Bones: Growth and Regrowth

Bone Development The process of bone development is similar to the process of bone healing Specifically, one of the two types of bone development Bones can be developed in a fetus in one of two ways: Intramembranous ossification Endochondral ossification What can you deduce from the names?

Intramembranous Ossification Unpack the name! Intra=within Ossify=to become a bone “To become a bone within a membrane” A membrane forms, and then within an ossification center, osteogenic cells become osteoblasts and convert the membrane into a flat bone Only flat bones are developed this way

Intramembranous Ossification pt2 Once the ossification has initiated, the connective tissue membrane converts into a flat bone from the center outwards This is NOT how bone heals since it is converting fibrous connective tissue directly to bone Only takes place during fetal development of flat bones

Endochondral Ossification Endochondral ossification is the conversion of cartilage to bone Process begins when chondroblasts form new cartilage in the shape of the bone Chondroblasts that become trapped in the cartilage become chondrocytes Cartilage develops an ossification center that spreads outwards Due to what cells?

Endochondral Ossification Bone collar forms around hyaline cartilage model. Cartilage in the center of the diaphysis calcifies and then develops cavities. The periosteal bud inavades the internal cavities and spongy bone begins to form. The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5. The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. Hyaline cartilage Area of deteriorating cartilage matrix Epiphyseal blood vessel Spongy bone formation Epiphyseal plate cartilage Secondary ossification center Blood vessel of periosteal bud Medullary cavity Articular cartilage Childhood to adolescence Birth Week 9 Month 3 Spongy bone Bone collar Primary ossification center 12345

Further Notes Endochondral ossification is how most bones in the body are formed Some cartilage remains at the joints, creating the articular cartilage that pads the bones at the joints If this cartilage gets worn away, arthritis will result Long bones have three initial ossification centers (diaphysis and each epiphysis) so the growth plate is the cartilage that remains

Bone Healing Bones may break (suffer a fracture) and need to be healed The process of healing closely resembles endochondral ossification Chondroblasts and fibroblasts form a fibrocartilaginous callus Osteoclasts clean away rough edges of broken bone The cartilage callus essentially undergoes ossification by osteoblasts Becomes a bony callus and eventually reincorporated into bone

Fracture Healing The most important part of treating a broken bone is to: 1.Set the bone in the correct position 2.Use a cast or equivalent to keep it from slipping out of place Why? Because the bone will fuse back into place whether you want it to or not Pictured: archaeological finding of a tibia that healed without being set  Oh sweet Jesus that must have hurt

Bone Growth/Decay All bone growth and decay is managed by two competing types of cells Osteoblasts build new bone tissue Osteoclasts degrade/remove bone tissue Bone tissue is constantly being worn down and rebuilt Osteoblasts absorb blood calcium to build bone, osteoclasts release calcium back into the blood

Osteoblast Activity Osteoblast activity is stimulated by increased blood calcium and by force/stress on bone Bones thicken when stressed Forensic analysis can determine whether someone was right- or left- handed! Bone density increases with increased force applied to the bones What exercises are good/bad for bone density? Bone density decreases with age, so build it up when you’re young!

Osteoclast Activity Osteoclasts break down bone material Release calcium into bloodstream when active Low blood calcium stimulates osteoclasts to break down bones to replace calcium Chronic low bone density can lead to osteoporosis in old age This is due to osteoclast activity Don’t blame them, you were the one who didn’t drink milk and then put stress on your bones when you had the chance!

Calcium Homeostasis Osteoclasts are stimulated to wear down bone by a hormone, PTH Parathyroid hormone PTH also stimulates calcium absorption from the kidneys and intestines PTH is released by the Parathyroid gland when blood calcium becomes low When blood calcium becomes high, PTH is not released, and further absorption of calcium is not possible Also causes you to urinate out excess calcium Calcitonin is released from the thyroid as well to further block osteoclast activity (not a key factor)

Osteoclasts degrade bone matrix and release Ca 2+ into blood. Parathyroid glands Thyroid gland Parathyroid glands release parathyroid hormone (PTH). Stimulus Falling blood Ca 2+ levels PTH Calcium homeostasis of blood: 9–11 mg/100 ml BALANCE

Bone Density Bone density is increased by the following behaviors: Regular calcium intake (also vitamins) Heavy forces applied to bones Higher metabolism Being male (testosterone activity) Bone density is decreased by the following behaviors: Lack of weight-bearing exercise Lack of regular calcium and vitamins Aging

That’s our show! Friday: Joints! (Chapter 7)