OSSIFICAZIONE
Dal mesenchima (tessuto connettivo non differenziato), la formazione dell’osso
può avvenire secondo due modalità definite:
Ossificazione Indiretta
Ossificazione Diretta
Ossificazione diretta o membranosa
Si passa direttamente dalla cellula mesenchimale (indifferenziata)
all’osteoblasta.
Propria della maggior parte delle ossa della volta cranica.
a
b
c
d
-Ossificazione indiretta o condrale
Comune nelle ossa lunghe, richiede un modello cartilagineo, sostituito da tessuto osseo mediante due fasi simultanee:
Ossificazione Pericondrale
Superficiale: formazione di un manicotto osseo periostale che procede dalla diafisi
verso le epifisi e
dalla superficie in profondità.
Ossificazione Encondrale
Comparsa
di nuclei di ossificazione situati profondamente nella diafisi e
nelle epifisi.
Durante lo sviluppo delle ossa lunghe, tra diafisi e epifisi (metafisi) è presente un disco di cartilagine, detto cartilagine di coniugazione.
La cartilagine di coniugazione si accresce in direzione delle epifisi, venendo progressivamente sostituita da tessuto osseo, lungo la sua superficie rivolta verso la diafisi.
Colorazione con alizarina per evidenziare il tessuto osseo
Blu:cartilagine
Rosso-arancio :osso e centri di ossificazione Giallo:midollo osseo
metafisi
Fratture
A haematoma is formed.
Acute inflammation of the injured area, marrow cavity and surrounding soft tissues occurs.
Blood vessels from the periosteum invade where normal blood supply has been interrupted, organising it into a granulation tissue. Cellular proliferation begins throughout the affected bone within 24 hours of injury, after a few days it is confined to the fracture area.
Formation of a primary callus. During the first months after injury a fibrocartilaginous callus is formed. The dead bone is resorbed and immature woven bone appears. The size of the callus is affected by the immobility of the fracture site; the smaller the amount of movement, the smaller the callus.
Woven bone is gradually replaced by lamellar bone, and the fracture is united by this bony secondary callus.
Many people, in particular elderly people and post- menopausal women, suffer from osteoporosis, a condition where the bones become more 'porous' and brittle.
Osteoporosis occurs when the rate of bone resorption by osteoclasts over takes bone formation, resulting in a decrease in bone mass.
The process can be clearly seen in the trabeculae of spongy bone. The trabeculae thin down and some are resorbed resulting in a less resilient structure.
Osteoporosis can occur when a bone is under used, for example, during treatment in a plaster cast, or it may have a hormonal basis. As men and women age they produce less of the sex hormones that stimulate bone formation.
Osteoporosis is common in post-menopausal women due to the drop in oestrogen levels which accompanies the menopause. Hormone replacement therapy - HRT can be used to halt, but not reverse, the process.
Osteoporosis makes the bones fragile and unable to deal with the loads and stresses they are subjected to normally. It affects the entire skeleton and increases the risk of fracture.
The bones particularly at risk are those with a relatively large amount of trabecular bone, such as the proximal femur, the
Normally, there is loss of bone mass with aging, perhaps 0.7% per year in adults. However, bone loss is greater in women past menopause than in men of the same age. The process of bone remodelling from resorption to matrix synthesis to mineralization normally takes about 8 months--a slow but constant process. Bone in older persons just isn't as efficient as bone in younger
persons at maintaining itself--there is decreased activity of osteoblasts and decreased production of growth factors and bone matrix.
This diagram illustrates changes in bone density with aging in women. The normal curve (A). A woman who begins with diminished bone density (B) before menopause.
