Cartilage
Characterized by
1. an extracellular matrix enriched with GAG and proteoglycans.
2. firm consistency of the extracellular matrix allow the tissues to be
mechanical stresses without permanent distortion.
3. cartilage is essential for the development and growth of long bones before
and after birth.
4. cartilage consists of cells, chondrocytes and chondroblasts
Three types of cartilage : Hyaline cartilage
elastic cartilage
fibrocartilage
cell nests : are the chondrocytes which found in little clusters , consisting of
two or more cells in one lacuna
function of chondrocytes: synthesize and secrete the extracellular matrix, in
hyaline cartilage chondrocytes
synthesize type II collage, proteoglycan, hyaluronic acid and chondroitin
perichondrium: is essential for the growth and maintenance of cartilage , it is
rich in collagen type I fibers and contains numerous fibroblasts. The cells in the
inner layer of the perichondrium are chondroblasts.
Collagen , hyaloronic acid , proteoglycans and small amount of several
glycoproteins are the principal macromolecules present in all types of cartilage
matrix .
Embryology of Cartilage:
Cartilage arises from mesenchyme. Some mesenchyme cells aggregate to form a blastema.
The cells of the blastema begin to secrete cartilage matrix and are then called chondroblasts.
They move apart as they deposit matrix, and when they are completely surrounded by matrix
they are called chondrocytes. The mesenchymal tissue surrounding the blastema gives rise to
the perichondrium.
Elastic cartilage : characterized by
1. great pliability
2. contain significant amounts of the protein elastin
Cartilage cells can give rise to benign chondroma
Or malignant chondrosarcoma tumors .
Growth
1. interstitial growth
2. appositional growth
by chondrocytes
by chondroblasts
regeneration :\
except in young children , damaged cartilage regenerate with difficulty and
often incompletely why ?
because of the activity of the perichondrium in fracture , chondroblasts from the
perichondrium invade the fractured area and regenerate new cartilage
in extensively damage areas , the perichondrium generate a scare of dense C.T.
instead of cartilage
White fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in
various proportions. It owes its flexibility and toughness to the former of these constituents,
and its elasticity to the latter. It is the only type of cartilage that contains type I collagen in
addition to the normal type II.
Fibrocartilage is found in the pubic symphysis, the annulus fibrosus of intervertebral discs,
meniscus, .
Intervetebral disks : the disks have two components
1. fibrous annulus fibrosus
2. nucleus pulposus the intervertebral disc acts as a lumbricated coushion that
prevent adjacent vertebrae from being eroded
by abrasive forces during movement annulus fibrosus: has an external layer of
dense C.T. the nucleus pulposus is situated in the center of the annulus fibrosus
.
repulsion of the annulus fibrosus , results in explosion of the nucleus pulposus
and concomitant flatting of the disk. Frequently slips from its position between
the vertebrae , it moves towards the spinal cord compress the nerves and result
in sever pain and neurological disturbance chondrocyte function depends on a
proper hormonal balance , the synthesis of sulfated GAG is accelerated by
growth hormones , thyroxin and testosterone slowed by cortisone and
hydrocortisone.
Bone (osseous tissue)
Bone is specialized c.t. composed of intercellular calcified material.
Matrix composed:
1. organic compound: protein- polysaccharides containing condroitin sulfate
2. Inorganic component: calcium, phosphate and calcium carbonate with
small amount of mangnesium, fluoride, sulfate. The organic matter in bone
matrix is type I collagen and ground substance
Cells of bone:
1. Osteoblasts: are
responsible for synthesis of the organic components of
bone matrix (type I collagen) .
2. Osteocytes: which derive from osteoblasts lie in the lacunae situated
between matrix, cylindrical matrix canaliculi house cytoplasmic processes of
osteocytes.these cells use activetly involved in the maintenance of the bony
matrix. Their death is followed by resorption of this matrix.
