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Zusammenfassung:
At gastrulation secreted organizer molecules induce neural cell fates in the neighboring ectodermal cells. The neuroectoderm becomes subdivided along the anterior–posterior (a-p) axis. In the trunk regions this process is governed by the hox genes. In the head three domains can be distinguished in a-p progression. The fore- brain is bordered by the anterior neural rigde (ANR) from which— among others—the stomodeum/pituitary develop. Sandwiched be- tween the ANR and the ventral epidermis lies the cement gland, the anterior-most larval organ, which is induced by the organizer and consists of nonneural ectoderm. The positioning of the cement gland depends upon a graded BMP signal and the homeobox gene otx2. Because otx2 is expressed in the cement gland and the neuroectoderm, region-specific cofactors were postulated which would select for specific anterior ectodermal cell fates. The Pitx genes Pitx1 and Pitx2c qualify for such cofactors. They are neces- sary and sufficient for cement gland differentiation. We show that (1) Pitx genes are expressed in the cement gland and the ANR during patterning and organogenesis and induced by otx2; (2) misexpression results in ectopic cement gland formation in whole embryos and induction of cement gland markers in animal cap explants; (3) loss of Pitx represses ectopic cement gland formation by otx2; and (4) Pitx gene transcription is modulated by a graded BMP signal in animal caps. They are induced at moderate BMP levels, but not transcribed at low or high BMP concentrations. We propose that Pitx genes are part of a network of regionally active homeobox transcription factors and hypothesize that their specific combinatorial action controls the process of anterior ectodermal patterning in a code-like manner.
