Lecture #18: Histology of the Heart

What are the three layers of the heart wall?

Endocardium, myocardium, and epicardium. The endocardium lines the chambers, the myocardium contains the contractile cardiac myocytes, and the epicardium forms the outer surface of the heart.

What structures are found in the endocardium?

The endocardium contains endothelium, collagen, fibroblasts, smooth muscle, nerves, and Purkinje cells.

What structures are found in the myocardium?

The myocardium contains cardiac myocytes, capillaries, blood vessels, small nerves, lymphatics, and minimal connective tissue located primarily around vessels and between myocytes.

How does myocardial thickness vary among the heart chambers?

Myocardial thickness follows LV > RV > atria. The left ventricle has the thickest myocardium because it must generate the pressure required for systemic circulation.

What structures are found in the epicardium?

The epicardium contains a mesothelial surface with underlying connective tissue, adipose tissue, coronary vessels, nerves, and autonomic ganglia.

What is the histologic composition of the epicardial surface?

The outer surface is simple squamous epithelium called mesothelium with underlying connective tissue containing elastin and collagen.

What is an important function of epicardial adipose tissue?

Epicardial adipose tissue acts as a cushion within the pericardial cavity and surrounds structures such as coronary vessels and nerves.

Where are the blood vessels and nerves supplying the heart located?

Blood vessels and nerves supplying the heart are present in the epicardium; autonomic structures include postganglionic parasympathetic ganglia.

How does atrial endocardium differ from ventricular endocardium?

The lecture histology images show that atrial endocardium is substantially thicker than ventricular endocardium, representing an important regional variation in heart-wall morphology.

What lines the ventricular cavity?

The ventricular cavity is lined by endothelium. Beneath it is a collagen-rich subendothelial/endocardial layer that can also contain nerves and Purkinje cells.

Where are Purkinje cells found within the heart wall?

Purkinje cells are located in the subendocardial region and form the terminal subendothelial branches of the cardiac conduction system.

What are the major histologic characteristics of cardiac myocytes?

Cardiac myocytes are specialized striated muscle cells with complex approximately cylindrical shapes, central nuclei, abundant mitochondria, and connections to neighboring myocytes through intercalated discs.

Where does branching primarily occur in cardiac myocytes?

Branching occurs primarily at the ends of cardiac myocytes at the intercalated discs. A given myocyte connects with many other myocytes; Y-shaped myocytes are rare.

How are cardiac myocytes oriented across the ventricular wall?

Myocyte direction changes across the wall somewhat like a Japanese fan, rotating approximately 180° from the endomyocardium to the epimyocardium.

What is the approximate capillary-to-myocyte ratio in the adult heart?

The capillary-to-myocyte ratio is approximately 1:1 in adults, including humans, reflecting the myocardium's extensive vascularization.

How much of a cardiac myocyte's volume is occupied by mitochondria?

Mitochondria occupy approximately one-third of cardiac myocyte volume; the lecture also describes cardiac muscle as containing about 25–35% mitochondria.

Why do cardiac myocytes contain so many mitochondria?

Cardiac muscle is highly dependent on aerobic metabolism and therefore requires abundant mitochondria to continuously generate the energy needed for contraction.

How does mitochondrial content compare between cardiac and skeletal muscle?

Cardiac muscle contains approximately 25–35% mitochondria by cell volume compared with approximately 3–5% in skeletal muscle, reflecting cardiac muscle's greater dependence on aerobic metabolism.

What is the primary function of intercalated discs?

Intercalated discs join adjacent cardiac myocytes and help synchronize contraction by providing electrical coupling with relatively unrestricted ionic passage between adjoining cells.

How can intercalated discs be recognized on routine H&E?

Intercalated discs appear as darker transverse lines crossing cardiac muscle fibers at the boundaries between adjacent cardiac myocytes.

What happens to cardiac myocyte proliferation after birth?

Cardiac myocyte proliferation ceases shortly after birth. Continued heart growth occurs primarily through myocyte hypertrophy rather than proliferation; non-myocytes continue to proliferate.

What is the histologic effect of pressure-overload cardiac hypertrophy?

Pressure overload increases cardiac myocyte diameter, producing wall thickening in areas of viable myocardium.

What cellular change produces cardiac chamber dilation?

Chamber dilation results from increased myocyte length caused by addition of sarcomeres in series.

How does hypertension alter myocardial capillary density?

Hypertension causes myocytes to enlarge and push capillaries apart, decreasing LV capillary density from approximately 4,000 capillaries/mm² normally to approximately 2,500 capillaries/mm².

