To provide further details of the development of the sacroiliac joint, which might help to explain some of its unusual adult features, the development of the bones, joint cavity, synovial membrane, articular cartilages, joint capsule, and ligaments has been examined by light microscopy in a series of 13 human embryos and fetuses ranging from 21 mm greatest length (G.L.) to 185 mm crown-rump length (C.R.).
Perichondral ossification of the ilium was first seen at 39 mm C.R. and led to the formation of bone on the external and internal surfaces of the ala of the ilium. In the sacrum, centers of endochondral ossification were first recognized at 100 mm C.R. within the bodies and arches of the sacral vertebrae.
Cavitation commenced at around 39 mm C.R. as separate, small spaces within the middle layer of the interzone in the cranial region of the joint, which was avascular prior to the onset of cavitation and showed evidence of pyknotic cells around the sites of cavitation. The joint cavity spread caudally involving the entire joint by 185 mm C.R., but fibrous septa bridging across between the sacral and iliac articular surfaces were still present.
Vascularization of synovial mesenchyme was first observed at 155 mm C.R. and synovial villi at 185 mm C.R. The outer layers of the interzone contributed fibrous layers to the articular surfaces of the sacrum and ilium, which were still present at 185 mm C.R., that on the ilium being the thicker and showing evidence of continuing proliferation and differentiation into cartilage, responsible for further bone growth of the ilium.