What characterizes intramembranous ossification?

Intramembranous ossification is a specific process during bone development that occurs directly within a connective tissue membrane. This type of ossification is primarily responsible for the formation of flat bones, such as those in the skull and the clavicles. In this process, mesenchymal cells differentiate directly into osteoblasts, which then begin to produce bone matrix.

The characteristic of occurring within a fibrous membrane accurately captures the essence of intramembranous ossification, as it does not require a cartilage model unlike endochondral ossification, which involves replacing a cartilage model with bone. This is why the choice indicating it happens within a fibrous membrane is the correct answer.

The other options do not correctly represent intramembranous ossification: it is not specifically associated with long bones, is not limited to cartilaginous structures, and does not require cartilage as a precursor, as these features are relevant to endochondral ossification instead.