General Structural Plan of the Wall
Unlike arteries and veins, the wall of blood capillaries is not divided into tunics (tunicae). Its structural framework is formed by three cell types:
- Endothelial cells (Endotheliocytes). Form the continuous inner lining and rest directly on the basal lamina.
- Pericytes. Branched connective tissue cells located within the split of the endothelial basement membrane. They do not form a continuous layer, but partially wrap around the capillary like a basket. Functionally, pericytes occupy an intermediate position between fibroblasts and myocytes. They are progenitor (cambial) elements capable of differentiating into other cell types during angiogenesis.
- Adventitial cells. Undifferentiated connective tissue elements located on the outer aspect of the vessel. Like pericytes, they do not form a continuous layer and serve as a cambial reserve.
Capillary permeability depends directly on the structure of two main components: the endothelial layer and the basement membrane.
Classification by Wall Structure
Based on the structural characteristics of the endothelium and basement membrane, capillaries are divided into three types (in order of increasing permeability):
- Continuous (Somatic). The most common type. Both the endothelium and basement membrane are continuous. Tight junctions and gap junctions (nexuses) form between endothelial cells and pericytes. The basement membrane is not interrupted at contact sites but splits to enclose the pericyte. The primary transport mechanism is pinocytosis.
- Fenestrated (Visceral). The basement membrane remains continuous, but the cytoplasm of endothelial cells contains local thinning regions known as fenestrae. These are characteristic of organs with active metabolic exchange and macromolecule transport: renal glomeruli (where primary urine is formed), intestinal villi, and endocrine glands.
- Discontinuous (Sinusoidal). The endothelium and basement membrane feature large, open, gap-like pores. This allows the passage not only of large molecules but also of entire blood cells. Pericytes are completely absent in their walls, and the basement membrane can be highly discontinuous (e.g., in the liver).
Classification by Diameter
The dimensions of blood capillaries vary significantly depending on their type and localization. Histology recognizes the following diameter gradations:
- Narrow: 4–7 µm;
- Standard (Somatic): 7–11 µm (6 to 11 µm);
- Sinusoidal: 20–40 µm (typical of hematopoietic organs, where standard capillaries drain into them, and the sinusoids transition into small veins);
- Lacunar: 50–70 µm.
Note: Renal glomeruli may feature capillaries of typical (standard) diameter that simultaneously possess fenestrae and large pores.
Regulation of Permeability
The ability of capillaries to transmit various substances depends on the state of the basement membrane and the extracellular matrix surrounding adventitial cells. Under the action of specific enzymes (e.g., hyaluronidase), the degree of polymerization of matrix macromolecules decreases. As a result, the permeability of the vascular bed at the capillary level increases significantly.
Differences from Lymphatic Capillaries
Lymphatic capillaries have a fundamentally different organization. Their cellular composition includes only a single cell type — endothelial cells. They completely lack a basement membrane. To prevent vessel collapse, endothelial cells are firmly anchored to the surrounding connective tissue by collagen anchoring filaments.