Planaria

I heard a talk today. The speaker was a junior professor at the Whitehead Institute at MIT. He works with Schmidtea mediterranea, a flatworm commonly known as Planaria.

These Planaria look very funny. They are 3-8 mm long, and look like penises with eyes. Well, those dotted eyes are real photoreceptors. There are quite some levels of differentiation of tissues with them. They have brains, literally.

The most funny thing about Planaria is that they can regenerate themselves if they are cut into two halves, or eight. If you decapitate them, they regrow their head in days. If you cut them longitudinally, each of the two halves regrow into two individuals. If you play around with the chopping-up thing, you can cut the worm from its head half way into its body, the cut half of the worm will regrow into 2 full heads and you will end up with a Y-shaped worm with 2 head and one tail.

Planaria can actually reproduce asexually simply by splitting themselves into two, and the regrow into two adults. This amazing regeneration ability had attracted scientists such as Thomas. H. Morgan, who also pioneered the genetics of fruit flies, as early as in 1898.

These worms seem never die - they keep splitting themselves. In fact, they are very busy rebuilding themselves all the time. The turn-over rate of the cells within these worms are very high. The worm you see today will make up of a completely different set of cells a few weeks later.

The cool thing the speaker's lab is doing is, they now manage to use RNAi to knock down the genes in these worms. They have the genome of this S. meditteranea sequenced and designed RNAi to screen for genes that show phenotypic effects on regeneration. Basically they fed the flatworms with E. coli expressing clones of RNAi and chopped them up after 2 days. The worms that showed impaired ability to regenerate would have the gene important for regeneration knocked down.

So they have opened the treasure chest that holds the whole new world of stem cell and cancer research. The way the neoplast grow, differentiate and migrate parallels that of stem cells. The whole worm can be labeled by antibodies and BrdU, and the cells can be easily dislodged for FACs. Another goody: the differentiation is slow - it takes days to show phenotype. With such a simple system, they will be able to pick up signals that trigger regeneration/differentiation in different tissues quite easily. Also, with rapidly dividing neoplast, these worms rarely develop cancer. They must have some unique and tight control for cell division, as well as DNA repair and replication checkpoints to make sure that they do everything correctly during cell division and pass on their own genome very accurately.

The speak was Peter Reddien. He did his post-doc in Alejandro Sanchez's lab in University of Utah. Being an HHMI investigator and invited to write review articles in the Annual Review of Cell and Developmental Biology and Nature Reviews Genetics, Sanchez must be a respected figure in this field. He looks he is in his 40's only.