General Anatomy (osteogenesis And Bone Growth) Flashcards
What are the four types of bone cells?
- Osteoprogenitor (osteogenic) cells – undifferentiated cells – can divide to replace themselves & can become osteoblasts – found in inner layer of periosteum and endosteum
- Osteoblasts – form matrix & collagen fibers but can’t divide
- Osteocytes – the principal cells of bone tissue. – mature cells that no longer secrete matrix
- Osteoclasts – huge cells from fused monocytes (WBC) – serve to break down bone tissue – function in bone resorption at surfaces such as endosteum
What is ossification?
- All embryonic connective tissue begins as mesenchyme
- Bone formation is called osteogenesis or ossification
What are the two types of ossification?
– Intramembranous ossification is the formation of bone directly from fibrous connective tissue membranes (dermis)
– Endochondral ossification is the formation of bone from hyaline cartilage models
What happens during intramembranous ossification?
Step 1: An ossification center appears in the fibrous connective tissue membrane.
- Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center.
Step 2: Bone matrix (osteoid) is secreted within the fibrous membrane.
- Osteoblasts begin to secrete osteoid, which is mineralized within a few days.
- Trapped osteoblasts become osteocytes
Step 3: Woven bone and periosteum form
- Accumulating osteoid is laid down between embryonic blood vessels, which form a random network. The result is a network (instead of lamellae) of trabeculae.
- Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum.
Step 4: Bone collar of compact bone forms and red marrow appears.
- Trabeculae just deep to the periosteum thicken, forming a woven bone collar that is later replaced with mature lamellar bone.
- Spongy bone consisting of distinct trabeculae persists internally and its vascular tissue becomes red marrow.
What is the process of endochondral ossification?
- chondrocytes at the center of the growing cartilage model enlarge and then die as the matrix calcifies.
- newly derived osteoblasts cover the shaft of the cartilage in a thin layer of bone
- blood vessels penetrate the cartilage. New osteoblasts form a primary ossification center
- the bone of the shaft thickens,and the cartilage near each epiphysis is replaced by shafts of bone
- blood vessels invade the epiphyses and osteoblasts form secondary centers of ossification.
Process of bone growth in lenght
- The epiphyseal plate is the area of growth in a long bone.
- It is a layer of hyaline cartilage where ossification occurs in immature bones.
- On the epiphyseal side of the epiphyseal plate, cartilage is formed.
- On the diaphyseal side, cartilage is ossified, and the diaphysis grows in length.
- The epiphyseal plate is composed of four zones of cells and activity.
- Between ages 18 to 25, epiphyseal plates close – cartilage cells stop dividing and bone replaces the cartilage (epiphyseal line) (Growth in length stops by age 25)
What are the zones of the epiphyseal plate?
- proliferation zone - chondroblasts quickly divide and push the epiphysis away from the diaphysis, lengthening the bone.
- hypertrophic zone - older chondrocytes enlarge and signal the surrounding matrix to calcify
- calcification zone - matrix becomes calcified; chondrocytes die, leaving behind trabeculae-shaped calcified cartilage NOT YET BONE
- ossification zone - osteoclasts digest the calcified cartilage, and osteoblasts replace it with actual bone tissue in the shape of the calcified cartilage-resulting in bone trabeculae
What is the process of bone growth in thickness?
- While bones are increasing in length, they are also increasing in diameter; growth in diameter can continue even after longitudinal growth ceases.
- This is called appositional growth.
- Osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum.
- The erosion of old bone along the medullary cavity and the deposition of new bone beneath the periosteum not only increase the diameter of the diaphysis but also increase the diameter of the medullary cavity.
- This process is called modeling.
What is bone remodeling?
Remodeling is the ongoing replacement of old bone tissue by new bone tissue
- old bone is constantly destroyed by osteoclasts, whereas new bone is constructed by osteoblasts
- several hormones and calcitriol control bone growth and bone remodelling
Ongoing since osteoclasts carve out small tunnels and osteoblasts rebuild osteons.
– osteoclasts form leak-proof seal around cell edges – secrete enzymes and acids beneath themselves
– release calcium and phosphorus into interstitial fluid – osteoblasts take over bone rebuilding
- Continual redistribution of bone matrix along lines of mechanical stress – distal femur is fully remodeled every 4 months
What factors affect bone growth?
Nutrition – adequate levels of minerals and vitamins o - calcium and phosphorus for bone growth
- vitamin C for collagen formation
- vitamins K and B12 for protein synthesis
Sufficient levels of specific hormones
– during childhood need insulin-like growth factor - promotes cell division at epiphyseal plate
- need GH(growth),thyroid(T3&T4)
– at puberty the sex hormones, estrogen and testosterone, stimulate sudden growth and modifications of the skeleton to create the male and female secondary characteristics
Where are growth hormones secreted?
Secreted from the anterior pituitary and is regulated by the hypothalamus and the mediator of GH actions
What are the regulatory factors for the growth hormone?
- GHRH
- SRIF
- ghrerin
- IGF-I
What is the hormonal abnormalities affecting bone growth?
- Oversecretion of hGH (human growth hormone) during childhood produces giantism
- Undersecretion of hGH or thyroid hormone during childhood produces dwarfism
- Both men or women that lack oestrogen receptors on cells grow taller than normal – oestrogen is responsible for closure of growth plate
How does a fracture repair itself?
Formation of fracture hematoma – damaged blood vessels produce clot in 6-8 hours, bone cells die – inflammation brings in phagocytic cells for clean-up duty – new capillaries grow into damaged area
Formation of fibrocartilagenous (soft) callus – fibroblasts invade & lay down collagen fibers – chondroblasts produce fibrocartilage to span the broken ends of the bone
Formation of bony (hard) callus – osteoblasts secrete spongy bone that joins 2 broken ends of bone – lasts 3-4 months
Bone remodeling – compact bone replaces the spongy bone in the bony callus – surface is remodeled back to normal shape
