Histology self-study 3: Muscle and Cartilage

Skeletal muscle diameter and length

Diameter: 10-100µm

Length: 1mm-30cm

Cardiac muscle diameter and length

Diameter: 15-30 micrometres

Length: 85-120 micrometres

Smooth muscle diameter and length

Diameter: 3 - 15µm

Length: 20 - 200 µm

Skeletal muscle structure

long, cylindrical cells

cytoplasm filled with contractile filaments

multinucleated with many peripheral nuclei

exhibit cross-striations

skeletal muscle control?

voluntary control - innervated by motor neurones from the somatic nervous system

respond quickly to stimuli

skeletal muscle stem cell name

Satellite cells

Cardiac muscle structure?

Short branched cells

Form a syncytium

Single, central nucleus (occasionally binucleate)

Intercalated discs join cells end-to-end

Cardiac muscle control?

Involuntary - innervated by the autonomic nervous system

Cardiac muscle pigment?

Lipofuscin - end-stage lysosomes containing undigested material

Cardiac muscle lifespan?

Poor capacity for regeneration

Automatic rhythmic conditions for life

Smooth muscle purpose

Apply pressure to organs (e.g. stomach, intestines, uterus)

Blood vessels

Smooth muscle structure

spindle-shaped cells of various size

single centrally located nucleus

non-striated

retain ability to divide

smooth muscle control

Involuntary control - innervated by the autonomic nervous system

Also under hormonal control

responds slowly to stimuli - capable of long, sustained contractions

What muscle cell is this and how can you tell? (Transverse section)

Skeletal muscle cell

Polygonal cross-sections

Nuclei at periphery

What muscle cell is this and how can you tell? (Longitudinal section)

Skeletal muscle cells

Can see sarcomere striations from thick and thin filaments

Satellite cell nuclei of mostly heterochromatin can be seen on the surface of muscle cells

Bundle of muscle cells

Fascicle

What part of the skeletal muscle is this?

Epimysium - dense connective tissue that surrounds the entire muscle and is usually continuous with a tendon

What are the white-line structures shown in the skeletal muscle?

Endomysium - thin layer connective tissue that surrounds each muscle cell

Capillaries seen at the corners of the muscle cells

What thick tissue layer is seen in the skeletal muscle?

Perimysium - thick layer of connective tissue that surrounds a group of muscle cells to form fascicles.

What type of muscle are the dark muscle fibres and what are the light ones in this skeletal muscle?

Type I - dark, smaller muscle cells that specialise in long, slow contraction

Type II - light, larger muscle cells, specialise in fast contraction (majority of muscle in picture)

What feature is shown in the skeletal muscle?

Muscle spindle - sensory receptors within a muscle that detect change in length of the muscle.

Main composition of tendons

Dense, collagenous connective tissue

What is sarcoplasmic reticulum?

Smooth endoplasmic reticulum but in muscle cells

Thick filament

Myosin arranged as bipolar filaments anchored at the M-line.

Thin filaments

Actin arranged as unipolar filaments anchored at the Z-line.

A band

Dark blue - thick myosin filaments and parts of thin actin filaments where they overlap.

I Band

Light blue - only thin actin filaments, narrows with contraction

Z line

Purple - thin actin filaments anchor to Z lines

H bands

Light blue - only myosin filaments without overlapping actin filaments

M line

cyan - holds myosin filaments together

I meaning

isotropic

A meaning

anisotropic

What 2 internal membrane systems play a role in excitation-contraction coupling?

The sarcoplasmic reticulum

Transverse tubular system (T-tubules)

T system

Tubular invaginations of sarcolemma

Terminal cisternae

Flattened vesicular structures formed by sarcoplasmic reticulum on either side of each T tubule.

Triad

Formed by a pair of terminal cisternae and a T tubule

Describe the steps of skeletal muscle stimulation

Action potentials propagate from the sarcolemma to the interior of the muscle fibre via the T tubules

At the triads, depolarisation of the T tubules triggers the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm surrounding the myofibrils.

Rise in calcium concentration - interaction between myosin motors and actin filaments initiated

Myofibril contraction

What allows for a rapid response during skeletal muscle stimulation?

T tubule and skeletal muscle arrangement reduces diffusion pathway for calcium.

Where is calcium released from during skeletal muscle contraction?

Terminal cisternae

Where is calcium accumulated during skeletal muscle contraction?

Fenestrated network of sarcoplasmic reticulum - largest surface area for calcium pumps.

