Heart & Blood Vessels Histology By Dr. Nabil, Khouri

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Heart & Blood Vessels Histology

By Dr. Nabil, Khouri

The Cardiovascular System IV


The Heart Wall


The myocardium. Note the endocardial layer, which consists of endothelium supported by a

rather thick layer of sub-endocardial connective tissue (green). The ventricular lumen is indicated.

Lumen


Cardiac muscle orientation

The tissues that comprise the myocardium, as well as the adjacent tissues of the endocardium and pericardium, are continuous, which means that the cardiac muscle is one single tissue that wraps around itself to form the heart.

The myocardial tissue in normal heart spirals up from the base to the apex, causing a series of clear intersections of cardiac muscle tissue.

The double spiral formation of the myocardial tissue allows a 60% increase in ejection fraction with a fiber shortening of 15%.


Different Cardiac muscle layers & Orientations

Tricuspid & bicuspid Valves


Cardiac Muscle Longitudinal Section

Cardiac muscle consists of Branched muscle cells with one centrally placed nucleus.

The Nuclei are oval, rather pale and located centrally in the muscle cell which is 10 - 15 µm wide.

Cardiac muscle exhibits cross-striations.

Cardiac muscle is for these reasons also called involuntary striated muscle.

X40 Magnification

cell nucleus

Intercalated Discs

One cell


Cardiac Muscle

N


Cardiac Muscle Cross section & Purkinje fibers

X40 Magnification

X20 Magnification


This medium power view of the inner wall of the left ventricle shows a cluster of Purkinje fibers, that are partof the conduction system of the heart. The fibers run within the subendocardial connective tissue.


At high power, the structure of individual Purkinje fibers can be seen. Much larger than normal cardiac muscle fibers, these cells still

contain centrally-placed nuclei and, although they do still contain myofibrillar elements, their cytoplasm appears somewhat vacuolated

Purkinje fibers

Cardiac Cells

Sub-endocardial CT


Note the coronary vessels surrounded by epicardial fat. Can you determine which are arteries and which are veins


Blood Vessels

Blood is carried in a closed system of vessels that begins and ends at the heart

The three major types of vessels are arteries, capillaries, and veins

Arteries carry blood away from the heart, veins carry blood toward the heart

Capillaries contact tissue cells and directly serve cellular needs

www.lab.anhb.uwa.edu.au/.../Vascular.htm


Generalized Structure of Blood Vessels

Arteries and veins are composed of three tunics –

tunica externa tunica media tunica interna

Capillaries are composed of endothelium with sparse basal lamina

Lumen – central blood-containing space surrounded by tunics


Tunics Tunica interne (tunica intimae)

Endothelial layer that lines the lumen of all vessels

In vessels larger than

1 mm, a sub-endothelial

connective tissue basement

membrane is present Tunica media

Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system

Controls vasoconstriction/ vasodilatation of vessels

Tunica externa (tunica adventitia) Collagen fibers that protect

and reinforce vessels Larger vessels contain vasa

vasorum


Histological Structure of Blood Vessels


Large Elastic (Conducting) Arteries

Thick-walled arteries near the heart; the aorta and its major branches Large lumen allow low-resistance conduction of blood Contain internal elastic lamina and fibers in all three

tunics Withstand and smooth out large blood pressure

fluctuations Allow blood to flow fairly continuously through the body


Elastic (Conducting) Arteries

Tunica intima: Large arteries often have a large subendothelial layer. The border of the

intima is delineated by the internal elastic membrane. The internal elastic membrane may not be conspicuous because of the abundance of elastic material in the tunica media.

Tunica media:Both connective tissue and smooth muscle are present in the intima. It is

the thickest of the three layers. The smooth muscle cells are arranged in a spiral around the long axis of the vessel. They secrete elastin in the form of sheets, or lamellae, which are fenestrated to facilitate diffusion. These lamellae, and the large size of the media, are the most striking histological feature of elastic arteries. In addition to elastin, the smooth muscle cells of the media secrete reticular and fine collagen fibers and proteoglycans (all not identifiable). No fibroblasts are present.

Tunica adventitia:This is a relatively thin connective tissue layer. Fibroblasts arethe

predominant cell type, and many macrophages are also present. Collagen fibres predominate and elastic fibres (not lamellae) are also present. Blood vessels supplying the adventitia and outer media are also present, these are called vasa vasorum ("vessels of the vessels").


{Media

Adventitia

{Intima

{

Vasa vasorumSmooth muscle cells

Elastic laminae

Endothelium

ELASTIC ARTERY

Collagen


Elastic artery Aorta

el = elastic lamellaeend = endothelial cell nuclein = smooth muscle cell nucleiTA = tunica adventitiaTI = tunica intimaTM = tunica media


Elastic fibers

Lumen

Endothelium

vasa vasorum


Muscular Arteries and Arterioles

Muscular arteries – distal to elastic arteries; deliver blood to body organs Have thick tunica media with more smooth muscle

and less elastic tissue Active in vasoconstriction

Arterioles – smallest arteries; lead to capillary beds Control flow into capillary beds via vasodilation and

constriction


Medium Arteries: Tunica intima:

The tunica intima is thinner than in large arteries, there are fewer smooth muscle cells and less elastic tissue. The outermost part of the intima is defined by a very prominent internal

elastic membrane (not obscured by elastic lamellae as in large arteries). The basement membrane of the endothelium may rest directly on the

internal elastic membrane, or be separated by a sub-endothelial layer of CT.

