Wall Structure Features
The wall of a lymphatic capillary is extremely simplified to ensure maximum permeability. Unlike blood capillaries, there are no pericytes and no basement membrane. The entire structure is formed exclusively by a single layer of cells—endothelium.
Lymphatic capillary endothelial cells have several unique morphological features:
- Size: The cells are significantly larger than their counterparts in blood capillaries.
- Surface: On the basal side facing the interstitium (surrounding tissue), there are numerous microvilli.
- Cytoplasm: Contains a massive number of pinocytotic vesicles and lysosomes. The function of lysosomes here is critically important—they degrade ballast components entering along with the tissue fluid.
- Junctions: Intercellular junctions between neighboring endothelial cells are very loose, which is the primary condition for high wall permeability.
Anchoring Filaments
Due to the lack of a prominent basement membrane and strong intercellular junctions, the capillary requires an external supporting framework. This supportive function is performed by anchoring filaments—specialized structures unique to lymphatic capillaries.
Their main task is to provide a secure mechanical connection between the vessel and the surrounding interstitium. Their topographical attachment mechanism is as follows:
- Proximal end: Attaches to the outer surface of endothelial cells, most commonly in the areas of intercellular junctions.
- Distal end: Interweaves firmly into bundles of connective tissue collagen fibers running parallel to the capillary axis.
Under electron microscopy, these filaments appear as thin fibrous structures extending from the basal surface of the endothelial cell into the surrounding tissue.
Physiology of Tissue Fluid Drainage
Anchoring filaments are not just static supports; they play a key role in ensuring the drainage function, i.e., the entry of interstitial fluid into the lumen of the lymphatic capillary. The mechanism of their action depends directly on the volume of fluid in the tissues.
Mechanism of opening "valves/gaps":
- Fluid accumulation: During tissue edema, the volume of tissue fluid increases, creating intense mechanical pressure on the surrounding matrix.
- Tension: Under pressure, collagen fibers shift, causing tension in the attached anchoring filaments.
- Opening: Strongly stretched filaments mechanically pull on the endothelial cell wall, literally "pulling apart" the loosely connected cells.
- Result: Intercellular gaps form or significantly widen between endothelial cells. Through these open spaces, fluid flows freely from the interstitium into the lumen of the lymphatic capillary.