Neuromeres

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.

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. 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. 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. 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. 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. 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. The genes that are being expressed fall into two categories, extracellular signaling proteins and intracellular transcription factors. 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. 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 cord anatomy

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:

8 cervical segments

Cervical nerves exit above C1 segment and below C1-C7.

Note that this creates 8 nerves and segments corresponding to only 7 cervical vertebra.

12 thoracic segments

Nerves exit below T1-T12.

5 lumbar segments

Nerves exit below L1-L5.

5 sacral segments

Nerves exit below S1-S5.

1 coccygeal segment

Originally, during development there are two segments S1 and S2 which fuse.

The nerves in this case exit at the coccyx.

In more detail

The spinal cord is the main pathway for information connecting the brain and peripheral nervous system. The length of the spinal cord is much shorter than the length of the bony spinal column. The human spinal cord extends from the foramen magnum and continues through to the conus medullaris near the second lumbar vertebra, terminating in a fibrous extension known as the filum terminale.

It is about 45 cm (18 in) long in men and around 43 cm (17 in) in women, ovoid-shaped, and is enlarged in the cervical and lumbar regions. The cervical enlargement, located from C3 to T2 spinal segments, is where sensory input comes from and motor output goes to the arms. The lumbar enlargement, located between L1 and S3 spinal segments, handles sensory input and motor output coming from and going to the legs.

The spinal cord is protected by three layers of tissue, called spinal meninges, that surround the canal. The dura mater is the outermost layer, and it forms a tough protective coating. Between the dura mater and the surrounding bone of the vertebrae is a space called the epidural space. The epidural space is filled with adipose tissue, and it contains a network of blood vessels. The arachnoid mater is the middle protective layer. Its name comes from the fact that the tissue has a spiderweb-like appearance. The space between the arachnoid and the underlying pia mater

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