Elastin Fibers in the Developing Mouse Ossicular, Stapediovestibular, and Mandibular Joints

Objective: The development of the mouse stapediovestibular joint (SVJ) and mandibular joint (MJ) were studied in an attempt to elucidate elastogenesis in the articular tissue; the histology was compared with elastogenesis in the auditory ossicular joints. Methods: Foetuses at stages between E11 and E20 from pregnant Slc-ICR mice and mice aged from 0 day to 28 days after birth (b.x.d) were employed in the present study. Head portions containing the MJ and the labyrinthine capsule were dissected out and prepared for light microscopy and conventional TEM. Results: The following findings were obtained: 1) Articular cartilages of the MJ and SVJ were derived from the embryonic articulating mesenchymal tissue. 2) Completion of the incudomalleal joint (IMJ) and incudostapedial joint (ISJ), the SVJ and the MJ were observed during b.0-2.d, b.6.d and b.10-12.d, respectively. 3) Maturation of the IMJ and ISJ, and the SVJ was subsequently observed during b.5-6.d and b.21.d, respectively. 4) Maturation of the IMJ and ISJ was closely related to pneumanization of the middle ear; the SVJ matured later than the ossicular joints. The MJ was complete while the incisor erupted. 5) The MJ articular cartilage transformed showing atypical cellular zonation, but was still hypertrophic, immature and distributed with arborizing blood vessels during b.21-28.d. 6) Elastin fibres in the subsynovial layer of the mature IMJ and ISJ were mainly elastic fibres, while the mature SVJ and the immature MJ contained elaunin fibres. 7) Age changes in cellular and fibrous components of the articular tissue was not evident in the functioning mature SVJ and completed MJ. Conclusions: The present study reveals histological differences of the extracellular matrix (ECM) between joints at different developmental states, and elucidates that maturation of the ECM elastin fibres reflects the functional requirements in the articular tissue of different joints.