3. osteoclasts: are very large branched motile cells. Contain from 5 to 50
nuclei. Osteoclasts are devrived from the fusion of bone marrow- derived
monoucleated cells. Osteoclasts lie within enzymatically atched depressions in
the matrix known as howships lacunae .
periosteum and endosteum
enternal and external surfaces of bone are covered
by layers of bone- forming cells and c.t.
sharpeys fibers : bundles of periosteal collagen
fibers penetrate the bone matrix, binding the
periosteum to bone
osteoprogenitor cell : cells located in the inner
layers of periosteum it is like fibroblast with the
potential to divide by mitosis and differentiate into
osteoblasts.
Types of bone:
1. compact bone
2. cancellons ( spongy) bone microscopic examination of
bone shows two varieties primary bone is the first bone tissue
to appear in embryonic development and in fracture repair it
is characterized by random disposition of fine collagen fibers.
This tissue is usually temporary except in few places in body,
primary bone are lower mineral content and a higher
proportion of osteocytes than in secondary bone t.
Sharpey's fibres (bone fibres, or
perforating fibres) are a matrix of
connective tissue consisting of
bundles of strong collagenous fibres
connecting periosteum to bone. They
are part of the outer fibrous layer of
periosteum, entering into the outer
circumferential and interstitial
lamellae of bone tissue.
Histogenesis
1. interamembranous ossification
a. some of the embryonic c.t. (mesenchyme) transforms
into a highly vascular sheet
b. cells enlarge and differentiate into osteogenic cells then
to osteoblasts
c. mesenchyme condenses into a network of soft trabeculae
d. osteoblast deposit an organic matrix (osteoid tissue ) lack
of ca++ salts
e. calcium phosphate is deposited in the matrix
Endochondral ossification
Take place with a piece of hyaline cartilage whose shape
resmbles model
1. The bone tissue appears as a hallow bone cylinder that
surrounds the mid portion of the cartilage model [bone collar]
2. the local cartilage undergoes a degenerative process this
processes begins at the central portion of the cartilage
diaphysis where B.V. penetrate through the bone collar
1. proliferation
2. hypertrophy
3. calcified zone
4. chondrocyte die
5. ossification zone
epiphyseal plate : connect the two
epiphyses and diephysis and responsible for the growth in
length of the bone
oseopetrosis : a disease
caused by a defect in osteoclast
function that results in overgrowth , thickening , and hardening of
bones.
Obesity imposes significant strain on the articular cartilage ,
accelerating its degeneration , joint problems are far more frequent
in obesity individuals.
Chondroplastic dwarfism : results from the chondrocytes in
multiplication and hypertrophy zones fail to multiply, the long
bone grow slowly and stop growing early
Bone Fracture
There are three processes involved in the healing of fractures inflammatory, reparative and remodelling phases or 6 stages
- the hematoma stage, inflammatory stage, formation of
granulation tissue, soft and 'hard' callus formation, and
remodelling.
Their duration depends on age, health and nutritional status.
Hematoma Stage:
Hemorrhage, clot formation - within hours to days.
Inflammatory Stage:
Begins within 48 hours, inflammatory cells appear.
Organization and resorption of
Granulation Tissue:
From 2 days. Presence of - 12 mesenchymal cells, fibroblasts,
new capillaries.
Soft Callus:
One week to several months. Callus grows and bridges the
fracture site; cartilage and trabelcular bone laid down.
Hard Callus:
One week to several months. When callus has sealed the bone
ends.
Trabecular bone.
Remodelling:
Continues for several months. Reorganization of bon.
Bone marrow
There are two types of bone marrow: red marrow (consisting
mainly of hematopoietic tissue) and yellow marrow
(consisting mainly of fat cells). Red blood cells, platelets and
most white blood cells arise in red marrow. Both types of
bone marrow contain numerous blood vessels and capillaries.
At birth, all bone marrow is red. With age, more and more of
it is converted to the yellow type. About half of adult bone
marrow is red. Red marrow is found mainly in the flat bones,
such as the hip bone, breast bone, skull, ribs, vertebrae and
shoulder blades, and in the cancellous ("spongy") material at
the epiphyseal ends of the long bones such as the femur and
humerus. Yellow marrow is found in the hollow interior of the
middle portion of long bones.
In cases of severe blood loss, the body can convert yellow
marrow back to red marrow to increase blood cell production.
