• The first somites appear on day 20 in the region of the future base of the skull
•          the 8^th, 9^th, and 10^th somitomeres differentiate into the first, second, and third pairs of somites on day 20. 
•          The rest of the somites form in cranial/caudal progression at a rate of about three or four a day.
•          In the human about 42-44 pairs of somites form, flanking the notochord from the occipital region to the embryonic tail.
•          The caudalmost somites eventually disappear giving a final count of 37 pair.

The somites establish the segmental organization of the body

•          the somites give rise to the axial skeleton, to the voluntary musculature of the neck, body wall, and limbs, and to part of the dermis of the neck and trunk.
•          The first four pairs of somites form in the occipital region
•          Contribute to the development of the occipital region of the skull, the bones that form around the nose, eyes, and the inner ears, to the intrinsic ocular muscles, and to the muscles of the tongue.
•          the next eight pairs form in the cervical region
•          contribute to the occipital bone, and others form the cervical vertebrae, the associated muscles, and part of the neck dermis
•          the next twelve pairs form in the thoracic region
•          contribute to the thoracic vertabrae, the musculature and bones of the thoracic wall, part of the thoracic dermis, and part of the abdominal wall.
•          there are five lumbar somites next.
•          Form abdominal dermis, the abdominal muscles, and the lumbar vertabrae.
•          Five sacral somites
•          Form the sacrum with its associated dermis and musculature
•          Three coccygeal somites
•          Form the coccyx

The axial mesoderm induces the overlying mesoderm to form the neural plate

•          the first event in the formation of the future central nervous system is on day 18, when a thickened neural plate appears in the epiblast along the midsaggital axis cranial to the primitive pit.
•          The neural plate forms in response to inducing substances secreted by the underlying axial mesoderm structures
•          These substances diffuse to the overlying epiblast cells, in which they activate specific genes that cause the cells to differentiate into a thick plate of columnar, pseudostratified neurectoderm.
•          the neural plate appears first at the cranial end of the embryo and differentiates craniocaudally
•          it is broad cranially and tapered caudally
•          the expanded cranial portion gives rise to the brain.
•          Even at this stage, there are three visibly divided regions
•          Future forebrain, midbrain, and hindbrain
•          the narrow caudal portion gives rise to the spinal cord.

Beginning of the Fourth Week

The somites differentiate into sclerotome, myotome, and dermatome which gives rise to the specific mesodermal components.

•          The Sclerotome cells surround the notochord and neural tube
•          Develop into the vertebrae
•          Arise as follows.
•          Each newly formed somite cell develops a central cavity that becomes occupied by a population of loose core cells.
•          The somite then ruptures on its medial side, and the core cells + some other cells from the ventromedial wall of the somite, form the sclerotome.
•          Ventral portion- surrounds the notochord and forms the vertebral body
•          Dorsal portion- surrounds the neural tube and forms the vertebral arch.
•          DEFECTS
•          Scoliosis- defective induction of vertebral bodies on one side of the vertebral bodies
•          Spina bifida, myeloschisis, and anencephaly- caused by abnormal development of the neural tube and vertebral arches
•          The segmental sclerotomes split and recombine to form intersegmental vertebral rudiments
•          The spinal nerves develop  segmentally (each spinal nerve emerges at the same level as the corresponding somite)
•          How do the spinal nerves sprout from the developing vertebral canal?
•          Sclerotomes split and recombine to form vertebral rudiments that lie intersegmentally.
•          The sclerotome splits into a cranial half and a caudal half which then split and recombine with the adjoining caudal and cranial half
•          As a result of sclerotomal resegmentation, the segmental spinal nerves exit between the vertebrae.

**** It is important to remember that even though there are seven cervical vertebrae, there are eight cervical spinal nerves because one exist between the base of the skull and the first cervical vertebrae

•          The fibrous intervertebral discs form between the vertebral bodies at segmental levels.
•          The inner core, the nucleus pulposis, is composed of cells of notochordal origin
•          The surrounding annulus fibrosis develops from sclerotomal cells that are left in the region of the sclerotome splitting during resegmentation

The ribs develop from costal processes of the developing thoracic vertebrae

•          Costal Processes form in association with the vertebral arches of all the developing neck and trunk vertebrae.
•          Only in the thoracic region do the distal tips of the costal processes lengthen to form ribs.
•          The ribs begin to lengthen and form on day 35.
•          The first seven ribs connect ventrally to the sternum via costal cartilages by day 45 and are called true ribs
•          The five lower ribs do not articulate directly with the sternum and are called false ribs.
•          The ribs develop as cartilaginous precursors that later ossify, a process of endochondral ossification.
•          The costal processes of the cervical vertebrae gives rise to the lateral boundary, and the transverse processes give rise to the medial boundary of the foramina transversaria that transmit the vertebral arteries.
•          In the lumbar region, the costal processes become the transverse processes of the lumbar vertebrae.
•          The costal processes of the first two or three sacral vertebrae contribute to the development of the lateral sacral mass or ala of the sacrum.
•          The sternum develops from a pair of longitudinal mesenchymal condensations, the sternal bars, that form in the ventrolateral body wall.  

The myotomes and dermatomes develop at segmental levels

•          The portion of the somite that is translocated laterally while the sclerotome remains in its original medial location is called the dermomyotome.
•          The dermomyotome quickly differentiates into two structures: a dermatome and a myotome
•          The dermatomes with lateral plate mesoderm differentiate into dermis of the neck, back, and the ventral and lateral trunk.
•          The myotomes differentiate into myogenic cells (muscle producing)
•          Each myotome splits into two structures: a dorsal epimere and a ventral hypomere.
•          The epimeres give rise to the deep epaxial muscles of the back, including the erector spinae and the transversospinalis groups.
•          The hypomere gives rise to the hypaxial muscles of the lateral and ventral body wall in the thorax and the abdomen.  These include the three layers of intercostal muscles in the thorax, the homologous three layers of the abdominal musculature, and the rectus abdominus muscles that flank the ventral midline.
•          In the cervical region, hypaxial myoblasts form the strap muscles of the neck
•          In the lumbar region, the hypomeres form the quadratus lumborum muscles.
•          Myotomes DO NOT contribute to the formation of the tendons and inter connective tissue of the body wall muscles.  These structures arise from somatopleuric later plate mesoderm. 

Tags: dermatome, epimere, hypomere, mesoderm, myotome, sclerotome, scoliosis, somatopleuric mesoderm, somiomeres, somite

This entry was posted on Friday, January 16th, 2009 at 12:49 pm and is filed under Embryology. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.