Hemodynamics and Barrier-Exchange Role
Endothelial cells, characterized by elongated nuclei, line the inner surface of blood vessels. At the level of the microvasculature, the geometry of blood flow changes: due to the minimal diameter of capillaries, erythrocytes are forced to move one after another (line up in a single file). As the capillary transitions into a venule and its lumen enlarges, red blood cells begin to move in two rows, after which the number of rows increases rapidly. The thin capillary wall allows for clear visualization of erythrocytes.
Together with the basement membrane, the endothelium isolates the blood from the interstitial environment. The integrity of this barrier is maintained by intercellular junctions:
- Interdigitations.
- Tight junctions.
- Gap junctions.
Metabolic exchange between the blood and tissue fluid in capillaries occurs via diffusion (through fenestrae or pores) and active transport using pinocytotic vesicles. In larger vessels, the endothelial layer is responsible for supplying nutrients to the inner layers of the vascular wall itself.
Regulation of Hemostasis and Tone
The effect of the endothelium on blood coagulation is dual in nature and depends entirely on the state of the vessel. In an intact (undamaged) state, potent anticoagulant activity predominates. The cells synthesize prostacyclin and express heparin (a sulfated glycosaminoglycan) on their surface. In addition, the glycocalyx possesses a negative charge, which physically repels platelets. However, upon vessel wall injury, a procoagulant program is triggered: cells release thromboplastin, initiating the coagulation process.
A multitude of receptors for various hormones and biologically active substances are expressed on the cell surface (especially in arterioles). The binding of ligands to these receptors prompts the endothelium to synthesize factors that diffuse to the layer of smooth myocytes. As a result, smooth muscle cells relax or contract, altering the vessel lumen.
Immune Responses and the Mechanism of Inflammatory Edema
The endothelial lining actively participates in immune processes. Physiological lymphocyte migration—"homing"—occurs in lymphoid organs. The endothelium of postcapillary venules begins to express specific markers known as vascular addressins. Blood lymphocytes recognize them and target their return to lymphoid tissue.
In the focus of inflammation, the picture changes:
- Additional adhesion proteins appear on the surface of endotheliocytes.
- Circulating leukocytes (primarily neutrophils) recognize these proteins, attach, and migrate into the tissues.
- Under the influence of inflammatory mediators, the cytoskeleton of endothelial cells undergoes reorganization: they deform, becoming taller and shorter.
- The widening of intercellular spaces leads to a sharp increase in permeability, causing fluid to rush into the tissues and cause edema.
Pericyte Functions and Angiogenesis
Pericytes are cells surrounding the capillary externally that form its outer supporting framework. They participate in the production of basement membrane components and respond sensitively to humoral and neural stimuli. They can alter hemodynamics in two ways: either through the active contraction of their structural elements or by changing their own volume (releasing fluid or swelling).
Pericytes are indispensable in the processes of angiogenesis, which is triggered when tissues lack nutrients and oxygen. Endothelial cells begin to divide and migrate, forming branches. Pericytes exhibit high plasticity: during the remodeling of a capillary into a larger vessel, they are capable of differentiating into smooth myocytes and fibroblasts. At the same time, direct contact between a pericyte and an endothelial cell inhibits the division of the latter, ensuring stabilization of the new vessel. Pericytes also serve as an important link in inflammation, acting as a mediator that transmits signals from tissues to the endothelium and stimulates the appearance of adhesion molecules on its surface.
Briefly: answers to topical questions
What vasoactive substances does the endothelium synthesize to regulate smooth myocyte tone?
To regulate smooth muscle cell tone, the endothelium synthesizes a number of biologically active substances. These include:
- Nitric oxide (NO) — endothelium-derived relaxing factor, a potent vasodilator causing vascular relaxation.
- Prostacyclin (and other prostaglandins, e.g., $PGE_2, PGD_2$) — prevents vasoconstriction, ensuring vasodilation.
- Endothelin-1 — a peptide with a potent vasoconstrictor effect.
What adhesion molecules are expressed on the endothelial surface in the focus of inflammation?
In the focus of inflammation, adhesion molecules appear/are expressed on the endothelium. Sources explicitly name:
- cell adhesion molecules, including integrins;
- ICAM-1;
- VCAM-1.
These molecules ensure the binding of leukocytes to the endothelium and their migration into tissues.
What procoagulant activity factors does the endothelium release upon vascular injury?
Upon vascular injury, the endothelium exhibits procoagulant activity through the release of thromboplastin — tissue factor, factor III, which initiates blood coagulation. Sources also indicate that injured endothelium activates proconvertin — factor VII.
Specifically in the platelet link, it is indicated that endotheliocytes produce von Willebrand factor, which binds to platelets and promotes their aggregation.
What types are capillaries classified into depending on the structure of the endothelium and basement membrane?
Capillary classification is based on the structure of the endothelial wall and permeability:
- Continuous capillaries — have a continuous wall. Located in muscles, skin, lungs, and the central nervous system.
- Fenestrated capillaries — characterized by the presence of "windows" (fenestrae) in the endothelium. Found in renal glomeruli, the intestine, and endocrine glands.
- Sinusoidal capillaries — have a discontinuous endothelial lining, large intercellular spaces, and fenestrated pores in both the endothelium and the basement membrane. Located in the spleen, liver, and bone marrow.
What growth factors trigger endothelial proliferation during angiogenesis?
Sources explicitly indicate that during angiogenesis, endotheliocytes proliferate and migrate, with tissue demand for oxygen and nutrients acting as the stimulus.
Among the angiogenic factors whose increased expression is associated with angiogenesis and new vessel formation are:
- VEGF — vascular endothelial growth factor;
- FGF — fibroblast growth factor.
Class and exam questions
Why do blood clots not form in an undamaged vessel?
Healthy endothelium possesses anticoagulant properties. Its glycocalyx has a negative charge that repels platelets, and the cells themselves secrete prostacyclin and carry heparin on their surface.
What is the mechanism of edema formation in inflammation?
Inflammatory mediators cause endotheliocytes to alter their cytoskeleton — they become shorter and taller. As a result, intercellular spaces widen, vascular permeability increases, and the liquid part of the blood escapes into the tissue.
Into which cells can pericytes differentiate?
Due to pronounced cellular plasticity, pericytes can transform into fibroblasts and smooth myocytes. This is necessary for the formation of the muscular layer during the transformation of a capillary into a larger vessel.
What else to study on this topic
- Types of endothelial intercellular junctions: structure and functional significance.
- Biochemical mechanisms of lymphocyte homing and the role of vascular addressins.
- Factors initiating coagulation: the mechanism of action of thromboplastin.
- Humoral regulation of angiogenesis during tissue hypoxia.
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