What is the process of intramembranous ossification primarily responsible for forming?

Intramembranous ossification is a key process in the formation of certain types of bones, particularly flat bones like the bones of the skull, clavicles, and some facial bones. This process involves the direct transformation of mesenchymal tissue into bone without a cartilage intermediary. Mesenchyme, which is a type of connective tissue, condenses and differentiates into osteoblasts, the cells responsible for producing new bone. As osteoblasts secrete the bone matrix, it eventually mineralizes and becomes ossified, forming the flat bone structure.

This process is distinct from endochondral ossification, where bones such as long bones are formed from a cartilage template. Understanding that intramembranous ossification is specifically tied to flat bone formation helps to clarify why this choice is the correct answer. Other options refer to various other bone types and processes that do not involve intramembranous ossification.