Neuromer

At the early stages of studying the segmental organization of the developing nervous system in vertebrate embryos, it was suggested that the formation of neuromeres in them occurs in three stages, as a result of three successive waves of intensive cell division and differentiation, propagating from the rostral (anterior, head) end of the embryo to the caudal ( posterior, tail), and that at each stage after passing these waves dividing anatomical - histological boundaries future neyromerov becoming sharper and determines nnym, and the fate of cells differentiating neuroepithelial within these future neyromerov - more and more unique. ^{[3] [4]}

These embryonic structures were called by the early authors, respectively, proneiromeres (or prerenyromers, “prototypes” of future neuromeres), then neuromeres proper, and then postneuromeres, or metaneomeres (“mature” neuromers). ^{[3] [4] The} disappearance of the neuromeres and their borders as the nervous system of the embryo ripens and the neuromeres are replaced by structures of the future “mature” or “adult” brain, as suggested by the early authors, occurs in the opposite direction, from the caudal (posterior, caudal) end to the rostral (anterior, head). ^{[3] [4]} At the same time, neuromeres themselves at each stage of development were supposed to arise de novo , independently from one another, from the cells of the corresponding part of the neural tube, and not as a result of the separation of an existing neuromer into two or more smaller neuromers. ^{[3] [4]}

However, it was later shown that, even if such an order of emergence, strengthening of boundaries and the subsequent disappearance of brain neuromeres is generally true for embryos of non-mammalian vertebrates (it was true for fish , birds , reptiles , amphibians , model organisms studied by early authors) generally not true for mammalian embryos. ^{[5] [6]} In particular, in rat embryos, although brain neuromeres arise, develop and disappear in a strictly defined, programmed order, this order itself is neither rostro-caudal nor caudo-rostral. Both the appearance and disappearance of neuromeres in rat embryos occurs and is controlled in a more complex way. ^{[5] [6]}

In addition, it turned out that no strictly three consecutive waves of cell division and hardening of the histological boundaries of future neuromeres characteristic of embryos of all those species of fish, birds, reptiles and amphibians that were studied by early authors in mammalian embryos, and in particular in embryos, turned out to be rats are not observed. ^{[5] [6]} The determination and strengthening of the histological boundaries of future neuromeres occurs in mammalian embryos in several stages, and the number of these stages is different for different types of mammals. It was also found that some neuromers in mammalian embryos do not arise de novo , from scratch, but as a result of dividing an existing neuromer into two (for example, the primary mesomer M, mesencephalon, is later subdivided into two mesomers M1 and M2, and the primary prosomer D, diencephalon , later subdivided into secondary prozomers D1 and D2). ^{[5] [6]}

Moreover, later, using modern equipment ( electron microscopes ), it was found that the assumptions made by early authors regarding the three mandatory waves of cell division and strengthening the boundaries of future neuromeres, regarding the indispensable emergence of neuromeres at every stage from the very beginning, de novo , and regarding the occurrence of them in the strict order from the rostral end of the embryo to the caudal, and their disappearance in the reverse order, from the caudal end to the rostral - in the general case, apparently, are incorrect and To fish, birds, reptiles and amphibians. They can also have a different number of waves of cell division and hardening of the boundaries of neuromers, different for different species, and the possibility of dividing the existing neuromer into two or more, and not strictly rostral-caudal order of emergence of neuromeres, and not strictly the reverse order of their disappearance (but, however, the order of their appearance and disappearance during embryogenesis is hard-coded). Thus, the mechanism of the formation and disappearance of neuromeres even in fish, birds, reptiles and amphibians is more complex than previously thought by the authors. ^{[5] [6]} Moreover, this is true for a human embryo. ^{[1] [2]} In this regard, the terms “proneiomer” or “preneiromer” and “postneuromer” or “metaneiomer” are proposed to be considered obsolete, and the terms “primary neuromer” and “secondary neuromer” should be used instead. ^{[1] [2]} It is proposed to use the term “subneuromer” or “tertiary neuromer” for transverse subunits that briefly arise within the framework of secondary neuromeres, or consider them after subdivision as the final number of secondary neuromeres, assuming, therefore, that the number of secondary neuromers may stage to stage embryo development. ^{[1] [2]}

