Re: How are blood cells released from the bone marrow?

Bone marrow is a mesh of interconecting cells called stromal cells, along 
with collagen fibres and fat cells.   You could think of it as a big 
sponge.  Blood vessels called blood sinuses are there too - the escape 
route for cells that are made in the bone marrow.  People think that one 
cell in the bone marrow can change itself and make all types of blood 
cells that you see.  This is called the pluripotent stem cell.  The 
pluripotent stem cell can become a neutrophil, a lymphocyte, a red blood 
cell or a platelet or one of several other cell types depending on the 
signals in the bone marrow.  These are all blood cells that perfrom 
special functions and are important if not essential to stay healthy.  

The stem cells then commit to becoming one type of cell or another and 
they divide several times so that one stem cell will make many of the 
final cell type.  The most common type of blood cell is the red blood 
cell or ethryocyte.  These cells are made in the bone marrow from a 
reticulocyte and when they are ready to leave, they squeeze through the 
blood sinuses with special molecules called adhesion molecules.  Adhesion 
molecules let cells stick and release themselves to blood vessels (as 
well as to each other).  When the cell has squeezed through, it gets into 
the blood.  All the other blood cells do the same thing, except the 
platelet which isn't really a cell at all but a fragment of a cells.  The 
precursor (the cell which makes the platelet) doesn't leave the bone 
marrow but stays there until it dies, making platelets.  

Hope that answers your question.