Haematology: Disorders of the system
Angiogenesis is the formation of new blood vessels, this can be arteries, veins, capillaries and lymphatics. Angiogenesis occurs as a consequence of various reasons. It could be due to pathological process such as embolism, neoplasia, diabetes or trauma. Angiogenesis can occur due to regeneration of the endometrium following menstruation or it can just happen simply due to growth. Exercise can stimulate angiogenesis. Why is this useful? Well it allows for more routes for blood and therefore oxygen to be delivered to tissues to help deal with strenuous activities.
There are two types of angiogenesis, sprouting and intussusceptive. Sprouting angiogenesis begins with a single capillary, endothelial cells grow outwards (like a sprout) and then a lumen forms inside the sprout, and thus, we have two capillaries. Intussusceptive angiogenesis begins with a single capillary, then an endothelial grows in the centre of the capillary, a new lumen forms and again we have two capillaries. This requires matrix metalloproteases.
The stimulus for angiogenesis is ischaemia, which is a lack of blood supply, and therefore a lack of oxygen supply. Lack of oxygen supply is known as hypoxia. Ischaemia occurs in growing tissues where they exceed the blood supply or when blood supply is compromised. Hypoxia stimulates the production of pro-angiogenic factors as well as erythropoietin (EPO) production from the kidneys. EPO synthesis drives the production of new red blood cells to therefore increase oxygen carrying capacity. But anyway, back to pro-angiogenic factors. Hypoxia stimulates hypoxia-inducible factor to activate the pro-angiogenic factor VEGF. VEGF stands for vascular endothelial growth factor and does a pretty good job! VEGF binds to its receptor expressed only on endothelial cells and drives its proliferation.
Arteriogenesis is the formation of new arteries, common causes of this are obstructed blood flow through embolism (clot blockage) or narrowing of vessels (stenosis).
What is anaemia? Anaemia is the lack of red blood cells. This can lead to a sufferer experiencing shortness of breath and being quite pale. Aplastic anaemia is a result of too few blood cells. Iron deficiency anaemia leads to defects in haemoglobin as iron binds to haemaglobin. Pernicious anaemia is due to vitamin B12 deficiency which makes the red blood cells larger than normal. Anaemia can be causes by a variety of factors such as: bleeding, haemoglobin synthesis defects, destruction of normal cells, and defects in the making of blood cells (haematopoiesis.) Cytopenias is the word used to describe too few blood cells, therefore anaemia is a type of cytopenia.
So what else can go wrong?
Well you can get abnormal new growth, new growth in an uncontrolled manor is what causes cancer. Examples of cancers of the blood are leukaemia, lymphoma and myeloma. Leukaemia is a cancer of the white blood cells, these cells proliferate uncontrollably in the bone marrow (where blood cells are made), these cells then spill out into circulation. When in circulation, the leukaemia cells do not divide as it is not the appropriate environment for cell division. The appropriate environment is the stem cell niche in the bone marrow. Myeloma is a cancer of plasma cells (red blood cells). Lymphoma is a cancer of T and B lymphocytes as well as natural killer cells.