Histology of Great Vessels

Question 1

describe layers of blood vesses

Answer 1

3 layer structure;
inner layer - tunica intima - single layer of squamous epithelial cells supported by basal lamina and thin layer of connective tissue
internal elastic membrane (layer of elastic tissue) separates tunica intima from tunica media
middle layer - tunica media - smooth muscle, thickness varies
external elastic membrane (layer of elastic tissue) separates tunica media from tunica adventitia
outer layer - tunica adventitia - supporting connective tissue

elastic fibres do not stain with H&E

Question 2

describe elastic arteries

Answer 2

largest arteries
e.g. aorta
many sheets of elastic fibres in their tunica media to provide elastic recoil

Question 3

describe arterioles

Answer 3

1 or 2 layers of smooth muscle in their tunica media and almost no tunica adventitia
important in controlling blood flow in tissue

Question 4

describe capillaries

Answer 4

composed of endothelial cells and a basal lamina
they have pericytes at intervals just outside the basal lamina - these are connective tissue cels that have contractile properties

Question 5

describe the 3 types of capillary

Answer 5

continuous - most common. Found in muscle, connective tissue, lung, skin and nerves
fenestrated - pores in wall. Found in mucosa of gut, endocrine glands, glomeruli of kidney
sinusoidal or discontinuous - large gaps through which macromolecules, or cells, can pass. Basal lamina is discontinuous or absent . Found in liver, spleen and bone marrow

Question 6

describe microvascular networks

Answer 6

small arterioles connect to a post-capillary venule through a network made up of metarterioles, throughfare channels and capillaries
precapillary sphincters, composed of smooth muscle, at the beginning of the capillary help control flow thorough the network

Question 7

describe postcapillary venule

Answer 7

capillary networks drain into here
endothelial cell lined and contain a thin layer of connective tissue and occasional pericytes
important sites for exchange (e.g. cell moving into tissue in inflammation)
once vessels acquires intermittent smooth muscle cells into tunica media, they are referred to as venules

Question 8

describe veins

Answer 8

in addition to tunica intima, they have a relatively thin but continuous tunica media typically consisting of a few layers of smooth muscle
tunica media is thinner than in a muscular artery
largest veins (vena cava and hepatic portal vein) have thick tunica adventitia which incorporates bundles of longitudinally oriented smooth muscle
veins are flexible and can accommodate expansion and contain most of the blood in the body

Question 9

describe valves

Answer 9

most small to medium sized veins have valves that are inward extensions of the tunica intima

Question 10

describe the layers of the heart

Answer 10

3 layers
endocardium - inner (including valves)
myocardium - middle
epicardium - outer

Question 11

describe endocardium

Answer 11

made up of;
endothelium
basal lamina
thin layer of collagen fibres
layer of denser connective tissue
may be subendocardium of loose connective tissue containing small blood vessels and nerves and the branches of the impulse conducting system

Question 12

describe myocardium

Answer 12

thick
made up of;
bundles and layers of contractile cardiac muscle fibres
individual muscle fibres are surrounded by delicate, collagenous connective tissue with rich network of capillaries

cardiac muscle cells have single central nucleus (sometimes 2)
have intercalated discs passing across the fibres at irregular intervals - attaching cells and allowing spread of electrical activity

Question 13

describe epicardium

Answer 13

surface . of heart - single layer of flattened epithelium => mesothelium
basal lamina
fibroelastic connective tissue and in some places, adipose tissue where coronary arteries are embedded

Question 14

describe pericardium

Answer 14

2 parts;
fibrous - sac of tough fibrocollagenous connective tissue
serous - layer of simple squamous epithelium (mesothelium), backed by basal lamina and connective tissue. Lines the inner surface of the fibrous pericardium (parietal serous pericardium) and covers the surface of the heart (visceral serous pericardium)
2 mesothelial layers are apposed to each other and separated by thin pericardial cavity which contains small amount of pericardial fluid that provides lubrication for heart movement

Question 15

describe fibrous skeleton of the heart

Answer 15

formed by thick bands of fibrous connective tissue around the heart valves, between the atria and between the ventricles
connective tissue supports the valves but provides attachment for the cardiac muscle fibre
electrically isolates the atria from the ventricles

Question 16

describe heart valves

Answer 16

outer endothelial layer with basal lamina
layer of collagen and elastin fibres
core of dense connective tissue, lamina fibrosa, that is continuity with fibrous skeleton
leaflets of valves separating atria from ventricles are anchored by papillary muscles in the wall of the ventricle by collagenous strands called the chordae tendineae, which merge with lamina fibrosa
lamina fibrosa is the core of a heart valves and is dense irregular connective tissue that is continuous with the fibrous skeleton of the heart
no blood vessels in valves

Question 17

describe cardiac muscle cells

Answer 17

contractile cells
pacemaker cells
conducting cells

Question 18

describe pacemaker cells

Answer 18

highly specialised
smaller than contractile cardiac myocytes
embedded in more extensive matrix of connective tissue
appear pale (P cells) because of the paucity of organelles within them
few myofibrils (irregularly arranged), little glycogen and no proper T-tubule system
small modified nodal muscle fibres act as pacemakers

Question 19

describe purkinje fibres

Answer 19

Unlike small pacemaker and bundle fibres, Purkinje fibres are larger than normal cardiac muscle cells
found in the subendocardial layer just deep to the endocardium
abundant glycogen, no T-tubules, no intercalated discs and sparse actin and myosin filaments, which tend to be found at the periphery of the cells
in histological sections they appear pale, and often exhibit a very pale/clear centre
Purkinje fibres distribute the excitatory activity such that ventricular contraction generally occurs from inferior to superior

Question 20

describe lymph vascular system

Answer 20

lymphatic vessels that drain tissue fluid, returning it to veins in the base of neck
lymphatic capillaries begin as blind sacs

system of relatively thin walled vessels that drain excess interstitial fluid (lymph) into the blood stream
interstitial fluid contains ions, lipids, proteins and occasional cells
most of excess interstitial fluid returns to capillaries and venous vessels, but a portion returns to the circulatory system by entering lymphatic vessels
Along the way the lymph passes through lymph nodes, providing for immunological surveillance
Lymphatic vessels have no central pump, but smooth muscle in walls, hydrostatic pressure in the tissue and compression of the vessels by voluntary muscle, combined with valves in the vessels, produces flow.