The elementary unit of all life on Earth is a cell. It is the formation of new cells that allows the body to grow and develop. The vital activity and structure of these units is very complex and depends on the specifics of the destination.
The emergence of the term "zygote"
The appearance of the term “zygote” is a merit of the German scientist Edward Strasburger, who devoted his whole life to the study of cytology and the chromosome theory of heredity. It was he who, at the end of the 19th century, first came to the conclusion that cell division in a plant, animal and human body occurs approximately according to the same pattern.
Zygote: definition
Sex cells (gametes) perform a reproductive function, that is, due to their presence in the body, sexual reproduction is possible. The female gamete is the egg, the male is sperm. Each germ cell has a haploid (single) set of chromosomes.
The concept of “ontogenesis” includes all stages of the development of an organism from birth to death. Its beginning is fertilization, that is, the fusion of the sperm and the egg, resulting in the formation of a zygote. It contains hereditary information received from both parents. What is a zygote? Biology gives an exact definition of this concept. We will consider it in detail.
So, zygotes are diploid cells that are formed by the fusion of gametes. Diploidy is due to a complete (double) set of chromosomes, equivalently obtained from the paternal and maternal gametes. The development of the zygote begins immediately after the fertilization process, which is also called fertilization.
Zygote Crushing
Upon completion of the fertilization process, the resulting cell moves into the uterus. Along the way, the development and fragmentation of the zygote is already beginning. This is a complex process consisting of several stages.
About 30 hours elapse between the moment of fertilization and the first mitotic division. This is the time when all the preparatory processes take place in the female body, without which the development of a new organism is impossible. A variety of factors affecting the state of a woman's body can impede further embryo formation. These factors include chromosomal mutations, the wrong lifestyle of the mother (alcohol, drugs, smoking), the use of certain drugs, severe viral diseases, etc.
An embryo develops from a zygote. The first stage of this process is crushing. This is not a random name. Indeed, between the divisions there is no stage of cell growth, that is, as a result of each of them, the cells exactly in half decrease in size, becoming smaller. The daughter cells formed by crushing the zygote are called “blastomeres”. The crushing process lasts until the nucleus is formed from it. Thus, we can say that the human body appears as a result of the multibillion-dollar division of the zygote.
Blastula phase and gastrulation
Blastomeres consist of two layers. The outer one is adjacent to the transparent shell. It is called trophoblast (nutrient leaf), and the outer - embryoblast (germinal leaf). Gradually, in the process of multiple division, a cavity is formed between these two layers, inside which a liquid accumulates. This moment is the phase of the blastula.
Reproduction of embryoblast cells is accompanied by its growth on the wall of the cavity. As a result, the appearance of the second layer of the embryoblast, which forms the vitelline vesicle. Simultaneously with this process, the embryo of the fetus is formed from the bulk of the embryoblasts, grouping at one of the poles. It is at this stage that the fetus reaches the uterus. In the future, it is released from the transparent shell and implantation.
Trophoblast in this process plays an important role, because its cells, having proteolytic properties, melt the epithelium at the site of attachment of the embryo to the uterine mucosa. As a result, the epithelium becomes a milk-like mass.
The developmental stages of the zygote quickly succeed each other. Blastula turns into a gastrula, which is a two-layer ball. The outer layer is the ectoderm, the bottom is the endoderm.
Organogenesis
Zygotes are the first cells of new organisms. The whole set of organs necessary for life is initially formed from them. The process of organ formation is called organogenesis. Its beginning is marked by the formation of the neural plate, which occurs in the ectoderm on the dorsal side of the embryo. Then the rudiments of other organs and systems arise: the skin and nervous system (from the ectoderm); digestive and respiratory systems (from the endoderm); musculoskeletal system, circulatory, excretory and reproductive systems (from the mesoderm). Further, until the moment of birth, these systems develop and improve.
Postembryonic development
This period begins from birth to puberty. Depending on the level of organization of organisms, two types of embryonic development are distinguished .
- Direct development. In this case, the child is similar to the parents in terms of external and internal characteristics. The differences are in the size and underdevelopment of some organs. It is characteristic of birds and mammals, including humans.
- Indirect development. With this type of development, the child (larva) has many differences with the parents. It is characteristic of frogs and insects.
Zygotes are cells that duplicate the parental genotype. But in the process of embryo development, cells begin to vary in structure and perform various functions. This is due to the fact that some types of genes function in some cells, and others in others. Thus, the body is a complex system based on a zygote.