Species: rat (Rattus norvegicus).
Technique: A, B) paraffin embedding, sections stained with haematoxylin-eosin, C) paraffin embedding, sections stained with Masson trichrome; D) paraffin embedding, sections stained with haematoxylin-eosin.
Capillaries are blood vessels with a very narrow diameter, measuring only a few micrometers. They are made up of squamous non-stratified epithelium, referred to as the endothelium. The other component of capillaries is the basal lamina, a layer of distinct extracellular matrix located between the endothelium and the underlying connective tissue. The very low thickness of the endothelial cells makes possible an effective interchange of molecules between the blood and the surrounding tissues. Capillaries are arranged in nets to thoroughly supply the organs with blood.
The endothelium can be classified as continuous, fenestrated, and discontinuous according to its cellular features and organization. Continuous endothelium is the more abundant capillary type. In this type, the endothelial cells seal the intercellular spaces so tightly that only the smallest molecules can cross the endothelium through the intercellular spaces. These endothelial cells show many cytoplasmic vesicles that indicate active endocytosis and exocytosis processes (Figure 1). Fenestrated endothelium shows endothelial cells with channels or passages that connect blood directly with basal lamina and inner tissues. This type is frequent in endocrine glands and the intestine, places where the flux of substances to the blood is very intense. Discontinuous or sinusoidal endothelium is less frequent. Its endothelial cells do not completely seal the intercellular spaces, so substances and even cells can cross the endothelium easely. This type is found in the liver, bone marrow, and spleen.
The endothelium performs many functions, including thrombosis regulation, facilitating thrombolysis, promoting platelet adhesion, maintaining vascular tone, and blood conduction. Furthermore, leukocytes need to be attached to the endothelium before leaving the blood stream toward other tissues. Endothelial cells release many signaling molecules targeting different types of cells.
The capillary network is malleable, adapting to organ and tissue specific demands. Therefore, proliferation of endothelial cells and their arrangement in capillaries are under the control of external signals. This plasticity is essential for repairing tissue and organ repair, as well as during embryo development. Angiogenesis is the formation of new blood vessels from pre-existing ones, with new endothelial cells arising from proliferation and differentiation of already existing endothelial cells.
Nearly all cells of the body are at a distance of 50 to 100 µm away from a capillary. It means that cells in growing organs should release molecular signals for the development of the capillary network. Vascular endothelial growth factor (VEGF) is a key molecule that promotes the formation of new capillaries. It is important to know how the molecular mechanism of angiogenesis works to understand, for example, the development of tumors, as angiogenesis is essential for tumor growth. If there is no angiogenesis, the internal cells of the tumor will die by starvation or by lack of oxygen as the tumor expands.
Pericytes are cells associated to capillaries. They can regulate the capillary diameter and, therefore, the blood flux, due to their contractile properties. Pericytes arise from differentiation of mesenchymal cells located near the endothelial cells.