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Bone Tissue
BIO 137 Anatomy & Physiology I
The Skeletal System
• Organs
• Functions
• Bones
• Support
• Cartilages
• Protection
• Tendons
• Movement
• Ligaments
• Hematopoiesis
• Mineral Store
• Energy Store
Tissues of the Skeletal System
• Bone is a highly vascularized C.T. with a hard, mineralized
extracellular matrix. It is found in the body in two different
arrangements:
• Compact bone – 80% of bone in skeleton
• Spongy bone - 20% of bone in skeleton
• is seen as
Compact Bone
• Most rigid connective
tissue
• Continuous, solid
extracellular matrix
found between cells
• Matrix composed of
mineral salts and
collagen
• Hardness due to mineral
salts in matrix
• Collagen provides
strength and resilience
Spongy Bone
• Made of Bony plates
called trabeculae
• Space between
trabeculae filled with red
bone marrow
• Found in flat bones
between sheets of
compact bone
• Found in epiphyses of
long bones
• Spongy bone reduces a
bone’s weight
Long Bone Structure
• Epiphysis - Expanded ends of a long bone, articulates
with other bones
• Articular cartilage (hyaline cartilage) on outer surface
• Composed of mostly spongy (cancellous) bone surrounded by
thin layers of compact bone
• Red bone marrow with
Hematopoietic stem cells
located here
Hyaline cartilage is the
articular cartilage of
this long bone
Long Bone Structure
• Diaphysis – shaft of bone
• Central medullary cavity
• Hollow tube in diaphysis filled with
yellow marrow
• Medullary Cavity is lined by
endosteum
• Osteoclasts located in endosteum
• Metaphysis – region between
diaphysis and epiphysis
• Growth plate here
Lone Bone Structure
• The medullary cavity is a space
within the diaphysis of long bones
that contains fatty yellow bone
marrow in adults.
• The endosteum is a membrane that
lines the medullary cavity .
• The endosteum is composed of
osteoclasts, osteoblasts, and
connective tissue.
• Nutrient foramen – canal where vessels
enter/exit a bone
Long Bone Structure
• The periosteum is a tough sheath of dense, irregular
connective tissue on the outside of the bone.
• It contains osteoblasts that
help the bone grow in thickness,
but not in length.
• Continuous with tendons and ligaments
• It also assists with fracture repair
Chemical Constituents of Bone
• Bone is 15% water, 30% organic proteins, 55% mineral salts
(hydroxyapatite crystals).
• Organic constituents
• Collagen fibers provide flexibility and tensile strength.
• Inorganic hydroxyapatite crystals (mineral salts)
• Calcium Phosphate (Ca3PO4)2
• Calcium Carbonate (CaCO3 – marble)
• Other trace elements: magnesium, fluoride, sulfate
Bone Cells
Osteogenic Stem Cell
Osteoclasts
• Derived from mesenchyme
• Forms osteoblasts
•Bone breakdown
(resorption)
• Only Mitotic bone cell
Osteoblasts
•Bone forming cells,
deposit bony matrix
around themselves
Osteocytes
• matured osteoblasts
•Completely surrounded by bony
matrix
•Carries out daily cellular activities
Histology of Bone Tissue
• Compact bone is composed of repeating structural
units called Osteons or Haverisan Systems
• Osteons have concentric circles of matrix called
lamellae surrounding a Central (Haversian) canal
• Blood vessels and nerves here
• Osteoblast cells mature into
Osteocytes after matrix is
laid down
Histology of Bone Tissue
• Lacunae spaces in osetons hosue osteocytes
• Canaliculi – small channels that connect lacunae of
osteocytes
• Nutrient and waste transport
• Gap junctions for communication
• Perforating (Volkmann’s) canals – transverse canals
that connect vessels of central (Haversian) canals
Haversian System of Compact Bone
Blood and Nerve Supply of Bone
• Bone is richly supplied with blood; Periosteal
arteries and veins supply the periosteum and
compact bone.
