14.2 Histogenesis

• Histogenesis as exemplified by striated musculature
  
• From premyoblasts to the myotubes
  
• The neuromuscular junction
  
• Adult musculature and its structures
  

Histogenesis as exemplified by striated musculature

The first indication of the beginning myogenesis is the lengthening of the premyoblasts. Responsible for this are several genes on chromosome 11. They belong to the myogenic regulatory factors (MRF) family and activate the transcription of muscle-specific genes.

From premyoblasts to the myotubes

Premyoblasts are the first specific and determining cells of myogenesis. At the place where muscles are being generated they transform themselves into myoblasts and begin to synthesize the actin and myosin myofilaments.

Fig. 3
A premyoblast is transformed into a myoblast by the activity of a gene on chromosome 11. It is now subject to a cell division and begins synthesizing actin and myosin myofilaments.

Very soon the myoblasts begin to divide and several cells merge to form multi-cellular myotubes 18-20 in which the myofibers are still arranged around the cell nucleus.
At the end of the embryonic period the cell nuclei move to the periphery and slowly lengthen. One can now clearly recognize the striations.

Fig. 4
Several myoblast lie on each other and fuse to become multi-cellular myotubes
18-20.
In this stage the cell nuclei still lie in the middle of the individual cells.

Fig. 5
At the end of the embryonic period the myotube cell nuclei move into the periphery below the basal membrane. The typical striation pattern of the muscle fibers are clearly visible.