Histo Lecture 10 II
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Intramembranous Ossification is responsible for formation of for:
- Bone healing
- Growth of flat bones (i.e. skulls, carpals, tarsals)
- Thickening of long bones
What are the steps of osteoblasts in intramembranous ossification?
- Osteoblasts secrete organic matrix (osteoid)
- Osteoblasts then directly mineralize the matrix with hydroxyapatite crystals
- Once surrounded by calcified matrix, osteoblasts become osteocytes
Osteoblasts are formed by differentiation of cells of the:
Periosteum and endosteum or bone marrow stem cells (mesenchyme)
What function do chondrocytes perform?
Produce cartilage and calcify the matrix
Where do osteoblasts deposit bone matrix (osteoid)?
Over the pre-existing cartilage matrix
Function of osteoblasts/osteocytes
Remove remnants of cartilage matrix resulting in typical bone matrix
Endochostral ossification is responsible for
- Growth of long bones
- Bone healing
Area of normal hyaline cartilage and chondrocytes
Area of intestine mitosis (proliferation) of chondrocytes
What happens in the Hypertrophic Zone?
- Area of chondrocyte hypertrophy due to glycogen uptake
- Cartilage ECM is partially resorbed
- Remnants appear as septa of matrix material between hypertrophic chondrocytes
What happens in the Calcified cartilage zone?
- Thin septa of cartilage matrix become calicified
- Chondrocytes in this zone die after matrix calcification
The Ossification zone is in charge of being
- The osteoprogenitor cells from the bone marrow are delivered to the calcified cartilage
- -Osteoprogenitor cells differentiate into osteoblasts
Osteoblasts deposit bone matrix over calcified cartilage matrix
Following ossification, the _____ cartilage is resorbed, leaving a calcified bone matrix, which is ______ bone tissue
Overall, the thickness of the _________ _______ does not change as that rate of bone production and cartilage growth are equal at opposite ends.
Endochondral ossification during fetal development is a ______ process
Bone under goes ________ remodeling througout life.
Bone remodeling is dependent on ________ placed on bones
Stresses placed on bones affect:
Deposition and resorption
State Wolff's Law
Every change in form and function of bone, or in its function alone, is followed by certain, definite changes in its internal architecture and external form.
Describe the process of how bones are repaired from a fracture
- Damaged blood vessels produce a localized clot
- Clot material is later removed by macrophages
- Periosteum and Endosteum repond with intense proliferation of cells
- Periosteal/endosteal fibroblasts differentiate into chondroblasts and hyaline cartilage model = soft callus
- Mesenchyme cells differentiate into osteoblasts and form osteoid
- Endochondral and intramembranous ossification occur to form primary (woven) bone = hard callus
- Hard callus is replaced by lamellar bone
Healing Time Table
- Soft callous - 2 wks
- Hard callous - 4 wks
- Lamellar bone - 6 wks
Does stress affect healing bone?
Yes. Healing bone is influenced by stresses placed upon it during the healing process
How does inactivity affect bones?
- Inactivity or decreased loading of bone results in reduced osteoblast activity but does not affect osteoclast activity
- Overall result is increased degradation of bone and increase in serum CA++ levels
What 3 qualities of bone change with increased loading?
- Hypertrophy of bone
- ↑ bone mass/density
- ↑ load to failure (strength)
Repetitive loading on bone creates:
What aides the bone healing process?
What causes stress fratures?
Stress fractures result when loading outpaces bone repair/remodeling
What is Osteopenia?
- Reuction in bone mineral density below normal levels, results in decreased bone strength
- (Often a precursor to the development of osteoporosis)
What causes Osteopenia?
Inactivity, aging, low intake of vitamin D and calcium, and smoking
What is the definition of Osteoporosis?
Defined as decrease in bone density and decrease in overall volume of bone
How is Osteopenia classified?
1 to 2.5 standard deviations from the norm
How is Osteoporosis classified?
Greater than 2.5 standard deviations from normal BMD
How many Americans have osteoporsis?
10 million (80% females)
Name some risk factors for Osteoporosis
- Older age
- Small frame
- Caucasian, Asian, Hispanic
- Poor diet (Low Vit D % calc; excess protein, sodium, caffeine)
- Smoking, alcohol abuse
- Steroid medications
- Anorexia, RA, GI pathology
What percent of the body's calcium is stored in bone?
Calcium levels are controlled by what system?
What are 3 regulators in calcium metabolism?
- -Parathyroid hormone
- -Vitamin D
How is Hypercalcemia determined?
- >10.5 mg/dL of blood
- Causes mild to severe proximal weakness in extremities
How is Hypocalcemia determined?
- <8.5 mg/dL of blood
- neurmosucular excitability and muscular tetany (especially UE flexion)
What is secreted in response to low levels of Ca++ in the plasma?
PTH promotes what action in the bone?
Osteoclast resorption and liberation of Ca++ into the blood
Where does PTH bind?
- -Osteoblasts stop producing bone
- -Osteoblasts secrete an osteoclast-stimulating factor
Where else does PTH act to affect calcium levels?
- Enhances calcium absorption from intestines
- Decreasese calcium excretion by kidneys
The thyroid gland synthesizes what hormone in response to elevated levels of Ca++ in the blood?
What is the function of Calcitonin?
- Inhibits matrix resorption by osteoclasts
- Inhibits new osteoclast formation
What vital bone cell does Calcitonin NOT interact with?
What is essential for intestinal absorption of Ca++ and PO4- from the kidney?
Vitamin D is necessary for active reabsorption of Ca++ and PO4- from what organ?
What stimulates the formation of Vitamin D?
What processes inative Vitamin D? What activates it?
4 characteristics of Diarthrodial Joint
- Outer fibrous layer
- Inner synovial layer (synovial membrane)
- Joint cavity
Define synovial fluid
Viscous, colorless & transparent fluid formed from a dialysate of plasma
The outer fibrous layer is comprised of what 3 things
- Dense irregular CT
- Continuation of periosteum
- Continuation with tendons and ligaments
Functions of Joint Capsule
- Nourishment (Direct vascular supply)
- Innervation (Mechanoreceptors, Free nerve endings, GVA/GVE to vasculature)
Cell types of synovium
- F cells - Synthesize large amounts of hyaluronic acid that is secreted into the synovial fluid
- M cells - Differentiated blood monocyte that serves as phagocytic cell of synovial membrane
Functions of the synovium
- -Highly vascularized
- -No neural innervation
- Produces synovial fluid for waste/nutrient exchange
- Synovial fluid to improve frictionless motion
Main constituents on Synovial Fluid
- Hyaluronic acid
- Electrolyte composition similar to plasma
Function of Synovial Fluid
- Delivers O2 to articular cartilage
- Medium for removal of waste products
- Reduces friction on articular surfaces
What are syndesmosis?
- When bones are united by dense connective tissue
- -ex. interosseus ligament between radius and ulna
What are Synchondrosis?
- Bones are united by hyaline cartilage
- -ex. epiphyseal plates, sternocostal joints
What is Symphysis?
- Bones united by fibrocartilage. Type of Synchondrosis
- -ex. pubic symphysis and IV disks
What are syntosis?
- Bones are united by bony tissue that has replaced normal synarthroses
- -Ex. cranial sutures in adults, epiphyseal plates upon maturity
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