In a developing baby, how is cartilage formed from mesenchymal cells?
The cells are induced by proteins like Activin A, Bone Morphogenic Proteins and Retinoic Acid-Sensitive Proteins. Interstitial growth begins as the cells divide, secrete collagen protein, push each other apart and the cartilage grows.
What is different about this developing collagen from mature collagen of its type?
It is avascular. Type II (hyaline cartilage) is avascular in early development.
Where is type II collagen mostly found?
Framework in developing embryo, articular surfaces and respiratory tract
What are the different cells seen in this image. What boundary do they form?
The dense layer is composed of pre-chondroblasts. The less dense area is composed of chondrocytes. The border seen is the perichondrium
In what ways does hyaline cartilage grow?
Appositional (prechondroblasts that look like fibroblasts become chondroblasts) and interstitially (chrondrocytes secrete type II collagen)
Which of the cells seen here are chondrocytes?
The "puffy" cells in the top of the picture.
How does ground substance change the characteristic of the cartilage you are looking at?
This cartilage has a lot of elastic fibers in it and is elastic cartilage. Note the darker staining pink due to charge of elastin in the H&E stain.
Where is elastic cartilage mostly found in the body?
Ears, nose and areas that require a lot of flexibility
Where is this type of cartilage mostly found?
This is fibrocartilage. It is found mostly in transitional tissue (bridges gap between cartilage, bone and tendon). It is mostly type I collagen
What genes are largely responsible for bone development?
How does bone develop from mesenchymal tissue?
The mesenchymal cell differentiates and begins to lay down a collagen matrix that eventually encases them. They are enclosed by the solid calcium-phosphate matrix.
How does this type of tissue grow?
Appositional growth by osteoblasts. The matrix is hard and interstitial growth is impossible by osteocytes.
Which of these cells is surrounded by osteoid and which is surrounded by bone?
The bottom cell is an osteoblast and is surrounded by osteoid because it is making bone. Note the euchromatic nucleus in the bottom cell. Note the darker bone matrix in the top cell indicating more mature bone.
What type of bone is likely being formed in this image?
Bones of the skull formed by intramembranous ossification. Note the isolated islands of bone formation.
How does endochondral bone begin to form from its hyaline cartilage framework?
Induction of chondrocytes begins formation of bone via calcification in the primary ossification center. Blood vessels begin to infiltrate. Chondrocytes atrophy and osteogenic cells infiltrate. Secondary ossification centers form in the head of the bone and the epiphyseal plate (growth plate) forms between the primary and secondary ossification centers.
What is the difference between the two hands in these images?
The one on the left is an infant and has cartilaginous areas where bone will be.
What is this cell and what is its function?
An osteoclast. It is a fusion of monocyte cells. It is involved in erosion of bone (via H+ and proteases) and remodeling so new bone can be laid.
What are the scrubbing bubbles of the bone? What follows them?
Osteoclasts. They erode canals in bone, are followed by endothelial cells making the aversion canals and osteoblasts laying down fresh bone where bone was eaten out.
You break your arm. How is healing induced in the bone?
Fibroblasts migrate to the area and reform the hyaline cartilage framework (callous) which holds the bone together. The hyaline cartilage then mineralizes during ossification. Then osteoclasts remodel the bone and the fracture is healed.