Regulated expression of nullo is required for the formation of distinct apical and basal adherens junctions in the Drosophila blastoderm.

During cellularization, the Drosophila embryo undergoes a large-scale cytokinetic event that packages thousands of syncytial nuclei into individual cells, resulting in the de novo formation of an epithelial monolayer in the cortex of the embryo. The formation of adherens junctions is one of the many aspects of epithelial polarity that is established during cellularization: at the onset of cellularization, the Drosophila beta-catenin homologue Armadillo (Arm) accumulates at the leading edge of the cleavage furrow, and later to the apicolateral region where the zonula adherens precursors are formed. In this paper, we show that the basal accumulation of Arm colocalizes with DE-cadherin and Dalpha-catenin, and corresponds to a region of tight membrane association, which we refer to as the basal junction. Although the two junctions are similar in components and function, they differ in their response to the novel cellularization protein Nullo. Nullo is present in the basal junction and is required for its formation at the onset of cellularization. In contrast, Nullo is degraded before apical junction formation, and prolonged expression of Nullo blocks the apical clustering of junctional components, leading to morphological defects in the developing embryo. These observations reveal differences in the formation of the apical and basal junctions, and offer insight into the role of Nullo in basal junction formation.

Local delivery of TGF beta2 can substitute for placode epithelium to induce mesenchymal condensation during skin appendage morphogenesis.

Development of skin appendages requires interactions between the epithelium and mesenchyme. Without the epithelium, dermal condensations cannot develop, and those already formed will disintegrate. Here we explored the molecular basis of this epithelial requirement and tried to identify the molecule(s) responsible by using the chick feather bud development as a model. TGF beta2 is a likely candidate because its message is predominantly expressed in the feather bud epithelium, and the protein is enriched in the dermal-epidermal junction within the bud. We tested this hypothesis by placing TGF beta-soaked beads on skin explants. We found that TGF beta2, but not TGF beta1, beads placed on top of epithelially stripped mesenchymes can induce dermal condensations. NCAM and tenascin-C (Tn-C) are expressed and protein kinase C is suppressed in the normal feather bud domain. This molecular organization is lost in denuded mesenchyme but can be restored by TGF beta2-coated beads. Subsequently, the TGF beta2-induced dermal condensations can induce nascent epithelium to form skin appendages. Together with our recent findings that ectopic Sonic hedgehog (Shh) expression causes wider TGF beta expression and larger dermal condensation, these results strongly suggest that TGF beta2 produced by epithelial placode is downstream to Shh and plays a key role in the induction of dermal condensation by activating the expression of NCAM and Tn-C, and by suppressing PKC expression.

Formation of the periodontal attachment in developing rabbit permanent incisors.

Permanent incisors from fetal New Zealand white rabbits aged 25-30 days in utero and from 1-20 days postnatally were processed for light microscopy. Examination of longitudinal and cross-sections revealed that cementum deposition and fiber attachment first occurred at 27 days in utero. A tooth-to-bone course of principal fibers confined to lateral aspects of the teeth was observed in 29-day fetal incisors, whereas lingually the periodontal ligament remained undeveloped as late as 5 days postnatally. In 20-day postnatal incisors, the development of the functional periodontal ligament was complete as evidenced by the fiber arrangement on all aspects of the root-analogue tooth surface. This study shows that 25-26-day fetal rabbit permanent incisors represent the end of the follicular phase, whereas 27-day fetal to 20-day postnatal teeth demonstrate distinct histologic characteristics of initial, incomplete and complete periodontal fiber attachment.