Neurula is the stage of embryo development
One of the periods of individual developmentorganisms - ontogenesis, is embryogenesis. It includes the time from the beginning of the formation of the zygote to the birth of the organism and consists of several successive phases. One of them is the neurula. This is the stage of development of vertebrate animals, leading to the formation of an embryo that has three germ layers, as well as a neural tube. In addition, within a few weeks the embryo has the formation of the basic structural elements of all parts of the central and peripheral nervous system. In this article, we will consider the features of both the stage itself and the physiological processes taking place in it.
Stages of embryonic development
Complete and uniform crushing of the zygoteplacental mammals, which includes man, leads to the appearance of a structure that looks like a mulberry or raspberry berry. It is called morula. Then its cells line up in one layer, and a blastula forms. It is a hollow sac whose walls are formed by blastomere cells. Then the process of invagination occurs, leading to the appearance of a two-layered embryo. This is the stage of gastrula. It consists of ectoderm and endoderm and has a primary mouth, called the blastopore lip. Later, its edges are closed, and the embryo has the appearance of a two-layer bag. Subsequent complications in the structure of the embryo concern the appearance of yet another, third, embryonic layer - the mesoderm, lying between the ecto- and endoderm. His education in human embryogenesis has its own specifics. Let's study it in more detail.
Formation of mesenchyme
The human embryo forms in its cavityisolated lateral outgrowths - somites or pockets. They are derived from the wall of the primary intestine. Of the somites, the mesoderm develops and this serves as a signal for the onset of organogenesis. A system of important structures is formed: the chord, the intestine, and the neural tube. They received the name of axial organs. The embryo complicates its structure, forming a new stage - the neurula. This occurs at 3-4 weeks of pregnancy. The mechanism of formation above this stage of the embryo will be considered below.
How does an embryo develop a neural tube
External embryonic leaf - ectoderm, onthe dorsal side of the fetus becomes denser and thicker, and turns into a nerve plate. Its edges rise upwards, forming two platens. Between them appears a through channel, which later transforms into a cavity of the brain and spinal cord, filled with liquor. The nerve plate closes its edges first in the cervical region of the embryo, then back along the axis of the embryo's body. The closure of the neural plate ends in the cranial part, where brain bubbles develop - the rudiments of the brain regions. By the fourth week of pregnancy, the neural tube is completely detached from the ectoderm. From the neural tube formed nerve cells, as well as trophic tissue - neuroglia and neurula occurs. This is the stage of embryo development, which consists of three embryonic sheets and a neural tube. How does the human nervous system form out of it?
The role of the neural crest in the formation of the nervous system
Immediately after the closing of the rollers and the appearancethe nerve tube in the areas of the central and dermal ectoderm, a group of cells is restored. It is located along the axis of the body of the embryo between the outer embryonic layer and the neural tube and is called the neural crest. Its cellular elements have a unique property. It is the ability to move to different parts of the human embryo. For example, some cells move deep into the trunk of the embryo and form neurocytes and neuroglia of the ganglia of the sympathetic and parasympathetic parts of the peripheral part of the nervous system. Other cells remain in the most nervous crest, initially forming ganglionic plates, which are further transformed into nodes of 31 pairs of spinal nerves. Neurulation processes do not end there, and further parts of the central and peripheral nervous system are improved.
Complication of the structure of the embryo in the neurula stage
Migrating cells of the neural crest, starting fromsection of the future intermediate brain to the sacral divisions at the level of 27 to 28 somites, have the ability to differentiate, that is, in essence, identical to the half-barreled cells of the red bone marrow. Starting from the region of the future intermediate brain to the sacral part of the embryo at the level of 27 to 28 pairs of somites, mature neurocytes and glial cells of the cranial nerves are formed, as well as the nerve nodes of the auditory and vestibular apparatus. At this stage of embryonic development in the region from 1 to 7 somite, the derivatives of the neural crest initiate the formation of the ganglia of the autonomic nervous system, innervating the heart, lungs, intestine and pelvic pelvic organs. Complete maturation of his nervous system occurs up to 40 weeks of pregnancy.
Causes of migration of neural crest cells
Based on the above processesthe formation of the nervous system, a natural question arises: how to explain the reason for the movement of cells of the neural crest over long distances in the body of the embryo? Modern embryology explains them as follows. First, the neurula is the stage of embryogenesis, on which the cells of the neural crest lose the ability to adhere, i.e., the connection with each other. The second reason lies in the chemical composition of the intercellular matrix of the embryo. It consists of hyaluronic acid and proteins: collagen, laminin, fibronectin, which possess tropism to the cells of the neural crest.
Their relationship to the extracellular matrix and migrationoccur due to signaling molecules - integrins, as well as the presence in the somites of the neurula of special glycoproteins: tenascin and T-cadherin. All this ensures the development of the basic parts of the nervous system of the embryo.