Distinct requirements for beta-catenin in pancreatic epithelial growth and patterning.

Pancreatic exocrine and endocrine lineages arise from multipotent pancreatic progenitor cells (MPCs). Exploiting the mechanisms that govern expansion and differentiation of these cells could enhance efforts to generate ß-cells from stem cells. Although our prior work indicates that the canonical Wnt signaling component ß-catenin is required qualitatively for exocrine acinar but not endocrine development, precisely how this requirement plays out at the level of MPCs and their lineage-restricted progeny is unknown. In addition, the contribution of ß-catenin function to ß-cell development remains controversial. To resolve the potential roles of ß-catenin in development of MPCs and ß-cells, we generated pancreas- and pre-endocrine-specific ß-catenin knockout mice. Pancreas-specific loss of ß-catenin produced not only a dramatic reduction in acinar cell numbers, but also a significant reduction in ß-cell mass. The loss of ß-cells is due not to a defect in the differentiation of endocrine precursors, but instead correlates with an early and specific loss of MPCs. In turn, this reflects a novel role for ß-catenin in maintaining proximal-distal patterning of the early epithelium, such that distal MPCs resort to a proximal, endocrine-competent "trunk" fate when ß-catenin is deleted. Moreover, ß-catenin maintains proximal-distal patterning, in part, by inhibiting Notch signaling. Subsequently, ß-catenin is required for proliferation of both distal and proximal cells, driving overall organ growth. In distinguishing two distinct roles for ß-catenin along the route of ß-cell development, we suggest that temporally appropriate positive and negative manipulation of this molecule could enhance expansion and differentiation of stem cell-derived MPCs.

Arrested development: high-resolution imaging of foveal morphology in albinism.

Albinism, an inherited disorder of melanin biosynthesis, disrupts normal retinal development, with foveal hypoplasia as one of the more commonly associated ocular phenotypes. However the cellular integrity of the fovea in albinism is not well understood - there likely exist important anatomical differences that underlie phenotypic variability within the disease and that also may affect responsiveness to therapeutic intervention. Here, using spectral-domain optical coherence tomography (SD-OCT) and adaptive optics (AO) retinal imaging, we obtained high-resolution images of the foveal region in six individuals with albinism. We provide a quantitative analysis of cone density and outer segment elongation demonstrating that foveal cone specialization is variable in albinism. In addition, our data reveal a continuum of foveal pit morphology, roughly aligning with schematics of normal foveal development based on post-mortem analyses. Different albinism subtypes, genetic mutations, and constitutional pigment background likely play a role in determining the degree of foveal maturation.