Block 2: Skeletal System - Lecture 2.4 Flashcards
What is Bone Modeling?
Construction of bone by Osteoblasts without prior bone resorption
a. occurs during growth
b. produces changes in bone size and shape
What is Bone Remodeling?
Reconstruction of bone that is first resorbed by Osteoclasts and then formed in the same locations by Osteoblasts
a. happens throughout entire life
b. maintains healthy skeletal tissue
What are the 2 major goals of Bone Modeling and Remodeling?
- Achieve strength for loading
a. response to mechanical stressors
b. peak skeletal strength comes from bone modeling - Maintain lightness for mobility
a. keeps strength in response to life experiences from bone remodeling
What are the 2 types of Bone Modeling?
Intramembranous Ossification: in membrane
Endochondral Ossification: from cartilage
What is Osteogenesis?
Bone formation throughout the body
Why are newborns born with about 100 more bones that a full grown adult?
- Facilitates brain growth
- Soft spots (not yet calcified) allow passage through birth canal
- Growth is not complete when born
What are the major differences between Prenatal and Postnatal Osteogenesis?
Prenatal:
a. Intramembranous Ossification: from mesenchyme
b. Endochondral Ossification: hyaline cartilage to bone
Postnatal:
a. Increase length (longitudinal growth)
b. Increase width (appositional growth)
sometimes sesamoid bones form
How does Intramembranous Ossification start?
2 ways:
a. with mesenchymal tissue (forms all CT)
b. with underdeveloped fibrous CT
What are some examples of bones that are formed through Intramembranous Ossification?
Cranial Bones, most Facial Bones, and parts of Clavicles
Creates layers: compact –> spongy –> compact
What is the first step in Intramembranous Ossification?
Mesenchymal cells form many ossification centers
a. mesenchymal cells congergate in areas with lots of blood vessels to increase tissue to bring nutrients signals, etc.
Process:
mesenchymal cells line blood vessels –> differentiation into osteogenic cells –> osteoblasts (can make proteins) –> secreting osteoid (organic: collagen fibers & GAGs)
What is the second step in Intramembranous Ossification?
Calcium salts crystallize on osteoid
a. forms spicules
Process:
calcium salts (brought by osteoblasts) crystallize on osteoid –> calcification –> formation of spicules –> some osteoblasts get trapped and differentiate into osteocytes
What is the third step in Intramembranous Ossification?
Mesenchyme differentiates
a. forms periosteum (outside of bone)
Process:
mesenchymal differentiation –> formation of periosteum –> all ossification centers meet –> formation of branching plates (trabeculae)
What is the fourth step in Intramembranous Ossification?
Osteoblasts form Compact Bone Layers
Process:
osteoblasts within periosteum create outside layers of compact bone –> continuation of remodeling spongy bone by osteoblasts & osteoclasts
What are the four basic steps to Intramembranous Ossification?
- Mesenchymal cells form many ossification centers
- Calcium salts crystallize on osteoid
- Mesenchyme differentiates
- Osteoblasts form Compact Bone Layers
What disease is a special form of Intramembranous Ossification?
Heterotropic Bone Formation
a. abnormal stresses stimulate bones to form where they shouldn’t
b. classified as sesamoid bones
What are the key differences between calcification and ossification?
Calcification: depositing of calcium
Ossification: replacement of CT by bone tissue
Calcification is a part of Ossification
How does Endochondral Ossification start?
Hyaline Cartilage
What bones are formed through Endochondral Ossification?
Most bones below the skull: humerus, femur, vertebrae, etc.
a. long bones develop diaphysis, epiphyses, and medullary cavity
What is the first step to Endochondral Ossification?
Mesenchyme differientiates into hyaline cartilage
a. covered by the perichondrium, creating a boundary for the cartilage
What is the second step to Endochondral Ossification?
Formation of a single primary ossification center
a. in the middle of future diaphysis
Process:
increased vasculature/blood flow –> brings nutrients, chemical signals, and osteogenic cells –> osteoblast formation (before matrix) –> perichondrium –> periosteum –> bony collar formation –> matrix begins to calcify –> chondrocytes at primary ossification center grow in size –> no diffusion of nutrients –> chondrocyte death leaves behind large cavities
What occurs as the first portion of step three in Endochondral Ossification?
Periosteal bud causes primary medullary formation
a. includes nutrient artery/vein, nerve fibers, osteogenic cells, and osteoclasts
Process:
periosteal bud invades medullary cavity –> cavity remodeling via osteoblasts & osteoclasts –> spongy bone formation –> primary medullary cavity formation
What occurs as the second portion of step three in Endochondral Ossification?
Secondary ossification center(s) form
a. chondrocytes go through similar process as chondrocytes for the primary ossification center
b. separated by epiphyseal plate
What is the fourth step in Endochondral Ossification?
Secondary medullary cavities form in epiphyses
a. one secondary center will be delayed
b. most long bones have 2 secondary centers
c. more distal center = faster growth
What is the fifth step in Endochondral Ossification?
Further growth
a. when growing the epiphyseal plate will ALWAYS be cartilage (no cartilage in adult bones = growth plates!)
Process:
increase cartilage at joints and epiphyseal plate –> ossification
What are the key differences between Primary Ossification and Secondary Ossification in Endochondral Ossification?
Primary Ossification:
a. marrow cavity formed
b. replaces all cartilage with bone
c. located in diaphysis
d. occurs before birth
Secondary Ossification:
a. no marrow cavity
b. some cartilage left for epiphyseal (growth plates) plates and articular cartilage
c. located in epiphyses
d. occurs around the time of birth
