Developmental Biology of Frogs
This lesson was developed by Dr. Cindy Timmerman, who has a PhD in developmental biology from the University of Florida
|
All of these cells appear the same and are totipotent, but as they multiply further, within 7 hours, they start to become specialized and migrate. Depending on the chemical and electrical signals from the environment, and neighboring, embryonic cells start to migrate and specialize into specific germ line. The blastula is created when these cells arrange to form a fluid-filled ball. When the blastula begins to flatten and form a hollow cup-shape, it is called a gastrula. It looks a bit like a deflated basketball.The edges of the cup start to curve toward each other to form a new hollow center called the archenteron. This chamber will eventually become the gut of the animal. The tiny pore leading to the archenteron chamber is called the blastopore. This will eventually become the anus. It is during this gastrula stage that the three germ cell layers are created: endoderm, mesoderm and ectoderm. These 3 germ cell lines give rise to specific organs and tissues within the body that are the same for all chordates.
Although there are other kinds of animals whose development is based on two germ cell types, all chordates have a body plan that is based on these three basic cell types, and therefore they are called triploblastic.
Now that we have these three germ cell types, neurulation and organogenesis can begin.The outermost layer of cells (ectoderm) of the gastrula begins to form two parallel ridges that rise up and converge on each other. The top of these two ridges curve to meet each other and the result is a dorsal hollow tube that will eventually become the brain and notochord. This whole process takes less than 48 hours.
After 3 to 7 days (depending on the water temperature), the zygote has most of its basic body plan laid down and is ready to hatch. In frogs, they don’t really hatch, as much as the jelly-like covering begins to break down when the zygote begins to wiggle about, trying out its new muscles and nerves.
The hatchling’s first meal may be the remnants of this egg coating. Ectoderm eventually gives rise to the skin. Mesoderm will form the notochord, muscles, skeleton, kidneys, gonads and most of the circulatory system. The endoderm will become the lining of the digestive tract, liver, pancreas and lungs.
Many species of frogs are being affected by water pollution, and in some areas there are such severe population crashes that species may be in danger of extinction. You can expand on this lesson with a discussion of abnormalities in frog development caused by (1) increases in parasites in nutrient enriched waterways, (2) from agricultural chemicals, and (3) from pharmaceuticals in livestock and human waste. Such a discussion will provide a bridge between Tadpole Tales and Caring for the Kaw, which explores the impact of stormwater runoff on water pollution.
Here are a few resources that can be used for discussion on this topic:
Frog deformities linked to farm pollution
Pesticide Atrazine Can Turn Male Frogs Into Females
Sterility in Frogs Caused by Environmental Pharmaceutical Progestogens, Study Finds