In the bone marrow, growth factors bind to proteoglycans in the stromal matrix and are thus concentrated in convenient proximity for presentation to adherent progenitors. Also, some of the stromal cells synthesize growth factors which cause paracrine stimulation of progenitors. There are receptors specific for each growth factor and their presence on the cell surface varies both with the cell lineage and the stage of differentiation in that lineage. Receptors determine which cells respond to a given growth factor. The changes brought about by the change in the cytoplasmic domain, results in activation.
Activation usually involves phosphorylation of a tyrosine kinase.
This initiates the signal that is ultimately transmitted to the nucleus, leading to gene expression, and gene products cause an appropriate cell response, proliferation, differentiation, etc. The best indication of normal marrow function is a normal haemogram, i.e. all blood cells are within normal limits. The simplest and most clinically useful method for assessing the marrow is to either aspirate or biopsy tissue from the marrow cavity. The stained aspirate permits excellent cellular identification of the cell lines and their stages of differentiation, and overall cellularity can be assessed. The iron stores can be evaluated using the Prussian blue stain.