Histology
Stanley Shostak
BioSc
1450
Spring 03
Lecture 8a. Circulatory System
tunica intima: endothelium with basement
membrane
+ delicate collagenous tissue
tunica media: muscular layer; vasa
vasorum
from adventitia may extend into; predominant influence on blood flow.
tunica adventitia supporting layer;
contains
vasa vasorum
Heart
tunica intima: endocardium; ct layer (subendocardial
supporting layer) continuous with perimysium;
fibro-elastic supporting layer: contains
conduction myofibers; thick subendocardial elastin layers;
tunica media: myocardium; comprises almost
whole
mass of heart;
tunica adventitia: epicardium = visceral
pericardium
= serosa: lined by mesothelium; coronary arterial system
heart valve
endocardium + subendothelial elastin layer +
thick
lamina fibrosa = fibrous lamina; merged fibro-elastic
supporting
layers.
papillary muscle: extensions of myocardium;
cordae tendinae: stabilize cusps of mitral
and tricuspid valves; merge with fibrous lamina.
valve anulus: fibrous ring at margins of
valve;
cardiac skeleton: central fibrous ct of
heart:
valve anulus of four cardiac valves continuous with collagenous tissue
of myocardium, endocardium and epicardium
Conducting system:
sino-atrial node: excitation originates
=
pacemaker region of right atrium; rate modulated by autonomic
nervous system; excitatory wave spreads throughout atria; spreads to
atrioventricular
node
excitatory cells sm specialized
myocardial
fibers; electrochemical stimuli transmitted via gap junctions; little
contractile
protein or glycogen; embedded in dense vascular collagenous tissue with
numerous autonomic nerve fibers.
internodal bundle: conducts impulse to
atrioventricular
node probably at somewhat slower pace.
atrioventricular node:
excitatory cells (same)
atrioventricular bundle = bundle of His:
spreads
stimulus throughout ventricular myocardium; divides within
interventricular
septum into smaller branches of
conduction myofiber = Purkinge fibers;
larger
than cardiac muscle fibers, may be binuclear; myofibrils beneath plasma
membrane; irregular; rich in glycogen & mitochondria; no T tubule
system;
no intercalated disks; desmosomes and gap junctions
Arteries
Expansion and recall: function of elastic tissue
within walls; regulation by varying diameter of distributing vessels
via
circumferentially (circularly) disposed smooth muscle; under control by
sympathetic nervous system and adrenal medullary hormones; elastin and
smooth muscle wall thick relative to diameter of lumen; gradual
transition
between types; decreasing amount elastic tissue with decreasing
diameter;
increasing amount smooth muscle.
Elastic arteries: major
distribution
vessels; aorta, innominate, common carotid, subclavian and pulmonary
arterial
vessels; blood flow highly pulsatile
tunica intima: elastin disposed in
discontinuous
sheets and fibers;
myointimal cells: smooth muscle-like
cells
in subendothelial supporting layer; lack basement membrane; accumulate
lipid with age; extreme form = early changes of atherosclerosis; along
with fibroblast, produce ECM.
tunica media: broad and extremely elastic;
concentric
fenestrated sheets of elastin separated by collagenous tissue;
relatively
few smooth muscle; outer half penetrated by vasa vasorum
tunica adventitia: collagenous; contains
vasa
vasorum.
Muscular arteries: main
distributing
branches of arterial tree: radial, femoral, coronary and cerebral
arteries
tunica intima:
internal elastic lamine: separates t.
intima
from t. media
tunica media: thick layer circumferentially
arranged
smooth muscle
external elastic lamine: less well
defined;
separates t. media from t. adventitia
tunica adventitia: collagen with
considerable
elastin.
