“Notes:
Bone Growth & Development”
Know This First!!
•
•
•
•
•
Osteo = Bone
Intra = Between
Perio = Outer
Endo = Inner
-blast, -cyte, -clast = types of cells
• Mesenchymal Cells = Stem Cells for bone,
cartilage, lymphatic structures and circulatory
structures.
(1) Bone Development
• Ossification = Bone Formation.
• Part 1: Prenatal Formation of Bony Skeleton
• Part 2: Postnatal Growth
(2) Prenatal Formation of
Bony Skeleton
• Bone Tissue will replace Cartilage Tissue
• Initial Cartilage Tissue is resilient and flexible,
allowing for increased mitosis.
• THE STEPS:
– Intramembranous Ossification:
– Endochondral Ossification:
(3) Intramembranous Ossification
•
•
•
•
•
[~Begins Week 8]
Cells produce fibrous membranes surrounding cartilage.
Ossification Center forms within fibrous membrane.
Osteoid (Nutrient + Osteoblast Mixture) secreted from
membrane.
Osteoblasts develop into Osteocytes.
•
Osteoid mineralizes around blood vessels, forming spongy
bone trabeculae.
Remaining external Mesenchymal Cells form periosteum.
•
•
Outer trabeculae lament and form thicker compact bone.
Cranial Bones + Clavicles formed.
(4) Endochondral Ossification
[~Begins Week 10-12]
•
•
Ossification Center forms within cartilage.
Blood Vessels infiltrate, allowing for periosteum formation.
•
•
Bone Collar forms around diaphysis of cartilage model.
Osteoblasts within the Periosteum secrete osteoid, encasing
cartilage.
Ossification Center calcifies, cartilage cells enlarge and die.
•
•
•
•
•
Inner cavity invaded by periosteal bud (blood vessels, nerves,
osteoblasts).
Osteoblasts surround dead cartilage cells, spongy bone trabeculae
formed.
Innermost trabeculae devoured by osteoclasts, medullary cavity
formed.
Outer trabeculae lament and form compact bone.
(5) Endochondral Ossification:
The Epiphyses
[~Begins Week 27-29]
•
•
•
•
Center (both inner and outer) of bone model
continues to ossify.
Ends remain as cartilage, continue to undergo
mitosis  Elongation.
Ossification was “chasing” Cartilage
Formation to the ends of the bone.
Epiphyses ossify…. Cartilage remains on
outer surface of epiphyses and at Epiphyseal
Plate.
(6) Postnatal Bone Growth
• Long bones lengthen from the center  ends.
– Cartilage Cells w/in Epiphyseal Plate continue to divide =
Growth + Lengthening.
– Epiphyseal Cartilage Cells replaced by osteoblasts (the
chase continues).
– Eventually the “Plate” is replaced by bone.
• All bones become thicker.
– Periosteum continues to secrete osteoid on top of existing
bone.
– Osteoclasts on inside devour some bone.
– Increased bone deposition, Decreased bone removal.
– Thickness can increase due to increased bone stress!
• Some facial bones continue to grow… past
adolescence.
(7) Hormone Regulation
• During Infancy & Childhood:
– Growth Hormone stimulates epiphyseal growth
– Growth Hormone released by pituitary gland, but
controlled by thyroid hormones.
• During Puberty:
– Testosterone and Estrogen are released slowly, in
increasing amounts
• Initially promotes growth spurt and masculinization /
feminization
• Later induce epiphyseal closure, ending growth