The bone
The cartilage
Muscle tissue
Dr. Makarchuk Iryna
Bone is a connective tissue characterized by a
mineralized extracellular matrix.
• Bone is a specialized form of
connective tissue that, like other
connective tissues, consists of cells and
extracellular matrix.
• The feature that distinguishes bone from
other connective tissues is the
mineralization of its matrix, which
produces an extremely hard tissue
capable of providing support and
protection.
• The mineral is calcium phosphate in the
form of hydroxyapatite crystals
• Bone matrix contains mainly type I
collagen along with other matrix
(noncollagenous) proteins.
Bone matrix contains lacunae connected by a
network of canaliculi.
Within the bone matrix are spaces called lacunae (sing., lacuna), each of which
contains a bone cell, or osteocyte.
The osteocyte extends numerous processes into small tunnels called canaliculi.
In addition to osteocytes, four other cell types are associated with bone.
• Osteoprogenitor cells are cells derived from mesenchymal stem cells; they
give rise to osteoblasts.
• Osteoblasts are cells that secrete the extracellular matrix of bone; once the
cell is surrounded with its secreted matrix, it is referred to as an osteocyte.
• Bone-lining cells are cells that remain on the bone surface when there is no
active growth. They are derived from those osteoblasts that remain after bone
deposition ceases.
• Osteoclasts are bone-resorbing cells present on bone surfaces where bone
is being removed or remodeled (reorganized) or where bone has been
damaged.
Bones are the organs of the skeletal system;
bone tissue is the structural component of bones.
• Typically, a bone consists of bone tissue and
other connective tissues, including hemopoietic
tissue, fat tissue, blood vessels, and nerves.
• If the bone forms a freely movable or synovial
joint, hyaline cartilage is present. The
ability of the bone to perform its skeletal
function is attributable to the bone tissue and,
where present, the hyaline or articular cartilage.
• Bone tissue is classified as either compact (dense)
or spongy (cancellous).
Bones are classified according to shape; the
location of spongy and compact bone varies with
bone shape.
Spongy and compact bone tissues are located in specific parts of bones. It
is useful, then, to outline briefly the kinds of bones and survey where the
two kinds of bone tissue are located.
On the basis of shape, bones can be classified into four groups:
• Long bones are longer in one dimension than other bones and consist
of a shaft and two ends (e.g., the tibia and the metacarpals).
• Short bones are nearly equal in length and diameter (e.g., the carpal
bones of the hand).
• Flat bones are thin and platelike (e.g., the bones of the calvaria and
the sternum). They consist of two layers of relatively thick compact
bone with an intervening layer of spongy bone.
• Irregular bones have a shape that does not fit into any one of the three
groups just described; the shape may be complex (e.g., a vertebra), or
the bone may contain air spaces or sinuses (e.g., the ethmoid bone).
Long bones have a shaft, called the
diaphysis, and two expanded ends, each
called an epiphysis.
The articular surface of the epiphysis is
covered with hyaline cartilage.
The flared portion of the bone between the
diaphysis and the epiphysis is called the
metaphysis.
A large cavity filled with bone marrow,
called the marrow or medullary cavity,
forms the inner portion of the bone.
Short bones possess a shell of compact
bone and have spongy bone and a marrow
space on the inside.
Elsewhere, periosteum, a fibrous
connective tissue capsule covers the outer
surface of the bone.
BONE FORMATION
The development of a bone is traditionally
classified as endochondral or intramembranous.
The distinction between endochondral and
intramembranous formation rests on whether a cartilage
model serves as the precursor of the bone
(endochondral ossification) or whether the bone is
formed by a simpler method, without the intervention of
a cartilage precursor intramembranous
ossification).
The bones of the extremities and those parts of the axial
skeleton that bear weight (e.g., vertebrae) develop by
endochondral ossification. The flat bones of the skull and
face, the mandible, and the clavicle develop by
intramembranous ossification.
Schematic diagram of developing long bone.
PHYSIOLOGIC ASPECTS OF BONE
• Bone serves as a reservoir for body calcium.
The maintenance of normal blood calcium levels is critical to health and life.
Calcium may be delivered from the bone matrix to the blood if the
circulating blood levels of calcium fall below a critical point (physiologic
calcium concentration in the human ranges from 8.9 to 10.1 mg/dL).
Conversely, excess blood calcium may be removed from the blood and
stored in bone. These processes are regulated by parathyroid hormone
(PTH), secreted by the parathyroid gland, and calcitonin, secreted by the
parafollicular cells of the thyroid gland .
 PTH acts on the bone to raise low blood calcium levels to normal.
 Calcitonin acts to lower elevated blood calcium levels to normal.
