Re: How are muscle cells built into tissues?

Hello Kristy,


Thank you for your question.  I am wondering weather you are asking what is 
the arrangement of muscle cells in tissues, or whether you are asking how 
the muscle cells originate in the tissue during muscle development, so I 
will tackle both areas.

Striated muscle fibres are made from terminally differentiated cells.  This 
means that these cells are not able to transform into other cell types as 
some cells are able to.  The muscle cells have been formed from stem cells 
during development of the embryo and foetus when they have differentiated 
into muscle cells and developed into muscle fibres.  These muscle fibres 
are unable to divide and produce more muscle fibres, so the number of 
muscle fibres that you have is determined during the eary stages of life.  
The formation of muscle is called 'myogenesis' and muscle is made from 
'myogenic' cells.

Myogenesis is a biphasic process.  The first phase is the generation of 
primary fibres, which occur during the very early embryonic stage of life 
and are distributed throughout the muscle forming regions of the animal.  
They are formed by the ordered fusion of embryonic myogenic cells to 
produce the primary fibres.  After the primaries have stopped forming the 
second phase of myogenesis starts with the production of secondary fibres. 
 This occurs during the next stage of development, termed the foetal stage. 
These are smaller (diameter) fibres than the primaries and use the surface 
of the primary fibres as a scaffold for the attachment and fusion of the 
foetal myogenic cells to generate the secondary fibres.  Several secondary 
fibres grow on each primary so there are many more secondary than primary 
fibres produced (>20:1 for humans).  In the later stages of foetal life 
adult myogenic cells are formed, and these are still around after birth.  
These adult myogenic cells are responsible for growth and regeneration of 
the muscle.

So the muscle fibres are formed by the fusion of a number of myogenic cells 
in a co-ordinated fashion to produce muscle fibres.  Each muscle fibre is a 
single cell, but since they are made from the fusion of many myogenic cells 
each muscle fibre contains many nuclei.

Skeletal muscle tissue is made up of a large number of skeletal muscle 
fibres organised into a muscle trunk.  Each muscle fibre runs the entire 
length of the muscle and is attached at each end to a tendon, and each 
tendon is attached to the bone that it moves.  The muscle trunk is made up 
of a number of bundles, called 'fascicles'. Each fascicle consists of a 
bundle of muscle fibres surrounded by connective tissue.  The nerves and 
blood vessels that supply the muscle pass through the connective tissues of 
the trunk.

Hopefully this has given you some insight into how the muscle fibres are 
formed and arranged in the muscle.  A general physiology text book should 
contain more information if you are interested in reading more.

Thank you for your interesting question and good luck in the future.

Dave Burton