objective 4 and 5 review

Compact (cortical) bone

Dense, rigid outer shell of bone that provides structural support, protection, and mechanical strength against compressive and bending stress

Periosteum

Dense irregular connective tissue membrane covering the outer surface of bones, consisting of an outer fibrous layer and an inner osteogenic layer

Three critical functions of the periosteum

1. Protects the bone and anchors blood vessels & nerves 2. Provides osteoprogenitor cells

for oppositional growth & fracture repair 3. Server as attachment point for tendons & ligaments via Sharpey's fibers

Osteon (Haversian system)

Functional unit of compact bone, consisting of concentric lamellae surrounding central canal

central canal (Haversian canal)

Longitudes channel running through the center of an osteon containing blood vessels, lymphatic vessels, and nerves

Perforating canal (volkman’s canal)

Transverse or oblique channel connecting central canals to each other, to the periosteum, and to the medullary cavity

concentric lamellae

rings surrounding the central canals to inside osteons

Interstitial lamellae

Remnants of partially reassigned older osteons between active osteons

Circumferential lamellae

Rings extending around the outer and inner perimeter of the bone shaft

Lacuna (lacunae)

Small spaces or cavities within the calcified matrix containing an osteocyte

Canaliculus (plural:caniculi)

Microscopic canals radiating from lacunae containing osteocytic cytoplasmic processes, enabling cellular communication & nutrient or waste transport

Sharpey's fibers (perforating fibers)

Bundles of collagenous fibers that extend from the periosteum into the outer circumferential lamellae, firmly anchoring the periosteum to bone

Spongy bone /trabecular/cancellous bone

Porous bone of tissue composed of a lattice of thin of columns called trabeculae, designed to resist stress along lines of force while reducing bone weight

Location of spongy bone relative to compact bone

Spongy bone is deep or internal to compact bone, forms epiphyses & metaphyses and flat bones

Inter-trabecular tissue

Red bone marrow, site of hematopoiesis, fills the spaces between trabeculae in spongy bone

Trabeculae vs. Osteon structure

Trabeculae- consists of aligned lamellae with osteocytes in lacunae but lack central/perforating canals and true osteons; nutrients diffuse from surrounding endosteum

Endosteum

Thin vascular membrane lining the medullary cavity, trabeculae of spongy bone, and internal central/perforating canals

Endosteum vs. Periosteum

Both: osteoprogenitor cells, osteoblasts, and osteoclasts for remodeling

Periosteum - covers external surfaces and has a fibrous outer layer

Endosteum - lines internal surfaces, lacks a dense fibrous outer layer

Bone remodeling

Osteoclasts reabsorb old/damaged bone matrix and osteoblasts deposit new bone matrix to maintain structural integrity & blood Ca

Relative proportions of compact and spongy bone

Spongy bone - epiphyses/metaphyses, minimal compact bone

Compact bone - diaphysis consists predominantly of thick compact bone with minimal spongy bone lining the cavity

Epiphyseal growth plate

Layer of hyaline cartilage in juvenile long bones responsible for longitudinal interstitial growth

Epiphyseal line

Upon growth completion of E plate, ossifies into line in adults

Medullary cavity

Central hollow cavity in long bones, shaft lined by endosteum, contains red bone marrow

Medullary age transition

After 20 so years, red bone marrow converts to yellow bone marrow (adipose storage)

Osteoprogenitor cells

Mesenchymal cells located in the periosteum and endosteum that undergo mitosis to produce osteoblasts

Osteoblasts

Bone-building cells, secrete organic bone matrix ( osteoid= 90% type 1 collagen, 10% ground substance), promote mineralization using alkaline phosphate

Osteocytes

Mature bone cells inside lacunae that act as mechanosensors to regulate bone remodeling and maintain matrix homeostasis

Osteoclasts

Carve out reabsorption pit, Howship's lacuna (where bone starts to break down)

Multi Nucleated, bone -destroying cells formed from the fusion of monocyte/macrophage lineage cells

Bone cell lineage relationships

Ostooprogenitor cells → osteoblasts → osteocytes

Osteoclasts arise separately from hematopoietic monocyte or macrophage stem cells

Osteocyte cellular interconnectedness

Osteocyte cytoplasmic processes extend through canaliculi

Gap Junctions - direct cell-to-cell communication and facilitate interstitial fluid movement between osteocytes

Mechanism of osteoclastic matrix dissolution (bone recycling)

Podosomes - make sealed environment

Carbonic anhydrase - aids in H+ secretion via proton pump to dissolve inorganic hydroxyapatite

Cathepsin K- degrades organic collagen

Transcytosis in osteoclasts

Endocytic uptake (transport) of degraded bone mineral and matrix fragments at the ruffled border, transport across cell cytoplasm, and exocytosis into interstitial fluid and blood