Embryogenesis is a physiological process during which the formation and development of an embryo takes place. Embryogenesis is the initial stage of ontogenesis of living things . Although the phenomenon of embryogenesis is known both in the plant and animal kingdoms, as well as in the kingdom of multicellular fungi , this article is devoted to the general features of embryogenesis only in animals, with some emphasis on the particular embryogenesis of chordates and vertebrates , especially mammals .
The process of embryogenesis begins with conception - the fertilization of the female gamete - the egg with the male gamete - the sperm . A fertilized egg forms a zygote , the only diploid cell in a new living organism. Under favorable conditions, the zygote then undergoes several consecutive mitotic divisions , without a significant increase in the total volume or mass of the formed cells. This process is called crushing . It leads to the formation of a multicellular embryo. Then, the processes of cell differentiation begin, thanks to which the embryo becomes multilayer (bilayer or, first, bilayer, then trilayer). Under unfavorable conditions, in many species of living creatures, the zygote in the early stages (in many animals in the stages up to the blastula or blastocyst inclusive) falls into temporary “hibernation”, temporarily stopping further development. This is called embryonic diapause . In plants, a similar phenomenon is known as the "resting phase of seeds."
In placental mammals, the term "embryogenesis" refers only to the early stages of intrauterine development , until histogenesis , morphogenesis and organogenesis are completed , and in particular, the maturation of the placenta is completed. In humans, the embryonic period covers the first 8 weeks (60 days) of fetal development, or Carnegie’s 23 stages . The subsequent stages of intrauterine development of mammals, in contrast to embryogenesis proceeding already within the framework of the created morphological structures and organs, are called fetal development, or development of the fetus . A fetus at this stage is called a fetus, or fetus. After birth, he is called a newborn .
Content
Sperm Fertilization, Conception, and Zygotes
The maternal ovum has an initial longitudinal asymmetry . It distinguishes the animal pole in which the nucleus is located (from the cells formed as a result of crushing at this end of the embryo, ectoderm and mesoderm cells subsequently form), and the vegetative pole or yolk pole (from the cells formed as a result of fragmentation at this end of the embryo, subsequently endoderm cells form). Thus, the egg initially has an axis that sets the cells of the future embryo in the direction of differentiation into the external and internal embryonic leaves, and then the anteroposterior axis (head-tail axis). Asymmetric ovum both in the transverse direction and in the direction “from top to bottom” - in its body there are special bodies defining the transverse asymmetry and asymmetry of the top and bottom, and concentration gradients of the corresponding proteins , specifying the future embryo of the axis “up / down” and “right / left. " These gradients of protein concentration along the axes of the maternal ovum play a very important role in the early stages of embryogenesis, before the start of the mid-blast transition and the associated start of expression of specific embryonic genes that control the further development of the embryo.
The egg is covered with protective shells that have several layers. The first protective layer that is in direct contact with the egg membrane consists of glycoproteins and is called the vitelline membrane , vitelline membrane, or yolk membrane . In mammals, this layer is called a transparent membrane ( lat. Zona pellucida ). Different animal taxa have different cell and cell-free membranes that cover the egg over this first membrane.
The process of fertilization , or fertilization (also known as the process of conception , or syngamy ) is the fusion of male and female gametes to produce a new living organism. In animals, this process involves the fusion of an egg with a sperm , which leads to the formation of a zygote , or, in other words, a fertilized egg. Depending on the specific type of animal, this process can occur either inside the body of the female - the so-called internal fertilization , or in the external environment, outside the body of the female - the so-called external fertilization , or external fertilization .
Zygote Crushing
In the early stages of embryo development, multiple fertilization of the zygote occurs without or almost without any increase in the total size of the embryo (that is, each of the formed cells is as much smaller than the initial size of the zygote as the cells formed at this point). This process is called zygote crushing . As a result, a cluster of small germ cells is formed, the so-called morula . In mammals, after crushing, already at the stage of 8 or 16 cells (three or four consecutive divisions), a sharp expansion of the formed ball of cells occurs due to an increase in the intercellular space, followed by an equally sharp decrease in the intercellular space and the total size of the ball to values are less than the original, and the formation of close intercellular connections, which will subsequently play a role in the exchange of intercellular signals. For the duration of this process, cell division is suspended and resumed after its completion. This process is called “compaction of the embryo,” or compaction of the embryo, its compression. The compaction process was observed in mammalian embryos only in vitro (in vitro). It is not known whether a similar process takes place in vivo (in a living organism). A similar phenomenon is not observed in animals standing at the lower steps of the evolutionary ladder. The cells that begin to differentiate, formed as a result of fragmentation, up to the stage of a blastula or blastocyst , are called blastomeres , that is, progenitor cells of cells of a future blastula or blastocyst.