Muscle tissue
The three types of muscle:
Three types of muscle tissue can be identified histologically:
skeletal muscle, cardiac muscle and smooth muscle. The
fibres of skeletal muscle and cardiac muscle exhibit cross
striations at the light microscope level and they are both
referred to as striated muscle.
Skeletal muscle
Skeletal muscle constitutes the muscle that is attached to and upper
part of the esophagus. (Some people use that moves the term visceral
striated muscle in the foregoing examples, but since it is identical in
structure to the the skeleton and controls motor movements and
posture. There are a few instances where this type of muscle is
restricted to soft tissues: the tongue, pharynx, diaphragm muscle the
skeleton,.)
Skeletal muscle fibres (cells) are actually a multinucleated formed by
the fusion of individual small muscle cells or myoblasts, during
development. They are filled with longitudinally arrayed subunits
called myofibrils. The myofibrils are made up of the myofilaments
myosin (thick filaments) and actin (thin filaments). The striations
reflect the arrangement of actin and myosin filaments and support
structures. The individual contractile units are called sarcomeres. A
myofibril consists of many sarcomeres arranged end to end. The
entire muscle exhibits cross-striations because sarcomeres in adjacent
myofibrils and muscle fibers are in register. The most obvious feature
in longitudinal sections of skeletal muscle is the alternating pattern of
dark and light bands, called respectively the A (anisotropic) and I
(isotropic) band. The I band is bisected by a dense zone called the Z
line, to which the thin filaments of the I band are attached.
The nuclei are located peripherally, immediately under the plasma
membrane (sarcolemma). The thickness of individual muscle fibres
varies (depending for example on location in the body and exercise)
but each fibre is of uniform thickness throughout its length. Skeletal
muscle fibres do not branch.
Connective tissue elements surround muscle fibres. Individual muscle
fibres are surrounded by a delicate layer of reticular fibres called the
endomysium. Groups of fibres are bundled into fascicles by a thicker
CT layer called the perimysium. The collection of fascicles that
constitutes one muscle is surrounded by a sheath of dense CT called
the epimysium, which continues into the tendon
. Blood vessels and nerves are found in the CT associated with
muscle. The endomysium contains only capillaries and the finest
neuronal branches.
Summary: Skeletal muscle fibres bear obvious striations, have
many peripherally located nuclei, are of the same thickness
throughout their length and do not branch.
.
.
.
Cardiac muscle
Cardiac muscle is the type of muscle found in the heart, and at the base of the
venae cavae as they enter into the heart. Cardiac muscle is intrinsically
contractile but is regulated by autonomic and hormonal stimuli.
Cardiac muscle exhibits striations because it also has actin and myosin
filaments arranged into sarcomeres. Generally these striations do not appear as
well-defined as in skeletal muscle. Cardiac muscle also has a much greater
number of mitochondria in its cytoplasm.
At the light microscope level, a number of features distinguish cardiac from
skeletal muscle. Cardiac muscle cells have only one or two nuclei, which are
centrally located. The myofibrils separate to pass around the nucleus, leaving a
perinuclear clear area . This clear area is occupied by organelles, especially
mitochondria (which are of course not visible in LM). As in skeletal muscle,
individual muscle fibres are surrounded by delicate connective tissue.
Numerous capillaries are found in the connective tissue around cardiac muscle
fibres.
Cardiac muscle cells are joined to one another in a linear array. The boundary
between two cells abutting one another is called an intercalated disc.
Intercalated discs consist of several types of cells junctions whose purpose is to
facilitate the passage of an electrical impulse from cell to cell and to keep the
cells bound together during constant contractile activity. Unlike skeletal muscle
fibres, cardiac muscle fibres branch and anastomose with one another. Although
made up of individual fibres, heart muscle acts as a functional syncytium during
contraction for the efficient pumping of blood.
Summary: Cardiac muscle fibres are striated, have one or two centrally
located nuclei, branch and anastomose with other fibres, and are joined to
one another by intercalated discs.
Figure 3
shows a longitudinal section of cardiac muscle.
