
The birth of a new organism begins at the moment of fertilization - fusion of gametes or germ cells: egg and sperm. Each of these cells contains 23 chromosomes, that is, half of those that contain the cells of the body, therefore, the combination of two such cells forms a new cell - a zygote with 46 chromosomes, from which, thanks to division, the organs and systems of the new organism are formed.
Conception begins with copulation during the reproductive period, that is, during the period of ovulation. With ejaculation, from 300 to 500 million sperm contained in the sperm enter the woman's vagina: thanks to their tail, sperm move, and some of them enter the uterus, and the most mobile ones are capable of getting from the uterus into the fallopian tubes, in which they can meet the egg. Less than 100 sperm reach the third section of the fallopian tube: colliding with the egg, they surround it, try to pass through all its membranes and get inside, but only one sperm succeeds.
The sperm cell enters the egg as seen under an electron microscope.
After fertilization, the nuclei of the egg and sperm merge and form a zygote - a fertilized egg.
The process begins immediately division of the zygote, or segmentation: after the first division of the zygote two cells - blastomeres also divide - four cells are obtained that continue to divide. Three days later, the zygote already consists of 16 cells that form a cluster that resembles a berry - morula. By the fifth day, morula cells, continuing to divide, begin to organize. Inside the morula, fluid accumulates, due to which the cells are forced out from its inner part to the outside: the morula is transformed into a blastula, consisting of two parts: an embryoblast, cells from which an embryo will subsequently form; and trophoblast, a thin layer of cells that separates a space filled with fluid, or blastocoel, - from this layer subsequently the placenta is formed.
As it divides, the zygote moves along the fallopian tube towards the uterus due to muscle contractions and rhythmic movements of small cilia of cells in the mucous layer of the fallopian tubes. The zygote moves along the fallopian tube, reaches the uterus, which receives the blastula and in which a new organism is formed over a period of nine months. Morula, getting into the uterus, continues to be in it for some time, until the endometrium is fully formed to accept it. On the seventh day after conception, the blastula is attached to the surface of the endometrium in search of a place to stop and receive nutrients - this process is called implantation.
Growth, maturation of gametes and other circumstances leading to the meeting of male and female germ cells are only of preliminary importance on the path to their union. The penetration of the sperm into the egg and the resulting fusion of the nuclear substances of both cells is the culmination of the fertilization process and heralds the beginning of a new individual's life.
Direct observations over the junction of gametes in mammals are very insignificant and fragmentary. Nevertheless, comparing these observations with more extensive data obtained in the study of aquatic animals, in which fertilization takes place outside the mother's organism, it is not difficult to imagine the whole course of events.
If sexual intercourse between healthy men and women happened around the time of ovulation, it will take only a few hours for the egg, which entered the fringed end of the fallopian tube, to be surrounded by a large number of sperm, of which only one penetrates the egg. Immediately after the introduction of the sperm, the egg undergoes changes aimed at preventing other sperm from entering it.
This phenomenon can it is easy to observe under a microscope in many marine species by performing experiments in a bowl of seawater. Once the sperm are injected into the dish containing the eggs, you can immediately see them swarming around each egg in droves. Despite the relatively huge volume of the egg, it can even be brought into rotation under the influence of the combined efforts of the spermatozoa.
When alone sperm has penetrated into the egg, its surface shell immediately thickens and stops less permeable; at the same time, the rest of the spermatozoa lose their directed activity, and soon only single spermatozoa remain in the vicinity of the fertilized egg. That these changes are related to the fertilization of the egg, and not to the loss of activity by other sperm, can easily be proven by adding unfertilized eggs to the dish and observing their fertilization with the remaining sperm.
V ovum only the sperm head (which consists almost exclusively of nuclear matter) and the neck (containing the centrosomal apparatus) penetrate. The tail disappears when the sperm is introduced. In the egg, the nuclear substance contained in the sperm head immediately loses its compact appearance and chromosomes are revealed in it. In this state, it is called the male pronucleus.
Usually at mammals in oogenesis, the first division of maturation is observed immediately after ovulation, and the second division of maturation is probably delayed until the sperm enters the egg. However, as soon as the sperm enters the egg, all processes are quickly activated and by the time the male pronucleus is formed, the second maturation division ends. The nucleus of the egg is from this moment called the female pronucleus.
Fertilization ends only when the chromosomes of the male and female pronuclei merge together. Since each pronucleus contains a haploid set of chromosomes, the full diploid set of chromosomes characteristic of a given species is restored in a fertilized egg.
