Bone (Osseous) Tissue

Osteo = prefix for words involving bones.
Osteology = study of bones
Osteocyte = a bone cell proper - principle cell type of bone.
Osteoblast = cell that generates bone. Osteoclast = cell that destroys bone.
Osteoporosis = disease in which bone material is lost.
Osteoma or osteoblastoma = bone tumor
Osteoid = bone material secreted by osteocyte.
Osteopath = doctor trained in osteopathy (conventional medicine which includes skeletal manipulation)

Bones on Star Trek

There are typically 206 bones in adult human body, but we are born with 270.
Some bones are "lost" by fusion.

"Extra" Bones: Vary from person to person
Wormian or Sutural: form in sutures or fontanels. Interparietal or "Inca bone."
Sesamoid: develop in tendons; patella and pisiform are common examples.
Small sesamoid bones are quite variable. Examples: hand, feet

Axial vs. Appendicular Skeleton

Axial = axis of body. Skull, vertebral column, and rib cage. Also includes the auditory ossicles and hyoid bone.

Appendicular = appendages and girdles (pectoral and pelvic) which anchor the appendages.

Functions of Bone:

Support & Protection
Body Movement
Hemopoiesis (RBC production)
Mineral Storage (Calcium and Phosphorus/Phosphate -- Hydroxyapatite)

Bone Structure:

Each bone is an organ since many different tissues are found in bones. Tissue types include: bone (osseous), cartilage (developing bone and articular cartilage), blood vessels (with blood, endothelial lining, muscle) nervous.

General classifications based on shape:

Long bones: much longer than wide. Most common in appendages, e.g., the femur, humerus, tibia, fibula, metacarpals, metatarsals.
Short bones: cuboidal. Force transference. E.g., tarsals and carpals.
Flat bones: most of bone is a flat surface. E.g., Skull bones such as parietals and frontal; scapula(?).
Irregular: Complex bones having many regions, articulations and processes. E.g., vertebrae; temporal and sphenoid bones of skull.

Surface Features of Bones:

Articulating surfaces: condyle, facet, head.

Non-articulating surfaces: crest, epicondyle, process, spine, trochanter, tubercle, tuberosity.

Depressions or Openings: alveolus, fissure, foramen (pl. foramina); fossa, fovea, meatus (canal), sinus, sulcus.

Gross Structure:

Haversian systems (osteons) were introduced in Chapter 4: Histology

Histology Overview

Compact vs Cancellous (spongy) bone. Example (femur)

Long bones typically have shaft or diaphysis with epiphyses on either end. Medullary cavity = central region containing yellow bone marrow. Spongy bone contains red bone marrow (blood cell production). Medullary cavity is lined with endosteum (connective tissue lining).
Bone Marrow Histology

Periosteum surrounds outside of bone: connective tissue; site of tendon and ligament connection.

Bone Growth: Two types of "ossification"

1) Intramembranous ossification.

Flat bones form by this method. Sheet of mesenchymce is laid down in embryo, ossification center forms in center, bone matrix laid down (all woven or trabecular at first). Periosteum forms. Compact bone (lamellar) forms on outside while woven (spongy) bone remains in center.

2) Endochondral ossification.

Mesenchyme forms and some cells differentiate into chondrocytes. A cartilaginous template or model for each bone is formed. Chondrocytes die after laying down calcium salts (not true bone). Osteoblasts invade from perichondrium and become osteocytes and lay down bone. This first happens in center of bone (primary ossification center), then in the both epiphyses (secondary ossification centers). Bones grow by continued division of chondrocytes and subsequent ossification. Epiphyseal plate is zone at epiphysis at which this is happening. When chondrocytes stop dividing, bone will become totally ossified and bone growth (length) stops. Epiphyseal line replaces plate.

Bone Histology and endochondral ossification (see parts 9 and 10 in this site)

Craniosynostosis: abnormalities in cranial (interosseous) bone formation

cleidocranial dysplasia - defect in intramembranous ossification

Miscellaneous bone histology images

Remodeling

Osteoclasts enzymatically digest old bone from inside; osteoblasts add new bone tissue to sides from periosteum. Bone is constantly being remodeled.

Wolff’s Law: Bone density is influenced by stress placed on it.

Bone Physiology

Plasma calcium levels must be carefully regulated. Bone is major source of calcium and phosphate.

Hypocalcemia (low blood calcium) results in muscle spasms and increased excitability of nerves.

Regulation of plasma calcium: if Calcium levels fall, parathyroid glands are stimulated to release parathyroid hormone (PTH). PTH stimulates osteoclasts (bone resorption) and stimulates calcium conservation by kidney and gut (stimulates vitamin D conversion).

When calcium levels rise, PTH is inhibited and another hormone may be released. The thyroid releases calcitonin in response to elevated calcium levels. Calcitonin lowers blood calcium by inhibiting osteoclasts and stimulating excretion of calcium by kidneys.

Bone growth is influenced by growth hormone and sex steroids (androgens and estrogens)

Osteoclast and Osteoblast Quicktime movie go to "Allelix Biopharmaceuticals" Quicktime movie link.

Hypercalcemia - general info and cancer links

Thyroid & parathyroids

Hyperparathyroidism - general medical info

Animations of Bone Metabolic Units

Giantism and Dwarfism

Excessive production of growth hormone in a child can result in giantism.

Low production of growth hormone in a child can result in dwarfism. Since growth hormone is produced by the pituitary gland, this would be "pituitary dwarfism."

Dysfunctions in endochondral ossification and cartilage formation, e.g., achondroplasia, can also produce dwarfism. Growth hormone levels are normal.

Achondroplasia and Dwarfism Web Sites:

Acromegaly

Excessive growth hormone production after long bone growth has been halted results in thickening of bones.
Thickened facial bones and hands are most noticeable changes.

Acromegaly--Hand comparison

"Andre the Giant" an early wrestling celebrity had both giantism and acromegaly. Andre the Giant wrestling site.

Osteoporosis: inadequate activity of osteoblasts (and/or overactivity of osteoclasts) results in loss of bone density (both mineral and organic components). Bones become prone to fracture. Increased frequency in post-menopausal women.

Estrogen helps to regulate PTH (inhibits PTH activity). After menopause PTH activity is less restrained and osteoclasts increase their activity. Exercise, diet (esp. soy & calcium rich foods), and hormonal (ERT) therapy help.

Fosamax (Alendronate) used to treat osteoporosis -- inhibits osteoclasts directly.

Calcitonin: injected or inhaled via nasal spray is used to treat osteoporosis.

Osteoporosis and Bone Physiology: Great Site!

Osteoporosis:

Why is there a concern that "modest dress" can promote osteoporosis in women?

Paget's Disease: Disorganized bone metabolism/remodeling
Bone may be too thick in some areas and too thin in other areas. Unknown cause.