Incudomalleal joint formation: the roles of apoptosis, migration and downregulation.
BACKGROUND
The middle ear of mammals is composed of three endochondrial ossicles, the stapes, incus and malleus. Joints link the malleus to the incus and the incus to the stapes. In the mouse the first arch derived malleus and incus are formed from a single Sox9 and Type II collagen expressing condensation that later subdivides to give rise to two separate ossicles. In contrast the stapes forms from a separate condensation derived from the second branchial arch. Fusion of the malleus and incus is observed in a number of human syndromes and results in conductive hearing loss. Understanding how this joint forms during normal development is thus an important step in furthering our understanding of such defects.


RESULTS
We show that the developing incudomalleal joint is characterised by a lack of proliferation and discrete areas of apoptosis. Apoptosis has been suggested to aid in the removal of pre-cartilaginous cells from the joint region, allowing for the physical separation of the cartilaginous elements, however, we show that joint initiation is unaffected by blocking apoptosis. There is also no evidence of cell migration out of the presumptive joint region, as observed by labelling of joint and ossicle cells in culture. Using Type II collagen lacZ reporter mice, however, it is evident that cells in the presumptive joint region remain in place and downregulate cartilage markers.


CONCLUSIONS
The malleus and incus first appear as a single united condensation expressing early cartilage markers. The incudomalleal joint region forms by cells in the presumptive joint region switching off cartilage markers and turning on joint markers. Failure in this process may result in fusion of this joint, as observed in human syndromes such as Branchio-Oto-Renal Syndrome or Treacher Collins Syndrome.
Department of Craniofacial Development and Orthodontics, King's College London, London, UK. samin@uchc.edu
Susan
S
Amin
Eva
E
Matalova
Carol
C
Simpson
Hiroki
H
Yoshida
Abigail S
AS
Tucker
D000818:Animals; D017209:Apoptosis; D053477:Apoptotic Protease-Activating Factor 1 / Q000037:antagonists & inhibitors; D015415:Biomarkers; D002358:Cartilage, Articular / Q000166:cytology / Q000196:embryology*; D053148:Caspase 3; D053453:Caspase 9; D061945:Caspase Inhibitors; D002465:Cell Movement; D015536:Down-Regulation; D004432:Ear, Middle / Q000166:cytology / Q000196:embryology*; D004622:Embryo, Mammalian; D020287:In Situ Nick-End Labeling; D007596:Joints / Q000166:cytology / Q000196:embryology; D051379:Mice; D008817:Mice, Mutant Strains; D009924:Organ Culture Techniques; D018809:Proliferating Cell Nuclear Antigen / Q000032:analysis
C506593:Apaf1 protein, mouse; D053477:Apoptotic Protease-Activating Factor 1; D015415:Biomarkers; D061945:Caspase Inhibitors; D018809:Proliferating Cell Nuclear Antigen; C505409:Casp3 protein, mouse; C506434:Casp9 protein, mouse; D053148:Caspase 3; D053453:Caspase 9