• Nerves accompany the blood
vessels
• The periosteum is rich in
sensory nerves sensitive to
tearing or tension
Bone Formation
• Ossification or osteogenesis is the process of
forming new bone. Bone formation occurs in four
situations:
• Formation of bone in an embryo
• Growth of bones until adulthood
• Remodeling of bone
• Repair of fractures
Bone Formation
• Osteogenesis occurs by two different methods,
beginning about the 6th week of embryonic
development.
• Intra-membranous ossification produces
spongy bone.
• This bone may subsequently be remodeled to form
compact bone.
• Endochondral ossification is a process
whereby cartilage is replaced by bone.
• Forms both compact and spongy bone.
Bone Formation
• Intra-membranous ossification is the simpler of
the two methods.
• It is used in forming the flat bones of the skull, mandible,
and clavicle.
• Bone forms from mesenchymal cells that develop within a
membrane – without going through a cartilage stage
• Many ossification centers.
Intramembranous Bone Formation
Bone Formation
• Endochondral ossification is the method used in
the formation of most bones, especially long bones.
• It involves replacement of hyaline cartilage bone models.
• There are one primary and two secondary centers of
growth.
Endochondral Bone Formation
Bone Formation
• Ossification contributing to bone length is usually complete
by 18-21 years of age.
• Bones can still continue to thicken and are capable of repair
even after the epiphyseal growth plates have closed.
Bone Structure
• In adolescents, through the end
of active growth, the epiphysis
of the long bones contains
hyaline cartilage and forms an
“epiphyseal growth plate”.
• The growth plate is always
actively dividing causing the
bone to elongate from each end.
Bone Structure
• In adults, the epiphyseal
cartilage is no longer present
and elongation of bones has
stopped.
• The epiphyseal growth plate
becomes an “epiphyseal line”,
as growing cartilage is
replaced by calcified bone.
Epiphyseal plates
Long Bone Growth at Epiphyseal Plate
• 4 zones of cartilage at epiphyseal plate
• Zone of Resting cells
• Closest to epiphysis, don’t participate in growth, anchors plate to
bony tissue of epiphysis
• Zone of Mitotic cells
• Rows of many cells undergoing mitosis
• As ECM forms around cartilage thickens
• Zone of Hypertrophic cells
• Older cells enlarge and thicken plate, lengthening bone
• Calcifying older cells begin to die
• Zone of Calcified cartilage
• Thin layer of dead cartilage
• Osteoblasts lay down bone to replace calcified cartilage
Articular cartilage
Epiphysis
EPIPHYSEAL (GROWTH)
PLATE:
Zone of resting cartilage
Zone of proliferating cartilage
Zone of hypertrophic cartilage
New chondrocytes are
formed
Old chondrocytes are
replaced by bone
New
diaphysis
Zone of calcified
cartilage Diaphysis
(c) Lengthwise growth of bone at epiphyseal plate
Long Bone Growth at Epiphyseal Plate
• 2 MAJOR EVENTS
1. New cartilage cells are produced by mitosis at
epiphyseal end of plate
2. Replacement of cartilage with bone on diaphyseal end
of plate by endochondral ossification
• Epiphyseal plate temporarily increases in width
• Bones continue to lengthen until ossification centers
meet and the epiphyseal plate ossifies under control of
sex hormones at end of puberty
Growth At Epiphyseal Plate
Bone Growth in Thickness:
Appositional Growth
• A bone thickens as Compact bone is deposited below
the periosteum
• Osteoclasts will resorb (break down) bone on the
inside to form the medullary cavity
• The diaphysis of a long bone grows in thickness on
the outside while the medullary cavity is enlarged on
the inside
Bone Growth & Remodeling
• Ongoing process that includes bone resorption by
osteoclasts and deposition of bone by osteoblasts
