Bone Development & Growth Bone Growth Video Segment

Hyaline Cartilage – the most common cartilage Chondrocytes = cartilage cells Lacunae = pockets that house chondrocytes Cartilage is avascular (without a blood supply)

The Early Embryonic Skeleton Bone develops later → First skeleton composed of cartilage Replaced with bone (ossification); starting 6 weeks after fertilization More than 300 bones at birth → fuse to 206

Bone Growth 2 types: Interstitial – in length at epiphyseal plate; when growth ceases, bone shows epiphyseal lines Appositional – in width; new bone is added to the outer surface by osteoblasts from periosteum

Bone Remodeling = replacement of old bone by new bone is very metabolically (chemically) active bone is remodeled along the lines of mechanical stress (remember to exercise; exercise increases bone mass) different rates in different regions distal head of the femur is replaced ~ every 4 months

Bone Nutrition Minerals Vitamins Calcium (dairy, leafy vegetables, tofu; the most abundant mineral in the human body) Phosphorus (dairy, lean meat, fish) Vitamins A – stimulates osteoblasts C – needed for collagen synthesis D – needed to make calcitriol (hormone that increases intestinal absorption of Ca++) B12 & K – needed for synthesis of bone proteins

Homeostatic Imbalance: Rickets Homeostatic Imbalances Disease of children due to lack of vitamin D Calcium is not deposited in bones Bones become soft & deformed Rickets

Hormonal Regulation Growth hormone responsible for general growth of all body tissues Sex hormones (testosterone and estrogen) stimulate osteoblasts Calcitonin & parathyroid hormone regulate Ca++ level (calcitonin ↓ [Ca++ ] while parathyroid hormone ↑ [Ca++ ] in the blood) Calcitriol ↑ Ca++ level

Calcium Homeostasis Bone serves as a “buffer” to prevent sudden changes in blood Ca++ levels too much blood Ca++ (hypercalcemia) or too little (hypocalcemia) can cause the heart to stop working

Another Example of Homeostatic Imbalance: Osteoporosis Homeostatic Imbalances Decline in bone density Increased risk of fracture Possible causes: Lack of estrogen in women Lack of exercise to stress the bones Inadequate intake of calcium and phosphorus Abnormalities of vitamin D metabolism Loss of muscle mass

Osteoporosis

Bone Fractures Fracture = break of the bone Repair of bone: Spongy bone forms in first few days Blood vessels regrow and spongy bone hardens Full healing takes 1-2 months

Steps in the Repair of a Fracture Fig. 6.9