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How does nearly perfect radial symmetry arise in an animal like the starfish? My confusion lies in the fact that cells are fluid & amorphous, & that these characteristics seem to make it difficult to form symmetric structures. Many of the sources I've read describe the formation of the blastula but not the development of the five arms following that. Also, at one point in the development it is bilaterally symmetric. Furthermore, some species of starfish can regrow from a single arm.

What process mediates the formation of radial symmetry in a developing organism? Is it that arms grow individually & there are cascades of cellular signaling that mediate the production of new arms until the desired amount is reached or is it something else? How do the arms of the starfish remain evenly spaced?

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  • $\begingroup$ The morphogenesis of symmetry is an enigma in biochemistry, especially for echinoderms. Biologists can't agree totally and tell you a specific reason. here's all I can find(2018) "Over the last 20 years, an answer for how vertebrate embryos can tell their left from right has emerged. This model involves midline-located L-R organizers (LROs) – the node in mice. " dev.biologists.org/content/141/8/1603 some wiki info: en.wikipedia.org/wiki/NODAL wiki says that echinoderms also have the Nodal development gene. $\endgroup$ – DeltaEnfieldWaid Nov 8 '20 at 10:00
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That is the field of developmental biology which deals with organizers. Cell organizers are processes, genes and signalling proteins cells use to communicate / send and receive information from their neighbors, so that their spacial distribution in the body regulates their development.

The cells also have gradients of signaling proteins inside the cytoplasm, and cell polarity, so the cells know which way round they are:

"A spatially distributed signalling cascade can create step-like activation profiles, which decay at successive distances from the cell surface, assigning digital positional information to different regions in the cell. Feedback and feedforward network motifs control activity patterns, allowing signalling networks to serve as cellular devices for spatial computations."

All metazoans use the same types of organizers to develop different patterns of cells, so starfish use the same genes as mice to organize their axes. They are early metazoan genes/ Precambrian. Here is an in depth answer to your question: www.ncbi.nlm.nih.gov/pmc/articles/PMC5307799/

The cells know where they are, because they are receiving multiple signalling proteins from further away in the animal. Here is a picture of Bicoid morphogen in fruitflies:enter image description here

The signalling proteins are received from cells far away, and that signals to other cells to make different kinds of signalling proteins to organize the local neighbors. enter image description here

Given the complexity of metazoan morphogenesis, there are very many organizers and we only know a few of them. The axis formation is related to the Nodal gene which uses some kind of axial/mid-line located organizers to control symmetry, there are documentd about LRO: left right organisers, in starfish its the same chemistry in a different geometry. The Nodal biochemistry: https://youtu.be/kO8k9aiQJsQ?t=189

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