Endothelial cells are the fundamental elements of the vascular system. They form a single cell layer that lines all blood vessels and lymphatics and regulates exchanges between the bloodstream and the surrounding tissues. New vessels originate as endothelial sprouts from the walls of existing small vessels. A specialized motile endothelial tip cell at the leading edge of each sprout puts out filopodia that respond to gradients of guidance molecules in the environment, leading the growth of the sprout in much the same way as the growth cone of a neuron is led. The endothelial stalk cells following behind become hollowed out to form a capillary tube. Signals from endothelial cells organize the growth and development of the connective-tissue cells that form the surrounding layers of the vessel wall.
A homeostatic mechanism ensures that blood vessels permeate every region of the body. Cells that are short of oxygen increase their concentration of hypoxia-inducible factor 1α (HIF1α), which stimulates the production of vascular endothelial growth factor (VEGF). VEGF acts on endothelial cells, causing them to proliferate and invade the hypoxic tissue to supply it with new blood vessels. As new vessels enlarge, they recruit increasing numbers of pericytes—cells that cling to the outside of the endothelial tube and mature into the smooth muscle coat that is needed to give the vessel strength.
a hieRaRChiCal Stem-Cell SyStem: blood Cell FoRmation
The function of blood vessels is to carry blood, and it is to blood itself that we now turn. Blood contains many types of cells, with functions that range from the transport of oxygen to the production of antibodies. Some of these cells stay within the vascular system, while others use the vascular system only as a means of transport and perform their function elsewhere. All blood cells, however, have certain similarities in their life history. They all have limited life-spans and are produced throughout the life of the animal. Most remarkably, they are all generated ultimately from a common stem cell, located (in adult humans) in the bone marrow. This hematopoietic (blood-making) stem cell is thus multipotent, giving rise to all the types of terminally differentiated blood cells as well as some other types of cells, such as the osteoclasts in bone, as mentioned earlier. The hematopoietic system is the most complex of the stem-cell systems in the mammalian body, and it is exceptionally important in medical practice.
Red blood Cells are all alike; white blood Cells Can be Grouped in three main Classes
Blood cells can be classified as red or white. The red blood cells, or erythrocytes, remain within the blood vessels and transport O2 and CO2 bound to hemoglobin. The white blood cells, or leukocytes, combat infection and in some cases phagocytose and digest debris. Leukocytes, unlike erythrocytes, must make their way across the walls of small blood vessels and migrate into tissues to perform their tasks. In addition, the blood contains large numbers of platelets, which are not entire cells but small, detached cell fragments or “minicells” derived from the cortical cytoplasm of large cells called megakaryocytes. Platelets adhere specifically to the endothelial cell lining of damaged blood vessels, where they help to repair breaches and aid in blood clotting.
All red blood cells belong in a single class, following the same developmental trajectory as they mature, and the same is true of platelets; but there are many distinct types of white blood cells. White blood cells are traditionally grouped into three major categories—granulocytes, monocytes, and lymphocytes—based on their appearance in the light microscope.