All bone formation is a replacement process. Embryos develop a cartilaginous skeleton and various membranes. During development, these are replaced by bone during the ossification process. In intramembranous ossification, bone develops directly from sheets of mesenchymal connective tissue. In endochondral ossification, bone develops by replacing hyaline cartilage. Activity in the epiphyseal plate enables bones to grow in length. Modeling allows bones to grow in diameter. Remodeling occurs as bone is resorbed and replaced by new bone. Osteogenesis imperfecta is a genetic disease in which collagen production is altered, resulting in fragile, brittle bones.
endochondral ossification
process in which bone forms by replacing hyaline cartilage
epiphyseal line
completely ossified remnant of the epiphyseal plate
intramembranous ossification
process by which bone forms directly from mesenchymal tissue
modeling
process, during bone growth, by which bone is resorbed on one surface of a bone and deposited on another
ossification
(also, osteogenesis) bone formation
ossification center
cluster of osteoblasts found in the early stages of intramembranous ossification
osteoid
uncalcified bone matrix secreted by osteoblasts
perichondrium
membrane that covers cartilage
primary ossification center
region, deep in the periosteal collar, where bone development starts during endochondral ossification
proliferative zone
region of the epiphyseal plate that makes new chondrocytes to replace those that die at the diaphyseal end of the plate and contributes to longitudinal growth of the epiphyseal plate
remodeling
process by which osteoclasts resorb old or damaged bone at the same time as and on the same surface where osteoblasts form new bone to replace that which is resorbed
reserve zone
region of the epiphyseal plate that anchors the plate to the osseous tissue of the epiphysis
secondary ossification center
region of bone development in the epiphyses
zone of calcified matrix
region of the epiphyseal plate closest to the diaphyseal end; functions to connect the epiphyseal plate to the diaphysis
zone of maturation and hypertrophy
region of the epiphyseal plate where chondrocytes from the proliferative zone grow and mature and contribute to the longitudinal growth of the epiphyseal plate
1. Why is cartilage slow to heal?
A) because it eventually develops into bone
B) because it is semi-solid and flexible
C) because it does not have a blood supply
D) because endochondral ossification replaces all cartilage with bone
C
2. Why are osteocytes spread out in bone tissue?
A) They develop from mesenchymal cells.
B) They are surrounded by osteoid.
C) They travel through the capillaries.
D) Formation of osteoid spreads out the osteoblasts that formed the ossification centers.
D
3. In endochondral ossification, what happens to the chondrocytes?
A) They develop into osteocytes.
B) They die in the calcified matrix that surrounds them and form the medullary cavity.
C) They grow and form the periosteum.
D) They group together to form the primary ossification center.
B
4. Which of the following bones is (are) formed by intramembranous ossification?
A) the metatarsals
B) the femur
C) the ribs
D) the flat bones of the cranium
D
5. Bones grow in length due to activity in the ________.
A) epiphyseal plate
B) perichondrium
C) periosteum
D) medullary cavity
A
6. Bones grow in diameter due to bone formation ________.
A) in the medullary cavity
B) beneath the periosteum
C) in the epiphyseal plate
D) within the metaphysis
B
7. Which of the following represents the correct sequence of zones in the epiphyseal plate?
A) proliferation, reserved, maturation, calcification
B) maturation, proliferation, reserved, calcification
C) calcification, maturation, proliferation, reserved
D) calcification, reserved, proliferation, maturation
C
1. In what ways do intramembranous and endochondral ossification differ?
In intramembranous ossification, bone develops directly from sheets of mesenchymal connective tissue, but in endochondral ossification, bone develops by replacing hyaline cartilage. Intramembranous ossification is complete by the end of the adolescent growth spurt, while endochondral ossification lasts into young adulthood. The flat bones of the face, most of the cranial bones, and a good deal of the clavicles (collarbones) are formed via intramembranous ossification, while bones at the base of the skull and the long bones form via endochondral ossification.
2. Considering how a long bone develops, what are the similarities and differences between a primary and a secondary ossification center?
A single primary ossification center is present, during endochondral ossification, deep in the periosteal collar. Like the primary ossification center, secondary ossification centers are present during endochondral ossification, but they form later, and there are two of them, one in each epiphysis.