Red fibre properties

Slow twitch upon stimulation

Sustained contraction

Aerobic metabolism

Numerous mitochondria

Rich in myoglobin

Profuse capillary network

Found in muscle that maintains body posture

White fibre properties

Fast twitch response

Cannot sustain contractions for long periods of time

Anaerobic metabolism

Few mitochondria

Little myoglobin

Abundant in muscle for intense but sporadic contraction

What type of cartilage is in this tendon and how can you tell?

Fibrocartilage

Collagen fibres have irregular arrangement

Chondrocytes round/oval with a clear space surrounding the nuclei

External lamina function

Contractile forces from internal contractile proteins transmitted to external lamina via link proteins

Link proteins on muscle cell membrane

Binds individual cells into single functional mass

Satellite cells function

Enter mitosis after damage to muscle

Fuse to form differentiated muscle fibres

Muscle fibres formed after damage often have nuclei in the centre of the fibre rather than at the periphery

What type of muscle is this and how do you know? (transverse section)

Cardiac muscle cells have rounded cross-sections

Centrally located nucleus

What type of muscle is this and how do you know? (longitudinal section)

Cardiac muscle - cells are joined end to end and often branched

Intercalated discs perpendicular to direction of muscle fibres

Lipofuscin pigment can be seen near nucleus of some cells.

What is the purpose of intercalated discs?

Enable synchronisation and contraction of cardiac fibres by providing both mechanical and electrical coupling.

3 types of membrane to membrane contact

Adherens junctions

Desmosomes

Gap junction

Adherens junctions

Contain cell adhesion molecule cadherin

Anchor actin filaments at the end of terminal sarcomeres

Allows contractile forces to be transmitted from cell to cell

Desmosomes

Provide anchorage for intermediate filaments in cytoskeleton

Prevent cardiac muscle detaching during contraction

Gap junctions

intercellular channels

allow ions and small molecules to pass from cell to cell

structural basis for electrical transmission between cells

Diad (or dyad)

Single terminal sac of sarcoplasmic reticulum closely apposed to T tubule.

Purkinjie fibres

conducting system of modified cardiac muscle fibres

larger than cardiac muscle cells

sometimes binucleated

contain numerous mitochondria

few, irregularly arranged myofibrils

lack T tubules and intercalated discs

connect via gap junctions and desmosomes

cytoplasm rich in glycogen

Unitary (visceral) smooth muscle

Individual muscle fibres linked by gap junctions

act as a unit

predominantly in walls of hollow viscera

Multi-unit smooth muscle

individual units without gap junctions

unlinked fibres - each contract independently

found where fine graded contractions occur, e.g. iris

Which muscle is this and how can you tell? (longitudinal section)

relaxed smooth muscle - nuclei elongated with rounded ends

when contracted nuclei spiral, kink or twist

cytoplasm pink, non-striated, little detail

What muscle is this and how can you tell? (transverse section)

smooth muscle - individual cells vary in diameter

cross sections have centrally located nuclei surrounded by unstained region

Caveolae

small invaginations of the plasma membrane involved in the regulation of ion channels and calcium signalling.

Dense bodies

Points of attachment for actin filaments and intermediate filaments of the cytoskeleton.

found in cytoplasm and areas adjacent to the plasma membrane

What forms cartilage

chondroblasts, chondrocytes, collagen fibres, hydrated proteoglycans.

Collagen in cartilage

provides strength to resist stretch

proteoglycan trapped water in cartilage

generates turgor pressure that allows cartilage to spring back to shape following deformation

Components of cartilage

perichondrium

chondroblasts

chondrocytes

perichondrium

connective tissue sheath that surrounds cartilage

vascularised

rich in collagen

inner layer contains fibroblasts/mesenchyme that can differentiate to form chondroblasts

chondroblasts

immature cartilage cells

secrete extracellular matrix

not rigidly embedded in matrix

chondrocytes

mature cartilage cells

embedded in extracellular matrix

reside in small spaces within the matrix called lacunae

may be more than one cell per lacuna

chondrocytes in hyaline cartilage grouped together called isogenic groups

3 types of cartilage

hyaline, elastic, fibrocartilage

hyaline cartilage

type II collagen and chondromucoprotein

found in nose, trachea, where ribs join sternum and at epiphyseal plate of long bones

contains no blood vessels or nerves

elastic cartilage

matrix contains abundant branching and anastomosing elastic fibres

type II collagen

found in external ear and epiglottis

contains no blood vessels or nerves

fibrocartilage

large amounts of well-ordered type I collagen fibres

found where tendons attach to bone, intervertebral discs, within pelvic pubic symphysis

mixture of regular dense connective tissue and hyaline cartilage

chondrocytes dispersed singularly, in columns or in isogenous groups

there is no perichondrium