At the Carnegie’s stage 9, in the future brain of the human embryo, six primary neuromers can be distinguished, listed in order from the rostral (head) end to the caudal (caudal) end: the future prosencephalon (one brain) , consisting of one primary prosomer, also also consisting of one primary mesomer M the future mesencephalon (midbrain) , and the future rhombencephalon (rhomboid brain) , consisting of four primary rhombomeres , denoted by the letters A, B, C and D. ^{[1] [2]}

At the Carnegie stage 14, the formation of secondary neuromers in the brain of a human embryo is completed. At this stage, five secondary cerebral vesicles can be distinguished, and in them there are a total of sixteen secondary neuromers: five secondary prozomers (one prozomer T1 in the telencephalon, and four prozomer in the diencephalon - D1 and D2, the last with the division into the rostral partsephalon, caudal partsephalon and synencephalon, comprising three separate secondary prozomers), two secondary mesomers M1 and M2 in the mesencephalon, and eight secondary rhombomeres Rh1-Rh8, plus the isthmus I, which is also a separate rhombomere. ^{[1] [2]}

The cerebral hemispheres of the brain are not, in the strict sense of the word, either prozomer proper or the direct derivatives of any prozomer. Initially, they are formed as an outgrowth from the Prosomer T1 far beyond, forward, then expanding on the sides, in both directions. They do not have a specific neuromeric organization, segment structure. Nevertheless, for the convenience of classifying neuromeres, it was proposed to consider the cerebral hemispheres as the T2 pseudoprosoomer, not included, however, in the total score of 16 “true” secondary brain neuromers or five “true” secondary prozomers in a human embryo. ^[2]

Concrete brain structures of adult chordates are formed from specific neuromeres. So, for example, from the 2nd prozomer of the diencephalon (D2), the thalamus and epithalamus are formed . ^[7]

The neuromeres of the future spinal cord are located exactly on the borders of the somites, and control the formation of the corresponding vertebrae and intervertebral discs through which future spinal roots will pass. In the human embryo of the spinal neuromer, after the formation of somites, thirty-two, according to the number of spinal somites and their corresponding vertebrae.

The neuromeres of the future spinal cord of the developing embryo are closely correlated both in their number and in the anatomical location and functions with the segments of the spinal cord of a newborn vertebrate. Anterior and posterior (ventral and dorsal) roots of the spinal cord depart from them. The spinal cord itself in newborns or adult vertebrates (including humans) is not segmented, in contrast to the abdominal arthropod nerve chain, in which each segment of the body (or rather, each somite of the arthropod embryo, which may be larger than the segments of the adult body animal, since some somites subsequently grow together and merge) corresponds to its own separate nerve node or ganglion. Vertebral spinal cord segmentation is carried out along the vertebrae and the corresponding spinal roots extending between them, and their innervation zones.

A person has 31 segments of the spinal cord, in accordance with 30 vertebrae, and 31-32 spinal somites of the human embryo at the stage of completion of somite formation. These segments are grouped into five zones: cervical, thoracic, dorsal, lumbar and coccygeal zones, respectively, the division of the vertebrae into the same subgroups.

Eight Cervical Segments

Cervical spinal roots extend above the first cervical vertebra (C1) and below the cervical vertebrae C1-C7. Thus, in the cervical segment in humans there are eight spinal roots, despite the fact that there are only seven cervical vertebrae in humans.

Twelve Thoracic Segments

The spinal roots of the twelve thoracic segments of the human spinal cord extend below the thoracic vertebrae T1-T12.

Five dorsal segments

The spinal roots of the five spinal segments of the human spinal cord extend below the spinal vertebrae L1-L5.

Five lumbar segments

The spinal roots of the five lumbar segments of the human spinal cord extend below the five lumbar vertebrae S1-S5.

One coccygeal segment

Initially, during the embryonic development, there are two coccygeal vertebrae, S1 and S2, which then fuse together to form a fixed coccyx . The radicular nerves in this case exit the lower opening of the coccyx, forming the so-called ponytail .

More details