MUSCULAR TISSUE

Objectives:Objectives:

By the end of this lecture you should be By the end of this lecture you should be able to:able to:Identify and describe the histological Identify and describe the histological structure of the three types of muscle structure of the three types of muscle cells and list the differences between cells and list the differences between them.them.

MUSCULAR TISSUEMUSCULAR TISSUE

MUSCULAR TISSUEMUSCULAR TISSUE Made of elongated Made of elongated

muscle cells (fibers).muscle cells (fibers). 3 types of muscles 3 types of muscles

(muscle fibers):(muscle fibers):1-1- Skeletal:Skeletal: striated, striated,

voluntary.voluntary.

2-2- Cardiac:Cardiac: striated, striated, involuntary.involuntary.

3-3- Smooth:Smooth: non- non-striated, involuntary.striated, involuntary.

SKELETAL MUSCLESKELETAL MUSCLE The whole muscle The whole muscle

is covered by a is covered by a C.T. covering, the C.T. covering, the epimysiumepimysium..

Consists of parallel Consists of parallel skeletal muscle skeletal muscle fibersfibers, arranged in , arranged in bundles, separated bundles, separated by C.T. septa, the by C.T. septa, the perimysiumperimysium..

The individual fibers The individual fibers are separated by are separated by C.T., C.T., endomysiumendomysium,,

Skeletal Muscle FibersSkeletal Muscle Fibers L.M. PictureL.M. Picture::

– CylindricalCylindrical in shape. in shape.

– Non-branchedNon-branched..

– Covered by a clear cell Covered by a clear cell membrane, the membrane, the sarcolemma.sarcolemma.

– MultinucleatedMultinucleated: nuclei are : nuclei are multiple and are peripherally multiple and are peripherally located (close to the located (close to the sarcolemma).sarcolemma).

– CytoplasmCytoplasm ((sarcoplasmsarcoplasm)) is is acidophilic and shows clear acidophilic and shows clear transverse striationstransverse striations..

Skeletal Muscle FibersSkeletal Muscle Fibers

E.M. PictureE.M. Picture::

Sarcoplasm contains:Sarcoplasm contains:

– Parallel Parallel myofibrilsmyofibrils..

– Numerous Numerous mitochondriamitochondria, , arranged in rows between arranged in rows between the myofibrils.the myofibrils.

– Well developed smooth Well developed smooth endoplasmic reticulum endoplasmic reticulum ((sarcoplasmic reticulumsarcoplasmic reticulum).).

– Myoglobin pigment.Myoglobin pigment.

– Glycogen.Glycogen.

Skeletal Muscle FibersSkeletal Muscle Fibers E.M. Picture of E.M. Picture of

MyofibrilsMyofibrils::

– Contractile threads Contractile threads ((organellesorganelles), arranged ), arranged longitudinally in the longitudinally in the sarcoplasm.sarcoplasm.

– Each myofibril shows Each myofibril shows alternating dark (alternating dark (AA) and light ) and light bands (bands (II).).

– The A band shows a pale The A band shows a pale area in the middle (area in the middle (H bandH band) ) which is divided by a dark which is divided by a dark line (line (M lineM line).).

– The (I) band shows a dark The (I) band shows a dark line in the middle (line in the middle (Z line)Z line)..

Skeletal Muscle FibersSkeletal Muscle Fibers E.M. Picture of MyofibrilsE.M. Picture of Myofibrils::

– The The sarcomeresarcomere is the is the distance between 2 distance between 2 successive Z lines.successive Z lines.

– The myofibrils are formed The myofibrils are formed of myofilaments (thick of myofilaments (thick myosin and thin actin).myosin and thin actin).

– The (A) band is formed of The (A) band is formed of myosinmyosin myofilaments myofilaments mainly and the terminal mainly and the terminal ends of actin myofilaments.ends of actin myofilaments.

– The (I) band is formed of The (I) band is formed of actinactin myofilaments. myofilaments.

Sarcomere

AMyosinActin

II

ZZ

AAAAA

Skeletal Muscle FibersSkeletal Muscle Fibers The TRIAD tubular The TRIAD tubular

systemsystem::– The sarcolemma sends The sarcolemma sends

transverse invaginations into transverse invaginations into the sarcoplasm, the the sarcoplasm, the T-tubulesT-tubules. . They form collars around the They form collars around the myofibrils at the level of themyofibrils at the level of theA - I junctions.A - I junctions.

– The SR forms transverse The SR forms transverse wider cisternae (wider cisternae (terminal terminal cisternaecisternae) on either side of the ) on either side of the T-tubule.T-tubule.

– The 2 terminal cisternae of the The 2 terminal cisternae of the SR and the T-tubule in-SR and the T-tubule in-between them form the triad between them form the triad tubular system, which plays an tubular system, which plays an important role during muscle important role during muscle contraction.contraction.