Tunica media: Smooth muscle cells predominate in the tunica media, and little elastic

material is present. As in large arteries, no fibroblasts are present. lastic fibres (few), collagen, and ground substance are produced by the

smooth muscle cells. In tissue preparation, the internal elastic membrane of the intima appears

wavy due to the contraction of the smooth muscle of the media. Tunica adventitia:

The main constituent of the adventitia is collagen fibres, secreted by fibroblasts. Elastic fibres are also present, a concentration of such fibres at the inner boundary of the adventitia is called the external elastic membrane.

The external elastic membrane is not as prominent as the internal, and as arteries get smaller (see small arteries, below) disappears much earlier.

The tunica adventita is relatively larger than in elastic arteries,


VESSEL-WALL LAYERING

Tunica INTIMA

Endothelium

Lamina propria

Tunica MEDIA

Tunica ADVENTITIA

}Internal elastic lamina

This layering scheme works well for muscular arteries, but becomes strained when applied to other vessels, e.g., in regions of the vena cava there may be no T media; in arterioles, the intima has a convincing identity only in TEM

External elastic lamina


Muscular Artery and Vein

vein

artery

tunica externa thickest layer of

vein

A

V


Artery and vein


Endothelial cell

Smallest ARTERIOLE

Smallest arteriole, is a capillary with smooth muscle cells wrapped around it, with modifications to the endothelial cells - less transport, more interaction with SMCs.

Smooth muscle cell SMC/ VSMC

Reticular fibers

Contraction regulates flow by need

For fast flow & non-stick, until clotting is needed

Controls passage through the wall

Helps control blood flow

Mechanical support


Arteriole

arteriole (center) and an accompanying venule (right).


arteriole and an accompanying venule.


Capillaries

Capillaries are the smallest blood vessels Walls consisting of a thin tunica interna, one cell

thick Allow only a single RBC to pass at a time Pericytes on the outer surface stabilize their walls

There are three structural types of capillaries: continuous, fenestrated, and sinusoids


Capillary Structure

Figure 21.4


Capillary


Continuous Capillaries

Continuous capillaries are abundant in the skin and muscles, and have: Endothelial cells that provide an uninterrupted lining Adjacent cells that are held together with tight

junctions Intercellular clefts of unjoined membranes that allow

the passage of fluids Continuous capillaries of the brain:

Have tight junctions completely around the endothelium

Constitute the blood-brain barrier


Continuous Capillaries


Fenestrated Capillaries

Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys)

Characterized by: An endothelium riddled with pores (fenestrations) Greater permeability to solutes and fluids than

other capillaries


Fenestrated Capillaries


Sinusoids

Highly modified, leaky, fenestrated capillaries with large lumens

Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs

Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues

Blood flows sluggishly, allowing for modification in various ways


Sinusoids


An interconnected network of vessels consisting of Collateral arteries feeding an arteriole Metarterioles Arteriovenous anastomoses Capillaries Venules

Capillary Beds


Capillary Beds

A microcirculation of interwoven networks of capillaries, consisting of: Vascular shunts – metarteriole–thoroughfare

channel connecting an arteriole directly with a postcapillary venule

True capillaries – 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed


Capillary Beds


The Organization of a Capillary Bed


Capillary

Capillary


Collect blood from all tissues and organs and return it to the heart

Are classified according to size Venules Medium-sized veins Large veins

Veins


Vein

tunica externatunica media

tunica interna


{ Adventitia

{Intima

Bundles of longitudinal smooth muscle

Occasional circular SMC Numerous elastic fibers

LARGE VEIN Details


Large vien


Muscular vein


Medium size veins The tunica Externa

consists of the endothelium and a thin subendothelial layer with smooth muscle cells among the connective tissue elements. A thin internal elastic membrane may or may not be present. (If present, it is not nearly as prominent as in arteries).

The tunica media is much thinner relative to that of an artery, and consists mostly of circularly

arranged smooth muscle but also contains collagen fibres. The tunicas intima and media therefore tend to be less distinct from one

another than is the case in arteries. The tunica adventitia is usually thicker than the media and is made up

mostly of collagen fibres. It may contain longitudinally oriented smooth muscle bundles. (Remember

gradations between the vessels of different sizes are continuous.)


Medium size vein

Medium-sized vein with a much less compact muscle layer than you saw in the preceding arteries. The tunica media is indicated by bar "a". Bar "b" = adventitia, which is at least as wide as the media, and often even wider. There is no evident inner elastic membrane. (Blood in the lumen stains red here.) To the right, compare sizes and walls of one small artery (d) and two very small veins (c) and (e


The Function of Valves in the Venous System Venules and medium-sized veins contain valves Prevent backflow of blood


A Comparison of a Typical Artery and a Typical Vein

Heart & Blood Vessels Histology By Dr. Nabil, Khouri

Documents

Transcript of Heart & Blood Vessels Histology By Dr. Nabil, Khouri