• Bone can repair itself after injury.
The initial response to a fracture is similar to the response to any injury
that produces tissue destruction and hemorrhage. New loose connective
tissue, granulation tissue, is formed, and as this tissue becomes denser,
cartilage forms in parts of it.
Cartilage is a form of connective tissue composed of
cells called chondrocytes and a highly specialized
extracellular matrix.
• Cartilage is an avascular tissue that
consists of chondrocytes and an
extensive extracellular matrix.
More than 95% of cartilage volume
consists of extracellular matrix,
which is a functional element of
this tissue. The chondrocytes are
sparse but essential participants in
producing and maintaining the
matrix.
• The extracellular matrix in
cartilage is solid and firm but also
somewhat pliable, which accounts
for its resilience.
Three types of cartilage that differ in appearance and
mechanical properties are distinguished on the basis of
characteristics of their matrix:
Hyaline cartilage is characterized by matrix containing
type II collagen fibers, GAGs, proteoglycans, and
multiadhesive glycoproteins.
Elastic cartilage is characterized by elastic fibers and
elastic lamellae in addition to the matrix material of
hyaline cartilage.
Fibrocartilage is characterized by abundant type I
collagen fibers as well as the matrix material of hyaline
cartilage.
HYALINE CARTILAGE
• Hyaline cartilage is distinguished by
a homogeneous, amorphous matrix.
• The matrix of hyaline cartilage
appears glassy in the living state:
hence, the name hyaline [Gr. hyalos,
glassy]. Throughout the cartilage
matrix are spaces called lacunae.
Located within these lacunae are the
chondrocytes.
• Hyaline cartilage is not a simple,
inert, homogeneous substance but a
complex living tissue. It provides a
low-friction surface, participates in
lubricating synovial joints, and
distributes applied forces to the
underlying bone.
ELASTIC CARTILAGE
Elastic cartilage is distinguished by the presence of elastin in
the cartilage matrix.
• In addition to containing the normal
components of hyaline cartilage matrix,
elastic cartilage matrix also contains a
dense network of branching and
anastomosing elastic fibers and
interconnecting sheets of elastic
material.
• Elastic cartilage is found in the external
ear, the walls of the external acoustic
meatus, the auditory (Eustachian) tube,
and the epiglottis of the larynx. The
cartilage in all of these locations is
surrounded by a perichondrium similar to
that found around most hyaline cartilage.
Unlike hyaline cartilage, which calcifies
with aging, the matrix of elastic cartilage
does not calcify during the aging process.
FIBROCARTILAGE
Fibrocartilage consists of chondrocytes and their matrix material in
combination with dense connective tissue.
• Fibrocartilage is a combination of dense
regular connective tissue and hyaline cartilage.
The chondrocytes are dispersed among the
collagen fibers singularly, in rows, and in
isogenous groups.
• These chondrocytes appear similar to the
chondrocytes of hyaline cartilage, but they
have considerably less cartilage matrix
material.
• There is also no surrounding perichondrium as
in hyaline and elastic cartilage.
• Fibrocartilage is typically present in
intervertebral discs, the symphysis pubis,
articular discs of the sternoclavicular and
temporomandibular joints, menisci of the knee
joint, the triangular fibrocartilage complex of
the wrist, and certain places where tendons
attach to bones.
CHONDROGENESIS AND CARTILAGE GROWTH
Most cartilage arises from mesenchyme during chondrogenesis.
Chondrogenesis, the process of cartilage development, begins with
the aggregation of chondroprogenitor mesenchymal cells to form a
mass of rounded, closely apposed cells. In the head, most of the
cartilage arises from aggregates of ectomesenchyme derived from
neural crest cells. The site of hyaline cartilage formation is
recognized initially by an aggregate of mesenchymal or
ectomesenchymal cells known as a chondrogenic nodule.
Cartilage has limited ability for repair.
When hyaline cartilage calcifies, it is replaced by bone.
• The portion of articular cartilage that is in contact with bone tissue in
growing and adult bones, but not the surface portion, is calcified.
• Calcification always occurs in cartilage that is about to be replaced
by bone (endochondral ossification) during an individual’s growth
period.
• Hyaline cartilage in the adult calcifies with time as part of the aging
process.
Cartilage is capable of two kinds of growth,
appositional and interstitial.
With the onset of matrix secretion, cartilage growth
continues via a combination of two processes:
• Appositional growth, the process that forms new
cartilage at the surface of an existing cartilage; and
• interstitial growth, the process that forms new
cartilage within an existing cartilage mass
Muscle Tissue
Muscle tissue is responsible for movement of the body and its parts
and for changes in the size and shape of internal organs.
