“Highlights” is a new feature that calls attention to exciting advances in developmental biology that have recently been reported in Developmental Dynamics. Development is a broad field encompassing many important areas. To reflect this fact, the section will spotlight significant discoveries that occur across the entire spectrum of developmental events and problems: from new experimental approaches, to novel interpretations of results, to noteworthy findings utilizing different developmental organisms.

A coherent hypothesis (Dev Dyn236:2039–2049) Like oil and water, cells with differential intercellular adhesion properties do not mix. This concept is exemplified in animal pancreatic islets, which are composed of an aggregate of insulin-secreting β-cells surrounded by other endocrine cells. Until now, the presumed biophysical properties underlying this cellular arrangement had not been tested. Presented here, work from Foty and colleagues support the differential adhesion hypothesis (DAH), which predicts that β-cells have greater surface tension and, therefore, greater intercellular adhesiveness than surrounding cells. By measuring the binding energy among cells in pseudoislets assembled in vitro, they find that β-cells have greater surface tension than surrounding cells. Consistent with this result, β-cells also express higher levels of the adhesion molecule E-cadherin. Finally, when surrounding cells overexpress an exogenous adhesion molecule, P-cadherin, they “pull away” from the β-cell core. This finding shows that changing the adhesive relationship between cell types can influence their relative positions. DAH validation is an important step toward bioengineering islet cell substitutes, a potential means to control diabetes.

Boning up on osteoclastogenesis (Dev Dyn236:2181–2197) Although the skeletal system might appear to be permanent, it undergoes constant remodeling during an animal's lifetime. The dynamic process is mediated in part by osteoclasts, which remove and resorb bone. Aurora and colleagues suspected that many gene networks underlying development of these industrious cells had yet to be discovered. In their search, they used microarrays comparing gene expression in primary osteoclast precursors induced to undergo osteoclastogenesis, to mock-treated precursors. Crucial to their data analysis was generation of a coexpression network. Clusters of genes that show the same expression profile over multiple time points are thought to function together to regulate a common biological outcome. By identifying coexpressing clusters, processes never before attributed to osteoclasts were identified, including angiogenesis, reciprocal T-cell/osteoclast signaling, cholesterol regulation, and apoptosis. Potential roles for the pathways in osteoclastogenesis are discussed. Networks identified in developing osteoclasts may also predict gene interactions in other cell types.

Cracking the Hox code (Dev Dyn236:2454–2463) Although it will never achieve the notoriety of The Da Vinci code, the Hox code has been controversial in its own right. How do Hox, a cadre of genes with overlapping expression and redundant functions, pattern the vertebrate axial skeleton? In this review, Wellik presents supporting evidence that is used to build an up-to-date interpretation of how the Hox code operates in vertebrates. Of note is the observation that the same Hox genes use different mechanisms to pattern lateral plate and somitic derived axial skeletal elements. In addition, she presents evidence that debunks the “posterior prevalence model,” which states that the most posteriorly expressed Hox gene imparts relevant patterning information at a particular axial level. The data are key to her argument that coexpressed, active Hox proteins work together to pattern skeletal morphologies. In closing, Wellik discusses what additional experimental evidence must be gathered before the code can be definitively cracked. Where are Tom Hanks and Audrey Tautou when you need them?