23 Radial Growth of Muscle Fibres

23.1 Introduction

The basic concepts are relatively simple. Animals with bulging muscles have a high yield of meat. Muscles are composed of myofibres, and when these grow radially- so does the whole muscle. Here we will be concerned with how myofibres grow radially, and with complexities of muscle structure which complicate things

23.2 Myofibre diameters and myofibrillar hyperplasia

Formation of new myofibrils
  1. New myofilaments are added around the outside of the myofibril.
  2. This increases the length of the diffusion pathway for calcium ions as they turn contraction on and off. (Remember - calcium ions are released from the sarcoplasmic reticulum to initiate muscle contraction, then re-sequestered for relaxation).
  3. This causes mechanical stress in the myofibril - the outside contracts and relaxes before the central axis.
  4. Calcium ions building up in the interior of the myofibril activate enzymes (calpains) which release thin myofilaments from their Z-lines.
  5. Contraction causes the weakened myofibril to split.
  6. But, the myofibril is very long, and a split at one point may not match up to a split at another point.
  7. We end up with a complex structure.  Where once there was a single myofibril seen in transverse section, we now see many myofibrils in transverse section - but following them along the myofibril we find they are all linked.
  8. Thus, we have not really formed any new myofibrils, just increased their size and complexity.
  9. But, it appears we have formed new myofibrils, so we will keep on talking about their numbers!

To keep things simple in the diagram above - we are looking at transverse sections at the midlength of relaxed sarcomeres where we will only see thick myofilaments. The new new myofilaments are shown in green. You can see how the progressive addition of new myofilaments will increase the length of the pathway by which calcium ions move in and out of the myofibril.

23.3 New myofibre nuclei

As the myofibre grows radially, it adds new nuclei.

Myofibre nuclei have the following features.

Myofibre nuclei contain DNA combined with histones and other structural proteins to form chromatin. When DNA is used for protein synthesis, the chromatin is dispersed, only binds weakly to histological stains, and is called euchromatin. In non‑dividing cells, chromatin may form darkly stained irregular clumps called chromatin particles. Nuclei also contain RNA and darkly stained clumps of RNA form nucleoli. The number of nucleoli may vary between animal species. Condensed regions of darkly stained chromosomes sometimes persist between cell divisions and are called heterochromatin. In the mononucleated cells of the body, such as those of the skin or liver, darkly stained chromosomes composed of inactive DNA are seen when cells divide. But, as explained below, the situation in multinucleated myofibres is more complex, and distinct chromosomes are not seen by light microscopy within myofibres.

On or near the myofibre membrane are several types of nuclei.

The red line above is where the TWO membranes are located - one around the satellite cell and one lining the depression on the myofibre surface.

The existence of satellite cells became accepted in 1961, although they had been seen a hundred years earlier - but no one believed it. Pericytes are mesodermal cells found around very small blood vessels. They contain actomyosin and are probably capable of contraction and phagocytosis.

The features of satellite cells are:
The main two functions of satellite cells are to provide nuclei for growing myofibres and a source of myoblasts for postnatal muscle regeneration. These two functions may be independently regulated by factors such as insulin and IGF (insulin-like growth factor) controlling the growth function, and fibroblast growth factor controlling the regenerative function .

Why were mitotic chromosomes never observed inside myofibres ?  You would expect them if satellite cells  look like other nuclei inside the myofibre. The answer - satellite cells are too small to allow the chromosomes to separate sufficiently to be seen clearly by light microscopy.

Further information

Structure and Development of Meat Animals and Poultry.  Pages 389-396.