Myelogenesis is the medical term used to describe, first, embryonic spinal cord formation and, second, the formation of the medulla of all medullary nerves, which is carried out by oligodendroglia and Schwann cells. Both meanings of the term deal with developmental processes of the nervous system. Disorders of these developmental processes result in functional impairment of the central and peripheral nervous systems.
What is myelogenesis?
Myelogenesis is the medical term used to describe, first, embryonic spinal cord formation and, second, the formation of the medulla of all medullary nerves. The term myelogenesis is medically associated with two different meanings. Thus, on the one hand, the term refers to the embryonic development of the spinal cord and, on the other hand, to the myelination (of myelin) to form the medulla of medullary nerve fibers. The spinal cord arises from the caudal portion of the neural tube during embryonic development. In this context, myelogenesis is a subsequent step of neurulation. In the context of myelination, myelogenesis corresponds to the wrapping of medullary nerves. In the central nervous system, this wrapping is performed by the so-called oligodendroglial cells and in the peripheral nervous system by Schwann cells. The wrapping results in myelin sheaths, which are formed by a single glial cell in each of the central and peripheral nervous systems. Each Schwann cell wraps spirally around a nerve fiber segment. Each oligodendroglial cell forms outgrowths, and these outgrowths individually wrap around a single section of nerve fiber.
Function and purpose
During embryonic neurulation, the embryonic neural tube is formed. With this structure, the central nervous system manifests for the first time. The spinal cord emerges from the neural tube as part of the central nervous system. Its cranial end merges into the so-called rhombencephalon, which borders four occipital somites on each side. From the sixth week of development, the wall of the neural tube differentiates into three distinct layers. In addition to a ventricular zone, an intermediate zone and a marginal zone can be discerned. The spinal cord acquires its final shape around the tenth week of development. The cerebral and spinal cord membranes surround the structure, which itself lies in the vertebral canal. The spinal cord and spinal canal are developed by the fourth month. Their developmental steps occur in parallel. The growth of the spinal column progresses further and further from this time. However, myelogenesis in this context refers exclusively to neurulation and the medullary formation that builds on it. In terms of myelination and thus the marrow formation on medullary nerve fibers, the term myelogenesis refers to the wrapping of the nerves, which results in the isolation of the structures from their surroundings. Myelination electrically insulates the axons of the nerves, ensuring that signals in the nervous system can be transmitted at high speed and with almost no loss. The wrapping of the axons takes place on the fiber at regular intervals. Between the individual sheaths of myelin are gaps of approximately equal size. These gaps occur due to waist-shaped constrictions and are named as Ranvier’s lacing rings, which are histologically recognizable as tiny nodules. Because of their appearance, they are also called nodus. Between two Ranvier nodi lies the so-called internode. The structure of myelinated and unisolated sites ensures that nerve fibers are receptive to signals from the outside and action potentials can thus be communicated between the individual axons. Myelination already takes place during embryonic development. The process begins around the third embryonic month and comes to an end in the fourth year of life with the full myelination of the pyramidal tracts.
Diseases and disorders
Disruptions in myelogenesis can have fatal consequences for an organism. This is true for disorders during embryonic development of the spinal cord, as well as for those of myelination of medullary nerve fibers. If, for example, nerve fibers receive too little medulla due to disturbed myelination, they are insufficiently insulated from their environment. The result is signal loss during the transmission of action potentials. Such signal losses slow down transmission conduction or, in extreme cases, completely prevent transmission.At the pyramidal tracts, insufficient myelogenesis can thus cause paralysis, for example. Developmental disorders of the spinal cord can have similar consequences. We always speak of amyelia when the entire spinal cord is missing. Without spinal cord, however, humans are not able to survive. In hypoplasia or dysplasia, the spinal cord shows underdevelopment or maldevelopment. Both phenomena are due to external causes and are not genetic. Dysplasia or hypoplasia of the spinal cord occurs, for example, after mechanical, infectious, nutritional or toxic damage during early pregnancy. One of the conceivable malformations of the spinal cord is diastematomyelia. This is a congenital cleft formation of the spinal cord. The structure divides into unequal parts, each with its own membranes. In most cases, the division is localized in the lower thoracic region or begins at the upper lumbar spine. All malformations and underdevelopments of the spinal cord result in functional disorders of the nervous system. The location of the malformation determines exactly how these functional disorders manifest themselves. The individual spinal cord segments in their entirety supply all areas of the body with nerve fibers and are thus crucial elements for all bodily processes. Thus, a maldevelopment of the spinal cord can result in organic disorders as well as disorders of motor function or perception. The same consequences apply by and large to a disturbed medulla formation. However, since the spinal cord is part of the central nervous system, developmental disorders of the spinal cord usually have more serious consequences than disorders of medullary septum formation. The latter may, in some circumstances, relate exclusively to the peripheral nervous system.