Why can hypertension decrease exercise capacity and cause chest pain on exertion?

Hypertrophied myocardium has fewer capillaries and must perfuse a greater fraction of its remaining capillaries at rest to meet increased energy needs. This decreases capillary reserve, reducing exercise capacity and increasing chest pain on exertion.

What is myocardial interstitial fibrosis?

Interstitial fibrosis is collagen deposition between cardiac myocytes. It contributes to ventricular stiffness and impaired ventricular relaxation.

What type of fibrosis occurs after myocardial infarction?

Myocardial infarction causes myocyte necrosis followed by replacement fibrosis, producing a large fibrotic region where viable myocytes were previously located.

What type of myocardial fibrosis is associated with hypertension?

Hypertension produces perivascular fibrosis around arteries, which contributes to vessel stiffness.

What are papillary muscles histologically?

Papillary muscles are fingerlike projections from the inner heart surface containing a core of parallel cardiac myocytes and capillaries and covered externally by endocardium.

What connects papillary muscles to the valve apparatus?

Papillary muscles connect to chordae tendineae, which consist of dense connective tissue covered by endothelium.

What is the histologic structure of cardiac valves?

Cardiac valves have a dense connective tissue core extending from the fibrous ring and are lined on both sides by simple squamous epithelium called endothelium.

Are cardiac valves vascularized?

No. Cardiac valves are avascular and receive nutrition from blood circulating within the heart chambers.

What is the histologic composition of chordae tendineae?

Chordae tendineae consist of a dense collagen/connective tissue core covered by endothelium and have no direct vascular blood supply.

What is the cardiac skeleton?

The cardiac skeleton is dense fibrous connective tissue surrounding the four major cardiac valves, extending between the valves as fibrous trigones and into the membranous upper interventricular septum.

What structures make up the cardiac skeleton?

The cardiac skeleton includes dense fibrous rings surrounding the four major valves, the fibrous trigones between valves, and the membranous portion of the upper interventricular septum.

How does the cardiac skeleton structurally support the valves?

Its dense fibrous connective tissue forms strong rings around the valves and provides the connective-tissue framework from which the dense connective tissue cores of the valve leaflets extend.

How is the cardiac skeleton related to electrical conduction?

The fibrous skeleton separates atrial from ventricular myocardium; the cardiac impulse crosses this fibrous skeleton through the AV conduction pathway via the Bundle of His.

Where is the SA node located?

The sinoatrial node is a group of specialized cardiac muscle cells located near the junction of the superior vena cava and right atrium.

What is the function of the SA node?

The SA node is the pacemaker of the heart and normally generates electrical impulses at approximately 60–100 impulses per minute.

What is the conduction pathway from the SA node to the ventricles?

SA node → atrial cardiac muscle and internodal tracts → AV node → Bundle of His across the fibrous skeleton → right and left bundle branches → Purkinje fibers.

What types of cells form the cardiac conduction system?

Nodal cells, the AV bundle, bundle branches, and Purkinje fibers are modified cardiac muscle cells specialized for electrical impulse conduction.

How do SA and AV nodal cells compare in size with working cardiac myocytes?

Cells of the SA and AV nodes are smaller than normal working cardiac myocytes.

How do Purkinje fibers and bundle cells compare in size with working myocytes?

Purkinje fibers, AV bundle cells, and bundle branch cells are larger than normal working cardiac myocytes.

What histologic features distinguish conduction cells from working cardiac myocytes?

Conduction cells contain fewer myofibrils, few or no T-tubules, and abundant glycogen compared with regular working cardiac myocytes.

How quickly does the cardiac conducting system conduct compared with regular cardiac muscle?

Parts of the cardiac conducting system convey impulses approximately 4 times faster than regular cardiac muscle fibers.

What is the histologic structure of the pericardium?

The pericardium consists of dense fibrous connective tissue lined by mesothelium on its inner surface.

Why can acute accumulation of pericardial fluid cause cardiac tamponade?

The dense fibrous pericardium does not acutely expand or stretch. Rapid accumulation of fluid or blood in the pericardial cavity therefore compresses the heart and restricts cardiac function, producing cardiac tamponade.

How does the pericardium respond differently to chronic cardiac hypertrophy than to acute fluid accumulation?

The pericardium does not expand significantly during acute fluid accumulation, but with chronic cardiac hypertrophy it can slowly expand to accommodate the enlarged heart.