Lecture 8- Histology of Bone Flashcards
What are the different bone shapes
Long, short, irregular flat
What are the parts of the long bone
Epiphysis, metaphysis, diaphysis
What is the epiphysis
End of long bone, near articular cartilage
What is the metaphysis
Between articular cartilage and shaft
What is the diaphysis
Shaft of bone
Classifying bones by density/porosity
Compact vs cancellous
What are the functions of the bone
Structural framework for support and protection of body organs, levers for muscles to attain movement, house and make blood cells, fat storage, reservoir for minerals= calcium, phosphorus, magnesium, potassium
Bone is inelastic therefore bone is very good at ____ but less good at
Resisting tensile forces but less at resisting compressive and shearing forces
Organic component of bone matrix
Primarily made of type 1 collagen
Inorganic component of bone matrix
Calcium hydroxyapatite- mainly calcium and phosphorus
Periosteum and endosteum
Vascularized connective tissue that lines the surface of bone, nourish bone tissue and make bone cells for appositional growth
What does periosteum line
External surface of bone
What are the two layers of periosteum
Fibrous and inner cellular layer
Fibrous periosteum
Outer layer of dense connective tissue
Inner cellular layer
Contains osteogenic cells
What does the endosteum linen and do
Central cavity, loose CT, has monolayer of osteogenic cells and osteoblasts
Compact bone
Provides support to external surfaces of bone and makes up shafts of long bone, made of 4 lamellar systems =- outer circumferential, inner circumferential, osteons, interstitial
What is the outer circumferential lamellae
Deep to periosteum, outermost region of diaphysis, contains sharpey’s fibers anchoring periosteum to bone
Inner circumferential lamellae
Completely encircle the marrow cavity, trabecullae of spongy bone extend into marrow cavity
Osteons
Form bulk of compact bone
Interstitial lamellae
Remnants of osteons after osteoclast resorption
Secondary osteons
Form as bone remodels- replaces tissue damaged via micro fractures- form concentric lamellae of bone
Central canal
Contains blood vessels and nerves
Perforating canal
Connect central canals with each other and blood vessels in the periosteum and marrow cavity, perpendicular to central canals
Two different microtextures of bone
Woven (immature)and lamellar (mature)
Woven (immature) bone
Initial bone formation, fracture repair, poorly organized type I collagen, relatively weak
Lamellar (mature) bone
Replaces woven bone, very strong; well organized, type I collagen
Lamellar bone replaces woven bone during:
Formation of primary bone (primary osteons) and remodeling to form secondary bone (secondary osteons)
Cancellous (trabecular, spongy) bone
Formed by interconnecting bony struts or trabeculae that provide internal support to bones, high surface area that facilitates calcium ion exchange between bone and blood
Spaces between trabeculae contain either
Red marrow that produces blood cells and yellow marrow made of adipose tissue
Osteoblasts function
Secrete ECM, lower blood calcium levels
Osteocytes
Osteoblasts embedded in lacunae
What do osteoblasts and osteocytes derive from
Osteoprogenitor cells
Osteoclasts
Resort bone ECM
What are osteoclasts derived from
Progenitor cells from the monocyte-macrophage lineage located in bone marrow
What are inactive osteoblasts called
Bone lining cells
Identifying factors of osteoblasts
Located along surface of bone, extensive rER, golgi, polarized organelles- nucleus located away from secretory activity
Osteoblasts communicate via
Gap junctions
Osteoid
Initial organic component synthesized by osteoblasts
Calcify, slowly calcify if low vitamin D
Low calcium results in
Renal failure
What do osteocytes direct
Bone remodeling in response to mechanical stimuli, form gap junctions with each other and osteoblasts by sending cell processes through channels in bone called canaliculi
Organic portion of bone
Mostly type I collagen, histology looks pale before mineralized and called osteoid
Inorganic portion of bone
Added to to osteoid, made of hydroxyapatite crystals that are mostly calcium ions bound together by multiadhesive glycoproteins. Osteoblasts secrete bone specific alkaline phosphatase an enzyme that aids in hydroxyapatite formation
How do osteoclasts increase blood calcium levels
Resorb bone, endocytose into ECM, secrete into blood
Ruffled border
Portion of osteoclast in direct contact with bone
Clear zone
Ring of cytoplasm around restorative compartment, immediately surrounds ruffled border, lacks organelles
Actin ring
Inside the clear zone helps create sealing zone isolating subosteoclastic compartment, prevents damage to tissue
How is the inorganic portion of bone resorbed
Dissolved by pumping H+ ions into the resoprtive cavity. The ruffled border increases surface area for transport
How is the organic portion of bone resorbed
Degraded by lysosomal enzyme that are secreted into cavity. Clear zone around it forms a seal to prevent secretions from damaging surrounding tissue
What are the byproducts of resorption and what happens to them
Ca2+ are endocytosed by osteoclast and secreted into blood
What are the two hormones that regulate bone ECM resorption
Parathyroid hormone and calcitonin
Parathyroid hormone
Released by parathyroid gland, promotes resorption, increases blood calcium levels, receptors on osteoblasts, osteoblasts produce osteoclast stimulating factor
Effect: stimulate osteoclast to increase bone resorbing activity
Calcitonin
Secreted by thyroid gland, slow resorption, decrease blood calcium levels, receptors on osteoclasts
Effect: inhibit osteoclast to decease bone resorbing activity
What do oxygen levels determine
Differentiation of mesenchymal cells to facilitate either cartilage or bone formation
What happens in a high O2 environment
Mesenchymal cells differentiate into osteoprogenitor cells and osteoblasts that form bone. This process is called intramembranous ossification and requires vascularized mesenchyme to supply O2
What happens in low O2 environments
Mesenchymal cells differentiate into Chondrogenic cells and chondrocytes that form hyaline cartilage. As blood vessels grow into the cartilage, chondrocytes exposed to high O2 and start acting like bone cells by calcifying ECM. Eventually ECM is replaced with bone. Process called endochondral ossification. It begins in avascular mesenchyme and results in highly vascularized bone
Steps in intramembranous ossification
- Mesenchymal cells near blood vessels differentiate into osteoblasts
- Osteoblasts form bone trabeculae. The early sites of bone formation near blood vessels are called ossification centers
- Over time, the trabeculae enlarge, fuse and remodel to form bones and marrow cavities internally
Steps of endochondral ossification
- A low O2 environment induced mesenchymal cells to differentiate into chondrocytes that form hyaline cartilage model of the bone
- Blood vessels invade the cartilage inducing the formation of ossification centers bone cells migrate and replace the cartilage with bone
- Sheet of hyaline cartilage called a growth plate remains between the epiphysis and diaphysis. Growth plate is responsible for longitudinal growth. Growth ends when the growth plates are vascularized and replaced with bone
Zone of reserve
Directs proliferation
Zone of proliferation
Chondrocyte mitosis
Zone of hypertrophy
Chondrocytes enlarge brining in O2
Zone of calcification
Chondrocytes calcify the ECM
Zone of ossification
Cartilage is replaced with bone
