Work conducted in the laboratory of MOL faculty  Danelle Devenport, in collaboration with Ezster Posfai’s research group, has uncovered a crucial step in tissue organization of mammalian embryos. The paper describing this advance appeared March 21, 2025 in the journal Science.

The next time a house cat graces you with its presence to demand your admiration, you’ll have the Planar Cell Polarity (PCP) pathway to thank for that sleek and neat fur coat; the PCP pathway organizes skin so that hair follicles all orient in the same direction, rather than aiming every which way like a bad case of bed-head. And it goes more than skin deep. The PCP pathway directs the organization of all epithelial tissues, the two-dimensional sheets of cells that form the barriers between different tissues in the body, by giving epithelial cells the directional cues they need to organize themselves as the embryo develops.

The cells that make up epithelial sheets are organized along a shared axis that designates where the tissue’s leading and trailing edges are found. This axis is echoed within individual cells: a different set of proteins is found at the cell’s leading edge than at the trailing edge. Although this asymmetrical arrangement was first noticed years ago and has been the subject of intensive study ever since, it’s still unclear how it comes about. 

“Plenty of cell types that function as individuals can polarize their contents on their own – like yeast, C. elegans zygotes, and migrating cancer cells,” said Danelle Devenport, a Professor in the Department of Molecular Biology at Princeton.

Unlike cancer cells or yeast, cells in an epithelial sheet function as a collective. So, it stands to reason that they may therefore require external cues — such as interactions with other epithelial cells — in order to establish polarity. There are hints this may be the case. For example, the prominent PCP protein Celsr spans the cell membrane and binds to Celsr found on neighboring epithelial cells. This spurs the recruitment of other PCP proteins to the cell membrane. Are such interactions required to establish polarity or is epithelial cell polarization a solo act, something individual epithelial cells can accomplish on their own?

“Basically we set out to determine the minimal number of epithelial cells required to establish planar polarity,” said Devenport. “Polarization might require contacts between multiple cells, but if so, the number of cells required to generate asymmetry could be two, three or more.”

Working under Devenport’s supervision, graduate student Lena Basta investigated this question using special chimeric mouse embryos generated by Bradley Joyce, a postdoctoral researcher who works with Posfai. In these embryos, the majority of epithelial cells lack Celsr and so cannot form Celsr bridges with each other. The minority of cells that do express Celsr also sport fluorescently tagged versions of two asymmetrically positioned PCP proteins, Vangl and Fz6. In normal epithelia, Vangl concentrates at cells’ leading edge while Fz6 is found at the trailing edge, so tracking these proteins lets the researchers determine when polarization has occurred. The team used super-resolution microscopy to examine cell-to-cell contacts in their chimeric epithelia.

“Super-resolution imaging lets us resolve on which side of a cell-cell contact a protein resides. The distance between the cell membranes of neighboring cells is smaller than what can be resolved by conventional light microscopy,” said Devenport.

Basta and colleagues found that a single Celsr-expressing cell surrounded by cells that lack Celsr was unable to polarize any PCP proteins, but when two cells with Celsr abutted each other, the PCP proteins concentrated asymmetrically on either side of the interface between the two cells. This shows that a single contact is enough to get halfway to the polarized state. However, a cell could only fully polarize its PCP proteins — with Vangl at one end and Fz6 at the other — if it had formed contacts with at least two other cells. 

“Planar cell polarity is, by definition, a multicellular process. Our work demonstrates that it is also mechanistically multicellular,” said Devenport.

In epithelial sheets where all cells can form Celsr bridges with each other, it’s likely that another, currently unknown signal helps epithelial cells align with the body axis by introducing directional bias in the polarization process. But the formation of Celsr bridges is an essential first step in establishing planar cell polarity. Precisely how Celsr manages this is a question for future studies.

“We’re most immediately interested in how the formation of Celsr1 bridges initiates the sorting of opposing PCP complexes,” Devenport said.

Funding: This study was supported by funding from the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) (R01 AR066070, R01 AR068320, F31AR077407); the National Institute of Child Health and Human Development (NICHD) (R01 HD105009), and the National Institute of General Medical Sciences (NIGMS) NIH (T32 GM007388).