Bone Cells & Bone Development

Let’s meet Bob and Claude! Cells of Bone Tissue Let’s meet Bob and Claude!

Osteoprogenitor (Osteogenic) Cells Embryonic cells that divide to produce osteoblasts Are located in inner periosteum & endosteum Assist in fracture repair

Osteoblasts Immature bone producing cells that secrete matrix compounds not yet calcified to form bone Osteoblasts surrounded by osseous tissue become osteocytes

Osteocytes Mature bone cells that maintain the bone matrix Do not divide

Osteoclasts Breaks down bone Dissolve bone matrix and release stored minerals which is taken up by the blood Causes osteoporosis (loss of bone tissue) if bone is unable to repair

Bone Metabolism Bone stores 99% of total calcium in body Calcium ions involved with many body systems nerve & muscle cell function blood clotting enzyme function in many biochemical reactions

Bone Homeostasis and Metabolism

Bone Metabolism (Background) The thyroid gland is located in the front of the neck and wraps around the trachea; has a shape that is similar to a butterfly formed by two wings (lobes) and helps to regulate metabolism

Bone Metabolism: Hormones Parathyroid hormone (PTH) is secreted if Ca2+ levels falls PTH gene is turned on and more PTH is secreted from gland, osteoclast activity increased Calcitriol is a hormone that works with PTH & promotes absorption of calcium in digestive tract & blood, to prevent it from being released in the urine.

Bone Metabolism: Hormones Calcitonin is a hormone that produces the opposite effects: if calcium level gets too high, there is less absorption in the digestive tract, increased excretion by the kidneys, and movement of calcium from the blood into the bones inhibits osteoclast activity increases bone formation by osteoblasts

Bone Remodeling Ongoing process Osteoclasts secrete enzymes and acids to break down bone This releases calcium and other minerals of bone for resorption by blood Osteoblasts rebuild osteons

Bone Tissue and Exercise Pull on bone by skeletal muscle and gravity is mechanical stress Lack of mechanical stress results in bone loss reduced activity while in a cast or bedridden person Weight-bearing exercises build bone mass

Osteoporosis Prevention Calcium supplements, weight-bearing exercise, & hormone replacement therapy (including a form of calcitonin) behavior when young may be most important factor!

Disorders of the Skeletal System Failure of bone calcification Rickets Growing children Osteomalacia adults

Herniated Disc Ligaments of the intervertebral discs become weak or are injured, the surrounding fibrocartilage ruptures, and the material inside protrudes

Bunion Inflammation of the joint, bone spurs, calluses Caused by wearing tight fitting shoes

Abnormal Spinal Curvatures

Fractures A fracture is break in a bone

Fracture Repair A fracture is break in a bone Stage 1 (Inflammation) Repair starts with formation of fracture hematoma damaged blood vessels produce clot, bone cells die, new capillaries grow into damaged area Stage 2 (Repair) Callus (healing tissue) formation- fibroblasts lay down collagen fibers, chondroblasts produce cartilage to span the broken areas of the bone

Fracture Repair Bone Formation: Bone remodeling: osteoblasts secrete spongy bone that joins the broken areas of bone lasts 3-4 months Bone remodeling: compact bone replaces the spongy in the callus surface is remodeled back to normal shape

Bone Formation Osteogenesis: Ossification: bone formation the process of replacing other tissues with bone

Ossification 1) Intramembranous ossification (dermal ossification) occurs in the dermis Produces bones such as mandible and clavicle 2) Endochondral ossification Bone replaces cartilage Most bones formed this way Observed easily in long bones

Ossification Chondrocytes in the center of hyaline cartilage enlarge, calcify, and die, leaving cavities in cartilage

Ossification 2. Blood vessels grow around the edges of the cartilage and osteogenic cells begin to change to osteoblasts. Produces layer of superficial bone around shaft which becomes compact bone

Ossification 3. Blood vessels enter the cartilage. Spongy bone develops at the primary ossification center in the shaft where bone tissue replaces cartilage. A marrow cavity is created.

Ossification 4. Capillaries and osteoblasts enter the epiphyses creating secondary ossification centers

Ossification 5. Epiphyses fill with spongy bone (no cavity in this region of the bone). On the ends, hyaline cartilage that remains is articular cartilage.

Epiphyseal Plate At the Epiphyseal plate the bone grows lengthwise as bone tissue replaces cartilage

Epiphyseal Line When long bone stops growing after puberty, epiphyseal cartilage disappears at the growing epiphyseal plate and an epiphyseal line is visible on X-rays.

Bone Growth New cartilage is formed continuously on external face of articular cartilage and on the surface of the epiphyseal plate that faces the bone end (further away from medullary cavity) Old cartilage on the internal face is broken down and replaced by bone

Bone Growth

Bone Growth Bone increases in diameter through appositional growth Appositional growth is controlled by hormones Osteoblasts in periosteum add bone tissue to external face of diaphysis Osteoclasts in endosteum remove bone from inner face of diaphysis wall