This tissue is characterized by aggregates of specialized, elongated cells
arranged in parallel array that have the primary role of contraction.
Myofilament interaction is responsible for muscle cell contraction.
Two types of myofilaments are associated with cell contraction.
•Thin filaments (6 to 8 nm in diameter, 1.0 m long) are composed
primarily of the protein actin. Each thin filament of fibrous actin (Factin) is a polymer formed from globular actin molecules (G-actin).
• Thick filaments (15 nm in diameter, 1.5 m long) are composed of the
protein myosin II. Each thick filament consists of 200 to 300 myosin II
molecules. The long, rodshaped tail portion of each molecule
aggregates in a regular parallel but staggered array, whereas the head
portions project out in a regular helical pattern.
The two types of myofilaments occupy the bulk of the
cytoplasm, which in muscle cells is also called sarcoplasm
[Gr. sarcos, flesh; plasma, thing ].
Actin and myosin are also present in most other cell types
(although in considerably smaller amounts), where they play
a role in cellular activities such as cytokinesis, exocytosis,
and cell migration. In contrast, muscle cells contain a large
number of aligned contractile filaments that the cells use for
the single purpose of producing mechanical work.
Muscle is classified according to the appearance of the
contractile cells.
Two principal types of muscle are recognized:
Striated muscle, in which the cells exhibit crossstriations at
the light microscope level, and
Smooth muscle, in which the cells do not exhibit
crossstriations.
Striated muscle tissue is further subclassified on the
basis of its location:
• Skeletal muscle is attached to bone and is responsible for
movement of the axial and appendicular skeleton and for
maintenance of body position and posture. In addition,
skeletal muscles of the eye (extraocular muscles) provide
precise eye movement.
• Visceral striated muscle is morphologically identical to
skeletal muscle but is restricted to the soft tissues, namely,
the tongue, pharynx, lumbar part of the diaphragm, and
upper part of the esophagus. These muscles play essential
roles in speech, breathing, and swallowing.
• Cardiac muscle is a type of striated muscle found in the wall
of the heart and in the base of the large veins that empty into
the heart.
SKELETAL MUSCLE
• A skeletal muscle cell is a multinucleated syncytium.
In skeletal muscle, each muscle cell, more commonly called a
muscle fiber, is actually a multinucleated syncytium. A
muscle fiber is formed during development by the fusion of
small, individual muscle cells called myoblasts.
• The nuclei of a skeletal muscle fiber are located in the
cytoplasm immediately beneath the plasma membrane, also
called the sarcolemma.
• A skeletal muscle consists of striated muscle fibers held
together by connective tissue.
• The connective tissue that surrounds both individual muscle
fibers and bundles of muscle fibers is essential for force
transduction.
The connective tissue associated with muscle is named
according to its relationship with the muscle fibers:
• Endomysium is the delicate layer of reticular fibers that
immediately surrounds individual muscle fibers. Only smalldiameter blood vessels and the finest neuronal branches are
present within the endomysium, running parallel to the
muscle fibers.
• Perimysium is a thicker connective tissue layer that
surrounds a group of fibers to form a bundle or fascicle.
Fascicles are functional units of muscle fibers that tend to
work together to perform a specific function. Larger blood
vessels and nerves travel in the perimysium.
• Epimysium is the sheath of dense connective tissue that
surrounds a collection of fascicles that constitutes the
muscle. The major vascular and nerve supply of the muscle
penetrates the epimysium.
• Three types of skeletal muscle fibers - red, white, and
intermediate - can be identified by color in vivo.
• Skeletal muscle fibers are characterized by speed of
contraction, enzymatic velocity, and metabolic activity.
• The three types of skeletal muscle fibers are type I
(slow oxidative), type IIa (fast oxidative glycolytic),
and type Iib (fast glycolytic) fibers. Three types of
fiber are typically found in any given skeletal muscle;
the proportion of each type varies according to the
functional role of the muscle.
Myofibrils and Myofilaments
• The structural and functional subunit of the muscle fiber
is the myofibril (A muscle fiber is filled with
longitudinally arrayed structural subunits called
myofibrils).
• Myofibrils are composed of bundles of myofilaments.
Myofilaments are the individual filamentous polymers
of myosin II (thick filaments) and actin and its
associated proteins (thin filaments). Myofilaments are
the actual contractile elements of striated muscle. The
bundles of myofilaments that make up the myofibril are
surrounded by a well- developed smooth-surfaced
endoplasmic reticulum (sER), also called the
sarcoplasmic reticulum.
Cross-striations are the principal histologic feature
of striated muscle.
