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Aggregates of stem cells can break symmetry and self-organize into embryo-like structures with complex morphologies and gene expression patterns. Mechanisms including reaction-diffusion Turing patterns and cell sorting have been proposed to explain symmetry breaking but distinguishing between these candidate mechanisms of self-organization requires identifying which early asymmetries evolve into subsequent tissue patterns and cell fates. Here we use synthetic ‘signal-recording’ gene circuits to trace the evolution of signalling patterns in gastruloids, three-dimensional stem cell aggregates that form an anterior–posterior axis and structures resembling the mammalian primitive streak and tailbud. We find that cell sorting rearranges patchy domains of Wnt activity into a single pole that defines the gastruloid anterior–posterior axis. We also trace the emergence of Wnt domains to earlier heterogeneity in Nodal activity even before Wnt activity is detectable. Our study defines a mechanism through which aggregates of stem cells can form a patterning axis even in the absence of external spatial cues.
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Growth and morphogenesis of the gastropod shell
Gastropod shell morphologies are famously diverse but generally share a common geometry, the logarithmic coil. Variations on this morphology have been modeled mathematically and computationally but the developmental biology of shell morphogenesis remains poorly understood. Here we characterize the organization and growth patterns of the shell-secreting epithelium of the larval shell of the basket whelk Tritia (also known as Ilyanassa ). Despite the larval shell’s relative simplicity, we find a surprisingly complex organization of the shell margin in terms of rows and zones of cells. We examined cell division patterns with EdU incorporation assays and found two growth zones within the shell margin. In the more anterior aperture growth zone, we find that inferred division angles are biased to lie parallel to the shell edge, and these divisions occur more on the margin’s left side. In the more posterior mantle epithelium growth zone, inferred divisions are significantly biased to the right, relative to the anterior–posterior axis. These growth zones, and the left–right asymmetries in cleavage patterns they display, can explain the major modes of shell morphogenesis at the level of cellular behavior. In a gastropod with a different coiling geometry, Planorbella sp., we find similar shell margin organization and growth zones as Tritia , but different left–right asymmetries than we observed in the helically coiled shell of Tritia . These results indicate that differential growth patterns in the mantle edge epithelium contribute to shell shape in gastropod shells and identify cellular mechanisms that may vary to generate shell diversity in evolution.
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- Award ID(s):
- 1656558
- PAR ID:
- 10097031
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 116
- Issue:
- 14
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- 6878 to 6883
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1073/pnas.1816089116