Two nuclei belonging to cardiac muscle fibres can be clearly
seen (lower right and middle left). Both have a prominent
nucleolus and a delicate pattern in the remainder of the
nucleus. The perinuclear area is also evident around both
nuclei. The other two nuclei are not very clear, but appear to
lie in connective tissue and probably belong to fibroblasts.
Fibroblast nuclei tend to be more flattened and darker staining.
Two intercalated discs are indicated , others, not quite so prominent, can also be seen. To the
left of the upper intercalated disc indicated, a muscle fibre appears to be branching.
.
Smooth muscle
Smooth muscle is the intrinsic muscle of the internal organs and blood vessels.
It is also found in the iris and ciliary body of the eye and associated with hair
follicles (arrector pili). No striations are present in smooth muscle due to the
different arrangement of actin and myosin filaments. Like cardiac muscle,
smooth muscle fibres are contractile but responsive to autonomic and hormonal
stimuli. They are specialized for slow, prolonged contraction.
Smooth muscle fibres are generally arranged in bundles or sheets. Each fibre is
fusiform in shape with a thicker central portion and tapered at both ends. The
single nucleus is located in the central part of the fibre. Fibres do not branch. , .
Smooth muscle fibres lie over one another in a staggered fashion (tapered part
of one fibre over thicker part of another). In longitudinal sections, it is often not
possible to distinguish the fibre boundaries, and smooth muscle may closely
resemble connective tissue (bundles of collagen). Where smooth muscle
bundles are interlaced with bundles of connective tissue (eg. in the uterus), one
can distinguish the smooth muscle by the orientation of the nuclei (all oriented
in the same direction), smooth muscle cell has a nucleus, fibroblast nuclei are
more scattered in bundles of CT). .
One distinguishing physiological feature of smooth muscle is its ability to
secrete connective tissue matrix. In the walls of blood vessels and the uterus in
particular, smooth muscle fibres secrete large amounts of collagen and elastin .
Summary: Smooth muscle fibres are fusiform with tapered ends,
have a single centrally located nucleus, and do not exhibit
striations.
.
Figure 6
shows a layer of smooth muscle in longitudinal
section lying between two layers in cross section. Notice how
the shape of the fibres in longitudinal section is harder to
distinguish than when individual fibres are seen,
Smooth muscle is an involuntary non-striated
muscle. It is divided into two sub-groups; the single-unit (unitary) and
multiunit smooth muscle. Within single-unit smooth muscle tissues,
the autonomic nervous system innervates a single cell within a sheet
or bundle and the action potential is propagated by gap junctions to
neighboring cells such that the whole bundle or sheet contracts as a
syncytium (i.e., a multinucleate mass of cytoplasm that is not
separated into cells). Multiunit smooth muscle tissues innervate
individual cells; as such, they allow for fine control and gradual
responses, much like motor unit in skeletal muscle.
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Troponin is a complex of three regulatory proteins that is integral to muscle contraction[1] in
skeletal and cardiac muscle but not smooth muscle
)
Tropomyosin and troponin bound to actin blocking myosin binding
Troponin binds to calcium, releasing Tropomyosin from actin allowing myosin
binding
Tropomyosin is an actin-binding protein that regulates actin mechanics. It is
important, among other things, for muscle contraction. Tropomyosin, along
with the troponin complex, associate with actin in muscle fibers and regulate
muscle contraction by regulating the binding of myosin. In resting muscle,
tropomyosin overlays the myosin binding sites on actin, with a single
tropomyosin molecule spanning 7 actin subunits, and is "locked" down in this
position by troponin T (tropomyosin binding troponin) and troponin I
(inhibitory troponin). Upon release of calcium from the sarcoplasmic reticulum
calcium binds to troponin C (calcium binding troponin). This "unlocks"
tropomyosin from actin, allowing it to move away from the binding groove.
Myosin heads can now access the binding sites on actin. Once one myosin head
binds, this fully displaces tropomyosin and allows additional myosin heads to
bind, initiating muscle shortening and contraction. Once calcium is pumped out
of the cytoplasm and calcium levels return to normal, tropomyosin again binds
to actin, preventing myosin from binding.