What can I learn from worms?June 2012
Lesson 3: How do planarians regenerate?
Cool Scientists: Randolph and Morgan
In the early development of most animals, an individual begins its life as a single cell. This cell divides and becomes two cells, then four, then eight and so on. Over time, a complex individual forms with billions of cells that have very different functions and appearances. One may wonder how all those cells manage to develop and organize themselves into the familiar shapes of animals. Why aren’t there a lot of cats with six toes or turtles with two heads running around? And if we do see these individuals, we wonder what went wrong? In other words, how do cells in an embryo“know” that they should form a leg, a brain, or an artery?
These are questions that scientists have been asking for hundreds of years and are still trying to solve today.You will have the opportunity to conductexperiments similar to those performed on planariansover a hundred years ago by scientists like Harriet Randolph and Thomas Hunt Morgan.
Harriet Randolph was born in Pennsylvania and had a late start in her science career, graduating from the women’s college Bryn Mawr at age thirty-three. She was trained by Edmund Beecher Wilson, the famous cell biologist who discovered that females and males havedistinct sex chromosomes. Randolph received her Ph.D. in Zurich, Germany, while researching the reproductive cells of insects. She returned to Bryn Mawr as a lecturer for biologyclasses and studied planariansduring the summer at the Marine Biological Laboratoryin Woods Hole, Massachusetts.
In his late twenties, Thomas Hunt Morganhad recently been hired at Bryn Mawr as a professor with his own research lab. When he wasn’t teaching, Morgan was usually experimenting at Woods Hole orin Italy.
In conjunction with another scientist, Morgan took very young sea urchin embryos and carefully divided each into two parts. The current belief at the time was that each half of the embryo could only develop into half a sea urchin, and not being completely functional, it would die. However, Morgan’s half-embryos developed into normal adults, showing that the fate of certain cells is not pre-determined during development. (See “Sea Urchins and Twins” box).
The idea that cells from a part of one organism have the ability to develop a whole new, different organism prompted Morgan to study regeneration. He considered Randolph’s experiments on planarians the “starting point” for his own work, and together they pioneered research on regeneration.
Whence (from what), planarians?
Scientists make observations that can lead to the formation of a hypothesis. Below are some drawings from Randolph and Morgan’s experiments. Can you guess how theseplanarians formed?In the box next to each picture, draw a planarian showing how you think the original planarian was cut. (Hint: the shaded portion is the original planarian while the white is the regenerated portion).
Whither (to what), planarians?
Below are some drawings from Randolph and Morgan that show some of the cuts they tested on their planarians. Can you guess what sort of planarian they will develop into? (The results may not be the same as the “Whence, planarians?” section). Draw other cuts you might like to perform on your planarian on a separate piece of paper. Perhaps you will produce something that has never been seen before!
1