Blood vessel histology

Capillary

Just has tunica intima; small vessels (5-10 micrometers in diameter)

Site of nutrient and gas exchange

Single layer of endothelium joined by tight junctions with basement membrane

Pericyte

Occasionally found in capillaries

Outside the endothelium

Progenitor cells with contractile ability

May form tumors

Continuous, fenestrated, sinusoid

Three types of capillaries

Endothelial cells

Line walls of CV system (even heart)

Present in every vessel including capillaries

Serves as a barrier

Produces basement membrane (further regulation of permeability)

Vasoactive factors

Growth factors: angiogenesis

Simple squamous

Type of cells of endothelial cells

Angiogenesis

Growth of new vessels when needed

Tunica intima

Endothelium, basement membrane, sub-epithelial connective tissue, +/- internal elastic lamina, +/- valves

Tunica media

Smooth muscle layer, collagen, +/- elastin, +/- internal elastic lamina

Tunica adventitia, tunica serosa

Types of outer layers

Tunica adventitia

Loose connective tissue (collagen and/or elastin, +/- blood vessels, +/- nerves)

Tunica serosa

Only if freely suspended from dorsal wall of peritoneum

Outer layer of mesothelium

Continuous capillaries

Most prevalent type of capillary —> Found in all tissues

Continuous tube

Tight junctions

Numerous vessels can be found in most tissues

Fenestrated capillaries

Type of capillary

Fenestrae (windows/pores)

Diaphragm (thin membrane) is found in most of them but absent in kidneys

Commonly found in tissues with substantial fluid transport (kidneys, GI tract, endocrine glands)

Sinusoids

Type of capillary

Lack uniformity in diameter —> Slows blood flow

More permeable

Basement membrane absent

Large pores and intercellular openings

Allow maximal exchange between blood and adjacent parenchyma

Found in liver, spleen, bone marrow, nasal cavity, some endocrine glands

Arteries

Round shape

Tunica intima: Distinct internal elastic lamina

Tunica media: Prominent elastin fibers and/or smooth muscle layers

Named after tunica media

Amount of smooth muscle in tunica media

What is the best criterion for distinguishing between arterioles and venules?

Veins

Collapsed or misshapen

Tunica intima: Valves are present

Tunica media: Fewer layers of smooth muscle, and elastin fibers are not always present

Usually named after the artery it accompanies

HEVs

Specialized blood vessels; special type of venule

Single layer of plump (cuboidal) endothelium

Incomplete basement membrane

Incomplete tight junctions

Found in lymphoid tissue (thymus): Allows WBC to exit tissue and enter circulation

Thin tunica adventitia: Connective tissue only, and can be absent in some species

Blood brain barrier capillaries

Components: Endothelial cells, extensive tight junctions, basement membrane, pericytes, astrocyte end-feet

Acts as a diffusion barrier for blood-brain exchange

Continuous capillaries (not fenestrated)

Low levels of transcytosis

Extensive transporters

Lymphatics

Drain excess fluid from connective tissue back to circulatory system

Absent in some tissues: eyes, bone marrow, CNS, cartilage

Architecture similar to small veins or venules

Contain valves

Thin walls, only endothelium

Typically collapsed or misshapen

Smooth muscle in larger vessels

Filled with plasma cells and WBC; no erythrocytes

Form lymph lacteal in GI tract