• Cross-striations are evident in
H&E–stained preparations of
longitudinal sections of muscle
fibers. They may also be seen
in unstained preparations of
living muscle fibers examined
with a phase contrast or
polarizing microscope, in
which they appear as
alternating light and dark
bands. These bands are termed
the A band and the I band.
In polarizing microscopy, the dark bands are birefringent (i.e.,
they alter the polarized light in two planes).
• Therefore, the dark bands, being doubly refractive, are
anisotropic and are given the name A band. The light bands
are monorefringent (i.e., they do not alter the plane of
polarized light). Therefore, they are isotropic and are given
the name I band.
• Both the A and I bands are bisected by narrow regions of
contrasting density.
• The light I band is bisected by a dense line, the Z line, also
called the Z disc.
• The dark A band is bisected by a less dense, or light, region
called the H band.
• Furthermore, bisecting the light H band is a narrow dense
line called the M line.
The functional unit of the myofibril is the sarcomere,
the segment of the myofibril between two adjacent
Z lines.
• The sarcomere is the basic contractile unit of
striated muscle. It is the portion of a myofibril
between two adjacent Z lines.
• A sarcomere measures 2 to 3 µm in relaxed
mammalian muscle. It may be stretched to more than 4
µm and, during extreme contraction, may be reduced
to as little as 1 µm . The entire muscle cell exhibits
crossstriations because sarcomeres in adjacent
myofibrils are in register.
The events leading to contraction of skeletal muscle can be
summarized as a series of steps.
The events involved in contraction can be summarized as follows:
1. The contraction of a skeletal muscle fiber is initiated when a nerve impulse
traveling along the axon of a motor neuron arrives at the neuromuscular
junction.
2. The nerve impulse prompts the release of acetylcholine into the synaptic cleft
that binds into ACh-gated Na+ channels causing local depolarization of
sarcolemma.
3. Voltage-gated Na + channels open, and Na + enters the cell.
4. General depolarization spreads over the plasma membrane of the muscle cell
and continues via membranes of the T tubules.
5. Voltage sensor proteins in the plasma membrane of T tubules change their
conformation.
6. At the muscle cell triads, the T tubules are in close contact with the lateral
enlargements of the sarcoplasmic reticulum, where gated Ca2 + -release
channels are activated by conformational changes of voltage-sensor proteins.
7. Ca2 + is rapidly released from the sarcoplasmic reticulum into the sarcoplasm.
8. Ca2 + binds to the TnC portion of the troponin complex.
9. The contraction cycle is initiated, and Ca2 + is returned to the terminal
cisternae of the sarcoplasmic reticulum.
CARDIAC MUSCLE
Cardiac muscle has the same
types and arrangement of
contractile filaments as
skeletal muscle. Therefore,
cardiac muscle cells and the
fibers they form exhibit crossstriations evident in routine
histologic sections.
In addition, cardiac muscle fibers
exhibit densely staining crossbands, called intercalated
discs, that cross the fibers
in a linear fashion or
frequently in a way that
resembles the risers of a
stairway
Structure of Cardiac Muscle
• The cardiac muscle nucleus lies in the center of the cell.
• Numerous large mitochondria and glycogen stores are
adjacent to each myofibril.
• The intercalated discs represent junctions between cardiac
muscle cells.
• The sER in cardiac muscle cells is organized into a single
network along the sarcomere, extending from Z line to Z
line.
• Passage of Ca2+ from the lumen of the T tubule to the
sarcoplasm of a cardiac muscle cell is essential to initiate
the contraction cycle.
• Cardiac muscle cells exhibit a spontaneous rhythmic
contraction.
SMOOTH MUSCLE
• Smooth muscle generally
occurs as bundles or
sheets of elongated fusiform
cells with finely tapered ends.
• The cells, also called fibers,
range in length from 20 µm in
the walls of small blood vessels
to about 200 µm in the wall of
the intestine; they may be as
large as 500µ m in the wall of
the uterus during pregnancy.
Smooth muscle cells are
interconnected by gap
junctions, the specialized
communication junctions
between the cells.
Structure of Smooth Muscle
• Smooth muscle cells possess a contractile apparatus of
thin and thick filaments and a cytoskeleton of desmin and
vimentin intermediate filaments.
• Dense bodies provide an attachment site for thin filaments
and intermediate filaments.
• Contraction in smooth muscles is initiated by a variety of
impulses, including mechanical, electrical, and chemical
stimuli.
• Smooth muscle cells lack a T system.
• Contraction of smooth muscle is regulated by the Ca2+–
calmodulin–myosin light chain kinase system.
• The force of smooth muscle contraction may be
maintained for long periods in a “latch state.”
• Smooth muscle is specialized for slow, prolonged
contraction.