Final answer:
The development of a baby's skull and bones involves intramembranous and endochondral ossification, where the initial cartilaginous skeleton is replaced by bone. The skull bones grow from mesenchymal cells and remain separated by fontanelles at birth, allowing for flexibility and brain growth. The vertebral column and thoracic cage form from mesenchyme cells, growing and ossifying during development.
Step-by-step explanation:
Development of Baby's Skull and Bones
The bones of the baby's skull and skeleton develop through two primary processes: intramembranous ossification and endochondral ossification. During fetal development, the skeleton is primarily cartilaginous, but gradually this cartilage is replaced by bone in a process known as ossification. The skull bones arise from mesenchyme, accumulating at the future bone sites where they differentiate into bone-producing cells. Intramembranous ossification is directly responsible for the formation of skull bones, which initially are separated by fontanelles, large areas of dense connective tissue that enable the skull to deform during birth and continue growing afterward. The fontanelles decrease in size and close typically by age 2.
In addition to this, the axial skeleton, including the vertebral column and thoracic cage, begins to form early in embryonic development. Here, mesenchyme cells from each sclerotome accumulate around the notochord, differentiate into a hyaline cartilage model, and through endochondral ossification, these cartilaginous models are replaced by bone.
At birth, the newborn skull is characterized by underdeveloped facial bones and the absence of the mastoid process. As the child grows, the process of bone development continues, with the arms and legs undergoing ossification from the center toward the ends of the bones. This ensures that long bones can increase in length during childhood, with growth plates at the ends of these bones facilitating this process.