Histogenesis of skeletal muscles
Striated muscles arise from intermediate part of somites called myotome.
Mononucleated mesodermal cells called promyoblasts migrate to definitive site, accumulate and show prominent mitotic activity and become myoblasts. Myoblasts are spindle-shaped cells with a single nucleus. They synthesize myofilaments. Myoblasts fuse to form myotubes, in which nuclei are peripheral. Fusion of myotubes leads to the establishment of a multinucleated syncytium, in which some striation can be seen by electron microscopy.
Some myoblasts do not fuse with others and remain mononucleated. They become satellite cells, which are internal to the basement membrane. Such cells are stem cells important in repair.
Regeneration of skeletal muscle. Regeneration after damage probably starts from satellite cells, which behave the myoblasts. They divide and initiate regeneration. An additional and essential factor for regeneration is adequate regeneration.
However muscle fibers practically are not regenerated completely because of disruption of the basal lamina cause death of myoblast. As a result fibroblasts repair of the injured site with scar tissue formation.
CARDIAC MUSCLE
The structure of cardiac muscle has many similarities to that of skeletal muscle; but there are important differences as well.
The cardiac myofibrils show transverse striations. There are same sarcoplasmic reticulum, T-system, numerous mitochondria and other organelles.
Unlike skeletal muscle fiber, the fiber of cardiac muscle is made up of individual cardiac muscle cells or cardiac myocytes. There are some types of cardiomyocytes. Most of them – contractile or typical. Untypical myocytes are conducting and secretory (see below).
The cardiac myocytes are joined to one another in linear array, forming chain of myocytes. That’s why cardiac muscle has appearance branching and anastomosing fibers.
Each myocyte has only one its own nucleus. The nucleus of each myocyte is located in the center of the cell (not peripherally, as in skeletal muscle).
The myofibrils are relatively few. There are large amount of glycogen and of myoglobin. In this respect cardiac muscle is closer to the red skeletal muscle than to the white.
Unlike skeletal muscle T-system lie opposite the Z-band. Hence typical triads are not present.
The junctions between adjoining cardiac myocytes are seen as dark staining transverse lines running across the muscle fibre. The lines are called intercalated discs. The intercalated discs are formed by cell membranes of adjacent myocytes.
There are numerous desmosomes, gap junctions, that allow electrical signals to pass from cell to cell.