• Opposing processes
• Total mass of bone in skeleton remains constant
• At any one time, 5% of total bone mas is being
remodeled
Factors affecting Bone Development,
Growth & Repair
• Normal bone growth depends on several
factors:
• Nutrition
• Hormonal secretions
• Physical exercise
Nutritional Requirements for Bone
Development
• Vitamin A
• Required for normal osteoblast and osteoclast activity during
normal development
• Vitamin C
• Required for collagen synthesis
• Vitamin D
• Promotes calcium absorption in the small intestines
Factors affecting Bone Development,
Growth & Repair
• Physical stress from exercise
• Stimulates bone growth (deposition)
• Lack of exercise makes bones atrophy
(weaker) and thinner
• Loss of bone mass
Hormones affecting Normal Bone
Growth
• Thyroid Hormones
• Secreted by the thyroid gland
• Stimulates osteoblasts
• Deficiency:
• Stunted growth
Hormones affecting Normal Bone
Growth
• Sex hormones: Estrogen and Testosterone
• Promote formation of bone tissue
• Abundant at puberty, stimulate long bone growth
• Growth Spurt
• Estrogen effect stronger than testosterone
• Responsible for closing epiphyseal plates
• Widening of female pelvis
Hormone Requirements for Bone
Development
• Growth hormone – secreted from the anterior
pituitary gland
• Regulates overall body growth
• IGFs released from liver and bone in response to
GH
• Stimulates division of cartilage cells in epiphyseal
plates during growing years and stimulates growth in
thickness
• Role in metabolism (discussed in Endocrine
System)
Growth Hormone Disorders
• Pituitary Dwarfism
• Pituitary gigantism
• Acromegaly
Growth Hormone Deficiency
• Pituitary Dwarfism
• GH deficiency in children
• Short stature from retarded skeletal growth
• Normal body proportions
Growth Hormone Excess
• Acromegaly
• GH excess in adulthood
• Bones become thicker & soft tissues proliferate
• Disfuguring
Growth Hormone Excess
• Pituitary Gigantism
• GH excess in
childhood before
epiphyseal plates have
closed
• Rapid growth in height
Blood Calcium Homeostasis
• Bone is the body’s major calcium reserve, storing
99% of calcium
• Inorganic salts in matrix of bone composed
mostly of calcium phosphate and calcium
carbonate
• Calcium required for blood clot formation, nerve
impulse conduction and muscle cell contraction
• Calcium homeostasis maintained by three
hormones:
• Parathyroid hormone
• Calcitonin
• Vitamin D
Blood Calcium Homeostasis
• Parathyroid Hormone
• Secreted by parathyroid glands in response to low
blood calcium
• Stimulates osteoclast activity = Bone resorption
• Secrete acids and enzymes that dissolve inorganic
mineral matrix of bone
• Releases calcium and phosphorous into the blood
Parathyroid Hormone
• Other PTH targets:
• Osteoclasts – Bone resorption
• Kidneys
• Reabsorb calcium during urine formation
• Secrete phosphate during urine formation
• Intestines
• Increases calcium absorption through activation of
Vitamin D regulation
• This is necessary for the intestines to absorb Ca++
• Functions to increase the calcium concentration of
the plasma (also increases concentration of
phosphate)
Blood Calcium Homeostasis
• Vitamin D
• Necessary for proper absorption of calcium in the
small intestines
• Lack of vitamin D decreases calcium absorption in
small intestines
• Decreases calcium content in bone matrix, making bones
soft and deformed
• Rickets in children
• Osteomalacia in adults
• Derivative of Cholesterol is converted by sunlight to
Vitamin D
Blood Calcium Homeostasis
• Calcitonin
• Released from thyroid gland in response to high
levels of blood calcium
• Inhibits osteoclast activity
• Stimulates osteoblasts to form bone tissue
• Stimulates the kidneys to secrete calcium
• Functions to decrease plasma calcium
concentration