section 1, chapter 7 The Skeletal System

The science of bones is called osteology Functions of bone Support & protect organs The brain is protected by the skull and the heart and lungs are protected the ribs & sternum Movement Muscles attach to skeleton Inorganic salt storage Stores calcium and phosphate Blood cell production Red bone marrow forms new blood cells

Components of bone The extracellular matrix of bones is composed of 1. hydroxyapatite – a calcium phosphate salt that provides the hardness of bones 2. collagen fibers – provides bone with some pliability The cells associated with bones include: Osteocytes = cells that maintain bone Osteoblasts = cells that deposit new bone. Once mature, osteoblasts become osteocytes. Osteoclasts = cells that dissolve bone. Osteoclasts originate from white blood cells and they secrete an acid that dissolves the inorganic salts of bone.

Bones may be classified by their shape. Long bones = elongated diaphysis humerus radius ulna femur tibia fibula metatarsals metacarpals phalanges Short Bones = cube-shaped carpals tarsals Flat Bones = plate-like sternum ribs scapula parietal and frontal bones

Bone Classification continued Irregular bones = variety of shapes vertebrae mandible maxilla ethmoid bone sphenoid bone sesamoid (or round) bone = develops within tendons patella

Parts of a long bone Diaphysis = shaft of long bone Lined with compact bone Epiphysis = expanded ends of bone Filled with spongy bone Proximal epiphysis & distal epiphysis Sites of articulation (joint) Epiphyseal plates Remnants of bone growth Articular cartilage Hyaline cartilage Covers epiphyses

Parts of a long bone Medullary Cavity Cavity within diaphysis Filled with bone marrow, blood vessels and nerves Endosteum Membrane that lines medullary cavity Contains osteoblasts Periosteum Tough membrane covering bone Continuous with tendons and ligaments Osteoblasts, blood vessels, and nerves

Parts of a long bone Compact bone Lines the Diaphysis Composed of osteons Spongy bone Fills the epiphyses Trabiculae = thin bony plates Osteocytes lie within trabiculae Figure 7.3

Compact Bone Osteon = Structural & functional unit of compact bone Lamella = concentric rings of bone Central Canal = blood vessels and nerves Lacunae = bony chamber that contains an osteocyte Canaliculi = canals with cellular processes Pathway for nutrient and waste diffusion Figure 7.5 Scanning electron micrograph of a single osteon in compact bone.

Osteon continued Perforating Canal = conveys blood from periosteum towards individual osteons

Figure 7.4 Compact bone is composed of osteons cemented together by bone matrix. Figure 7.4c Canaliculi allow nutrients and waste to diffuse between the central canal and individual osteocytes.

Bone Development and Growth Parts of the skeletal system begin to develop during the first few weeks of prenatal development Bone formation = ossification Bones replace existing connective tissue in one of two ways: As intramembranous bones As endchondral bones

Intramembranous Bones Broad, flat bones of the skull Formed by replacing layers of connective tissue (mesenchyme) with bone Osteoblasts within mesenchyme deposit bony matrix in all directions Osteoblasts become osteocytes once surrounded by bone

Endochondral Bones Endochondral Bones Most of the bones in the skeleton are endochondral Bone formation begins with a hyaline cartilage model Cartilage decomposes and is replaced by bone. Figure 7.6a stained bones of a 14-week fetus showing intramembranous and endochorndal bones.

Endochondral Ossification Hyaline cartilage forms model of future bone Cartilage degenerates and periosteum surrounds bone Osteoblasts from periosteum invade the degenerating tissue Osteoblasts beneath periosteum form compact bone at diaphysis = primary ossification center Later, Osteoblasts form spongy bone at epiphyses = secondary ossification center

Endochondral Ossification continued Figure 7.8 Major stages of endochondral ossification. (a-d fetal, e child, f adult)

Endochondral Ossification Two areas of endochondral bone retain cartilage after ossification. Articular cartilage surrounds the epiphyses for joints Epiphyseal plates retain cartilage for bone growth Articular cartilage

Growth at the Epiphyseal Plate Band of hyaline cartilage that remains between the two ossification centers Bone growth continues at epiphyseal plates until adulthood. New cartilage is added towards the epiphysis and cartilage is ossified towards diaphysis Once the epiphyseal plates ossify the bones can no longer be lengthened

