Development of the squamosomandibular articulation in the Mongolian gerbil (Meriones unguiculatus).

The early development of the squamosomandibular articulation in the Mongolian gerbil was investigated from its inception through the 8th postnatal day. Pregnant gerbils were sacrified beginning on day 18 through day 23 of gestation by chlorofrom inhalation and the fetuses were extirpated and decapitated. Neonatal gerbils, 1-8 days of age, were also sacrificed. The severed heads were frozen in a cryostat and sectioned at 14mum in frontal and transverse planes and stained in 1% aqueous Safranin 0. The anlage of the mandibular joint along with membranous bony deposits lateral to Meckel's cartilage in the vicinity of the presumptive body and ramus first appeared on the 18th gestational day. Condylar chondrogenesis occurred on the 20th prenatal day, and by day 22 of gestation chondrocytic hypertrophy was noted preparatory to osteogenesis. The presumptive articular disc was observed on the 21st gestational day while an upper and lower synovial cavity was discernible on the 22nd prenatal day. The squamosa and zygoma were first noted on the 20th postcoital day. There appeared to be a developmental lag from onset through completion of the gerbil squamosomandibular joint formation in comparison with that reported for the mouse and rat.