Re: On a cellular level, how do humans grow?

Hi Neil,
Thank you for your excellent question.  I hope that the information 
below will help you complete your project on Acromegaly.  You have 
probably done a lot of research on this disease already, but for the 
sake of anyone else who might read this answer once it’s archived, 
I’m going to cover a little bit of the basic information before I get to 
your specific question.  For more detail, a very good web page on the 
subject (including information about diagnosis and treatment, which I 
won’t discuss) can be found at:
 http://www.niddk.nih.gov/health/endo/pubs/acro/acro.htm

In brief, Acromegaly is a hormonal imbalance which results in an 
excess of growth hormone being produced.  The most common 
cause of this overproduction is a tumor of the pituitary gland, where 
GH is normally produced.  The results of sustained overproduction of 
GH vary, depending on the age of onset, but as one might imagine, 
excess GH in the body results in excess growth.  Usually, the 
overproduction of GH doesn’t begin until adulthood, but when the 
onset is during childhood, while the body is still actively growing, 
excess GH can result in GIGANTISM, or abnormal height.  

Now to your specific question, “On a cellular level, how to humans 
grow?” which one could rephrase as “what is the cellular response 
to the GH signal?”  

When GH is secreted into the bloodstream by the pituitary gland, it 
induces the production of another hormone, called Insulin-like 
Growth Factor 1, or IGF-1.  Cells that are capable of growth (i.e. cell 
division and proliferation) have receptor molecules attached to the 
cell membrane which can detect the presence of signaling 
molecules in the body.  Just as you have to have the right equipment 
to receive and process a radio, TV, or satellite signal, a cell must 
have the correct molecular receptor to receive a specific  signal.  
Once a cell receives the IGF-1 signal, the cell is stimulated to divide, 
and thus growth occurs.  In a normal system, IGF-1 and GH levels 
help to regulate each other:  For example, when IGF-1 levels are 
high, production of GH is reduced.  The levels of these and other 
hormones in the body at any given time are highly regulated by each 
other as well as external, environmental factors.  In the case of 
gigantism, excess levels of GH are produced and this system is 
upset.  The continuous presence of GH causes continuous 
production of IGF-1, which is able to act on any cells competent to 
receive the signal, and the result is uncontrolled growth. 

During childhood, and through puberty, the long bones of the body 
contain epiphyseal plates, commonly called “growth plates”.  These 
are populations of cartilage cells, which are located at each end of 
the bone.  The cartilage cells, called chondrocytes, are very 
responsive to the growth signals present in a normal growing body 
and continue to divide, which enables the bones to keep on growing 
until the person reaches their adult height.  Once this height is 
reached, the cartilage in the epiphyseal plates is replaced by rigid 
bone, which does not respond to GH.  In patients with gigantism, the 
chondrocytes are stimulated to divide more than in an unaffected 
person.   Thus, the bones of the child or adolescent grow much 
longer than they would normally, and the result is gigantism.   

I hope this information is what you were after, Neil.  Any biology or 
anatomy text at your school will probably have more detailed 
information (and pictures) of epiphyseal plates, which would 
probably enhance your project.  Please contact me again if I can be of 
further assistance.  

Cheers,

Jen