Skeletogenesis Flashcards
Axial skeleton parts
skull, spine, cranium and ribs
Appendicular skeleton
Limbs
Making of skeletal structures
Neural crest/mesoderm undergoes mesenchymal condensation, starts from centre and grows out (intramembranous ossification -> skull/mandible/clavicle) for bone or into cartilage. The cartilage template leads to persistent cartilage of joints or is removed and turned into bone
What is intramembranous ossification and where does it occur?
Intramembranous ossification is a type of bone development in which bone tissue is created directly from mesenchymal tissue without a cartilage template. This process primarily occurs in the flat bones of the skull, mandible, and clavicle. It is crucial for the formation of the cranial bones of the skull, the clavicles, and facial bones.
< 8 weeks in development
Key steps of intramembranous ossification
Mesenchymal stem cells in the connective tissue aggregate and differentiate into osteoblasts.
Osteoblasts secrete osteoid, the organic bone matrix made of collagen and other proteins.
The osteoid matrix calcifies (hardens) when the deposited calcium salts crystallize. Bony spicules are formed
Calcified areas form trabeculae, the spongy bone which eventually fuses to form the compact bone. Spicules join trapping BV
The surface layers of undifferentiated mesenchymal cells at the periphery of the bone develop into the periosteum, which contains the bone’s blood vessels and nerves.
Endochondral ossification
Endochondral ossification is a process by which bone tissue is created from pre-existing cartilage models. This is the predominant method for the formation of the bones in the body, particularly the long bones like femur, tibia, and humerus.
> 8 weeks in development
Describe the first step of endochondral ossification.
Chondrocyte Maturation: In the cartilage model, chondrocytes (cartilage cells) mature and enlarge to form a pattern that will shape the future bone.
What happens during the second step of endochondral ossification?
Cartilage Calcification: The mature chondrocytes die and disintegrate, leaving cavities within the cartilage. Simultaneously, the surrounding cartilage matrix calcifies, inhibiting nutrient diffusion and causing more chondrocytes to die.
What is the role of the periosteal bud in endochondral ossification?
Invasion of Periosteal Bud: A collection of blood vessels and osteogenic cells invades the disintegrating cartilage from the periosteum, bringing in precursors necessary for bone development, such as osteoblasts and osteoclasts.
Describe the process of bone formation in endochondral ossification.
Bone Formation: Osteoblasts, derived from the periosteal bud, begin depositing bone matrix over the remnants of the calcified cartilage, forming spongy bone. Simultaneously, osteoclasts resorb the calcified matrix, helping to shape the bone and form the medullary (marrow) cavity.
What happens during the final stages of endochondral ossification?
Continued Growth and Ossification: The growth continues from the epiphyseal growth plates, allowing for the elongation of bones until early adulthood. The cartilage at the growth plates eventually gets replaced by bone, completing the process.
Chondrogenesis
Formation of chondrocytes
Process of chondrogenesis
Skeletogenic mesenchymal cell develops into a condensed prechondrocyte by TGBFb and Wnt
Condensed prechondrocyte develops into an early chondrocyte by Sox9 and Sox5/6 which is from Wnt/b-catenin and Bmp respectively
Osteoprogenitor cell
Develops into an osteoblast known as a mesenchymal stem/stromal cell
Osteoblast
Forms bone extracellular matrix
