Vertebra
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224780055
2008-07-10T11:26:18Z
Yosri
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[[Image:Gray90.png|right|thumb|300px|A diagram of a thoracic vertebra. Notice the [[Bone#Terminology|articulation]]s for the [[rib]]s]]
[[Image:Spinal column curvature.png|thumb|150px|Different regions of the [[vertebral column]]]]
'''The [[Vertebral column]]''' (singular: '''''vertebra''''') are the individual [[irregular bone]]s that make up the spinal column (''aka'' ischis) — a flexuous and flexible column common to all [[Vertebrate]]s.
There are normally thirty-three (33) vertebrae in humans, including the five that are fused to form the [[sacrum]] (the others are separated by [[intervertebral disc]]s) and the four [[coccyx|coccygeal]] [[bone]]s which form the ''tailbone''.
The upper three regions comprise the remaining 24, and are grouped under the names ''cervical'' (7 vertebrae), ''thoracic'' (12 vertebrae) and ''lumbar'' (5 vertebrae), according to the regions they occupy.
This number is sometimes increased by an additional vertebra in one region, or it may be diminished in one region, the deficiency often being supplied by an additional vertebra in another.
The number of cervical vertebrae is, however, very rarely increased or diminished.
With the exception of the first and second cervical, the true or movable vertebrae (the upper three regions) present certain common characteristics which are best studied by examining one from the middle of the thoracic region.
==General structure==
[[Image:ACDF oblique annotated english.png|thumb|left|300px|Oblique view of cervical vertebrae]]
A typical vertebra consists of two essential parts: an anterior (front) segment, which is the [[vertebral body]]; and a posterior part – the [[Vertebral arch|vertebral (neural) arch]] – which encloses the [[vertebral foramen]].
The vertebral arch is formed by a pair of [[Pedicle of vertebral arch|pedicles]] and a pair of [[lamina of the vertebral arch|laminae]], and supports seven [[Process (anatomy)|processes]], four articular, two transverse, and one spinous, the latter also being known as the neural spine.
When the vertebrae are articulated with each other, the bodies form a strong pillar for the support of the [[head (anatomy)|head]] and [[Torso|trunk]], and the vertebral foramina constitute a canal for the protection of the ''medulla spinalis'' ([[spinal cord]]).
In between every pair of vertebrae are two apertures, the [[intervertebral foramina]], one on either side, for the transmission of the spinal nerves and vessels.
Two [[transverse processes]] and one [[spinous process]] are posterior to (behind) the vertebral body. The spinous process comes out the back, one transverse process comes out the left, and one on the right. The spinous processes of the cervical and lumbar regions can be felt through the skin.
Superior and inferior [[Bone#Terminology|articular]] [[Zygapophyseal joint|facets]] on each vertebra act to restrict the range of movement possible. These facets are joined by a thin portion of the neural arch called the ''[[pars interarticularis]]''.
==Regions==
[[Image:Orientation.PNG|thumb|right|300px|Orientation of vertebral column on surface. T3 is at level of medial part of [[spine of scapula]]. T7 is at [[inferior angle of the scapula]]. L4 is at highest point of [[iliac crest]]. S2 is at the level of [[posterior superior iliac spine]]. Furthermore, C7 is easily localized as a prominence at the lower part of the neck.<ref>Anatomy Compendium (Godfried Roomans and Anca Dragomir)</ref>]]
=== Cervical ===
''Note: For more detailed information, see [[Cervical vertebrae]]''
These are generally small and delicate. Their spinous processes are short (with the exception of C2 and C7, which have palpable spinous processes), and often split. Numbered top-to-bottom from C1-C7, [[atlas (anatomy)|atlas]] (C1) and [[axis (anatomy)|axis]] (C2), are the vertebrae that allow the [[neck]] and [[head]] so much movement. For the most part, the [[atlanto-occipital joint]] allows the [[skull]] to move up and down, while the [[atlanto-axial joint]] allows the upper neck to twist left and right. The axis also sits upon the first intervertebral disk of the spinal column. All [[mammal]]s except [[manatee]]s and [[sloth]]s have seven cervical vertebrae, whatever the length of the neck.
=== Thoracic ===
''Note: For more detailed information, see [[Thoracic vertebrae]]''
Their spinous processes have surfaces that [[Bone#Terminology|articulate]] with the [[rib]]s. Some rotation can occur between the thoracic vertebrae, but their connection with the rib cage prevents much ''flexion'' or other excursion. They may also be known as 'dorsal vertebrae', in the human context.
