172
UNIT 3
Organ Systems
Figure 9-15A.
Discontinuous capillaries, spleen.
H&E,
3
274
Discontinuous capillaries
, also called
sinusoidal capillaries
(or
sinuses
), are characterized by
large pores
, which per-
forate the cytoplasm of endothelial cells, and incomplete
or missing
basal laminae
. In addition, the sinusoids have
large
intercellular clefts
(gaps between endothelial cells).
These structures permit free movement of proteins, plasma
constituents, and other materials. In some cases, even cells
can pass through the capillary wall. This type of capillary
has a large diameter (30–60
μ
m) and may have an irregular
shape due to an incomplete basal lamina, spindle-shaped
endothelial cells, and fenestrated structures. Discontinu-
ous capillaries can be found in the liver, spleen, bone mar-
row, and lymph nodes.
A
Endothelial
Endothelial
cells
cells
Erythrocytes
Erythrocytes
Capillaries
Capillaries
(sinusoids)
(sinusoids)
Capillaries
Capillaries
(sinusoids)
(sinusoids)
Endothelial
cells
Erythrocytes
Capillaries
(sinusoids)
Capillaries
(sinusoids)
Figure 9-15B.
Discontinuous (sinusoidal) capillary.
EM,
3
5,200; inset
3
3,400
Sinusoidal capillaries
vary greatly in appearance, but they all have as common features, lumens of large diameter and gaps or
potential gaps between and sometimes even within endothelial cells. Objects as large as cells can pass through these discontinuities.
The sinusoidal capillary shown here is located in the red pulp of the spleen, and it is termed a
red pulp sinusoid
. The endothelial
cells are elongated with the long axis parallel to the long axis of the vessel. In cross section, as seen here, the endothelial cells are
nearly circular in shape. There are
gaps
between cells, such as the one seen at the top edge of this example, but many neighboring
endothelial cells are in close contact. Nevertheless, as seen in the
inset
, motile blood cells and macrophages can pass readily between
adjacent endothelial cells because of sparseness of intercellular junctions and the
incomplete basement membrane
.
T. Yang
Migrating cell
Migrating cell
Gap
Gap
Red pulp
Red pulp
cord
cord
Erythrocyte
Erythrocyte
Monocyte
Monocyte
Basement
Basement
membrane
membrane
Basement
Basement
membrane
membrane
Lumen
Lumen
Basement
Basement
membrane
membrane
Endothelial
Endothelial
cells
cells
Endothelial
Endothelial
cells
cells
Erythrocyte
Migrating cell
Monocyte
Basement
membrane
Basement
membrane
Lumen
Basement
membrane
Endothelial
cells
Endothelial
cells
Gap
Red pulp
cord
Endothelial cells
Gap
Incomplete basal lamina
Incomplete basal lamina
Incomplete basal lamina
B
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