Histo Block I: Capillaries, venules, veins Flashcards
WHat does the wall of capillaries consists of?
Capillaries consist only of endothelium with its basement membrane. There is no discernable (distinct) tunica media or tunica adventitia.
Usually link arterioles to venules
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micrograph of capillary in cerebellum
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Micrograph of capillary with pericyte
How is the lumen of capillaries?
The lumen is only one erythrocyte wide or smaller (~5 -10 microns)
What cells are associated with capillaries?
May have associated cells called pericytes (Rouget cells) which sit inside capillary basement membrane
What is the function of pericytes?
Pericyes help to regulate molecular exchange across capillary & can differentiate into endothelial or smooth muscle cells upon injury.
Function of capillaries
The thin wall allows easy exchange of molecules across the wall. Are approximately 50,000 miles of capillaries in body. They are the major site of movement of gases, metabolites, & waste products between the blood and the CT, i.e., body tissues.
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Capillary in longitudinal section
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capillary in cross section
What are the types of capillaries?
- Continuous capillaries
- Fenestrated capillaries
- Discontinuous (sinusoidal) capillaries
describe continuous capillaries
- no fenestrations(holes or pores) through the cytoplasm of the endothelial cell or discontinuities in or between cells.
- Tight junctions are usually present between the endothelial cells
describe fenestrated capillaries
- Presence of holes (fenestrations) through cytoplasm of capillaries.
- Diaphragms are present across the fenestrations in all cases except for kidney glomerular capillaries
describe discontinuous sinusoidal capillaries
- Distinct discontinuities (gaps) between endothelial cells.
- Fenestrations with diaphragms may also be present. Discontinuous basal lamina
How is the endothelium of continuous capillaries?
Endothelium is a continuous uninterrupted layer around the lumen with many tight junctions and without either fenestrations or discontinuities
What is the function of the continuous capillaries?
- Endothelium can be thick (200-300 nm in width) or thin (100 nm in width) depending upon selective permeability properties of wall. Tightly controls and regulates the passage of molecules to and from the lumen and the tissues. (least permeable)
How can molecules pass in continuous capillaries?
Molecules can pass across by diffusion, active transport, or by the transcellular pathway involving pinocytosis or receptor mediated endocytosis
Where can we find continuous capillaries?
lung, muscle, CNS, thymic cortex. Blood-brain-barrier, Blood-thymic barrier
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micrograph of
capillary in cerebellum
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EM of continuous capillary
how is the endothelium of fenestrated capillaries?
Typically a thinner endothelium which contains membrane lined transcellular channels (fenestrae) with diameters of 60 – 80 nm which run though the cytoplasm
-Fenestrae have a diaphragm everywhere except glomerular capillaries
-Fenestrae can be randomly distributed through the cell – may be many or few
What is the function of fenestrated capillaries?
Fenestrated capillaries are much more permeable than continuous capillaries and are found in areas where rapid exchange is needed e.g. endocrine glands and kidney glomerulus
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EM of fenestrated (arrows) capillary
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EM of wall of fenestrated capillary. The arrows point to the fenestrae with diaphragms
How is the endothelium of sinusoidal capillaries?
The endothelium is thin and irregular with large gaps and discontinuities between the cells which can be hundreds of nm in size.
-Lumen is often slightly larger than other capillaries (9 -12 um)
-Often find macrophages associated with these gaps
What is the function and location of sinusoidal capillaries?
Are the most porous and leaky of the capillaries
Found in areas where extremely free and open exchange of molecules across the capillary wall is required e. g. liver sinusoids, lymph node sinusoids and bone marrow sinusoids
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EM of sinusoidal capillary in liver