Between penetration into the sperm ovum and the union of the chromosomes of both pronuclei, the centrosomal apparatus delivered by the sperm forms the mitotic spindle. Chromosomes during this period prepare for the first mitotic division of a fertilized egg. This division usually occurs shortly after pronuclei join, but the mechanism of its activation is extremely complex and its nature remains unknown.
It is clear that it is not limited to just connecting male and female pronuclei, since in some of the lower animals, which have gametes readily available for experimentation, spermatozoa can start dividing in the cytoplasm of oocytes with removed nuclei. In other cases, the sperm, whose nuclear substance was irreversibly damaged by radium rays, could still penetrate the egg and induce it to divide.
Furthermore, egg cells many lower animals can begin development in the absence of spermatozoa, under the influence of appropriate mechanical or chemical stimuli, which has been called artificial parthenogenesis. However, as a rule, in such cases, development is much weaker and does not last long. The activation of cell division is insufficient without the presence of a full-chain sperm cell, which is necessary to maintain normal growth force.
In our article, you will learn what a gamete is. This is a special cell, the functions of which are strictly specialized. What are they all about? Let's figure it out together.
What is a gamete: definition
Translated from Greek, this term means "wife" or "husband". This determines its meaning as precisely as possible. Gamete is a reproductive cell. In nature, there are two types of it - male and female.
In any case, gametes are formed as a result of the division of the primary germ cells. At the same time, their diploid set of chromosomes is preserved. This leads to an increase in their number. The process of the formation of male and female gametes has its own significant differences. So, from one primary sperm, four full-fledged cells are formed that are capable of fertilization. In female gametes, only one ovum acquires this ability.
Ovum structure
What is a female gamete? It is always an immobile cell containing a sufficient supply of nutrients necessary for the development of the future organism. It has a rounded or spherical shape. The ovum is reliably protected by several membranes: yolk, transparent and outer. Its cytoplasm is a real storehouse of yolk inclusions.
Features of male germ cells
Now let's see what a male type gamete is. Sperm cells are always much smaller than eggs. This is because male gametes contain only genetic information. Why are they lacking nutrients? The fact is that the basis of the future organism is precisely the egg, which has enough of them.
Gametes of plants and animals: similarities and differences
Male gametes of animals are mobile. Sperm are made up of three parts: the head, neck and tail. The first one contains the core. Its chromosome set is haploid, or single. This structural feature is typical for all germ cells. The sperm head also contains the acrosome, or apical body. produces a special enzyme that is able to dissolve the protective membranes of the egg. The cervix contains centrioles and mitochondria. They generate the energy needed to propel the tail.
The male gametes of plants are called sperm. In the highest seed representatives of this kingdom, they are found in the anthers of the stamens. They move with the help of wind, insects or humans. The process of their transfer to the stigma of the pistil is called pollination.
What is a plant gamete and where is it located? If we are talking about an ovum, then, like in plants, it is an immobile oval-shaped cell. It is located in the lower expanded part of the flower pistil. In order for the gametes to merge, the two sperm travel to the female gamete as the germ tube grows. As a result of their fertilization, a seed is formed.
In higher spore plants, germ cells mature in specialized organs - gametangia. In these organisms, a clear alternation of generations is observed in the life cycle.
Let's consider this process using the example of mosses. His sexual generation is represented by a green rug. It consists of individual leafy plants. Gametophytes are formed on them, in which the sex cells mature. As a result of the fertilization process, for the implementation of which water is needed, an asexual generation grows - a sporophyte. It looks like a box with a dry stem. In it cells of asexual reproduction, which are called spores, mature. They enter the soil and again give rise to gametophyte. So the phases of the life cycle replace each other.
Fertilization result
A fertilized egg is called a zygote. Her chromosome set is already diploid, or double. In animals, fertilization is external and internal. In the first case, it occurs outside the body of the female. This method is typical for fish and amphibians. With the help, the male introduces sperm into the female's body. The development of the fetus also takes place there, so this method is more progressive.
In plants, the most complex process of gamete fusion is observed in flowering plants. It is called double because the female gamete and the central germ cell are connected to the sperm. The result is the formation of the embryo, the storage nutrient called the endosperm, and the rind. And all together is a seed.
The zygote begins to split up. In this case, the embryo is formed. At first, it consists of one layer. It is called blastula. Further in it begins the laying of tissues and future organs. During this period, it is called gastrula. The formation of the embryo continues by laying down three germ layers, from which certain organs and their systems develop.
So, in our article we examined what a gamete and a zygote are. These structures are carriers of hereditary information and give rise to a new organism.
