At the End of the Lecture, Student Should Be Knowing;

HISTOLOGY OFCARDIAC MUSCLE

Learning Objectives

At the end of the lecture, student should be knowing;

•  The basic structure of blood circulatory system

•  Different layers of the wall of heart

•  Characteristics of cardiac muscle cell

•  Structure of Intercalated disc

•  Junctional specializations making up the intercalated disk.

•  Identification of different microscopic views of cardiac muscle and its ultrastructures.

Blood Circulatory System

•  Comprises of a central muscular pump, the heart.

•  and a circuit of vessels through which the blood flows.

•  The blood circulatory system has three kinds of vessels

arteries

veins

capillaries

Circulatory System-Layout

•  The whole circulatory system has a common basic structure.

•  Composed of three layers

•  Tunica intima

•  Tunica media

•  Tunica adventitia

Heart

•  Three basic layers

•  Endocardium

(equivalent to tunica intima)

•  Myocardium

(equivalent to tunica media)

•  Epicardium /

Visceral pericardium

(equivalent to tunica adventitia)

Myocardium

•  Consist of cardiac muscle

•  Thickness varies

•  Thinnest in atria and thickest in left ventricle.

•  Two layers

•  Outer layer ; Superficial

•  Deeper layer ; Deeper layer

Cardiac muscle

•  Found only in the myocardium.

•  Involuntary but striated.

•  Cells are aligned in the form of chains

•  branched.

•  Fibers form an interconnecting network joined to each other by specialized intercellular junctions called intercalated disks/discs.

•  Endomysium CT containing a rich capillary network.

Cardiac muscle

They show a cross striated banding pattern.

•  Size – 100 µm & 15 µm.

•  Nucleus usually single large oval central

•  Sarcolemma – T- tubes, not regularly arranged, and lie at Z-lines

•  Sarcoplasm - more abundant.

In addition to myofibrils, it contains:

•  Organelles – numerous mitochondria, SPR & GA.

•  Inclusions – glycogen granules, fat droplets, lipofuscin pigment & membrane bound granules

•  Myofibrils course irregularly & frequently branch

•  Sarcosomes occupy 40% or more cytoplasmic volume

•  Sarcoplasmic reticulum - overall plexiform pattern is formed.

•  T- tubes are generally associated with one lateral expansion called “diads”.

•  Fatty acids major fuel of heart.

•  Glycogen granules small amount used in stress.

•  Lipofuscin pigment

•  Membrane-limited granules 0.2 – 0.3 µm diameter. 600/cell in Rt. Atrium, contain high MW precursor of a hormone atrial natriuretic factor (auriculin).

Cardiac muscle

Cardiac muscle

•  Longitudinal section; Under high power

•  Capillaries seen between the fibers.

•  Red blood cells are seen in single file

•  Longitudinal section; Under high power

•  Capillaries seen between the fibers.

•  Red blood cells are seen in single file

Drawing of a section of the heart muscle showing central nuclei, cross-striation, and intercalated disks

Cardiac muscle

Longitudinal section of portions of 2 cardiac muscle cells.

The transversely oriented parts of the intercalated disk

consist of a fascia adherens and numerous desmosomes.

The longitudinal parts (arrows) contain gap junctions.

Mitochondria (M) are numerous. Fibrils of reticular fibers

are seen between the two cells. x18,000.

INTERCALATED DISCS (DISKS)

•  Under L/M, L/S of cardiac muscle

•  shows 0.5 – 1 µm thick darkly staining transverse lines crossing the chains of cardiac cells at irregular intervals the “Intercalated discs”.

•  follows an irregular or step-like course.

•  Two regions can be distinguished:

•  Transverse portion.

•  Lateral portion.

INTERCALATED DISCS (DISKS)

•  Under E/M , Intercalated disks represent specialized intercellular junctions with a complex pattern and a variety of structural characteristics.

Ultra structure of heart muscle

•  In the region of an intercalated disk Contact between cells is accomplished by inter digitation in transverse region

•  contact is broad and flat in the longitudinal plane (LP)

A , A band

I , I band

Z , Z line

Ultra structure of heart muscleJunctional specializations making up the intercalated disk.

•  Fasciae (or Zonula) adherents (A) in the transverse

portions of disk anchor actin filaments of terminal

sarcomere to the plasmalemma.

•  Maculae adherentes, or desmosomes (B),

in the transverse portions of

disk, bind cells together, preventing their separation during contraction cycles.

•  Gap junctions (C)/Nexus

restricted to longitudinal

portions of the disk