The process of neural tube formation and developmental defects

The process of neural tube formation is called neurulation. Neurulation begins with the appearance of the neural plate, which invaginates into the spine to form a neural network with neural grooves on either side of the spine. Gradually, the neural grooves come closer to each other along the midline of the spine and merge, thus converting the neural grooves into a neural tube.

The formation of the neural tube begins in the area of the future neck at the level of the 4th somite (4 segments of the cervical spine). Neurulation begins 16 days after fertilization and ends on days 21-22. Neurulation in humans occurs in several stages. First, the dorsal part of the spine closes, then the closure occurs in the area from the forehead to the parietal tubercles, the third stage closes the facial part of the skull, then the area from the occipital lobes to the cervical spine, and the last stage closes the sacral part.

If neurulation is disrupted at the stages of neural tube closure, congenital malformations are detected, such as:

anencephaly (congenital absence or underdevelopment of the brain and cerebellum),
spina bifida - Spina Bifida,
exencephaly (absence of skull bones)

and other defects of the central nervous system. Neural tube closure defects are quite rare. With serious defects in the development of the neural tube, such as anencephaly, spina bifida, the fetus dies in the womb or is born nonviable and dies in the coming days after birth. Modern equipment helps detect neural tube defects during regular examinations during pregnancy. With early diagnosis of neural tube defects, termination of pregnancy is indicated.

The formation of the human neural tube is a complex interaction between genetic and environmental factors. Several genes are important for neural tube formation, but nutritional factors such as cholesterol and folate levels are also important for proper neural tube formation. Scientists have estimated that 50% of neural tube closure defects in the fetus can be prevented by prescribing folic acid and vitamin B 12 supplements to the pregnant woman.

WHO recommends that all women planning pregnancy , as well as pregnant women, take 400 mcg of folic acid daily to reduce the risk of neural tube defects.

All information is for informational purposes only. If you have any health problems, you need to consult a specialist.

Functions

Nervous tissue ensures the normal functioning of all organs and systems. It consists of 2 types of cells:

neurons are the main structural element; they participate in the reception, processing and transmission of external and internal impulses;
neuroglia - provide nutrition and protection to neurons, distinguish between groups of neurons with different functions.

Nervous tissue takes part in the formation of the main structures of the brain - central and peripheral.

Types of pathology

Spina bifida

The most common type of defect. At the base of the spine, in this area there is a partial protrusion of the spinal cord. With extensive non-union, over time, problems arise with trophism and sensitivity of the skin, improper formation of the feet, pathology of posture, and dysfunction of the pelvic organs.

Spinal cysts and hernias

Meningocele is a lesion of the membranes, a protrusion of the dura mater.
Meningoradiculocele - the opening enters the meninges and spinal nerves. Paresis or paralysis develops.
Meningomyelocele – the spinal cord along with roots and membranes emerges into the hernia. The child's condition is serious, there are no motor functions, and the functioning of the pelvic organs is disrupted.

Anencephaly

Develops in the upper part of the tube; parts of the brain are not fully formed. The anomaly is more common in girls. The pathology is incompatible with life.

Encephalocele

There is partial protrusion of the brain and membranes through the bones of the skull. Rarely seen.

Chiari malformation

The rarest disorder in which brain tissue leaks into the spinal canal.

The main symptoms of the disorders are physiological deformation, paralysis, convulsions, developmental delay.

Structure and layers

During the development of the nervous system, three layers appear in the tube:

ependymal or internal;
mantle or intermediate;
marginal veil or outer layer.

The first layer serves as the basis for neurons and gliocytes of the nervous system. Some neurons move into the peripheral part of the organ, forming the mantle layer, and the rest form glial cells.

The most important of these, ependymocytes, line the inside of the tube, which will then develop into the central spinal canal and the walls of the ventricles of the brain.

In the inner layer, separate functional zones are distinguished, which over time transform into one another. The division zone contains cells, among which those closer to the center extend processes to the outer edge and move the body of the neuron containing the nucleus there.

At this time, DNA synthesis occurs in it, after which the particles return to their place. The cells “sitting” on the lower edge are divided into two.

At the migration site, these halves interact. One of them begins to divide again, and the other, using it, moves to the intermediate layer.

The intermediate layer appears thanks to neuroblasts - migrant cells that have not lost the ability to divide and are attached to the inner part of the layer.

Neuroblasts gradually form axons, which are sent to the outer layer, thus forming a dendritic tree and turning into neurons. Unattached cells transform into spongioblasts. From the intermediate layer, such a part of the spinal tissue as gray matter is formed.

The layer on the outside consists of the processes of neurons contained in the mantle layer and blood vessels. It acts as the basis for the development of white matter in the brain.

Structure of the blood-brain barrier

The brain has a large network of arterial and venous vessels, respectively, bringing blood to and draining the brain tissue. However, metabolism occurs at the capillary level. Both the inner and outer surfaces of the vessel are lined with key structures that form selective exchange.

The capillary wall is formed by special endothelial cells; the inner surface of these cells comes into contact with the circulating blood and its components. The outer surface is in contact with a circumferentially continuous basement membrane.

