Cardiovascular Histology

• ALL components of the CV system have 3 layers
• 1. Tunica intima (inner, endocardium)
• 2. Tunica media (middle, myocardium)
• 3. Tunica adventitia (outer, epicardium)

• Lumenal (inner) layer
• Endothelium (simple squam.) contacts blood
• Subendothelium (loose CT / some smooth m. cells)

Longitudinally!

endothelium / subendothelial elements arranged on the axis of blood flow

• 1. Maintains selective permeability barrier.
• 2. Maintains non-thrombogenic barrier.
• 3. Modulates blood flow and vascular resistance.
• 4. Regulates cell growth.
• 5. Regulates immune response.
• 6. Maintains ECM.
• 7. Involved in lipoprotein metabolism

• Most prominent layer in arteries
• - Smooth muscle / Elastic membranes (concentric / relative)
• - Reticular fibers (type III collagen)
• - Proteoglycans

• Elements of media are oriented concentrically (spiral)
• (smooth muscle / elastic membranes)

ECM made by smooth muscle cells

Elastic lamellae (type III reticular fibers / proteoglycans)

Found in the tunica media

Elastin rebound maintains BP in diastole (relaxation)

• Internal elastic lamina
• (both; prominent in muscular arteries)

• External elastic lamina
• (muscular arteries)

• Most prominent layers in veins
• Connective tissue sheath
• - mainly type I collagen
• - elastic fibers
• - smooth muscle
• Vasomotor nervi vascularis
• - innervated smooth m. of media

large vessels may have vasa vasorum

Vasa vasorum found in t. adventitia of large blood vessels

t. adventitia is oriented longitudinally

• 1. Endocardium
• 2. Myocardium
• 3. Epicardium
• 4. Conducting system of specialized cardiac m. cells

• Innermost heart layer (analog = t. intima)
• Endothelial lining
• Subendocardium

Underlying layer of the endocardium

• Composed of loose CT / elastic fibers / some smooth m. cells
• Contains small veins / nerves

In some locations, also contains Purkinje fibers

• Specialized cardiac muscle cells (subendocardium)
• - larger than reg. cardiac m. cells
• - fewer myofibrils (stains lighter)
• - central nucleus (often binucleated) surrounded by glycogen (light area)

• - Impulse initiated at SA node (pacemaker)
• - Travels to AV node
• - AV bundle (of His) sends impulse to L/R bundle branches to apex of heart.

• Middle heart layer (analog = t. media)
• - Thicker in ventricles (inner circular / outer spiral layers)
• - Cardiac m. is anchored to cardiac skeleton (dense fibrous CT)
• - Cardiac m is postmitotic

• - Lots of inflammatory cells
• - Necrotic tissue

With time, inflammatory cells increase in number / lots of scar tissue

• Outermost heart layer (analog = T. Adventitia)
• - Thick layer of loose CT / adipose
• - Contains neurovasculature
• - Outermost layer = mesothelium (simple. squam)
• - forms visceral serous pericardium

• 1. Right AV (tricuspid)
• 2. Left AV (mitral)
• 3. Pulmonic
• 4. Aortic

• Tunica Intima
• - endothelial cells rest on thick subendothelium
• - longitudinal oriented CT
• - internal elastic lamina not clearly defined
• Tunica Media
• - concentric lamellae of elastin (40-70)
• - some smooth muscle
• Tunica Adventitia
• - thin, compared to vessel
• - vasa vasorum, extend into t. media

• Tunica Intima
• - endothelial cells rest on subendothelium (some smooth m.)
• - longitudinal orientation
• - internal elastic lamina, prominent identifying feature
• Tunica Media
• - concentric smooth muscle (3-40)
• - some elastin (special stain required)
• Tunica Adventitia
• - external elastic lamina
• - longitudinal oriented fibroblasts / collagen / elastic fibers
• - vasa vasorum / lymphatics

• Tunica Intima
• - endothelial cells rest on thin subendothelium
• - internal elastic lamina not usually visible
• Tunica Media
• - concentric smooth muscle (< 3)
• Tunica Adventitia
• - very thin

• - Continuous endothelium resting on basal lamina
• - Cells joined by tight junctions (zonula occludens)
• - Found in CNS / PNS, muscle, CT, exocine glands

• Surrounding basal lamina of continuous capillaries is layer of astrocytic endfeet. Endfeet induce formation of blood-brain barrier
• - endothelial tight junctions
• - paucity of pinocytotic vesicles

• - Continuous endothelium interrupted by pores (fenestrae)
• - Pores may have thin diaphragm over them (except renal glomeruli)

• Endocrine = fenestrated
• Exocrine = continuous

• - Specialized w/ relatively large lumens
• - Many open fenestrations
• - Absence of continuous basal lamina
• - Macrophages present along walls

Found in liver / lymphoid / hematopoietic organs

• - small, postcapillary vessels
• - paired w/ arterioles
• - very thin walls
• - functional characteristics of capillaries (exchange vessels)

• Tunica Intima
• - very thin
• - valves formed by subendothelial CT (not in large veins)
• - valves have covering of endothelium

• Tunica Media = poorly developed
• Tunica Adventitia = very well developed

• Artery = prominent media
• Vein = prominent adventitia

• Large Artery
• - thick intima
• - thick media
• - underdeveloped adventitia

• Large Vein
• - thick intima
• - thin media
• - well developed adventitia w/ longitudinal bundles of smooth m.

• - very thin-walled vessels
• - large irregular lumens (some valves)
• - larger lymphatics similar to veins w/o well defined tunics

Should not see RBC's in lymphatics