CARDIAC MUSCLECARDIAC MUSCLE Found in the myocardium.Found in the myocardium. Striated and involuntary.Striated and involuntary. L.M. Picture of Cardiac L.M. Picture of Cardiac

Muscle FibersMuscle Fibers::– CylindricalCylindrical in shape. in shape.– IntermediateIntermediate in diameter between in diameter between

skeletal and smooth muscle skeletal and smooth muscle fibers.fibers.

– BranchBranch and anastomose. and anastomose.– Covered by a thin Covered by a thin sarcolemmasarcolemma..– MononucleatedMononucleated. Nuclei are oval . Nuclei are oval

and central.and central.– SarcoplasmSarcoplasm shows shows non-clear non-clear

striationsstriations (fewer myofibrils). (fewer myofibrils).– Divided into short segments (cells) Divided into short segments (cells)

by the by the intercalated discsintercalated discs..

Cardiac Muscle FibersCardiac Muscle Fibers E.M. PictureE.M. Picture::

– Few myofibrils.Few myofibrils.

– Numerous mitochondria.Numerous mitochondria.

– Less abundant SR.Less abundant SR.

– T-tubules come in contact with T-tubules come in contact with only one cisterna of SR forming only one cisterna of SR forming “Diads” (not triads).“Diads” (not triads).

– Glycogen & myoglobin.Glycogen & myoglobin.

– Intercalated discsIntercalated discs: : are formed of are formed of the two cell membranes of 2 the two cell membranes of 2 successive cardiac muscle cells, successive cardiac muscle cells, connected together by connected together by junctional junctional complexescomplexes (desmosomes and (desmosomes and gap junctions).gap junctions).

SMOOTH MUSCLESMOOTH MUSCLE Present in walls of blood vessels Present in walls of blood vessels

and viscera (digestive, urinary, and viscera (digestive, urinary, genital .... etc).genital .... etc).

Non-striated and involuntary.Non-striated and involuntary. L.M. Picture of Smooth Muscle L.M. Picture of Smooth Muscle

FibersFibers::– FusiformFusiform in shape (spindle-shaped). in shape (spindle-shaped).

– Small diameterSmall diameter..

– Non-branchedNon-branched..

– Thin Thin sarcolemmasarcolemma..

– MononucleatedMononucleated. Nuclei are oval & . Nuclei are oval & central in position.central in position.

– SarcoplasmSarcoplasm is is non-striatednon-striated..

Smooth Muscle FibersSmooth Muscle Fibers E.M. PictureE.M. Picture::

– Sarcoplasm contains Sarcoplasm contains mitochondria and mitochondria and sarcoplasmic sarcoplasmic reticulum.reticulum.

– Myosin & actin Myosin & actin filaments are filaments are irregularly arranged irregularly arranged (that’s why no (that’s why no striations could be striations could be observed).observed).

– Cells are connected Cells are connected together by gap together by gap junctions for cell junctions for cell communication.communication.

REGENERATION OF MUSCLEREGENERATION OF MUSCLE

(1) Skeletal muscle cells:(1) Skeletal muscle cells:

- Can not divide.- Can not divide.

- Limited regeneration by satellite cells- Limited regeneration by satellite cells

(stem cells on the muscle cell’s (stem cells on the muscle cell’s surface).surface).

(2) Cardiac muscle cells:(2) Cardiac muscle cells:

-- No regenerative capacity.No regenerative capacity.

(3) Smooth muscle cells:(3) Smooth muscle cells:

- Can divide.- Can divide.

- Regenerate from pericytes.- Regenerate from pericytes.

→ → active regenerative response.active regenerative response.

Comparison between different types of Comparison between different types of muscle fibersmuscle fibers

   SKELETALSKELETAL CARDIACCARDIAC SMOOTHSMOOTH

SiteSite Muscle attached to Muscle attached to skeletonskeleton

Myocardium of the Myocardium of the heartheart

Viscera, e.g. stomachViscera, e.g. stomach

ShapeShape CylindricalCylindrical CylindricalCylindrical FusiformFusiform

DiameterDiameter LargestLargest Medium-sizedMedium-sized SmallestSmallest

BranchingBranching Non-branchedNon-branched BranchedBranched Non-branchedNon-branched

StriationsStriations ClearClear Not clearNot clear AbsentAbsent

Intercalated discsIntercalated discs AbsentAbsent PresentPresent AbsentAbsent

NucleiNuclei Numerous and Numerous and peripheralperipheral

One central nucleusOne central nucleus One central nucleusOne central nucleus

ActionAction VoluntaryVoluntary InvoluntaryInvoluntary InvoluntaryInvoluntary

RegenerationRegeneration LimitedLimited NoNo YesYes

Clinical ApplicationClinical Application

Cardiac hypertrophy:Cardiac hypertrophy: During cardiac hypertrophy the number During cardiac hypertrophy the number

of cardiac muscle cells is not increased; of cardiac muscle cells is not increased; instead, they become longer and larger instead, they become longer and larger in diameter.in diameter.

Thank youThank you