A&P: Bone Tissue (microanatomy and histology)

• responsible for growth and development of long bones (ex. femur, humerus)

• hyaline cartilage is replaced w/ bone tissue after it dies

• occurs in flat bones of the skull, mandible, and clavicle

• starts w/ a sheet of fibrous connective tissue (mesenchyme) → fibroblasts/mesenchymal cells are transformed into osteoblasts → secrete matrix that ultimately becomes spongy bone

• small branches/spicules found in spongy bone

• provide structural support and help distribute forces within the bone

• located at the ends of long bones and in the pelvis, ribs, skull, and vertebrae

1. Support

2. Protection

3. Movement

4. Electrolyte balance: stores calcium and phosphate ions and releases them into tissue fluid/blood according to physiological needs

5. Acid-Base balance: bone tissue buffers blood against excessive pH changes by absorbing/releasing alkaline phosphate and carbonate salts

6. Blood formation: red bone marrow

7. Hormone secretion: influences secretion and action of insulin and moderates the stress response

contains bone marrow in long bones

shaft of long bones — provides leverage

expanded head at each end of long bones — strengthens joints and provides added surface area for the attachment of tendons and ligaments

slightly denser spongy bone b/t the epiphysis and diaphysis in mature bones — remnant of the epiphyseal plate (childhood growth zone)

a layer of hyaline cartilage on the joint surface where one bone meets another

• a tough surface membrane of bones

• outer fibrous layer of collagen, inner osteogenic layer of bone-forming cells

• provides strong attachment and continuity from muscle to tendon to bone

• no periosteum over articular cartilage

a thin layer of reticular connective tissue that lines the internal marrow cavity, covers all the honeycombed surfaces of spongy bone, and lines a canal system found throughout compact bone

• shieldlike plates that protect delicate organs such as the brain and heart and form broad surfaces for muscle attachment

• two layers of compact bone (inner and outer tables) enclosing a middle layer of spongy bone (diploe)

• stem cells that develop from embryonic mesenchyme → most other bone cell types

• occur in endosteum and inner layer of periosteum

• multiply continually — only cells capable of dividing and producing more bone cells

• bone-forming cells that synthesize the organic matter of the bone and then promote its mineralization (osteogenesis)

• form rows in endosteum and inner layer of periosteum

• have abundant mitochondria and ER to support their role in osteogenesis

• former osteoblasts that have become embedded in the matrix they deposited

• 90-95% of bone cells

• reside in cavities called lacunae (interconnected by canaliculi)

• have dendrites that contact neighboring osteocytes (gap junctions), blood vessels, and osteoblasts on the bone surface

• some resorb bone matrix and others deposit it

• strain sensors

• bone-dissolving cells on the bone surfaces (osteolysis)

• develop from the same bone marrow stem cells as blood cells (not osteogenic cells)

• large (up to 150 µm) w/ 3-4 nuclei (up to 50)

• reside in pits called resorption bays

• ruffled border faces the bone surface

• osteon: functional unit of compact bone, concentric layers of bone tissue (lamellae — torsional strength) surrounding the Haversian canal (where oxygen/nutrients, blood vessels, nerves, and connective tissue are)

• lacunae: small spaces in each lamella where osteocytes are located

• canaliculi: tiny channels that connect lacunae and allow nutrient and waste product exchange b/t osteocytes

• in response to hormonal changes

• in response to mechanical stress

• bone turnover is constant

too much bone breakdown (more osteoclasts than osteoblasts)

inappropriate buildup of bone (more osteoblasts than osteoclasts)

excess growth hormone during development (before adulthood)

deficiency of growth hormone during development (proportional)

slow growth before birth

excess of growth hormone after epiphyseal plate closes → acts on cartilage (changes in nose and ears, square jaw)

source: pituitary gland

effects: increased muscle and bone growth through intermediate growth factors (insulin-like growth factors)

source: thyroid gland

effects: stimulates osteoblasts & promotes protein synthesis → bone growth

source: skin (sun) — produced by the sequential action of the skin, liver, and kidneys

effects: increases calcium in plasma — promotes reabsorption of calcium (filtered from the blood) from kidneys, promotes absorption of calcium from foodstuffs that’re in the digestive system

• without calcitriol = calcium and phosphate levels in the blood are too low for normal deposition → bone softness (rickets/osteomalacia)

source: parafollicular cells of the thyroid gland

effects: secreted when blood/plasma [Ca2+] is too high → lowers [Ca2+] through osteoclast inhibition & osteoblast stimulation

• more important in children b/c osteoclasts are more highly active

source: parathyroid glands (on posterior surface of the thyroid gland)

effects: secreted when blood calcium is low → raises [Ca2+] by

1. binding to osteoblast receptors → stimulate osteoclast population & promote bone resorption

2. promote Ca2+ reabsorption by the kidneys → less calcium is lost in the urine

3. promotes final step of calcitriol synthesis in kidneys → enhancing the calcium-raising effect of calcitriol

4. inhibits collagen synthesis by osteoblasts → inhibits bone deposition

source: ovaries of females; smaller amounts by adrenal glands and testes in males

effects: stimulates osteoblasts & adolescent growth; prevents osteoporosis

source: testes in males; smaller amounts in adrenal glands and ovaries in females

effects: stimulates osteoblasts and promotes protein synthesis, thus promoting adolescent growth and epiphyseal closure

• weakening of spongy bone tissue (osteoclast > osteoblast)

• common in women post-menopause in head of femur & vertebrae

fibrous connective tissue/mesenchyme

hyaline cartilage

flat bones: mandible, frontal bone, occipital bone, etc.

long bones: femur, humerus, rib, vertebrae, etc.

intramembranous ossification

endochondral ossification