Neuromeres are morphologically or molecularly defined transient segments of the early developing

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

Rhombomeres In the vertebrate embryo, a rhombomere is a transiently divided segment of the developing neural tube, within the hindbrain region (a neuromere) in the area that will eventually become the rhombencephalon. The rhombomeres appear as a series of sl ...

are such segments that make up the

rhombencephalon The hindbrain or rhombencephalon or lower brain is a developmental categorization of portions of the central nervous system in vertebrates. It includes the medulla, pons, and cerebellum. Together they support vital bodily processes. Metencephal ...

or hindbrain. More controversially, some argue that there exist early developmental segments that give rise to structures of the

midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...

( mesomeres) and

forebrain In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the Anatomical terms of location#Directional terms, rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hin ...

( prosomeres). Neuromeres are the segments of the

neural tube In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, a ...

that establish the embryonic brain during development. They can then be divided up so that each 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 Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, per ...

cells from each neuromere prompt the development of the nerves and arteries, helping to support the development of

craniofacial Craniofacial (''cranio-'' combining form meaning head or skull + ''-facial'' combining form referring to the facial structures grossly) is an adjective referring to the parts of the head enclosing the brain and the face. The term is typically used ...

tissues. If

gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

goes wrong, it can have severe effects on the developing embryo, causing abnormalities like craniofacial clefts, also known as

cleft palate A cleft lip contains an opening in the upper lip that may extend into the nose. The opening may be on one side, both sides, or in the middle. A cleft palate occurs when the palate (the roof of the mouth) contains an opening into the nose. The te ...

s. The anatomical boundaries of neuromers are determined by the expression of unique genes known as

Hox gene Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the cor ...

s in a particular zone. The Hox genes contain the 183-bp

homeobox A homeobox is a DNA sequence, around 180 base pairs long, that regulates large-scale anatomical features in the early stages of embryonic development. For instance, mutations in a homeobox may change large-scale anatomical features of the full- ...

, 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 will considered the precursore to the spinal cord and the rest of the nervous system, eventually becoming the Central Nervous System. The central nervous system can be divided into three classes of neuromeres: prosomeres, mesomeres, and rhombomeres. The forebrain forms the six prosomers, 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 The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

, from which ventral and

dorsal roots The dorsal root of spinal nerve (or posterior root of spinal nerve or sensory root) is one of two "roots" which emerge from the spinal cord. It emerges directly from the spinal cord, and travels to the dorsal root ganglion. Nerve fibres with the v ...

exit to form a specific pair of

spinal nerves A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the ...

.
The spinal chord 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 The coccyx ( : coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in all apes, and analogous structures in certain other mammals such as horses. In tailless primates (e.g. humans and othe ...

In more detail

The

is the main pathway for information connecting the brain and

peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brain ...

. The length of the spinal cord is much shorter than the length of the bony

spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordates ...

. The human spinal cord extends from the

foramen magnum The foramen magnum ( la, great hole) is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblon ...

and continues through to the

conus medullaris ''Conus'' is a genus of predatory sea snails, or cone snails, marine gastropod mollusks in the family Conidae.Bouchet, P.; Gofas, S. (2015). Conus Linnaeus, 1758. In: MolluscaBase (2015). Accessed through: World Register of Marine Species at ...

near the second lumbar vertebra, terminating in a fibrous extension known as the

filum terminale The filum terminale ("terminal thread") is a delicate strand of fibrous tissue, about 20 cm in length, proceeding downward from the apex of the conus medullaris. It is one of the modifications of pia mater. It gives longitudinal support to th ...

. 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 In anatomy, the meninges (, ''singular:'' meninx ( or ), ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in th ...

, that surround the canal. The

dura mater In neuroanatomy, dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous system. ...

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 In anatomy, the epidural space is the potential space between the dura mater and vertebrae (spine). The anatomy term "epidural space" has its origin in the Ancient Greek language; , "on, upon" + dura mater also known as "epidural cavity", "e ...

. The epidural space is filled with adipose tissue, and it contains a network of blood vessels. The

arachnoid mater The arachnoid mater (or simply arachnoid) is one of the three meninges, the protective membranes that cover the brain and spinal cord. It is so named because of its resemblance to a spider web. The arachnoid mater is a derivative of the neural c ...

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 Pia mater ( or ),Entry "pia mater"
in

Embryology

The spinal cord is made 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. Lastly, it is important to note that the past studies of Viktor Hamburger and Rita Levi-Montalcini in the chick embryo have been further proven by more recent studies which demonstrated that the elimination of neuronal cells by programmed cell death (PCD) is necessary for the correct assembly of the nervous system. Overall, spontaneous embryonic activity has been shown to play a role in neuron and muscle development but is probably not involved in the initial formation of connections between spinal neurons.

References

{{Development of nervous system Embryology of nervous system