Embryology and the Sea Anemone
Embryology is the science of the development of an embryo from the fertilization of the ovum to the fetus stage. How the newly borndevelops cell by cell is still a bit of mystery. The sea anemone, Nematostella vectensis, is a new study creature in embryology. Its career is being launched in part by the Stowers Institute for Medical Research Associate Investigator Matt Gibson, Ph.D., who is giving it equal billing with what has been his laboratory's leading player, the more traditional fruit fly. Gibson's lab investigates the cellular and molecular mechanisms used by cells to assemble into layers or clusters during embryogenesis. Those tissues, comprised of densely packed cells known as epithelial cells, shape the body not only of simple creatures but also of mammals, where they line every body cavity from lung to intestine and form hormone- and milk-secreting glands. Unfortunately these cells have a dark side too- over 80% of human cancers, carcinomas, are of epithelial origin.
A study of tentacle-formation in a sea anemone shows how epithelial cells form elongated structures and puts the spotlight on a new model organism.
The starlet sea anemone is a species of sea anemone native to the east coast of the United States, with introduced populations along the coast of southeast England and west coast of the United States. Populations have also been located in Nova Scotia, Canada. The starlet sea anemone has a bulbous basal end and a contracting column (usually less than two cm but no more than six cm) in order to burrow into the mud. At the top of the column is an oral disk containing the mouth surrounded by two rings of tentacles - typically 16 but up to 20.
The Gibson lab has historically used the genetic powerhouse Drosophila to investigate the control of epithelial cell shape and proliferation during wing, leg and eye development. Breaking with tradition, their new study published in the May 15th, 2013 issue of Development, explains how developing sea anemone larvae construct an even more basic epithelial appendage, the tentacle. The paper charts how epithelial cell shape changes drive tentacle development and is also the first to identify candidate genes driving those changes. Most of all, by putting a new model organism representing one of the simplest animals center stage, the study illuminates some of the most fundamental principles animals use to construct a body.
The Stowers study, led by first author Ashleigh Fritz, a graduate student at the University of Kansas School of Medicine working in the Gibson lab, began by imaging Nematostella larvae at the cellular level before, during, and immediately after juvenile tentacles sprang from their body. Freshly hatched Nematostella larvae are under intense pressure to get their tentacles up and running, as they use them to pull food toward their mouths. The question was, what kind of cellular reshuffling drove these survival-dependent changes in morphology?
"We thought tentacle outgrowth might be driven by cell proliferation," says Fritz, noting that some of Nematostella's freshwater cousins sprout appendages by constant cell division. "Instead, we observed that cells begin thickened and then thin out as tentacles elongate." In other words, the process was driven not by cell duplication along a tentacle axis but rather by stretching a stockpile of cells.
Embryologists call the embryonic thickening of epithelial cells that provides raw material for a mature structure a placode. "Placodes have appeared over and over throughout evolution," says Gibson, noting that placodes give rise to wings or eyes in flies and feathers and teeth in vertebrates. "Discovering that placodes are also utilized in animals as seemingly primitive as Nematostella shows how fundamental this strategy is in evolution."
The group also showed that activation of a cellular receptor known as Notch was mandatory for tentacles to emerge from a placode. Newly hatched Nematostella larvae swimming in lab seawater laced with a drug that blocks Notch receptor activity failed to sprout tentacles.
The researchers also constructed microarrays from tissue isolated at early, mid, and late stages of tentacle extension, allowing global comparison of the collection of mRNAs, or the transcriptome, at each stage. That effort, driven by Stowers Research Advisor Chris Seidel, Ph.D., and Ariel Paulson of the Stowers Computational Biology Core, is an obligatory step in pioneering any new model organism.
"Transcriptome analysis led us to identify novel tentacle markers," says Fritz, referring to molecular probes used to define a particular cell type. "Also gene expression patterns that we and others have identified allowed us to construct the first-ever molecular model of how tentacles are patterned."
In short, the study not only suggests universal principles underlying sculpting of epithelial structures from a placode, but also provides investigators with a toolkit to test whether specific genes drive the process.
An added bonus is that in 2007 a consortium of researchers sequenced the Nematostella genome and reported it to be more "human-like" in size and structure than that of Drosophila or another widely used model system, the nematode C. elegans. As a result, Gibson thinks that for many key questions, Nematostella may represent a better laboratory model than either.
For further information see Tentacles or Article.
Sea Anemone image via Wikipedia.