File:Stage7 paraxial-mesoderm.jpg: Difference between revisions

Carnegie Stage 7 showing the paraxial mesoderm region of the embryonic disc

Stage 7 Mesoderm: axial | paraxial | intermediate | lateral plate

Week 3, Carnegie stage 7, 15 - 17 days, 0.4 mm, embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side.

• Paraxial mesoderm lies lateral to the axial mesoderm.
• During week paraxial mesoderm begins to segment, at the level of the embryo body, into somites.
  • historic term was "primitive segments"
• This process is called Somitogenesis
• The paraxial mesoderm at the level of the embryo head remains unsegmented.

Carnegie Stage 7

Features: embryonic disc, primitive node, primative streak, primative groove, yolk sac

Facts: Week 3, 15 - 17 days, 0.4 mm

View 1: embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side.

Events: Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm.

Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes. These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract.

From the primitive node a tube extends under the ectoderm in the opposite direction to the primitive streak. This tube forms first the axial process then notochordal process, then finally the notochord.

The notochord is a key to embryonic folding and regulation of ectoderm and mesoderm differentiation. It lies in the rostrocordal axis and the embryonic disc will fold either side ventrally, pinching off a portion of the yolk sac to form the lining of the gastrointestinal tract.

Stage 7 Mesoderm: axial | paraxial | intermediate | lateral plate

Image Source: UNSW Embryology http://embryology.med.unsw.edu.au/wwwhuman/Stages/stage7.htm

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