General Plan of Wall Structure
Unlike arteries and veins, the wall of blood capillaries is not divided into tunics. Its structural framework is formed by three types of cells:
- Endotheliocytes. They form the continuous internal lining and lie directly on the basal lamina.
- Pericytes. Processed connective tissue cells localized in the clefts of the endothelial basal lamina. They do not form a continuous layer, but rather partially embrace the capillary like a basket. In their properties, pericytes occupy an intermediate position between fibroblasts and myocytes. These are cambial elements capable of differentiating into other cells during angiogenesis.
- Adventitial cells. Under-differentiated connective tissue elements located outside the vessel. Like pericytes, they do not form a continuous covering and serve as a cambial reserve.
Capillary permeability directly depends on the structure of two main components: the endothelial layer and the basal lamina.
Classification by wall structure
Depending on the structural features of the endothelium and basal lamina, three types of capillaries are distinguished (in order of increasing permeability):
- Somatic (continuous). The most common type. Both the endothelium and the basal lamina are continuous here. Contacts form between endotheliocytes and pericytes (tight junctions and gap junctions). The basal lamina is not interrupted at contact sites, but splits to enclose the pericyte. The main mechanism of substance transport is pinocytosis.
- Fenestrated (visceral). The basal lamina remains continuous, but local thinnings—fenestrae—are present in the cytoplasm of endotheliocytes. They are characteristic of organs with active metabolism and macromolecule transport: renal glomeruli (where primary urine is formed), intestinal villi, and endocrine glands.
- Perforated (sinusoidal). Large, full-thickness slit-like pores are present in the endothelium and basal lamina. This ensures the passage not only of large molecules, but also of entire formed elements of the blood. Pericytes are completely absent in their wall, and the basal lamina 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. The following diameter gradation is accepted in histology:
- Narrow: 4–7 µm;
- Regular (somatic): 7–11 µm (from 6 to 11 µm);
- Sinusoidal: 20–40 µm (typical of hemopoietic organs, where regular capillaries empty and the sinusoids themselves transition into small veins);
- Lacunar: 50–70 µm.
Note: capillaries of typical (regular) diameter may occur in renal glomeruli while simultaneously featuring fenestrae and large pores.
Regulation of permeability
The ability of capillaries to let various substances pass depends on the state of the basal lamina and the ground substance surrounding the adventitial cells. Under the action of specific enzymes (e.g., hyaluronidase), the degree of polymerisation 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 cell composition features only a single cell type—endotheliocytes. They completely lack a basal lamina. To prevent vessel collapse, endotheliocytes are firmly attached to the surrounding connective tissue via collagenous anchoring filaments.
Briefly: answers to topical questions
What are the functions of pericytes in the capillary wall?
Pericytes perform a complex of functions, ensuring the structural integrity and regulation of the microvasculature.
- Supportive-mechanical function — participate in the synthesis of basal lamina components and create the external framework of the capillary.
- Hemodynamic regulation — alter the capillary lumen through active contraction or changes in cell volume in response to neural and humoral signals.
- Participation in angiogenesis and regeneration — capable of differentiating into smooth myocytes and fibroblasts, and secrete factors that halt endothelial division to stabilize the vessel.
- Participation in inflammation — act as intermediaries in signaling to the endothelium and stimulate the expression of adhesion molecules.
How is substance transport carried out across the somatic capillary wall?
The main mechanism of substance transport across the somatic capillary wall is pinocytosis. The cytoplasm of endotheliocytes abounds with pinocytotic vesicles for metabolite transfer; they move from one side of the cell to the other.
From which embryonic source do capillary endotheliocytes develop?
Capillary endotheliocytes develop from embryonic mesenchyme. During embryogenesis, specific clusters of mesenchymal cells—blood islands—form in the wall of the yolk sac, the chorion, and subsequently in the body of the embryo itself. The peripheral cells of these islands gradually flatten and differentiate into endotheliocytes, forming the primary wall of future blood vessels.
S-F-P in order of increasing "leakiness": Somatic (Continuous) → Fenestrated (with Fenestrae/windows) → Perforated (with Perforated/Porous membrane).
Class and exam questions
What is the main difference between a capillary wall and an artery or vein?
The capillary wall lacks a division into three classic tunics. Its framework is formed by only three types of cells: endotheliocytes, pericytes, and adventitial cells.
Where in the body are perforated (sinusoidal) capillaries located?
They are located in hemopoietic organs (red bone marrow, spleen) and the liver. Their large pores are necessary for the migration of entire blood cells from tissues into the vascular bed.
What are anchoring filaments?
These are special collagen structures characteristic only of lymphatic capillaries. Through them, endotheliocytes attach to the surrounding tissue, which prevents the vessel from collapsing due to the absence of a basal lamina.
What else to study on this topic
- The mechanism of pinocytosis in somatic capillary endotheliocytes
- The role of pericytes and adventitial cells in angiogenesis
- Ultrastructural features of fenestrae and pores in electron micrographs
- The effect of hyaluronidase on the polymerisation of matrix macromolecules
- Morphology of capillary networks in renal glomeruli
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