BLF 103: Cardiac & Smooth Muscle Histology

• The module is Blood and Body Fluids (BLF) 103
• Safinaz Salah Eldin is the Professor of histology in the Histology Department

Objectives

• Understand the site and organization of cardiac muscle
• Recognize the histological structure (L.M., E.M.) of cardiac muscle fiber
• Identify purkinje fibers and their histological characteristics
• Understand the sites and organization of smooth muscle
• Recognize the histological structure (L.M., E.M.) of smooth muscle fiber.
• Identify regeneration and growth of smooth muscle
• Differentiate between skeletal, cardiac & smooth muscle

Cardiac Muscle

• Located in the myocardium of the heart
• Is involuntary
• Features branching and interconnected muscle fibers surrounded by delicate connective tissue (endomysium) rich in capillaries
• Each muscle fiber comprises numerous separate cells (cardiac myocytes)

Myocardium

• Forms the main bulk of the heart's wall
• Thicker in the wall of the ventricle than atrium
• Attached to the fibrous skeleton of the heart
• The fibrous skeleton of the heart is dense connective tissue between the atria and ventricles
• Is surrounded by the epicardium from the outside (visceral layer of pericardium)
• Is surrounded by the endocardium from the inside

Cardiac Myocyte

• Cylindrical, branched, anastomosing
• Intermediate in size at 15-30 µm in diameter
• 85-120µm in length
• Has a single, oval, and central nucleus
• Acidophilic with less clear striations
• Contains intercalated discs

Cardiac Muscle E.M

• Thinner Sarcolemma
• Wider T Tubule at Z line
• Fewer Myofibrils than skeletal muscle
• Numerous, larger, closely packed cristae in the Mitochondria
• Smooth endoplasmic reticulum is less developed, with only one terminal tubule Diad
• Has perinuclear Golgi region
• Glycogen is concentrated between myofibrils, as well as Lipid droplets
• Lipochrome pigments accumulate perinuclearly with old age

Atrial Granules in Muscle of Atria

• Membrane-bound dense granules (endocrine function)
• Contain atrial natriuretic polypeptide (ANP) affecting urinary secretion of sodium

Intercalated Disc

• Junction sites between the sarcolemma of two adjacent cardiac myocytes where muscle fibers are formed
• Under a Light Microscope (L.M.), they appear as densely stained clear transverse lines at irregular intervals
• They are formed of transverse and lateral regions with a Steplike appearance.
• The transverse component of the disc crosses the cardiac fiber at a right angle
• The lateral component of the disc lies parallel to the myofiber
• At the transverse part are Desmosomes and fascia adherens junctions, binding cardiac myocytes firmly together
• This binding prevents separation during repetitive contraction

Intercalated Disc At the Lateral Part

• Contains gap junctions
• These gap-junctions promote rapid impulse conduction through many cardiac muscle cells simultaneously
• This enables the contraction of many adjacent cells as a unit
• Their position protects cells from contraction forces

Valves of The Heart

• Endocardial folds
• Covered by simple squamous epithelium
• Have a middle supporting dense fibrous C.T.
• Rich in collagen & elastic fibers & macrophages
• Thickened at the base

Cardiac Regeneration

• Cardiac muscle injury results when coronary arteries become occluded and oxygen levels drop
• Cardiac muscle does not have progenitor cells
• Cannot regenerate after injury as in infarction
• Healing occurs via Fibrous scar

Purkinje Muscle Fibers

• Modified highly specialized cardiac muscle fibers.
• Conduct impulse 4-5x faster than ordinary cardiac muscle
• Contain Gap junctions
• Form the A-V bundle & its branches
• Present in the moderator band in the right ventricle.
• Grouped into bundles surrounded by C.T. sheath.
• Larger & paler than ordinary cardiac muscle
• Nucleus is eccentric in position
• No striations due to fewer myofibrils present peripherally
• Sarcoplasm is pale vacuolated due to ++++glycogen
• Do not have intercalated discs.

Smooth Muscle

• Non-striated and involuntary
• Connected via gap junctions
• The fiber is surrounded by thin reticular fibers (endomysium)
• Form sheets, layers, or bundles
• Bundle is surrounded by thin perimysium with B.V. & nerves
• Found the walls of viscera, blood vessels, and erector pilli muscle

Smooth Muscle Under Light Microscope (L.M.)

• Fusiform, non-branched, non-striated
• Small, with an 8 µm diameter
• Length varies from 20 µm in blood vessels up to 500 micron in the pregnant uterus
• Single, central, and oval Nucleus
• Corkscrew appearance during the contraction stage
• Acidophilic

Smooth Muscle E.M.

• Thin sarcolemma surrounded by basal lamina
• Contains Gap junctions
• Dense bodies are located on the inner aspect of the membrane and cytoplasm
• Has no T-tubules or a tubular system
• Has Caveolae (invagination from sarcolemma) instead

Sarcoplasm

• Contains numerous mitochondria, a small Golgi apparatus, free ribosomes, sarcoplasmic reticulum & glycogen

• Myofibrils show irregular arrangement, thus striations do not appear

• Formed of thick myosin & thin actin

• Actin inserts into cytoplasmic and plasmalemma-associated dense bodies

• Dense bodies contain α-actinin

• Correspond to the Z line of striated muscle

• Intermediate filaments desmin, also attached to the dense bodies

Smooth Muscle Contraction

• Controlled by autonomic innervation & hormones
• Affected by oxytocin causing contraction in labor
• Secrets C.T. matrix & fibers as in the wall of B.V.

Regeneration & Growth of Smooth Muscle

• Regeneration after injury occurs by mitosis
• Pericytes around small blood vessels participate in repairing vascular smooth muscle
• Smooth muscle grows in size (hypertrophy) & number (hyperplasia) in the pregnant uterus

Muscle Fiber Type Differences