4 Layers (zones) of growth at epiphyseal Plate Zone of resting cartilage Cartilage cells near epiphysis Do not participate in bone growth Anchor epiphyseal plate to epiphysis Zone of proliferating cartilage Young chondrocytes undergoing mitosis Adds new cartilage to plate

4 Layers (zones) of growth at epiphyseal Plate Zone of hypertrophic cartilage Older cells enlarge and thicken the epiphyseal plate Osteoblasts invade and calcify the cartilaginous matrix. Zone of calcified cartilage Dead cells & calcium matrix Ossified bone Osteoclasts dissolve and phagocytize the matrix Osteoblasts invade the region and deposit new bone. (b) End of Section 1, Chapter 7 Figure 7.9a

Section 2, Chapter 7 Bone Homeostasis

Homeostasis of Bone Tissue Calcium is constantly exchanged between the blood and bone. Bone resorption = Osteoclasts breakdown bone releasing calcium into the blood. Bone resorption occurs when blood [Ca2+] is low and it’s stimulated by parathyroid hormone (PTH). Bone deposition = Osteoblasts deposit new bone from calcium in the blood stream. Bone deposition occurs when blood [Ca2+] is high and it’s stimulated by the hormone calcitonin.

Nutrients that effect bone homeostasis Vitamin D – promotes Ca2+ absorption in small intestine Vitamin D deficiency = softened and deformed bones Osteomalacia in adults Rickets in children Vitamin A – balances bone resorption and deposition Vitamin A deficiency = retards bone development Vitamin C – is required for collagen synthesis. Vitamin C deficiency = results in fragile bones

Hormones that affect bone homeostasis Calcitonin Secreted from thyroid gland Promotes bone deposition Parathyroid Hormone (PTH) Secreted from parathyroid glands Promotes bone resorption Figure 7.13 Hormonal regulation of blood calcium and resorption

Hormones that affect bone homeostasis Growth Hormone (GH) Secreted from pituitary gland Promotes bone growth at epiphyseal plates Pituitary Gigantism over secretion of GH during childhood Pituitary Dwarfism insufficient GH during childhood Acromegaly Over secretion of GH as an adult Occurs after epiphyseal plates have sealed Enlargement of hands, feet, nose

Hormones that affect bone homeostasis Sex Hormones (testosterone & estrogen) Promotes long bone growth at puberty Sex hormones also stimulate ossification at epiphyseal plates. Effects of Exercise on bone homeostasis Contracting muscles pull on bones and promotes bone thickening Figure 7.12 The thickened bone on the left is better able to withstand forces from muscle contractions.

Bone Fractures Incomplete Fractures Greenstick fracture Fissured

Bone Fractures Complete Fractures Transverse fracture Oblique fracture Comminuted fracture Spiral fracture

Repair of a fracture Step 1. hematoma formation When a bone breaks blood vessels rupture and the periosteum tears. The repair of a broken bone occurs in 5 general steps. Step 1. hematoma formation Blood soon forms a hematoma (blood clot). Hematoma in foot

Repair of a fracture Step 2. temporary spongy bone Osteoblasts invade from periosteum and deposit temporary spongy bone. Step 3. cartilaginous callus Fibroblasts deposit a mass of fibrocartilage “cartilaginous callus” & Phagocytes remove hematoma Osteoclasts remove bony debris

Repair of a fracture Step 4. bony callus Step 5. bone remodeling Osteoblasts replace the cartilaginous callus with bone, forming a bony callus Step 5. bone remodeling Osteoclasts remove excess bone, remodeling the bone the bone close to its original shape.

Disorders of Bone Over time, osteoclasts outnumber osteoblasts, and more bone is resorbed than can be deposited. Bone mass decreases as a result. Bone loss is rapid in menopausal women due to reduced estrogen Osteopenia “low bone mass” Progresses towards osteoporosis Osteoporosis “porous bone” Bones develop spaces and canals Bones are fragile and easily broken Common in menopausal women (from the low estrogen levels)

End of Chapter 7, Section 2 Ways to delay or prevent osteoporosis: Exercise daily. Consume enough calcium and vitamin D every day. Do not smoke. End of Chapter 7, Section 2