Endothelial cells are attached to each other very tightly. Consequently, the endothelium functions as an impermeable barrier between the capillary lumen and brain tissue. Necessary substances (water, glucose, oxygen, ions, etc.) are transmitted through special channels. Other compounds that may be dangerous do not penetrate the blood-brain barrier.

Pericytes are considered integral components in the formation of the blood-brain barrier. They surround the endothelial cells of the capillaries and are capable of contracting to regulate capillary blood flow and the amount of blood flowing through the capillaries.

Astrocytes are highly branched cells with small bodies found in both the white matter (fibrous astrocytes) and the gray matter (protoplasmic astrocytes). Their “legs” surround the surface of the capillaries, helping to carry out metabolism.

History of the study of the BBB

For the first time, the BBB became known after the work of P. Ehrlich, who studied the penetration of dyes into tissues.

He noted in experiments with animals that when dye is injected into the blood, the brain structures are not stained. And when injecting paint into the spinal canal, coloring occurs, but the paint does not enter the blood and body tissues. A logical conclusion was made - there is a certain barrier that separates the cerebrospinal fluid and blood. In 1900, the term BBB was first used.

Subsequently, knowledge about the BBB expanded; it was determined that there is a barrier between blood and cerebrospinal fluid and between plasma and peripheral nerves.

Causes of impaired development of organisms

The body's ability to regulate vital functions and respond to negative environmental influences does not come immediately from birth.
On the contrary, at birth, the still unformed embryo is most susceptible to the influence of negative factors. Animals and plants have developed special adaptations to protect the embryo. This includes the nutritious tissue that envelops the embryo or the direct receipt of nutrients from the mother's body. But, despite this, environmental factors still affect the development of the embryo, stopping growth or, conversely, accelerating the process. Factors that cause disturbances in the development of the embryo are called teratogenic

. Depending on the cause of the disorders, biological, physical and chemical factors are distinguished.

Biological factors

most harmful to the body. These include various diseases caused by harmful macro- and microorganisms or impaired cell division. This also includes metabolic disorders. The impact of diseases of a pregnant mother on the embryo is especially dangerous.

Cerebellum
Structure of the blood-brain barrier
The main stages of individual human development
Symptoms of defects
Research methods
The leading role of the nervous system
Stages of embryo development
History of the study of the BBB
Structure and layers
Brain Development

Physical factors

that harm the body are various types of ionizing radiation, excessive exposure to ultraviolet rays, noise, vibration, etc.

To chemical factors

include harmful chemical compounds: heavy metals, phenols, benzopyrene, as well as some medications, alcohol, nicotine and drugs.

Particularly harmful to the development of the embryo is the mother's use of alcohol, drugs, and nicotine, which inhibit cellular respiration. Abuse of these substances often leads to irreversible damage to the embryo, and in some cases even death.

What is a neural tube?

The neural tube is the basis of the future nervous system that forms in the fetus, which develops from the outer cellular layer (germ layer - ectoderm) along with the skin, while the digestive system is formed from the endoderm.
The laying of the fetal neural tube begins in the third week of its development. First, the neural plate appears, then small elevations called neural folds form at its edges, in the center of which a groove appears. It will later serve as a cavity for the organ. Around day 24, the plate begins to curl and the rollers close, forming a tube shape.

In front it expands, forming the brain vesicles, and the rest will become the spinal cord. Particles of the ganglion plate are gradually separated from the ridges, from which the nodes of the spinal cord and branches of the autonomic part of the nervous system are then formed. You can see the appearance of spinal ganglia in the embryo after 1.5-2 months.

The cells formed in the ganglion plate gradually move to the ganglia in the sympathetic trunk, the adrenal medulla and the walls of the future intestine.

The final formation of the tube occurs at 5-8 weeks, when the development of all fetal organs begins. At this moment, the baby’s heart, lungs, sensory organs, limbs and other systems are formed. The tube itself also becomes more complicated.

Diagnostics

The main diagnostic method is genetic screening; abnormalities in the structure can only be seen on ultrasound. If a pathology is identified, it is necessary to undergo examination by several specialists to exclude an error in diagnosis.

Signs of pathology during ultrasound examination:

thickening of the vertebrae in the lumbosacral region;
deformation of the lower extremities;
hydrocephalus;
Arnold-Kari syndrome.

Other research methods are amniocentesis, 3D ultrasound. If a developmental pathology is suspected, a woman is prescribed a biochemical blood test for the content of alpha-fetoprotein and acetylcholinesterase.

Confirmation of developmental defects is usually grounds for termination of pregnancy. But, since even the most modern methods of prenatal diagnosis reveal only the presence of pathology, but do not determine the severity, the decision to keep the child is made by the parents.

Causes

Neural tube defects are structural anomalies caused by delayed or incomplete closure. Formed in the embryonic period.

Why does pathology develop:

hereditary factor;
Patau, Turner, Edwards syndrome;
radioactive radiation, poisoning with arsenic, lead, pesticides;
taking anti-epileptic drugs shortly before or immediately after conception;
unbalanced diet, vitamin A hypervitaminosis;
increased spinal pressure in the fetus;
viral diseases in pregnant women, rubella is especially dangerous;
significant and prolonged increase in temperature in the early stages;
obesity caused by diabetes.

But the main cause is considered to be folic acid deficiency during pregnancy.