vertebra vertebra the spinal column, a defining synapomorphy shared by nearly all vertebrates, is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic irregular bone whose complex structure is composed primarily of bone, and secondarily of hyaline cartilage. they show variation in the proportion contributed by these two tissue types; such variations correlate on one hand with the cerebral/caudal rank (i.e., location within the backbone), and on the other with phylogenetic differences among the vertebrate taxa. vertebraea typical vertebra, superior viewa section of the human vertebral column, showing multiple vertebrae in a left posterolateral viewdetailspart ofspinal columnidentifierslatinvertebraeta98a02.2.01.001ta21011fma9914anatomical terms of bone the basic configuration of a vertebra varies, but the bone is its body, with the central part of the body constituting the centrum. the upper (closer to) and lower (further from), respectively, the cranium and its central nervous system surfaces of the vertebra body support attachment to the intervertebral discs. the posterior part of a vertebra forms a vertebral arch (in eleven parts, consisting of two pedicles (pedicle of vertebral arch) two laminae, and seven processes. the laminae give attachment to the ligamenta flava, which are ligaments of the spine. there are vertebral notches, each constituted by the shape of the pedicles, which form the intervertebral foramina when vertebrae articulate. these foramina are the entry and exit conduits accommodating the spinal nerves. the body of the vertebra, and its vertebral arch, form the vertebral foramen, which is the larger, stable and central opening: this accommodates the spinal canal, and encloses and protects the spinal cord. vertebrae articulate with each other to give strength and flexibility to the spinal column, and the shape at their back and front aspects determines the range of movement. structurally, vertebrae are essentially alike across the vertebrate species, with the greatest difference seen between an aquatic animal and other vertebrate animals. as such, vertebrates take their name from the vertebrae that compose the vertebral column. structure in humans general structure in the human vertebral column the size of the vertebrae varies according to placement in the vertebral column, spinal loading, posture and pathology. along the length of the spine the vertebrae change to accommodate different needs related to stress and mobility. each vertebra is an irregular bone. side view of vertebrae every vertebra has a body (vertebral body), which consists of a large anterior middle portion called the centrum (vertebral centrum, plural centra) and a posterior vertebral arch, also called a neural arch. the body is composed of cancellous bone, which is the spongy type of osseous tissue, whose microanatomy has been specifically studied within the pedicle bones. this cancellous bone is in turn, covered by a thin coating of cortical bone (or compact bone), the hard and dense type of osseous tissue. the vertebral arch and processes have thicker coverings of cortical bone. the upper and lower surfaces of the body of the vertebra are flattened and rough in order to give attachment to the intervertebral discs. these surfaces are the vertebral endplates which are in direct contact with the intervertebral discs and form the joint. the endplates are formed from a thickened layer of the cancellous bone of the vertebral body, the top layer being more dense. the endplates function to contain the adjacent discs, to evenly spread the applied loads, and to provide anchorage for the collagen fibers of the disc. they also act as a semi-permeable interface for the exchange of water and solutes. anatomy of a vertebra the vertebral arch is formed by pedicles and laminae. two pedicles extend from the sides of the vertebral body to join the body to the arch. the pedicles are short thick processes that extend, one from each side, posteriorly, from the junctions of the posteriolateral surfaces of the centrum, on its upper surface. from each pedicle a broad plate, a lamina, projects backwards and medialwards to join and complete the vertebral arch and form the posterior border of the vertebral foramen, which completes the triangle of the vertebral foramen. the upper surfaces of the laminae are rough to give attachment to the ligamenta flava. these ligaments connect the laminae of adjacent vertebra along the length of the spine from the level of the second cervical vertebra. above and below the pedicles are shallow depressions called vertebral notches (superior and inferior). when the vertebrae articulate the notches align with those on adjacent vertebrae and these form the openings of the intervertebral foramina. the foramina allow the entry and exit of the spinal nerves from each vertebra, together with associated blood vessels. the articulating vertebrae provide a strong pillar of support for the body. processes there are seven processes projecting from the vertebra: one spinous process two transverse processes four articular processes a major part of a vertebra is a backward extending spinous process (sometimes called the neural spine) which projects centrally. this process points dorsally and caudally from the junction of the laminae. the spinous process serves to attach muscles and ligaments. the two transverse processes, one on each side of the vertebral body, project laterally from either side at the point where the lamina joins the pedicle, between the superior and inferior articular processes. they also serve for the attachment of muscles and ligaments, in particular the intertransverse ligaments. there is a facet on each of the transverse processes of thoracic vertebrae which articulates with the tubercle of the rib. a facet on each side of the thoracic vertebral body articulates with the head of the rib. the transverse process of a lumbar vertebra is also sometimes called the costal or costiform process because it corresponds to a rudimentary rib (costa) which, as opposed to the thorax, is not developed in the lumbar region. there are superior and inferior articular facet joints on each side of the vertebra, which serve to restrict the range of movement possible. these facets are joined by a thin portion of the vertebral arch called the pars interarticularis. regional variation segments of human vertebrae vertebrae take their names from the regions of the vertebral column that they occupy. there are thirty-three vertebrae in the human vertebral column—seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, five fused sacral vertebrae forming the sacrum and three to five coccygeal vertebrae, forming the coccyx. the regional vertebrae increase in size as they progress downwards but become smaller in the coccyx. cervical vertebrae main article: cervical vertebrae a typical cervical vertebra there are seven cervical vertebrae (but eight cervical spinal nerves), designated c1 through c7. these bones are, in general, small and delicate. their spinous processes are short (with the exception of c2 and c7, which have palpable spinous processes). c1 is also called the atlas, and c2 is also called the axis. the structure of these vertebrae is the reason why the neck and head have a large range of motion. 