Histology Of The Heart

Question 1

Pericardium

Answer 1

The heart lies in a fibrous sack lined by a layer of mesothelial cells called the pericardium

The cells lining the inner surface of this fibrous sack and the outermost layer of heart are also known as the pericardium

On the heart itself, the pericardium comprises a single layer of flattened or squamous mesothelial cells that rest on a thin fibrous layer of connective tissue

Question 2

Visceral pericardium

Answer 2

Refers to the layer on the outer surface of the heart

Question 3

Parietal pericardium

Answer 3

Refers to the layer of cells on the inner surface of the fibrous sack containing the heart

Question 4

Epicardium

Answer 4

Immediately beneath the pericardium is a layer of white adipose connective tissue known as the epicardium- present between the pericardium and myocardium

Contains a mixture of vessels (coronary arteries) and nerves

Question 5

Myocardium

Answer 5

Thickest layer of the heart

This is formed from specialised cardiac muscle

Cardiac muscle shares some features of smooth muscle and some features of skeletal muscle, and some unique features of its own

Question 6

Cardiac muscle

Answer 6

In common with skeletal muscle, cardiac muscle is striated because the myofibrils within the muscle cells are all in register

Like smooth muscle the nuclei of cardiac muscle are centrally located and there is a single nucleus for each cardiac muscle cell

Unique features include: a branching nature of the fibres and the prescience of intercalated discs (can be seen on H&E stain sections as pale areas between adjacent mayocytes)

Question 7

intercalated discs

Answer 7

Specialised connections between the myocytes which join them together to form but branching chains

They connect the actin filaments of adjacent myocytes

Not seen in any other form of muscle

Question 8

Intercalated discs contain

Answer 8

Gap junctions

Adhering junctions

Desmosomes

Question 9

Between cardiac myocytes…

Answer 9

There is loose fibrous connective tissue (endomysium)

This may contain a very small number of lymphocytes

Question 10

Where are the largest myocytes found?

Answer 10

In the wall of the left ventricle

Question 11

Where are the small myocytes?

Answer 11

The myocytes of the atria have to pump against a much lower pressure and so are much smaller than those in the ventricles

Question 12

Heart is am endocrine organ

Answer 12

Secretes the hormone atrial natriuretic peptide as myocardium contains perinuclear neuroendocrine granules

This can be seen ultra-structurally in the myocardium of the atria in the presence of small granules located near to the nuclei of the myocytes

Question 13

Conducting system

Answer 13

Initiated in the SAN, across the atrial myocardium and passes to the AVN

Passes to the bundle of His and then through purkinje fibres that are subendocardial

Question 14

Where can purkinje fibres be found?

Answer 14

In the ventricles immediately below the endocardium but difficult to see in H&E stained sections

These specialised myocytes contain glycogen- rich cytoplasm that’s why appear pale and overlooked in H&E stain

Question 15

Endocardium

Answer 15

Innermost layer of the heart

Thin layer of fibrous connective tissue lined on the its innermost surface by a single flare layer of endothelial cells

Endothelial cells- simple squamous epithelium

Question 16

Valves

Answer 16

Covered by endothelium

The valves of the heart are all connected to the central fibrous body which forms the skeleton of the heart and they comprise 3 separate layers

Both sides of the cardiac valve are covered by vascular endothelial cells

The vascular endothelial cells of the heart are the same as those that line the inside of blood vessels