A&P: Bone Tissue (microanatomy and histology)

Bone growth, remodeling, calcium homeostasis

endochondral ossification

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

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

intramembranous ossification

• 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

trabeculae

• 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

functions of the skeleton

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

marrow (medullary) cavity

contains bone marrow in long bones

diaphysis

shaft of long bones — provides leverage

epiphysis

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

epiphyseal line

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

articular cartilage

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

periosteum

• 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

endosteum

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

flat 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)

osteogenic cells

• 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

osteoblasts

• 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

osteocytes

• 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

osteoclasts

• 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

compact bone structure

• 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

bone remodeling occurs…

• in response to hormonal changes

• in response to mechanical stress

• bone turnover is constant

osteoporosis

too much bone breakdown (more osteoclasts than osteoblasts)

Paget’s disease

inappropriate buildup of bone (more osteoblasts than osteoclasts)

pituitary giantism

excess growth hormone during development (before adulthood)

pituitary dwarfism

deficiency of growth hormone during development (proportional)

seckel syndrome

slow growth before birth

acromegaly

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

growth hormone (source & effects)

source: pituitary gland

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

thyroxine (source & effects)

source: thyroid gland

effects: stimulates osteoblasts & promotes protein synthesis → bone growth

calcitriol/vitamin D3 (source & effects)

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)

calcitonin (source & effects)

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

parathyroid hormone (source & effects)

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

estrogen (source & effects)

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

effects: stimulates osteoblasts & adolescent growth; prevents osteoporosis

testosterone (source & effects)

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

osteoporotic bone loss

• weakening of spongy bone tissue (osteoclast > osteoblast)

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

During intramembranous ossification, what type of tissue is replaced with bone?

fibrous connective tissue/mesenchyme

During endochondral ossification, what type of tissue is replaced w/ bone?

hyaline cartilage

Which bones would be formed by intramembranous ossification?

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

Which bones would be formed by endochondral ossification?

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

The growth in bones in width most resembles _________

intramembranous ossification

The growth in bones in length most resembles __________.

endochondral ossification