Working With Sea Urchin Embryos in the Lab
Sea urchins are still one of the most useful model organisms for developmental biology, despite everything else that has come along. Their embryos develop externally, they are transparent, and they synchronize remarkably well when you trigger spawning correctly. The volume "Developmental Biology Of The Sea Urchin And Other Marine Invertebrates Methods And Protocols Methods In Molecular Biology" compiled in the Methods in Molecular Biology series is one of those references that actual lab people end up citing more than they admit out loud. The book is structured around practical protocols rather than sweeping reviews. You will find chapters on spawning induction, fertilization assays, embryo culture, RNA extraction from tiny samples, whole-mount in situ hybridization, antibody staining, microinjection, and a few chapters on non-urchin invertebrates like ascidians and annelids. The sea urchin protocols tend to be the most thoroughly tested, which makes sense given the history of the field. If you are setting up a new sea urchin workup from scratch, start with Arbacia punctulata or Strongylocentrotus purpuratus. Arbacia spawns more reliably under potassium chloride injection, and S. purpuratus eggs are large and robust for microinjection work. I have had better luck with S. purpuratus for anything involving manual injection because the yolk density makes the cytoplasm easier to see under phase contrast.
One specific problem I ran into was consistently low recovery of mRNA from urchin embryos at the gastrula stage. The standard column-based kits either clogged or gave yields so low that qPCR came back as noise. What actually worked was switching to a phenol-chloroform extraction followed by ethanol precipitation. I would harvest about five hundred embryos, resuspend them in Trizol, homogenize with a glass homogenizer, add chloroform, spin, take the aqueous phase, precipitate with isopropanol, wash with seventy percent ethanol, and resolubilize in RNase-free water. The yield jumped from roughly two nanograms per microgram of tissue to about forty nanograms. It is more steps, but it is also more reliable when your starting material is this small. For in situ hybridization, the probe penetration step is where most people lose patience. The vitelline membrane needs to be removed before fixation if you want good signal. I use a brief treatment with five percent cysteine hydrochloride at pH eight point five for about four minutes, then rinse in seawater. Without this step the probes basically sit on the surface and you end up chasing background. The protocol in the book assumes you have already dealt with the vitelline membrane, so if you are reading it for the first time you might miss that detail and wonder why your staining is patchy. Fertilization timing is another area where the literature and the bench disagree sometimes. The book will tell you to collect eggs and sperm separately, dilute them, and mix. In practice, sperm concentration matters far more than anyone admits. If your sperm stock is too concentrated you get polyspermy and the embryos arrest within an hour. If it is too dilute you get no fertilization at all. I keep a stock of sperm at roughly one milliliter per fifteen milliliters of seawater and add about one hundred microliters to fifty milliliters of eggs. That gives a monospermic fertilization rate above ninety percent on a good day.
The book also covers microinjection into blastulae and mesenchyme blastulae. A common failure point here is bubble formation inside the embryo. When you inject too fast or use a needle with too large a diameter, the pressure differential pulls seawater in around the probe and you get bubbles that disrupt the cytoskeleton. I solved this by backing off the injection pressure to roughly thirty kilopascals and using a needle with an tip diameter under five micrometers. It takes longer to deliver the volume, but survival rates doubled and the embryos continued normal development. For RNA interference experiments, morpholinos against sea urchin targets work but the knockdown efficiency varies by target and by batch. I once spent two weeks trying to knock down a Wnt ligand because my morpholino was not working. The problem turned out to be storage. The morpholino had been reconstituted and frozen three times, which degrades the oligo over time. I made a fresh stock, ran a dose response from ten to fifty micromolar, and found that twenty micromolar gave the cleanest knockdown with the least toxicity. The book mentions dose optimization in passing but does not emphasize how critical it is for marine invertebrate morpholinos. There are tradeoffs to everything in this field. Sea urchin embryos are seasonal. Even with induced spawning, the quality of gametes drops outside the natural breeding window for most species. Arbacia in particular can be kept in cold rooms and stimulated year round, but the embryo quality still degrades compared to spring runs. If your project depends on highly synchronous development, plan around the season. Another limitation is that some molecular tools that are standard in vertebrate models simply do not translate well. CRISPR knockouts in sea urchins are possible now, but the efficiency is low and you need to inject into one-cell embryos, which means the injection timing has to be within minutes of fertilization. The book covers CRISPR briefly, but the real learning happens when you miss the window and watch half your embryos arrest.
Get the Full Details
For people who want to move beyond urchins, the protocols for ascidians like Ciona intestinalis are included in the same series and share some of the same basic techniques. Embryo handling is different enough that you should not assume full interchangeability, but the staining and fixation methods overlap substantially. If you are planning a project that spans multiple invertebrate models, the book is worth keeping on the bench even if you only use half the chapters. The downloadable resources associated with this volume are mostly supplementary figures and primer sequences rather than full protocol downloads. If you are looking for a complete digital workflow, you will need to adapt the printed protocols to your own lab conditions. The book is a solid foundation, but it is not a replacement for hands-on troubleshooting. The methods work when you follow them, and they fail in predictable ways when you do not. That is how most of this work goes.
