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Studies on blood capillaries. I. General organization of blood capillaries in muscle
Insights
The study details the three-layered structure of blood capillaries in rat, guinea pig, and hamster muscles and heart. It highlights the endothelial cell vesicles and their role in transport across capillary walls.
Area of Science:
- Cell Biology
- Histology
- Microcirculation Research
Background:
- Blood capillaries are vital for nutrient and waste exchange.
- Understanding capillary wall structure is crucial for studying physiological and pathological processes.
Purpose of the Study:
- To provide a detailed ultrastructural analysis of blood capillary walls in skeletal muscle and myocardium.
- To characterize the layers, cellular components, and intercellular junctions of these capillaries.
Main Methods:
- Transmission electron microscopy was used to examine capillary walls.
- Three-dimensional reconstruction was employed to analyze endothelial cell cytoplasm.
Main Results:
- Capillary walls consist of endothelium, basement membrane with pericytes, and adventitia.
- Endothelial cells contain numerous vesicles (650-750 A) involved in transport.
- Cells are joined by membrane fusions (zonulae occludentes), with rare intercellular gaps.
Conclusions:
- The fundamental structure of blood capillaries is consistent across different muscle types and species studied.
- Endothelial vesicles and junctions play key roles in regulating capillary permeability and integrity.
Abstract:
THE WALL OF THE BLOOD CAPILLARIES OF SKELETAL MUSCLES (DIAPHRAGM, TONGUE, HIND LEGS) AND MYOCARDIUM OF THE RAT, GUINEA PIG, AND HAMSTER CONSISTS OF THREE CONSECUTIVE LAYERS OR TUNICS: the endothelium (inner layer), the basement membrane with its associated pericytes (middle layer), and the adventitia (outer layer). The flattened cells of the endothelium have a characteristic, large population of cytoplasmic vesicles which, within the attenuated periphery of the cells, may attain a maximum frequency of 120/micro(2) of cell front and occupy approximately 18% of the cytoplasmic volume; these values decrease as the cells thicken toward the perikaryon. The vesicles are 650-750 A in over-all diameter and are bounded by typical unit membranes. They occur as single units or are fused to form short chains of two to three vesicles. Each configuration may lie entirely within the cytoplasm or open onto the cell surface. In the latter case, the unit membrane of the vesicle is continuous, layer by layer, with the plasmalemma. Chains of vesicles opening simultaneously on both the blood and tissue fronts of the endothelial tunic have not been observed either in sections or in a tridimensional reconstruction of a sector of endothelial cell cytoplasm. Adjacent endothelial cells are closely apposed to one another and appear to be joined over a large part of their margins, possibly over their entire perimeter, by narrow belts of membrane fusion (zonulae occludentes). Except for tongue capillaries, patent intercellular gaps are rare or absent. The middle layer is formed by a continuous basement membrane ( approximately 500 A thick) and by pericytes which lie in between leaflets of this membrane. The tips of the pericyte pseudopodia penetrate through the inner leaflet of the basement membrane and join the endothelium in maculae occludentes. The adventitia is a discontinuous layer comprising cellular (macrophages, fibroblasts, mast cells) and extracellular (fibrils, amorphous matrix) elements. The same general type of construction appears to be used along the entire length of the capillary.