BMP Signaling is Required for Development of the Maxillary Process

Bone morphogenetic proteins (BMPs) mediate molecular processes in developing facial structures. Blocking BMP signaling with their antagonist NOGGIN, disrupts epithelial-mesenchymal interactions in the developing frontonasal process (FNP) and blocks skeletal development in the skull.

Objectives: To assess morphological effects of blocking BMP signaling at different time points within the ectoderm or mesenchyme of the avian maxillary process.

Methods: A replication competent retrovirus encoding Noggin (RCAS-Noggin) (control RCAS-Alkaline Phosphatase (RCAS-AP)) was injected into the mesenchyme or onto surface ectoderm of the right maxillary process of chick embryos at Hamburger-Hamilton stage 20 (HH20) or HH24. Embryos infected at HH20 were harvested at HH24, HH28, HH32, and HH39. Embryos infected at HH24 were harvested at HH32, and HH39.

Results: Infection of the ectoderm with RCAS-Noggin at HH20 (n=12/13) resulted in no aberrant phenotype. Similarly, control embryos infected with RCAS-AP appeared normal (HH20 n=28/28).

Infection with RCAS-Noggin in the mesenchyme at HH20, resulted in severe malformations at HH39 (n=3/3): absent/shortened FNP, maxillary, and mandibular processes; forward rotation of the eyes, underdeveloped eyelids, and malformation of feather buds. Control embryos showed no aberrant phenotype (n=1/1).

To elucidate the mechanism(s) underlying the aberrant phenotype, embryos were examined at earlier time points to determine when alterations were first apparent. Facial primordia appeared unaffected at HH24 (n=7/7), and showed a minor phenotype at HH28 (n=3/4).

Infection of the mesenchyme at HH24 resulted in a similar, but less severe phenotype at ~HH39: ossification was disrupted but facial prominences expanded distally.

Conclusions: Blockade of BMP signaling in the ectoderm does not result in abnormal phenotype. Infection of the mesenchyme of the maxillary process with RCAS-Noggin results in severe facial dysmorphologies. These malformations likely occur primarily as a result of disrupted skeletogenesis. Timing of BMPs signaling blockade affects the severity of phenotype.

Support: T32 DE07306-12, 1R01DE018234-01