26 - Vascular biology II Flashcards
What are the important features of a continuous capillary?
Continuous capillary
- Endothelial cells join together, and at the point of uncion, there will be an intracellular junction - there are two types of junctions that may exist here:
- Completely tight zonula occludens
- Interrupted incomplete fascia occludens
- When endothelial cells join, one of the cells will have a marginal fold, which extends from the endothelial cell and covers the connection
- Large volume of pinocytotic vesicles within the cytoplasm of endothelial cells
- Intact basal lamina external to the endothelium - this is essentially the capillary wall, which will be thin to allow for rapid exchange
Where will you find continuous capillaries?
Locations
- Areas where you would want specific transport of materials with no leakage
- Examples: lungs, CNS, muscle, adipose tissue
Why is there a large volume of pinocytotic vesicles in the cytoplasm of endothelial cells that comprise a continuous capillary?
Pinocytotic vesicles
- Pinocytosis is a mode of endocytosis in which small particles are brought into the cell, forming an invagination, and then suspended within small vesicles that have been formed by the plasma membrane of the cell it entered
Why is this needed in continuous capillaries?
- Tight junctions limit the movement of material across endothelial cells
- Pinocytic vesicles are able to make up for this restriction by ferrying materials across endothelial cells
- Vesicles can fuse, forming c_ontinuous temporal channels_ across an entire endothelial cell
- This allows for the movement of materials from the lumen to the extracellular environment
Describe a fenestrated capillary without diaphragms
Features
- Numerous permanent fenestrations in the endothelial cell wall
- This allows for rapid movement of material between the lumen and the extracellular environment
Where would you find fenestrated capillaries without diaphragms?
- Glomerular capillaries of the kidneys where blood is filtered in renal corpuscle
Describe a fenestrated capillary with diaphragms
Note that most fenestrated capillaries have diaphragms
- Some pinocytotic vesicles will still be present, but the fenestration alone will allow for rapid exchange
- On an image of a fenestrated capillary with a diaphragm, you will see small lines that block the gaps of the fenestrations
Where would you find fenestration capillaries with diaphragms?
- Glomerular capillaries in the kidney
- Elsewhere in the kidney
- Intestines (allows for rapid movement of absorbed nutrients into the blood stream)
- Endocrine organs (allows for rapid pickup of released hormones by the blood
Describe a sinusoidal capillary
Sinusoidsal capillary features
- Larger in diameter compared to continuous and fenestrated capillaries
- Huge pores, larger than fenestrations
- Basal lamina is absent, or discontinuous if it is actually present
- This allows for more rapid exchange of materials
Where would you find sinusoidal capillaries?
- Bone marrow (allows for movement of cells into the blood stream that were developing in the bone marrow)
- Liver
- Spleen
- Adrenal cortex
What are venules? What are the two types?
Venules
- The smallest structure in the venous circulation
- Two types
- Pericytic venules: these venules are bound in microcirculation and have pericytes around them
- Muscular venules: these are larger than pericytic venules and have smooth muscle in the tunica media
Describe the movement of lymphocytes from the blood into the extracellular environment
Lymphocyte movement
- Lymphocyte recognizes adhesion molecules on wall of endothelium
- Lymphocyte attaches to endothelium at region of marginal fold
- Lymphocyte disrupts junctional complex
- Lymphocyte moves between endothelial cell junction
- Lymphocyte crosses endothelial cell wall and enters extracellular environment
How much exchange occurs across the wall of a pericytic venule? A lot or a little?
Describe a dilated pericytic venule in the area of inflammation
Pericytic venules in inflammation
- Many nuclei will be visible at the margins of pericytic vesicles
- This indicates that neutrophils are undergoing margination
- This is the movement of margins so that they attach to endothelial cells
- This allows them to emigrate across the pericytic venule wall and enter the extracellular environment
- Upon doing so, they can clean up debris from the inflammatory state
- Note that many RBCs will also be visible
What constitutes a “small vein”?
Small vein
- A little larger and more muscular than a venule
- The following layers will be visible
- Endothelium (white)
- Muscle within the tunica media (thin, dark blue)
- In the low pressure venous system, the tunica media will be thin and not much smooth muscle will be present
- In the higher pressure venous system, more tunica media and smooth muscle will be present
-
Tunica adventitia collagen fibers (light blue)
- This will be the thickest layer
- You will see valve leaflets - these are extensions of the endothelium
- You need to know which direction the blood is flowing based on the orientation of the leaflets
- Remember, the leaflets look like arms that are down by your sides, but out at a 30 degree angle - your head would be the direction of blood flow
What constitutes a “medium vein”?
Medium vein
- Walls will be a little thicker
- Smooth muslce, elastic elements and connective tissue will be present, with the connective tissue predominating
- Layers
- Tunica intima
- Endothelium resting on the basal lamina
- The tunica media will only have about 2 layers of smooth muscle (still pretty thin-walled layer)
-
Tunica aventitia is the thickest layer
- Some smooth muscle components will be shown here
Would you see internal elastic lamina or external elastic lamina in a medium vein?
No!
- These are seen in arterial circulation, NOT venous circulation
Describe the characteristics of a “large vein”
Large veins
- Examples: protal vein, inferior vena cava
-
Unique tunica adventitia
- Contains longitudinally arranged smooth muscle bundles
- Tunica media will contain 2 smooth muscle layers that are circularly arranged
- Tunica intima will be similar in size to other veins
What is the purpose of having longitudinally arranged smooth muscle bundles in the tunica adventitia of large veins?
Longitudinally arranged smooth muscles
- Contract and force blood back to the heart against gravity
- Circularly arranged muscles (such as in the tunica media) do not do this as effectively
Define vasculogenesis
De novo vesell formation
- Making vessels “from scratch”
- NOT from existing vessels