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 disc of the spinal column. cervical vertebrae possess transverse foramina to allow for the vertebral arteries to pass through on their way to the foramen magnum to end in the circle of willis. these are the smallest, lightest vertebrae and the vertebral foramina are triangular in shape. the spinous processes are short and often bifurcated (the spinous process of c7, however, is not bifurcated, and is substantially longer than that of the other cervical spinous processes). the atlas differs from the other vertebrae in that it has no body and no spinous process. it has instead a ring-like form, having an anterior and a posterior arch and two lateral masses. at the outside centre points of both arches there is a tubercle, an anterior tubercle and a posterior tubercle, for the attachment of muscles. the front surface of the anterior arch is convex and its anterior tubercle gives attachment to the longus colli muscle. the posterior tubercle is a rudimentary spinous process and gives attachment to the rectus capitis posterior minor muscle. the spinous process is small so as not to interfere with the movement between the atlas and the skull. on the under surface is a facet for articulation with the dens of the axis. specific to the cervical vertebra is the transverse foramen (also known as foramen transversarium). this is an opening on each of the transverse processes which gives passage to the vertebral artery and vein and a sympathetic nerve plexus. on the cervical vertebrae other than the atlas, the anterior and posterior tubercles are on either side of the transverse foramen on each transverse process. the anterior tubercle on the sixth cervical vertebra is called the carotid tubercle because it separates the carotid artery from the vertebral artery. there is a hook-shaped uncinate process on the side edges of the top surface of the bodies of the third to the seventh cervical vertebrae and of the first thoracic vertebra. together with the vertebral disc, this uncinate process prevents a vertebra from sliding backwards off the vertebra below it and limits lateral flexion (side-bending). luschka's joints involve the vertebral uncinate processes. the spinous process on c7 is distinctively long and gives the name vertebra prominens to this vertebra. also a cervical rib can develop from c7 as an anatomical variation. the term cervicothoracic is often used to refer to the cervical and thoracic vertebrae together, and sometimes also their surrounding areas. thoracic vertebrae main article: thoracic vertebrae a typical thoracic vertebra the twelve thoracic vertebrae and their transverse processes have surfaces that articulate with the ribs. some rotation can occur between the thoracic vertebrae, but their connection with the rib cage prevents much flexion or other movement. they may also be known as "dorsal vertebrae" in the human context. the vertebral bodies are roughly heart-shaped and are about as wide anterio-posteriorly as they are in the transverse dimension. vertebral foramina are roughly circular in shape. the top surface of the first thoracic vertebra has a hook-shaped uncinate process, just like the cervical vertebrae. the thoracolumbar division refers to the thoracic and lumbar vertebrae together, and sometimes also their surrounding areas. the thoracic vertebrae attach to ribs and so have articular facets specific to them; these are the superior, transverse and inferior costal facets. as the vertebrae progress down the spine they increase in size to match up with the adjoining lumbar section. lumbar vertebrae main article: lumbar vertebrae lumbar vertebra showing mammillary processes a typical lumbar vertebra the five lumbar vertebrae are the largest of the vertebrae, their robust construction being necessary for supporting greater weight than the other vertebrae. they allow significant flexion, extension and moderate lateral flexion (side-bending). the discs between these vertebrae create a natural lumbar lordosis (a spinal curvature that is concave posteriorly). this is due to the difference in thickness between the front and back parts of the intervertebral discs. the lumbar vertebrae are located between the ribcage and the pelvis and are the largest of the vertebrae. the pedicles are strong, as are the laminae, and the spinous process is thick and broad. the vertebral foramen is large and triangular. the transverse processes are long and narrow and three tubercles can be seen on them. these are a lateral costiform process, a mammillary process and an accessory process. the superior, or upper tubercle is the mammillary process which connects with the superior articular process. the multifidus muscle attaches to the mammillary process and this muscle extends through the length of the vertebral column, giving support. the inferior, or lower tubercle is the accessory process and this is found at the back part of the base of the transverse process. the term lumbosacral is often used to refer to the lumbar and sacral vertebrae together, and sometimes includes their surrounding areas. sacrum sacrum main article: sacrum there are five sacral vertebrae (s1–s5) which are fused in maturity, into one large bone, the sacrum, with no intervertebral discs. the sacrum with the ilium forms a sacroiliac joint on each side of the pelvis, which articulates with the hips. coccyx main article: coccyx the last three to five coccygeal vertebrae (but usually four) (co1–co5) make up the tailbone or coccyx. there are no intervertebral discs. development development of