Cartilage

I. GROUND SUBSTANCE contains the following components:

A. Proteoglycans
B. Glycosaminoglycans (GAGs): chondroitin sulfate and keratan sulfate
C. Glycoprotein: chondronectin
D. Mineral (inorganic) component: none

II. FIBERS

A. Type I collagen is found in fibrocartilage.
B. Type II collagen is found in hyaline and elastic cartilage.

III. CELLS

A. Chondroblasts arise from the perichondrium and may undergo mitosis.
B. Chondrocytes reside in lacunae and form isogenous groups surrounded by a territorial matrix.
This matrix is ground substance that stains basophilic because of the higher local concentration of
chondroitin sulfate. Chondrocytes may undergo mitosis.

IV. BLOOD VESSELS AND NERVES are absent. Like epithelium, cartilage is avascular. It receives
its nutrients by diffusion through the ground substance.

V. CHONDROGENESIS occurs in the embryo when mesenchymal cells withdraw their processes and
condense into aggregations called centers of chondrification.
Cartilage may then grow in the following ways:

A. Interstitial growth occurs by mitosis of preexisting chondrocytes.
B. Appositional growth occurs by differentiation of cells in the perichondrium into chondroblasts.

VI. HORMONAL INFLUENCE

A. Chondrocytes are stimulated by triiodothyronine, thyroxine, testosterone, and growth hormone
(via somatomedin Q.
B. Chondrocytes are inhibited by cortisone, hydrocortisone, and estradiol.

VII. CARTILAGE REGENERATION. In the adult, damaged cartilage has limited capacity for
regeneration and may form scar tissue instead of cartilage. In young children, damaged cartilage
shows a greater capacity for regeneration.


Bone

I. GROUND SUBSTANCE (bone matrix) contains the following components:
A. Proteoglycans
B. Glycosaminoglycans (GAGs): chondroitin sulfate and keratan sulfate
C. Glycoproteins: osteocalcin, osteopontin, and bone sialoprotein. Alt of these are vita, min-D
dependent and bind calcium ions.
D. Mineral (inorganic) component: hydroxyapatite (calcium phosphate crystals), citrate ions, and
carbonate ions. A weak acid or chelating agent demineralizes bone.

II. FIBERS consist of type I collagen.

III. CELLS

A. Osteoprogenitor cells differentiate into osteohlasts and are active during osteogenesis and repair of
bone fractures. Osteoprogenitor cells undergo mitosis.
B. Osteoblasts are derived from osteoprogenitor cells and secrete osteoid (unmineralized bone matrix).
Osteoblasts have parathyroid hormone (PTH) receptors. In response to PTH stimulation, they secrete
osteoclast- stimulating factor, procollagenase, and plasminogen activator. Osteoblasts do not undergo
mitosis.
C. Osteocytes and their cytoplasmic processes reside in lacunae and canaliculi, which are surrounded by bone matrix. Cytoplasmic processes of neighboring osteocytes communicate via gap junctions.
Osteocytes do not undergo mitosis.
D. Osteoclasts are multinucteated cells that are derived from monocytes.

1. During bone resorption, osteoclasts reside in Howship's lacunae.

2. Osteoclasts contain numerous lysosomes whose contents are secreted extracellularly to digest the organic component of the bone matrix.

3. They have a ruffled border containing carbonic anhydrase, which produces H' ions that create an acid environment conducive to bone demineralization.

4. Osteoclasts have calcitonin receptors, but do not have PTH receptors.

IV. BLOOD VESSELS AND NERVES are present and run in haversian canals and Volk~ mann's
canals.

V. OSTEOGENESIS occurs by replacement of preexisting connective tissue. In the embryo, two types of osteogenesis occur:

A. Intramembranous ossification occurs in the embryo when mesenchyme condenses into sheets of highly vascular connective tissue, which then forms a primary ossification center.
B. Endochondral ossification occurs in the embryo when mesenchyme first forms a hyaline cartilage model, which then develops a primary ossification center at the diaphysis. Later, secondary ossification is centered at the epiphysis at each end of the bone.

