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SOMITES

     Many animals are segmented; their anatomical structure has been achieved through repeating a basic unit of structure.  Once established, these repeating units can then be modified, often in ways which make the embryological pattern of segmentation less obvious.   Although segmentation is not overly obvious in many adult vertebrates, it is much more apparent in their embryos.  A major feature of both vertebrate and invertebrate patterning is the development of segments along the longitudinal axis (Berwerff, 1999).  The repeating blocks of muscle and connective tissue which form along the longitudinal axis in vertebrate embryos are called somites.  

By the 3rd week of human embryonic development about 38 pairs cuboidal blocks of mesodermal tissue known as somites form.  By the 5th week there are 42-44 pairs of somites.  Most of the axial skeleton and skeletal muscles will be derived from these somites (Moore, p. 74).

DRAWING OF SOMITES DRAWING OF SOMITES
DRAWING OF SOMITES

By the 20th day, the first pair of somites have formed in the neck region.  After this, an average of about three pairs of somites form per day until the fifth week at which point there are 42-44 somites.  These somites can be classified as 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal somites.  The first occipital somite and the final five to seven coccygeal somites degenerate later in development (Sadler, p. 65).

     Although somites are structures which are found only in the embryos of chordates, some of the molecular mechanisms controlling the formation of somites are shared with the segmentation of Drosophila embryos.    Not only are the genes involved in segmentation shared between vertebrates and invertebrates, but also some of the expression patterns.  In Drosophila and lancelets, the gene engrailed is expressed in the posterior portion of each segment.  In zebrafish and Drosophila hairy is expressed in every other segment/somite (although this is not observed in other insects or vertebrates).  (McGrew, 1998).  FGFR1, Tbx6, Wnt3a, Notch1, Delta1, Presinilin1, RBP-jk, Mesp2, and Paraxis are involved in vertebrate somitogenesis and many of these genes have invertebrate homologs (McGrew, 1998).  The gene hairy controls embryonic segmentation as a pair-rule gene in invertebrates.  In vertebrates, the hairy homolog her1 is expressed in bands in developing somites, suggesting a conserved genetic mechanism determining segmentation in coelomates (Muller, 1996).  FGF signals and the Notch pathway are involved in the “segmentation clock” which forms somites along the antero-posterior axis of the embryo.  Some evidence exists that the mesoderm of the amniote head is derived from two ancestral somites (Dubrelle, 2002).

     Vertebrates vary in the number of somites which form in embryonic development.  The number of trunk vertebrae derived fro these somites varies in vertebrates.  Sharks may possess several hundred vertebrae.  While most teleosts possess about 48 vertebrae, eels can possess hundreds of vertebrae and platyfish possess only 6 trunk vertebtrae.  Amphibian vertebral counts vary from 6 in some frogs to 285 in some caecilians.  The extinct snake Archaeophis may have possessed 565 vertebrae.  Some dolphins possess almost 100 vertebrae.  The number of vertebrae in mammalian tails varies from 3 to 47 (Richardson, 1998)

Amphioxus

AMPHIOXUS AMPHIOXUS
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