Osteoblasts differentiate from (stem) osteoprogenitor cells and

secrete components of the initial matrix, called osteon, that allow

matrix mineralization to occur.

Important components of osteoid include type II collagen, the

protein osteocalcin, which binds Ca2+ and matrix vesicles with

enzymes generating PO4

-.

High concentrations of Ca2+ and PO4

- ions cause formation of

hydroxyapatite crystals, whose growth gradually calcifies the

entire matrix.

Osteocytes differentiate further from osteoblasts when they

become enclosed within matrix lamellae and act to maintain the

matrix and detect mechanical stresses on bone.

Osteoclasts are very small cells, formed by fusion of several blood

monocytes, which locally erode bone matrix during osteogenesis

and bone remodeling.

Periosteum is a layer of dense connective tissue on the inner surface

of bone, bound to bone matrix by bundles of type I collagen called

perforating (or Sharpey) fibers.

Regions of periosteum adjacent to bone are rich in osteoprogenitor

cells and osteoblasts that mediate much bone growth and

remodeling.

The endosteum is a thick layer of active and inactive osteoblasts,

which lines all the internal surfaces within bone; osteoblasts here are

also required for bone growth.

In long bones of the limbs, these two types of mature bone tissue

occur in both the knobby, bulbous ends, called epiphyses, and in

the intervening shaft or diaphysis.

Immature bone, called lamellar bone, is formed during osteogenesis

or repair and has a calcified matrix with randomly arranged collagen

fibers.

Most lamellar bone consists of lamellae organized concentrically

around small central canals containing blood vessels and nerves;

this organization is called an osteon or Haversian system.

Within each osteon osteocytic lacunae occur between the lamellae,

with canaliculi radiating through the lamellae, which allow all

cells to communicate with the central canal.

All other bones form by intramembranous ossification, in which

osteoprogenitor cells surround and then invade hyaline cartilage

models of the skeletal elements in the embryo.

The growth plates are the key to bone elongation during childhood

and are organized as an interrelated series of developing zones.

Most distally is a “resting” or reserve zone of typical fibrocartilage.

In the zone of ossification, woven bone is laid down initially by

osteoblasts and remodeled into lamellae bone.

Appositional bone growth increases the circumference of a bone by

osteoblast activity at the periosteum and is accompanied by enlargement

of the medullary marrow cavity.

Lamellae and osteons are permanent structures and are replaced

and rebuilt continuously in a process of bone remodeling by which

bones change size and shape according to changes in mechanical

stress.

Bone repair after fracture or other injury involves the activation of

periosteal fibroblasts to produce an initial hard callus of fibrocartilage-

like tissue.

Ca2+, a key ion for all cells, is stored in bone when dietary calcium

is adequate and mobilized from bone when dietary calcium is

deficient.

Maintenance of proper blood calcium levels involves activity of all

three major bone cells and is largely regulated by subtle apocrine

interaction among these and other cells.

Parathyroid Hormone indirectly stimulates osteoclasts to inhibit levels

of calcium in blood, whereas calcitonin, which can elevates osteoclast

activity, increasing blood calcium levels.

Joints with very limited or no movement are classified collectively as

diathroses and freely mobile joints are called synarthroses.

Intervertebral discs are example synarthroses

Each intervertebral disc consists of a thin outer layer of fibrocartilage

forming a tough annulus fibrosus and a shock-absorbing inner,

gel-like core, the nucleus pulposus.

Diarthroses have a joint cavity filled with lubricant synovial fluid,

enclosed within a tough, fibrous articular capsule; ends of the bones

involved are covered with hyaline articular cartilage.

Fibroblast-like synovial cells of the synovial membrane remove

wear-and-tear debris from synovial fluid.

Which of the following osteocyte-derived biomolecules is

the major structural component present in bone matrix?

Which of the following connective tissue cells initiates

the mineralization of the bone matrix during growth and

bone remodeling throughout life?

A 57-year-old woman undergoes spine fusion surgery.

Following the procedure, the surgeon prescribes osteogenic

protein-1 (OP-1) to promote bone growth and healing.

OP-1 (also referred as bone morphogenic protein-7)

has been shown to stimulate the differentiation of mesenchymal

stem cells to yield which of the following cell types?

Bone obtained at the autopsy of a 68-year-old man is examined by light microscopy (shown in the image). Identify the structures indicated by the circles

A ground (polished) section of bone from the patient described in Question 27 is examined under high magnification (shown in the image). Identify the features indicated by the arrows.

The function of the periosteum in supporting new bone

growth is determined fundamentally by paracrine/endocrine

signals that regulate the differentiation of which of

the following connective tissue cells?

Osteoclasts are derived from which of the following

types of cells?

A 6-year-old boy with mild hydrocephalus suffers chronic infections and dies of intractable chronic anemia. At autopsy, his bones are dense and misshapen. The femur in particular shows obliteration of the marrow space. Hematopoietic bone marrow cells are sparse. The diagnosis is osteopetrosis, a rare genetic disorder caused by dysfunction of which of the following cell types?

For the patient described in Question 39, impaired mineralization of the new bone matrix was most likely caused by a chronic deficiency of which of the following vitamins?

Which of the following bones forms via the intramembranous ossification pathway during embryonic and fetal development?

A 16-year-old girl is seen for her annual health checkup. Measurements taken in the office indicate that the patient has grown 15 cm over the past year. Which of the following cells in epiphyseal growth plates was chiefly responsible for the longitudinal growth of long bones in this patient?

A 28-year-old man with achondroplasia is admitted to the hospital for hip replacement due to severe osteoarthritis. He has short arms and legs and a relatively large head. Which of the following pathogenetic mechanisms is most likely responsible for this patient’s inherited condition?

A 15-year-old girl complains of uneven shoulders that she first noticed 3 months ago while trying on a new dress. Physical examination reveals uneven back musculature and a prominent left scapula caused by rotation of the thoracic ribcage. Radiologic studies confirm mild scoliosis. This developmental abnormality is most likely caused by asymmetric growth of which of the following musculoskeletal structures?

A 14-year-old girl fractures her right fibula playing soccer. The leg is immobilized in a cast. During healing, a soft callus forms at the site of bone fracture. Which of the following represents the primary tissue composition of the patient’s soft callus?