5 Somites & Limb Buds

The somites are the blocks of mesoderm along the sides of the future spinal cord . The limb buds are where the fore- and hindlimbs start to develop on the embryo. Our main purpose is to consider where meat and bones originate. Muscle is formed from cells called myoblasts (from the Greek; mys - muscle, and blastos - bud).  But myoblasts cannot divide by mitosis. Hence, the dividing cells giving rise to myoblasts are called premyoblasts.

5.1 Somites

In the micrograph above taken longitudinally from a 72-hour chick embryo, you can see the block-like somites developing  from the unsegmented mesoderm. The chick's head is to the left.  The pale line from left to right of the micrograph is the nerve cord.

This is what it really looks like!

Seen in a whole embryo, each somite appears as a cube of tissue, with left and right somites forming pairs in an anterior to posterior sequence. The division of continuous strips of mesoderm to become somites follows a pattern created by the arrangement of cells early in development,  Somite formation is determined by Pax genes, while the  antero-posterior axis is determined by the Hox gene.

Mesenchyme cells.  From the Greek, this literally means cells 'poured' into the middle; in other words, cells without any distinguishing features which fill a space.  More typically a histologist might say, "the sclerotome is composed of loosely packed and morphologically undifferentiated mesenchyme cells." The mesenchyme surrounds the notochord and neural tube and later differentiates to form cartilaginous precursors of the vertebrae. In farm animals, myotomes become elongated from anterior to posterior, and this obscures the original cuboidal shape of each myotome. Extensive changes in the shape and orientation of somitic cells occur during the development of the somites - in other words, the shape of the future animal is being determined by the shapes of individual cells and they way they are packed..

5.2 Origin of myoblasts

   Premyoblasts divide and their daughter cells become myoblasts (only some of them, otherwise there would be no more dividing premyoblasts). Myoblasts fuse to form myotubes. Myotubes mature into muscle fibres. This will be explained more fully in the next lecture.

5.3 Limb buds

     Limb bud formation is regulated by a dialogue between ectoderm and mesoderm.
  1. The ectoderm makes an apical ridge in response to a message from the mesoderm.
  2. The mesoderm grows to form parts of the limb in response to a message from the ectoderm.
  3. The mesoderm instructs the ectoderm to remain thick and to keep inducing mesodermal growth.

 Further information

W.H. Freeman & B. Bracegirdle. (1963). An Atlas of Embryology. Heinemann, London. The original and still the best (in my opinion) manual to identify what you are looking at in sections through the developing chick.

B.M. Carlson. (1988). Patten's Foundations of Embryology. McGraw-Hill Publishing, New York. This is an updated version of  Bradley M. Patten's "The Embryology of the Pig" and the "Early Embryology of the Chick". These originals are still easily obtainable at relatively low cost and have really nice pictures and diagrams. 

Structure and Development of Meat Animals and Poultry,  Chapter 6.