Neuromere information

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Neuromeres are distinct groups of neural crest cells, forming segments in the neural tube of the early embryonic development of the brain.^[1] 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.^[1]

Neuromeres can then be divided up so that each segment is carrying different and unique genetic traits, expressing different features in development.

Neuromeres were first discovered in the beginning of the 20th century.^[2] 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.^[2] Swedish neuroembyrologists Bergquist and Kallen clarified the role of neuromeres by conducting several studies showing that neuromeres are important in the development of the nervous system.^[2] These experiments consisted of studying the brains of different vertebrates during their development period.

During embryonic development, neural crest cells from each neuromere prompt the development of the nerves and arteries, helping to support the development of craniofacial tissues. If gene expression goes wrong, it can have severe effects on the developing embryo, causing abnormalities like craniofacial clefts, also known as cleft palates.^[1] The anatomical boundaries of neuromeres are determined by the expression of unique genes known as Hox genes in a particular zone. The Hox genes contain the 183-bp homeobox, which encodes a particular portion of the Hox proteins called the homeodomain. The homeodomain can then bind to other portions of DNA to regulate gene expression. These genes determine the basic structure and orientation of an organism after the embryonic segments have formed.^[1] The neural crest cells that are found outside of a given neuromere will express the same proteins as the cells that are found inside the neural tube.^[1] The genes that are being expressed fall into two categories, extracellular signaling proteins and intracellular transcription factors.^[1] The genes are able to perform different tasks at different times depending on the environment that may or not be changing as well as when they are being activated and expressed.

The neural crest was first discovered by Wilhelm His in 1868 when he was studying the embryo of a chick.^[3] He first named it Zwischenstrang, which literally translated to mean “intermediate cord.” The name neural crest develops from the neural folds during embryonic development.^[3] 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.^[1]