title: The mechanism of lens placode formation: a case of matrix-mediated morphogenesis.
abstract: Although placodes are ubiquitous precursors of tissue invagination, the mechanism of placode formation has not been established and the requirement of placode formation for subsequent invagination has not been tested. Earlier measurements in chicken embryos supported the view that lens placode formation occurs because the extracellular matrix (ECM) between the optic vesicle and the surface ectoderm prevents the prospective lens cells from spreading. Continued cell proliferation within this restricted area was proposed to cause cell crowding, leading to cell elongation (placode formation). This view suggested that continued cell proliferation and adhesion to the ECM between the optic vesicle and the surface ectoderm was sufficient to explain lens placode formation. To test the predictions of this "restricted expansion hypothesis," we first confirmed that the cellular events that accompany lens placode formation in chicken embryos also occur in mouse embryos. We then showed that the failure of lens placode formation when the transcription factor, Pax6 was conditionally deleted in the surface ectoderm was associated with greatly diminished accumulation of ECM between the optic vesicle and ectoderm and reduced levels of transcripts encoding components of the ECM. In accord with the "restricted expansion hypothesis," the Pax6-deleted ectoderm expanded, rather than being constrained to a constant area. As a further test, we disrupted the ECM by deleting Fn1, which is required for matrix assembly and cell-matrix adhesion. As in Pax6(CKO) embryos, the Fn1(CKO) lens ectoderm expanded, rather than being constrained to a fixed area and the lens placode did not form. Ectoderm cells in Fn1(CKO) embryos expressed markers of lens induction and reorganized their cytoskeleton as in wild type ectoderm, but did not invaginate, suggesting that placode formation establishes the minimal mechanical requirements for invagination.
authors.affiliation: Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, Saint Louis, MO 63110, USA.
authors.forename: Jie
authors.initials: J
authors.lastname: Huang
authors.forename: Ramya
authors.initials: R
authors.lastname: Rajagopal
authors.forename: Ying
authors.initials: Y
authors.lastname: Liu
authors.forename: Lisa K
authors.initials: LK
authors.lastname: Dattilo
authors.forename: Ohad
authors.initials: O
authors.lastname: Shaham
authors.forename: Ruth
authors.initials: R
authors.lastname: Ashery-Padan
authors.forename: David C
authors.initials: DC
authors.lastname: Beebe
mesh_terms: D000818:Animals; D003599:Cytoskeleton / Q000378:metabolism; D004475:Ectoderm / Q000254:growth & development; D005109:Extracellular Matrix / Q000378:metabolism*; D005136:Eye Proteins / Q000235:genetics / Q000378:metabolism; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism; D007908:Lens, Crystalline / Q000254:growth & development* / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D009024:Morphogenesis; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D005136:Eye Proteins; D018398:Homeodomain Proteins; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors; C000605732:Pax6 protein, mouse; D012097:Repressor Proteins