part of part of develops from develops from output of output of intramembranous ossification bone tissue intramembranous bone mesenchyme intramembranous bone tissue Bone tissue that forms directly within mesenchyme, and does not replace other tissues[TAO]. Intramembranous ossification is the formation of bone in which osteoblasts secrete a collagen-proteoglycan matrix that binds calcium salts and becomes calcified[GO]. Intramembranous ossification is the way flat bones and the shell of a turtle are formed[GO]. Unlike endochondral ossification, cartilage is not present during intramembranous ossification[WP]. MA:0002837 Wikipedia:Intramembranous_ossification note the distinction between intramembranous bone and intramembranous bone tissue. Some bones may be a mixture of both tissue types. The MA class most likely refers to a tissue type (it is under bone tissue and connective tissue; there are no child classes) UBERON:0004764