6. VASCULAR
RELATIONS: (see Figure 2. above)
The vascular arrangement of the spleen determines the distribution
and inter-relation of both the red and
white pulp. The presence of lymphatics in the spleen is controversial.
Blood enters the spleen via:
-
1. The
splenic artery at the hilus;
2. Branches of this
artery follow the trabeculae and subdivide into:
3. arterioles,
which enter the splenic parenchyma;
4. The adventitia
(C.T. covering) of arterioles becomes looser further in the parenchyma
and lymphocytes infiltrate this loose reticular tissue. This produces
the ensheathing white pulp. These arterioles surrounded by the
white pulp are called central arterioles and
the surrounding sheath of lymphocytes is referred to as the periarteriolar
lymphoid sheath or PALS for short.
5. With further decrease
in size, arterioles less than 50 µm in diameter leave the white
pulp and enter the red pulp where they branch and are called penicillar
arteries (=brush-like).
6. These arteries
branch into several capillaries; which are covered with layers
of macrophages; this covering is called the ellipsoid
and the capillaries sheathed capillaries;
7. The last division
is formed by the terminal capillaries;
their ending in the venous sinuses is disputed (Open v.s. closed
circulation; Open if empties into pulp cords;
Closed if connected to venous sinuses).
8. The venous
sinuses between pulp cords
are permeable to cells of the pulp cords;
their wall is composed of elongated endothelial cells
(see diagram), which are supported by very narrow
reticular lamina. Cells are thought to move through the
slits between endothelial cells.
9. Sinuses empty into
the trabecular veins, which
are tributary to the:
10. Splenic
vein at the hilus. (See Figure 2. above)
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LABORATORY EXERCISE: Virtual Slides
35 and 125. Spleen, H&E stain - Macroscopically
the spleen is separated into red and white pulps.
A dense FECT capsule encloses the substance of
the spleen. Large trabeculae, extend inward from
the capsule and some trabeculae, near the hilus surround arteries
or veins. What are trabeculae made of? White pulp:
This is a lymphocytic sheath around small arterioles called the
PALS. The arterioles the PALS surround are called the central
arterioles of the white pulp. Some of these arterioles
appear as a cluster of arterioles; this is due to section cutting
through the numerous branches of these arterioles. Near the central
arteriole(s), in the enlarged part of the PALS, secondary
lymphoid nodules can be seen. Secondary lymphoid nodules
(or follicles) contain germinal centers. Some GCs show
both light and dark regions,
others appear to be made of only light or only dark regions. Why?
Explain.
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