One of the periods of individual development of organisms - ontogenesis, is embryogenesis. It includes the time from the beginning of the formation of the zygote to the moment of birth and consists of several successive phases. One of them is neurula. This is the stage of development of vertebrates, leading to the formation of an embryo having three germ layers, as well as a neural tube. In addition, within several weeks, the embryo forms 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 occurring in it.
Embryonic developmental stages
Complete and uniform crushing of the zygote of placental mammals, which include humans, leads to the appearance of a structure that looks like a mulberry or raspberry. She is called morula. Then its cells line up in a single layer, and a blastula is formed. This is a hollow sac, the walls of which are formed by blastomere cells. Further, the process of invagination occurs, leading to the appearance of a two-layer embryo. This is the stage of gastrula. It consists of an ectoderm and endoderm and has a primary mouth called the blastopore lip. Subsequently, its edges are closed, and the embryo has the appearance of a two-layer bag. Subsequent complications in the structure of the embryo relate to the appearance of another, third, germinal layer — the mesoderm, lying between the ecto- and endoderm. His education in human embryogenesis has its own specifics. We will study it in more detail.
Mesenchyme formation
The human embryo in its cavity forms isolated lateral outgrowths - somites or pockets. They are derivatives of the walls of the primary intestine. Mesoderm develops from somites and this serves as a signal to the beginning of organogenesis. A system of the most important structures is formed: the chord, intestines and neural tube. They are called axial organs. The embryo complicates its structure, a new stage is being formed - the neurula. This occurs at 3 to 4 weeks of gestation. The mechanism of formation of the aforementioned embryo stage is considered below.
How a neural tube appears in an embryo
The external germinal leaf is the ectoderm, on the dorsal side of the fetus it becomes denser and thicker, and turns into a neural plate. Its edges rise up, forming two rollers. A through channel appears between them, later transforming into a cavity of the brain and spinal cord filled with cerebrospinal fluid. The neural plate closes with 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 in the cranial part ends, where the brain bubbles develop - the rudiments of the parts of the brain. By the 4th week of pregnancy, the neural tube is completely isolated from the ectoderm. Nerve cells are formed from the neural tube, as well as trophic tissue - neuroglia and neurula arises. This is the stage of development of the embryo, which consists of three germ layers and a neural tube. How is the human nervous system formed from it?
The role of the neural crest in the formation of the nervous system
Immediately after the closure of the rollers and the appearance of the neural tube in the central and cutaneous ectoderm, a group of cells is detached. It is located along the axis of the body of the embryo between the outer germ layer and the neural tube and is called the neural crest. Its cellular elements have a unique property. This is the ability to move to different parts of a human embryo. For example, some cells move deep into the body of the embryo and form neurocytes and neuroglia of the ganglia of the sympathetic and parasympathetic sections of the peripheral part of the nervous system. Other cells remain in the neural crest itself, initially forming ganglion plates, which are then transformed into nodes of 31 pairs of spinal nerves. The processes of neurulation do not end there, and further improvement of parts of the central and peripheral nervous system occurs .
The complication of the structure of the embryo at the stage of neurula
Migrating cells of the neural crest, starting from the site of the future diencephalon to the sacral regions at the level of 27 - 28 somites, have the ability to differentiate, that is, in fact, identical to the semi-stem cells of the red bone marrow. Starting from the site of the future diencephalon to the sacral section of the embryo at the level of 27 - 28 pairs of somites, the formation of mature neurocytes and glia cells of the cranial nerves, 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 somites, the derivatives of the neural crest initiate the formation of ganglia of the autonomic nervous system that innervate the heart, lungs, intestines, and pelvic organs of the embryo. Full maturation of its nervous system occurs up to 40 weeks of pregnancy.
Causes of neural crest cell migration
Based on the above processes of formation of the nervous system, a logical question arises: how to explain the reason for the movement of cells of the neural crest over large distances in the body of the embryo? Modern embryology explains them as follows. Firstly, neurula is the stage of embryogenesis, at which the cells of the neural crest lose their ability to adhere, i.e., bonds 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 have tropism for neural crest cells.
Their connection with the extracellular matrix and migration occurs due to signaling molecules - integrins, as well as the presence in neurula somites of special glycoproteins: tenascin and T-cadherin. All this ensures the development of the main departments of the nervous system of the embryo.