TBL4 - Long Bone Formation Flashcards
What germ layer develops into the vertebral column and how does it do so?
The solid bilateral columns of paraxial and intermediate mesoderm mesenchymal cells of paraxial mesoderm-derived somites generate chondroblasts and osteoblasts that form the vertebral column via endochondral ossification
What germ layer develops into the scapula, clavicle, and bones of the upper limbs?
1) The lateral plate mesoderm forms bilateral columns that split into parietal (somatic) and visceral (splanchnic) layers
2) Mesenchymal cells of the parietal layer generate chondroblasts and osteoblasts that form the scapula, clavicle, and bones of the upper limb
How does endochondral ossification of long bones begin?
1) Endochondral bone formation begins with hyaline cartilage replicas of future adult bone. Having developed from mesenchyme, cartilage templates take the shape of the later bone
2) The first of two or more ossification centers appears in the shaft, or diaphysis, of the cartilage template. A thin bony collar appears around the diaphysis by intramembranous ossification as bone is laid down directly by connective tissue perichondrium of the cartilage
3) After the delicate collar of bone forms around the center of the diaphysis, the perichondrium becomes a periosteum
Describe how bone and bone marrow formation begins below the periosteum in endochondral ossification of long bones
1) Deep to the new collar, cartilage matrix begins to calcify, and chondrocytes hypertrophy and die
2) Osteoblasts, angiogenic capillaries, and macrophages from the bony collar periosteum enter the diaphysis and replace eroded cartilage with trabecular bone that creates the primary ossification center in the diaphysis (From the periosteum, blood vessels, collectively termed the periosteal bud, invade the diaphysis interior and bring in associated mesenchymal and osteoprogenitor cells)
3) Erosion of cartilage in the center and formation of a primitive marrow cavity also occur. Incoming blood vessels carry in primitive bone marrow cells
Describe secondary endochondral ossification
1) Because the interior diameter of the diaphysis remains constant, interstitial proliferation of remaining chondrocytes causes the two ends, or epiphyses, to grow longitudinally
2) Chondrocytes are thus arranged in columns and appear as two fronts on both sides of the central region. They eventually form the epiphyseal growth plates at the junction between epiphysis and diaphysis. Growth plates of hyaline cartilage determine the longitudinal diaphysis growth. Toward the end of fetal life and continuing into puberty, ossification centers appear in the two epiphyses of long bones. After adolescence, growth plates close and growth ceases
Why is achondroplasia associated with skeletal dysplasia e.g., dwarfism?
1) A skeletal dysplasia is a disorder that results either in a bone growing too large (hyperplasia) or too small (hypoplasia)
2) Achondroplasia (ACH), the most common form of skeletal dysplasia, primarily affects the long bones. Other skeletal defects include a large skull (megalocephaly) with a small midface, short fingers, & an accentuated spinal curvature
Describe the transformation of primary osteons into mature osteons to form compact bone on the outer part of the diaphysis
1) The process of the formation of osteons and their accompanying Haversian canals begins when immature woven bone and primary osteons are destroyed by large cells called osteoclasts, which hollow out a channel through the bone, usually following existing blood vessels
2) Layers of bone-forming cells, or osteoblasts, follow the osteoclasts and lay down new bone on the sides of the channel; the layers of bone built up in this way slowly narrow the channel until a tunnel not much larger than the central blood vessel remains
3) The blood supply for the osteocytes then passes through these channels, the Haversian canals. The spaces between adjacent osteons are filled with interstitial lamellae, layers of bone that are often remnants of previous Haversian systems
4) Transverse vessels, which run perpendicular to the long axis of the cortex, are called Volkmann canals; Volkmann canals connect adjacent osteons and also connect the blood vessels of the Haversian canals with the periosteum, the tissue covering the bone’s outer surface
What is noticeable in neonates about the diaphysis and epiphyses formations?
1) Residual trabecular bone in the diaphysis forms the marrow cavity
2) Secondary ossification centers form in the epiphyses
What occurs in long bones by age 10?
1) By age 10, the epiphyses mature into cores of trabecular bone covered by compact bone while the growth plates remain active during puberty (before this age, long bones are made up more of trabecular bone and more prone to fracture)
Note: Every 10 years, the skeleton replaces itself entirely
2) Bony union of the epiphyses with the diaphysis occurs at skeletal maturity
