CHAPTER 9
â– 
Circulatory System
171
Figure 9-14A.
Fenestrated capillaries, kidney.
H&E,
3
680; inset
3
1,405
Fenestrated capillaries
have endothelial cells that are
perforated by
small pores
, often bridged by
diaphragms
(the glomerulus of the kidney is an exception). The
basal
laminae
beneath the endothelial cells are
continuous
. This
type of capillary allows a greater range of substances to
pass freely through the capillary wall. Examples of fenes-
trated capillaries are the capillary beds in the intestine
(where nutrients are absorbed), the glomerulus of the
kidney (where blood is F
ltered and urine is formed), and
the capillaries in the endocrine organs, such as in the pitu-
itary gland and endocrine pancreas (where hormones are
secreted and released into the capillaries). The glomerular
capillaries in the kidney are shown here.
Lumen of
Lumen of
capillary
capillary
Podocyte
Podocyte
Endothelial cell
Endothelial cell
Endothelial cells
Endothelial cells
Glomerular
Glomerular
capillaries
capillaries
Glomerular
capillaries
Podocyte
Endothelial cell
Endothelial cells
Lumen of
capillary
A
Figure 9-14B.
Fenestrated capillary, pituitary.
EM,
3
14,000
The wall of this
fenestrated capillary
consists of a thin layer of cytoplasm interrupted at frequent intervals by circular pores, termed
fenestrations
, of fairly consistent diameter (about 70 nm). This permits freer movement than occurs in continuous capillaries for
some molecules such as
peptide hormones
. In order to pass between the vascular compartment and the interstitial tissue compart-
ment, dissolved molecules still must pass through the
diaphragms
that bridge the pores, and the
endothelial cells
still produce a
complete basal lamina at the interface with the surrounding tissue. ±enestrated capillaries are found in the gastrointestinal tract, the
kidney, and in endocrine glands. The vessel shown here is located in the anterior pituitary.
T. Yang
Diaphragm
Basal lamina
Lumen of a
Lumen of a
fenestrated capillary
fenestrated capillary
Secretory granules
Secretory granules
containing hormones
containing hormones
Collagen
Collagen
fibers
fibers
Diaphragm of
Diaphragm of
fenestration
fenestration
Endothelial cell
Endothelial cell
cytoplasm
cytoplasm
Collagen
fibers
Diaphragm of
fenestration
Lumen of a
fenestrated capillary
Endothelial cell
cytoplasm
Secretory granules
containing hormones
B
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