Arterioles: terminal
branches
of arterial tree; supply capillary beds; elastic tissue reduced to well
defined, fenestrated elastic sheet; regulates flow of blood to
capillary
beds;
large arterioles: lumen less than 0.3 mm
in diameter;
tunica intima: very thin; has thin internal
elastic lamina
tunica media: almost entirely smooth muscle
in six or less concentric layers
tunica adventitia: almost as thick as t.
media; no external elastic lamina
small arterioles:
tunica intima: same
tunica media: two or one concentric layers
of smooth muscle
tunica adventitia: merges imperceptibly with
surrounding ct
Microcirculation:
smooth muscle activity regulated by autonomic nervous system and
circulating
hormones, e.g., adrenal catecholamines; autoregulation:
regulates
local flow by concentration of oxygen and metabolites such as lactic
acid.
metarterioles: large
diameter
capillaries; discontinuous outer layer of smooth muscle cells; direct
communications
betwn arterioles and venules; contraction of smooth muscle directs
blood
through network
precapillary sphincter: at
arteriolar-capillary junction; controls flow within capillary bed;
capillaries 3 to 4 µm
& sinusoidsto 30 to 40 µm =
lg
diameter capillaries in liver (endothelium discontinuous), spleen,
lymph
nodes, some endocrine glands & bone marrow.
tunica intima:
endothelium: nuclei bulge inward;
basement
membrane probably does not present barrier to exchange
tunica media: absent
tunica adventitia: surrounding ct
continuous capillaries: endothelium
uninterrupted
lining; most common type; scattered tight junctions of the fascia
occludens
type; marginal folds: extend across intercelular junctions at
luminal
surface
fenestrated capillaries: enhanced
permeability
(to macromolecules smaller than plasma proteins); endothelium
contains
fenestrations = large pores in endothelium traversed by
electron
dense line (i.e., diaphragm [except in kidney glomeruli]); where
exchange
with blood is extensive (sm intestine, endocrine glands, kidney);
pericytes
rare.
discontinuous endothelium: sinusoids of
liver.
pericytes: embrace capillary (Note:
nuclei
bulge outward rather than inward like endothelial nuclei); supported by
its own basement membrane.
Venal side of microcirculatory system
post-capillary venules:smallest;
formed by union of capillaries; blood flow sluggish; portal point for
whte
blood cells; margination: accumulation white blood cells
(principally
neutrophils) at margin of erythrocyte stream; intercellular
junctional
complexes relatively uncommon
collecting venules: larger
diameter and number of pericytes;
muscular venules:
arterio-venous shunts: direct
connections
btwn arterial and venous systems; contraction of smooth muscle in capillary
sphincter directs blood through network.
Veins: Note: more likely to contain
blood
then arteries in histological sections.
muscular venules and small veins:
tunica intima: devoid of elastic fibers;
little more than endothelium; may be thrown into small folds (in
veins
not distended with blood);
valves: semilunar projections of t.
intima;
fibro-elastic tissue lined by endothelium; two leaflets; only in veins
of more than 2 mm in diameter, particularly in extremities.
tunica media: one or three layers of smooth
muscle
fibers; lumen diameter relatively thick compared to wall; thin compared
to arteries;
tunica adventitia: broadest layer; composed
of longitudinally arranged thick collgen fibers; merges with
surrounding
collagenous tissue.
large muscular veins (e.g.,
femoral,
renal) & vena cavae
tunica intima: a few elastic fibers but
no distinct layer; little more than endothelium; may be thrown
into
small folds (in veins not distended with blood);
tunica media: several layers circular smooth
muscle (longitudinal in vena cavae) separated by layers of collagenous
ct; a few elastic fibers but no distinct layer; penetrated by vasa
vasorum
and lymphatics vessels
tunica adventitia: contains vasa vasorum
and neurovascular bundle: contains small arteriole, venules,
lymphatic,
and nerve
Lymph vascular System: lymph reenters
venal circulation through thoracic duct (on lft side) and right
lymphatic
duct
In all tissues except CNS, cartilage, bone,
bone
marrow, thymus, placenta, cornea and teeth; distinguished from
venus
vessels by absence of erythorocytes; greater permeability compared
to capillaries;
endothelium extremely thin; basement membrane
rudimentary or absent; no pericytes
valves: more numerous than in veins;
supporting
tissue core of reticulin fibers with little ground substance.
anchoring filaments: fine collagenous
filaments
link endothelium to surroundint ct (prevent collapse).
lymph nodes: interrupt lymph flow along
lymphatic
vessels (see lymphatic system)