=== Lumbar ===
''Note: For more detailed information, see [[Lumbar vertebrae]]''
These vertebrae are very robust in construction, as they must support more weight than other vertebrae. They allow significant ''flexion'' and ''extension'', moderate lateral flexion (sidebending), and a small degree of rotation. The discs between these vertebrae create a [[lumbar]] lordosis (curvature that is concave posteriorly) in the human spine.
[[Image:Illu vertebral column.jpg|thumb|right|250px]]
=== Sacral ===
''Note: For more detailed information, see [[Sacral vertebrae]]''
There are 5 vertebrae (S1-S5). They are fused in maturity, with no [[intervertebral disc]]s.
=== Coccygeal ===
''Note: For more detailed information, see [[Coccygeal vertebrae]]''
There are 3-5 vertebrae (Co1-Co5), with no [[intervertebral disc]]s. Many animals have a greater number of "tail vertebrae" and, in animals, they are more commonly known as "caudal vertebrae." Pain at the [[coccyx|coccyx (tailbone)]] is known as [[coccydynia]].
== Development ==
The striking segmented pattern of the human spine is established during embryogenesis when the precursor of the vertebrae, the somites, are rhythmically added to the forming posterior part of the embryo. In human, somite formation begins around the third week post-fertilization and continues until a total of around 52 somites are formed. The somites are epithelial spheres that contain the precursors of the vertebrae, the ribs, the skeletal muscles of the body wall and limbs, and the dermis of the back. The periodicity of somite distribution and production is thought to be imposed by a molecular oscillator or clock acting in cells of the presomitic mesoderm (PSM). Somites form soon after the beginning of gastrulation, on both sides of the neural tube from a tissue called the presomitic mesoderm (PSM). The PSM is part of the paraxial mesoderm and is generated caudally by gastrulation when cells ingress through the primitive streak, and later, through the tail bud. Soon after their formation, somites become subdivided into the dermomyotome dorsally, which gives rise to the muscles and dermis, and the sclerotome ventrally which will form the spine components. Sclerotomes become subvidided into an anterior and a posterior compartment. This subdivision plays a key role in the definitive patterning of vertebrae which form when the posterior part of one somite fuses to the anterior part of the consecutive somite during a process termed resegmentation. Disruption of the somitogenesis process in humans results in diseases such as congenital scoliosis. So far, the human homologues of three genes associated to the mouse segmentation clock (MESP2, DLL3 and LFNG) have been shown to be mutated in human patients with human congenital scoliosis suggesting that the mechanisms involved in vertebral segmentation are conserved across vertebrates. In humans the first four somites are incoporated in the basi-occipital bone of the skull and the next 33 somites will form the vertebrae. The remaining posterior somites degenerate. During the fourth week of [[embryonic development]], the [[sclerotome]]s shift their position to surround the [[spinal cord]] and the [[notochord]]. The sclerotome is made of [[germ layer#Mesoderm|mesoderm]] and originates from the ventromedial part of the [[somite]]s. This column of tissue has a segmented appearance, with alternating areas of dense and less dense areas.
As the sclerotome develops, it condenses further eventually developing into the [[vertebral body]]. Development of the appropriate shapes of the vertebral bodies is regulated by ''HOX'' [[gene]]s.
The less dense tissue that separates the sclerotome segments develop into the [[intervertebral disc]]s.
The notochord disappears in the sclerotome (vertebral body) segments, but persists in the region of the intervertebral discs as the [[nucleus pulposus]]. The nucleus pulposus and the fibers of the [[annulus fibrosus disci intervertebralis|annulus fibrosus]] make up the intervertebral disc.
The primary curves (thoracic and sacral curvatures) form during fetal development. The secondary curves develop after birth. The cervical curvature forms as a result of lifting the head and the lumbar curvature forms as a result of walking.
There are various defects associated with vertebral development. [[Scoliosis]] can result from improper fusion of the vertebrae. In [[Klippel-Feil anomaly]] patients have two or more cervical vertebrae that are fused together, along with other associated birth defects. One of the most serious defects is failure of the vertebral arches to fuse. This results in a condition called [[spina bifida]]. There are several variations of spina bifida that reflect the severity of the defect.
==Footnotes==
{{reflist}}
==References==
* [http://www.bartleby.com/107/ Gray's Anatomy]: [http://www.bartleby.com/107/19.html The Vertebral column] - The [[1917]] [[Gray's Anatomy]] is available via the [http://www.bartleby.com Bartleby] project. It is available with full colour diagrams, and provides an excellent starting point in [[anatomy]], as well as a relatively complete source for [[gross anatomy]]. This article was copied and pasted from the 1917 Gray's Anatomy, which is in the [[public domain]].
{{Vertebral column and spinal cord}}
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