Chapter Seven

Primary organs of the skeletal system

Relatively dense connective tissue that appears white, smooth, and solid

located internal to the compact bone, appears porous, and makes up approximately 20% of the total bone mass

a semi-rigid connective tissue that is more flexible than bone

• attaches ribs to the sternum, covers the ends of some bones, the cartilage within growth plates
• provides a model for the formation of most of the bones in the body

• weight-bearing cartilage that withstands compression
• forms the intervertebral discs, the pubic symphysis, and cartilage pads of the knee joints

• support and protection
• movement
• hemopoiesis (process of blood cell production)
• storage of mineral and energy reserves

• -Greater in length than width
• -Have an elongated, cylindrical shaft
• -Found in the upper limbs and lower limbs

• -Have a length nearly equal to their width
• -Include the carpals and tarsals

small, sesame seed-shaped bones along the tendons of some muscles

• -Have flat, thin surface that may be slightly curved
• -Provide extensive surfaces fro muscle attachment and protect underlying soft tissues
• -Form the roof of the skull, the scapulae, the sternum, and the ribs

• -Elaborate, sometimes complex shapes
• -The vertebrae, hip bones, and several bones of the skull

• -The shaft of the long bone
• -Provides the leverage and major weight support of a long bone

• -Knobby region at each end of a long bone
• -Composed of an outer thin layer of compact bone and an inner, more extensive region of spongy bone
• -Articular cartilage covers the joint surface of an epiphysis to reduce friction and absorb shock

the end of the bone closest to the body trunk

the end farthest from the trunk

• a typical long bone contains both compact and spongy bone
• Medullary cavity: the hollow, cylindrical space within the diaphysis
• The endosteum lines the internal surface of the bone along the edge of the medullary cavity     
• The periosteum lines the
• external surface of the bone shaft
• o  The outer fibrous layer
• of dense irregular connective tissue protects the bone from surrounding
• structures, anchors blood vessels and nerves to the surface of the bone, and
• serves as an attachment site for ligaments and tendons
• o  The inner cellular layer
• includes osteoprogenerator cells, osteoblasts, and osteoclasts

• stem cells derived from mesenchyme
• -when they divide through mitosis, another stem cells is produced along with a "committed cell" that matures to become an osteoblast
• -Located in both periosteum and endosteum

• Osteoblasts: synthesize and secretes the osteoid
• Osteocytes: mature bone cells derived from osteoblasts that have lost their bone-forming ability (maintain the bone matrix and detect mechanical stress on a bone)
• Osteoclast: large, multinuclear, phagocytic cells. involved in breaking down bone

• Composed of both organic and inorganic components
• Osteoid: composed of both collagen protein plus a semisolid ground substance of proteoglycans and glycoproteins that suspends and supports the collagen fibers
• Also, composed of salt crytals that are primarily calcium phosphate

• are derived from mesenchymal cells and produce the cartilage matrix
• once they become encased within the matrix they are called chondrocytes which maintain the matrix

the basic functional and structural unit of mature compact bone

a cylindrical channel that lies in the center of the osteon and runs parallel to it, blood vessels and nerves travel within this

rings of bone connective tissue that surround the central canal and form the bulk of the osteon

mature bone cells found in small spaces between adjacent concentric lamellae, maintain the bone matrix

small spaces that house an osteocyte

• tiny, interconnecting channels within the bone connective tissue that extend from each lacuna, travel through the lamellae, and connect to other lacunae and the central canal
• it houses osteocyte cytoplasmic projections that permit intercellular contact and communication

• an open lattice of narrow rods and plates of bones, found in the spongy bone
• Their orientation is important because their lattice structure provides great resistance to stresses applied in many directions by distributing the stress throughout the entire framework

It is important because it is resilient and flexible and is a good shock absorber. It is also a good a model for the formation of most of the bones in the body

• bone growth within a membrane
• produces the flat bones of the skull, some of the facial bones, the mandible, and the central part of the clavicle
• begins when mesenchyme becomes thickened and condensed with a dense supply of blood capillaries

• -ossification centers form within the thickened regions of mesenchyme beginning at the eighth week of development
• -osteoid undergoes calcification
• -woven bone and its surrounding periosteum form
• -Lamellar bone replaces woven bone as compact bone and spongy bone form

begins with a hyaline cartilage model and produces most bones of the skeleton, including those of the upper and lower limbs, the pelvis, the vertebrae, and the end of the clavicle

• -Fetal hyaline cartilage model develops
• -Cartilage calcifies, and a periosteal bone collar forms
• -Secondary ossification centers form in the epiphyses
• -Bone replaces almost all cartilage, except the articular cartilage and epiphyseal cartilage
• -Lengthwise growth continues until the epiphyseal plates ossify and form epiphyseal lines

• -Dependent upon growth of cartilage within the epiphyseal plate
• -Growth in bone length occurs specifically within zone 2 as chondrocytes undergo mitotic cell division, and in zone 3 as chondrocytes hypertrophy
• -Growth in length is due to growth in hyaline cartilage connective tissue that is later replaced with bone

• -Occurs within the periosteum
• -Osteoblasts in the inner cellular layer of the periosteum produce and deposit bone matrix within layers parallel to the surface, called external circumferential lamellae
• -as the lamellae increase in number, the structure increases in diameter and the bone becomes wider

constant, dynamic process of continual addition of new bone tissue and removal of old bone tissue

helps the body regulate calcium levels in the blood, helps produce proteins to transport calcium throughout the body

stimulates liver to produce the hormone somatomedin, which causes cartilage proliferation at epiphyseal plate and resulting bone elongation

stimulates bone growth by stimulating metabolic rate of osteoblasts

stimulates osteoblasts; promotes epiphyseal plate growth and closure, increases the rate of both cartilage growth and bone formation within the epiphyseal plate (increased length of long bones and increased height)

promotes calcium deposition in bone and inhibits osteoclast activity

increase bone loss and, in children, impair bone growth when there are chronically high levels of glucocorticoids

• inhibits osteoprogenitor cells from differentiating into osteoblasts when there are chronically high levels of serotonin
• plays a role in the rate and regulation of normal bone remodeling because of this

• -PTH and calcitriol work together to increase the release of calcium from bone into the blood, increases osteoclast activity resulting in calcium release from the bone into the blood
• -PTH and calcitriol work together to stimulate the kidney to excrete less calcium in the urine by increasing calcium reabsorption in the tubules of the kidney

• -it is released from the thyroid gland in response to high blood calcium levels; it is also secreted in response to stress from exercise
• -calcitonin primarily inhibits osteoclast activity so less calcium is released from the bone into the blood
• -calcitonin stimulates the kidneys to increase the loss of calcium in urine, thereby reducing calcium levels in the blood