basement membrane. In the blood-brain barrier, there is also a continuous
endothelium with a basal lamina and an absence of fenestrations. Sur-
rounding the basal lamina in the brain are the foot processes of astrocytes,
which form the glia limitans; however, it is important to note that the bar-
rier function of the blood-brain barrier is formed specifically by endothe-
lial cell occluding junctions with many sealing strands. Other capillary
endothelia(answers b → e)in the body are fenestrated (transcellular
openings) or discontinuous (sinusoids). The fenestrae are transcellular
openings that occur in many of the visceral capillaries. In hematopoietic
organs, there are large gaps in the endothelium, and the capillaries are clas-
sified as discontinuous. Diaphragms (thinner cell membrane) are present
in some fenestrated capillaries and produce an intermediate level of mole-
cular transit. Diaphragms contain proteoglycans with particularly high
concentrations of heparan sulfate. This results in numerous anionic sites
that repel anionic proteins. The diaphragms facilitate the passage of water
and small molecules dissolved in fluid. Physiologically, the large pores (50
to 70 nm) of endothelia are represented by pinocytotic vesicles. Intercellu-
lar junctions, particularly the tight junctions, function as the small
endothelial pores (approximately 10 nm in diameter) observed in physio-
logic studies. Plasmalemmal vesicles and channels are neutrally charged
and rich in galactose and N-acetylglucosamine. Vesicular and channel
pathways are required for transport of anionic proteins such as insulin,
transferrin, albumin, and low-density lipoprotein (LDL).
156.The answer is e.(Junqueira, pp 206–209. Moore and Dalley, p 160.
Kierszenbaum, pp 335–336.)The site at which coronary arteries become
occluded are: first, at the left anterior descending (thus affecting both ven-
tricles anteriorly); second, at the origin of the right coronary artery affect-
ing both the right atrium and ventricle and disrupting cardiac rhythm; and
third, the circumflex branch (affecting both left atria and ventricle). Ather-
osclerosis is initiated by damage to the endothelial cells, which exposes the
subjacent connective tissue (subendothelium). The loss of the antithrom-
bogenic endothelium results in aggregation of platelets. Atherosclerosis is
one form of arteriosclerosis (hardening of the arteries) that involves depo-
sition of fatty material primarily in the walls of the conducting arteries. The
intima and media become infiltrated with lipid. Intimal thickening occurs
through the addition of collagen and elastin with an abnormal pattern of
elastin cross-linking. Platelets release mitogenic substances that stimulate
Cardiovascular System, Blood, and Bone Marrow Answers 263