File:Stage7 intermediate-mesoderm.jpg

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

• Intermediate mesoderm lies as rostocaudal (cranio-caudal) pair of mesoderm strips, lateral to the paraxial mesoderm
• Called "intermediate mesoderm" because it lies between paraxial and lateral plate mesoderm
• This mesoderm does not segment like paraxial mesoderm nor form a cavity like lateral plate mesoderm
• This mesoderm will contribute components to development of the renal and genital systems.
• Developing in sequence 3 pairs of renal systems:
  • Initially pronephros - within the cranial end of this mesoderm, degenerates rapidly.
  • Then mesonephros - within the majority of this mesoderm, eventually degenerating leaving the mesonephric duct.
  • Finally metanephros - within the caudal end of this mesoderm, forming the adult kidney.

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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