Нейромеры и сегментация нервной системы в эмбриональном развитии.

Neuromeres are distinct groups of neural crest cells, forming segments in the neural tube of the early embryonic development of the brain. There are three classes of neuromeres in the central nervous system – prosomeres (for the prosencephalon), mesomeres (for the mesencephalon) and rhombomeres (for the rhombencephalon) that will develop the forebrain, midbrain, and hindbrain respectively. Although researchers have long since recognized the different signs of differentiation during embryonic development, it was widely thought that neuromeres held no relation to the structure of the nervous system. He first named it Zwischenstrang, which literally translated to mean “intermediate cord.” The name neural crest develops from the neural folds during embryonic development. This is where the neural plate folds in on itself, forming the neural crest. Neural crest cells will eventually become portions of the peripheral nervous system. During development, the neural tube is considered as the precursor to the spinal cord and the rest of the central nervous system. The forebrain forms the six prosomeres, p1 to p6, which are then divided into two more categories, dorsal and ventral. The telencephalon forms from the dorsal parts of p6 and p5, where p6 becomes the olfactory system and p5 will coincide with the visual system. Mesomeres, m1 and m2, become the midbrain, which contains the superior and inferior colliculi. The 12 rhombomeres, which are numbered from r0 to r11, construct the hindbrain. The myelencephalon is made from rhombomeres r2 to r11, which also form the medulla. These rhombomeres are also associated with the neural crest that supplies the pharyngeal arches, a set of visible tissues that are in line with the developing brain and give rise to the head and neck.

Spinal segments are the part of the spinal cord, from which ventral and dorsal roots exit to form a specific pair of spinal nerves. The spinal cord is not segmented itself, it is only made into segments by the spinal nerves as they leave. 31 deferent segments exists in a human spinal cord:

Cervical nerves exit above C1 segment and below C1 C7. Note that this creates 8 nerves and segments corresponding to only 7 cervical vertebra.

Nerves exit below T1 T12.

Nerves exit below L1 L5.

Nerves exit below S1 S5.

Originally, during development there are two segments S1 and S2 which fuse. The nerves in this case exit at the coccyx.

The spinal cord develops from part of the neural tube during development. As the neural tube begins to develop, the notochord begins to secrete a factor known as Sonic hedgehog or SHH. As a result, the floor plate then also begins to secrete SHH, and this will induce the basal plate to develop motor neurons. Meanwhile, the overlying ectoderm secretes bone morphogenetic protein (BMP). This induces the roof plate to begin to secrete BMP, which will induce the alar plate to develop sensory neurons. The alar plate and the basal plate are separated by the sulcus limitans. Additionally, the floor plate also secretes netrins. The netrins act as chemoattractants to decussation of pain and temperature sensory neurons in the alar plate across the anterior white commissure, where they then ascend towards the thalamus. The elimination of neuronal cells by programmed cell death (PCD) is necessary for the correct assembly of the nervous system.