Bone Unit

Matrix

65% inorganic material consisting primarily of a calcium phosphate crystal called Hydroxyapatite. 35% is organic material consisting primarily of collagen and proteoglycans. The collagen and mineral components are responsible for the major functional characteristics of bone. Bone matrix might be said to resemble reinforced concrete. Collagen, like reinforcing steel bars, lends flexible strength to the matrix, whereas the mineral components, like concrete, give the matrix compression (weight-bearing) strength.

Osteocyte

Are bone producing cells. Noted for their cytoplasmic extensions. Extensions found in the canaliculi, which are tiny canals. Cells are trapped in bony tunnels called lacunae. Tend to be mature cells and are slow producers. Are trapped by their own hardened secretions or mineralized matrix. Connect and share nutrients.

Osteoblasts

Bone producing cells. Are young and highly productive. Commonly found in the 2nd layer of the periosteum and endosteum. Responsible for the formation of new bone. Some become osteocytes. new bone - ossification/osteogenesis. Protein-collagen.

Osteoclasts

Cells of bone destruction. Note the expanded surface area (size), which is necessary due to the abundance of microvilli. These play a major role in the continuing remodeling of the skeleton. When bone destruction is greater than bone remodeling or new bone formation, it may lead to diseases such as osteoporosis, or brittle bones. The skeleton is completely remodeled every 2 years. Microvilli increase surface area for secretions. Multi-nucleate.

Osteoprogenitor cells

Cells that are genetically predisposed to becoming bone producing cells. Are in an inactive state. Typically found in great numbers in fetus, infants, and children. Stem cells, needs O2.

epiphyseal disc or plate

a band of hyaline cartilage; also called the growth plate; responsible for the growth of bone lengthwise

Periosteum

a double-layered connective tissue sheath which covers bone and particularly the shaft of the long bones: the outer fibrous layer 7s dense and has irregular collagenous ct that contains blood vessels and nerves; the inner layer is a single layer of bone cells, which include osteoblasts, osteoclasts, and osteoprogenitor cells. The collagen fibers of tendons or ligaments become continuous w/ those of the periosteum. *Sharpey's Fibers: Short bundles of collagen fibers which strengthen the attachment of tendons and ligaments to bone*

cancellous bone

also called spongy bone; consists of interconnecting rods or plates of bone called trabeculae; the spaces that are formed as a result of this type of bony arrangement are filled with red marrow and blood vessels (woven bone)

nutrient artery

an artery which enters the shaft of a bone through a foramen; will branch numerous times to contribute to the vascular nature of bone

nutrient foramen

an opening in bone which leads to a bony passageway for small arteries, veins, and nerves

Concentric Lamaelle

are circular layers of bone matrix that surround a common center, the central canal.

Canaliculi

are little canals through which cytoplasmic extensions of the osteocyte extend in order to "reach out and touch" other cells and blood vessels. Through this approach, nutrients can diffuse from the blood vessels and cell to cell. Bow refers to these VERY minute passageways as "microfractures." Overall, osteocytes receive nutrients and eliminate waste products through the canal system within compact bone.

Lacunae

are small canals or passageways that outline a lamellae and run parallel to the central canal in a circular fashion. The space within is occupied by the osteocyte cell bodies.

articulating cartilage

hyaline cartilage; thin layer of cartilage which covers the ends of bones; where bone meets bone to form a joint; very smooth; to help create a friction free environment

distal epiphysis

inferior; portion of bone furthest from the head; has spongy bone

yellow marrow

is mostly adipose tissue; children have red marrow in the medullary cavity. As children mature, the red marrow is replaced with yellow marrow

diasphysis

long, narrow portion of bone; the shaft; has compact bone for strength, made of repeating units

epiphyseal line

marks the site of the pre-existing growth plate; it is now a mere remnant of jagged line; growing has ceased; no cartilage, only bone

Osteon/Haversian system

microscopic, functional, and repeating unit of compact bone. It consists of a single central canal, its contents, concentric lamella (matrix), lacunae with osteocytes, and canaliculi.

Central/Haversian Canals

passageways that run parallel to the long axis of the bone (longitudinally). Central canals are lined with endosteum and contain blood vessels, nerves, and loose ct.

Volkmann's Canals/Perforating canals

run perpendicularly to the long axis of the bone. In other words, these canals are oriented transversely, or horizontally. They pass through concentric lamellae. Blood vessels of the central canal are branches of the blood vessels of the Volkmann's Canals. The blood vessels of the canals typically enter and exit the bone by way of nutrient foramens.

red marrow

site of blood cell production; red blood cells, white blood cells, and platelets

Proximal epiphysis

superior; portion of bone closest to the head; has spongy bone

Medullary cavity

the large space within the shaft of the long bone; is filled with yellow marrow

Endosteum

the membrane which lines the medullary cavity of the diaphysis and the smaller cavities in cancellous and compact bone

compact bone

very dense bone; very strong; found where strength is of great importance, like the shaft of long bones and the perimeter of all bones; is comprised of repeating, microscopic, and functional units called Haversian systems or Osteons

Osteocytes

were once osteoblasts, but now have become surrounded by bony matrix (a trapped cell). Although these cells may become somewhat inactive, they can produce enough matrix to assist in bone maintenance.