LEC5

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.

It is difficult to see where premyoblasts originate because they look very similar to many other types of stem cells destined to form other types of tissues. Below is a diagram of a greatly simplified plan of a transverse section through an embryo. To use the same diagram for both farm mammals and poultry, the bottom of the diagram only indicates the general relationships of the somites to the endoderm (gut), to the coelom (body cavity) and to the parietal lateral plate mesoderm (ventral body wall). In other words, something like the top of the diagram may be seen in both mammalian and avian embryos, whereas the bottom of the diagram is imaginary.

Each somite is composed of three zones around a cavity called the myocoele. The myocoele is of no great importance and may simply be where the tissues come apart as they are being prepared for microscopic examination.
The sclerotome forms the vertebrae.
The myotome forms MUSCLE.
The dermatome forms the dermis (deep layer of the skin under the epidermis).
The ectoderm forms the epidermis of the skin.
The endoderm forms the gut and associated organs.

This is what it really looks like!

Genes. 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.

The axial muscles of the body, including the tongue and extraocular muscles, are derived from the somitic mesoderm of the myotomes.
Mesenchyme cells from both the somites and parietal layer of the lateral plate mesoderm move into the limb buds, thus giving rise to the muscles of the limbs (but there is plenty of debate about the relative importance of somites and lateral plate in different animals)..

5.3 Limb buds

Limb bud formation is regulated by a dialogue between ectoderm and mesoderm.

The ectoderm makes an apical ridge in response to a message from the mesoderm.
The mesoderm grows to form parts of the limb in response to a message from the ectoderm.
The mesoderm instructs the ectoderm to remain thick and to keep inducing mesodermal growth.

The mesoderm of the limb bud separates into muscle‑forming and cartilage‑forming regions.
The chondrogenic (cartilage-forming) regions can be detected because they synthesize extracellular proteoglycan (the rubbery matrix of cartilage).
Molecular differentiation is preceded by differential vascularization (formation of blood vessels in different patterns). Perhaps vascular growth is involved in establishing metabolic gradients in the developing limb?
Little is known yet about the factors regulating the quantitative distribution of premuscle mesoderm and the potential for muscle growth in meat animals. For example, the maximum postnatal number of myofibres might be predetermined by the number of stem cells and by the number of times their offspring divide. At present it appears likely the numbers of mitotic divisions in a cell lineage are genetically programmed, but with environmental factors controlling the rate of division.
The point at which multipotential stem cells (cells capable of forming anything) differentiate to become myogenic stem cells (cells only able to form muscle) is genetically regulated by a single gene, myd.
During the evolution of limbs in ancient amphibians, it appears there was a primary division of the myogenic mesoderm into a dorsal and a ventral mass. This still occurs in the embryos of our farm animals.

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.