1. Growth in length of long bones occurs at the epiphyseal plate.

a. Zone of reserve contains resting chondrocytes.
b. Zone of proliferation contains chondrocytes in mitosis that form isogenous groups.
C. Zone of hypertrophy contains hypertrophied and vacuolated chondrocytes.
d. Zone of calcification contains dead chondrocytes and calcified cartilage matrix.
e. Zone of ossification contains osteoprogenitor cells that congregate on spicules of calcified cartilage
matrix and differentiate into osteoblasts.

2. Growth in diameter of long bones occurs by deposition of bone at the periphery of the diaphysis as
osteoprogenitor cells differentiate into osteoblasts in the periosteum.

C. Ossification of adult bone

VI. HORMONAL INFLUENCES

A. Parathyroid hormone acts directly on osteoblasts to secrete osteoclast-stimulating factor.
The stimulated osteoclasts increase bone resorption, thereby elevating blood calcium level.
B. Calcitonin acts directly on osteoclasts to inhibit bone resorption.
C. Growth hormone has a major influence on skeletal growth and bone remodeling.
D. Estrogens and androgens. Closure of the epiphyseal plates is closely related to the production of sex
hormones associated with the development of ovaries and testes. In cases of precocious sexual
development, skeletal growth is stunted because of premature closure of the epiphyseal plates.
In cases of gonadal hypoplasia, closure of the epiphyseal plates is delayed and arms and legs become
disproportionately long.

Table. Ossification of Adult Bone                                                                                                        
Ossification Process           Adult Bone                                                                                                 
Intramembranous              Membranous neurocranium (cranial vault): frontal bone, parietal bones,
                                         intraparietal part of occipital bone, 
                                         Membranous viscerocranium (skeleton of face): maxilla, zygomatic,
                                         squamous part of temporal bone, palatine, vomer, mandible
Enclochondral                  Cartilagenous neurocranium (base of skull): ethmoid, sphenoid, petrous
                                         and mastoid parts of temporal bone, base of occipital bone
                                         Cartilagenous viscerocranium: incus, malleus, stapes, styloid process,
                                         hyoid bone
                                         Appendicular skeleton: shoulder and pelvic girdles and all bones of
                                         ex tremities
                                         Vertebra, sternum, ribs                                                                                

VII. BONE REGENERATION. Bone has a high capacity for regeneration through the proliferation of
osteoprogenitor cells and formation of bony callus.

VIll. CLINICAL CONSIDERATIONS

A. Osteoporosis is a decrease in bone mass associated with low levels of either growth hormone (in old
age) or estrogen (in postmenopausal women).
B. Osteomalacia (rickets) is characterized by a diminished calcium content of bone, which may occur as a
result of vitamin D deficiency or pregnancy.
C . Acromegaly, characterized by thick bones, is a result of excess growth hormone.
D. Scurvy, characterized by poor bone growth or fracture repair, is a result of vitamin C deficiency.
E. Vitamin A. An excess of vitamin A causes a premature closure of the epiphyseal plates, resulting in
small stature.

IX. COMPARISON OF CARTILAGE AND BONE

Table. Characteristics of Bone and Cartilage                                                                                           
Characteristic               Cartilage                                                     Bone                                                 
Ground substance        Chondroitin sulfate, keratan sulfate,           Chondroitin sulfate, keratan sulfate,
        components         chondronectin; no mineral component       osteocalcin, osteopontin,
                                                                                                       sialoprotein, hydroxyapatite,
                                                                                                       citrate, carbonate
Collagen types             Type I (fibrocartilage)                                 Type I
                                    Type II (hyaline & elastic)
Blood vessels               Absent; nutrients received via                     Present
                                    diffusion
Nerves                          Absent                                                        Present
Repair                          Low                                                            High
(regeneration)
        capacity
Mitosis                          Chondroblasts: yes                                     osteoprogenitor: yes
                                     Chondrocytes: yes                                      Osteoblasts: no
                                                                                                       Osteocyts: no
Communication            No junctions between chondrocytes           Gap junctions between osteocytes
Hormonal                     Triiodothyronine, thyroxine,                       Parathyroid hormone, calcitonin,
          influence            testosterone, growth hormone,                   growth hormone, estrogens,
                                     cortisone, hydrocortisone,                           androgens
                                     estradiol