Somites


- See: limb development

- Discussion:
    - skeleton and skeletal muscle develop from the mesoderm;
          - thin layer of cells in early embryo located between ectoderm & endoderm & interrupted in midline by notochord;
    - mesoderm on either side of notochord forms longitudinal mass, paraxial mesoderm;
            - at 21st day of development, paraxial mesoderm forms paired, cuboidal masses, somites;
    - somites first form first occipital somites, at cranial end of the notochord;
    - subsequent pairs of somites appear caudally next to previous pair;
    - total of some 42 to 44 pairs of somites are formed between the 21st and the 30th days of development;
    - lateral plate mesoderm:
          - remains largely unsegmented
          - remains attached to ventrolateral angle of each somite by intermediate mesoderm, which later forms the nephrogenic cord;
    - in neck & trunk, each somite develops into three recognizable portions
          - laterally a dermatome, destined to differentiate into dermis;
          - medially & ventrally sclerotome, from which develop vertebrae, intervertebral discs, & ligaments of vertebral column



The Ectodermal Thickening and Ridge in the Limbs of Staged Human Embryos.



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Wednesday, April 11, 2012 12:30 pm