Depending on the amount of yolk in the egg , the fragmentation of the zygote can occur in different ways: as holoblastic, or complete, fragmentation, or as meroblastic, or incomplete, fragmentation. Holoblastic, or complete, fragmentation occurs in embryos of those animal species in which the eggs contain very little yolk, in particular in mammals , including primates and humans , in which the developing embryo soon begins to receive nutrition from the mother's body, through the placenta , or through secretion milk by the mammary glands in marsupials , in which the cubs are born very very prematurely, with organogenesis not yet completely completed, and matures in the mother’s bag, attaching to its inner wall and eating milk. Meroblastic, or incomplete, crushing occurs in the embryos of those animal species in which the eggs contain a lot or a lot of yolk, for example, in reptiles and birds . Since in these animal species crushing occurs more slowly at the vegetative or vitelline pole of the embryo, they have an uneven distribution of the number and size of cells at different poles of the embryo. At the animal pole, there are significantly more cells, and they are smaller in size. Such an embryo is called animated (that is, developing faster at the animal pole). On the contrary, an embryo that develops faster at the vegetative (yolk) pole is called vegetalized.
In those species of animals in which the fertilized egg undergoes holoblastic, or complete, fragmentation, the first fragmentation after fertilization, leading to the formation of two blastomeres, always occurs along the animal-vegetative axis. The second fragmentation also always occurs perpendicular to the animal-vegetal axis. However, from this moment - from the moment of formation of the four blastomeres - the further fragmentation and spatial organization of the resulting blastomeres occurs differently in different animal species. We can say that they have a different plan or program of crushing.
The end of crushing marks the beginning of the next phase of embryo development, the so-called midblastula transition . At this stage, for the first time, transcription of zygote DNA begins and expression of not only maternal, but also paternal genes .
In amniotes , morula cells are initially tightly linked or fused, but soon they reorganize to form the outer layer, the so-called trophoblast , which becomes the outer amniotic membrane of the embryo, or rather, the fetal egg (it does not contribute to the development and formation of the tissues of the embryo itself, and only serves to protect it from the external environment), and the internal cell mass from which the embryo itself develops. Between the formed trophoblast and most of the internal cell mass, the so-called amniotic fluid quickly begins to accumulate. As a result, the morula becomes a bubble (vesicle), which is called a blastula , or a blastodermal vesicle, a blastodermic vesicle, a blastocyst , as well as an embryonic vesicle or an embryonic, fetal vesicle. A developing amnion is also called blastoderm , or germinal skin, rudimentary skin, sprouting skin, germinal membrane or germinal plate. At one pole of the developing fetal egg, the inner cell mass continues to maintain contact with the trophoblast. This pole is called the embryonic pole , because it designates the place where the future embryo will continue to develop [1] .
Morula Education
The dense ball of cells resulting from zygote fragmentation is called morula . After completion of the compaction, it is called a “compacted morula”, or “dense morula”. Subsequent cell divisions quickly double the number of cells (2, 4, 8, 16, 32, 64, 128 ...). Up to the stage of 128 cells and the formation of a bubble -like structure filled with yolk and / or amniotic fluid - a blastula , that is, before blastulation , the embryo is still called morula.
Blastula or Blastocyst Formation
After the 7th zygote fragmentation , at the stage of 128 cells, the developing embryo is called a blastula . A blastula is a spherical layer of cells ( blastoderm ) surrounding a cavity filled with amniotic fluid and / or yolk . This cavity is called a blastocele . In mammals, at this stage, a blastocyst is formed, characterized by the presence of an internal cell mass , which is histologically different from the outer blastula surrounding it - the blastoderm . Blastocysts should not be confused with blastula: although they are externally similar in structure and blastocyst is a special case of blastula, its cells have a different fate depending on whether they belong to the outer layer (blastoderm or amnion) or to the inner cell mass. The structure of the blastula is more uniform.
Formation of germ layers (germ layers)
Gastrul Education
Neurulation
Formation of the beginnings of the nervous system — the neural plate, then the neural roller, then the neural tube and notochords
Formation of the rudiments of the mouth and pharynx
Formation of the germinal heart and other primitive germinal structures
Somitogenesis
Histogenesis and morphogenesis
Organogenesis
See also
- Drosophila embryogenesis
- Mammalian embryogenesis
- Human embryogenesis
- Neurogenesis
- Parthenogenesis
Notes
- What is a cell?
- Campbell, Neil A .; Reece, Jane B .; Biology Benjamin Cummings, Pearson Education Inc. 2002.