Yolk Sacs and Hidden Teeth: Embryos in a Zoo Program

Before Hatching, Before Birth

1The Lakeshore Zoo runs conservation programs for animals that are rare in the wild. Its staff hatch whooping crane eggs, raise young Blanding's turtles until they are big enough to release into protected wetlands, and breed Przewalski's horses, a wild horse that once survived only in zoos. This fall, eighth grader Ines Okwuosa joined the program as a teen volunteer. She worked with the zoo's reproductive biologist, Dr. Callum Ostrowski, who studies how the embryos in these programs develop.

2In the incubator room, Dr. Ostrowski held a crane egg in front of a bright light, a method called candling. The egg is about 100 mm long, and much of what is inside is yellow yolk. By the end of the first week, Ines could see red blood vessels spreading over the yolk. The vessels grow in a thin sac that wraps around the yolk, called the yolk sac. It carries food from the yolk to the growing embryo until the chick hatches about a month later. Candled turtle eggs showed the same kind of sac and vessels over a large yolk.

3Mammal embryos are checked a different way. The zoo's vet uses ultrasound on pregnant mares. About two weeks after mating, the screen shows a small, round, fluid-filled sac that at this stage is mostly the yolk sac. A few weeks later the placenta forms, and the embryo gets food and oxygen from its mother's blood. Dr. Ostrowski also showed Ines ultrasound measurements of human embryos from a medical textbook (Table 2). A human egg cell is only about 0.1 mm wide.

4Next, the two visited a university museum that keeps preserved whale fetuses from animals that died naturally. An adult humpback whale has no teeth. Instead, comb-like plates called baleen hang from its upper jaw and strain small animals from the water. In young humpback fetuses, scientists find rows of tiny tooth buds in both jaws. The buds stop growing and are absorbed before birth, and the baleen grows later. In dolphin fetuses, tooth buds form in the same way, but they keep growing into the cone-shaped teeth that adult dolphins use to grab fish.