Frog (amphibian) development has been studied in detail as a model system in developmental biology. This lesson also organizes the classic concepts of cellular totipotency and nuclear transplantation experiments.
Basic Knowledge
Sequence of frog development:
Fertilized egg → 2-cell stage → 4-cell stage → 8-cell stage → morula → blastula (blastocoel on the animal-pole side) → gastrula (blastopore forms on the vegetal-pole side and invagination occurs) → neurula (neural plate → neural tube formation) → tail-bud embryo → tadpole larva → metamorphosis → frog (adult)
Gray crescent: During fertilization in frogs, a gray crescent (gray crescent) forms on the side opposite the point of sperm entry. It determines the future dorsal side. The blastomeres on the gray-crescent side become dorsal, and the organizer forms there.
Cellular totipotency and nuclear transplantation: John Gurdon (1962) transplanted the nucleus of a frog intestinal cell into an enucleated oocyte and obtained a normal tadpole. This showed that the nucleus of a differentiated somatic cell does not lose totipotency (the ability to differentiate into all cell types). This discovery provided the theoretical background for the development of iPS cells and led to the 2012 Nobel Prize in Physiology or Medicine (shared with Shinya Yamanaka).
Answer: The common discovery is that even differentiated somatic cells retain the totipotency of their nuclei (genomes), and that pluripotency can be restored by reprogramming (resetting) them under appropriate conditions. The technical difference is that Gurdon reprogrammed a differentiated nucleus by placing it in the cytoplasmic environment of an oocyte, whereas Yamanaka achieved reprogramming in vitro by introducing transcription factors (Oct3/4, Sox2, Klf4, and c-Myc) into somatic cells using viral vectors.
- Frog development = fertilized egg → blastula (blastocoel: animal-pole side) → gastrula → neurula → tadpole
- Gray crescent = forms opposite the sperm-entry point and determines the dorsal side
- Gurdon (1962) = normal development after intestinal-cell nuclear transplantation → proof of totipotency
- Totipotency = even the nucleus of a differentiated somatic cell retains all genomic information
- Gurdon + Yamanaka = 2012 Nobel Prize in Physiology or Medicine
- Metamorphosis = tadpole → frog, directed by thyroid hormone
Practice
- Explain why the gray crescent forms during frog development and state its developmental significance.
- State why Gurdon's nuclear transplantation experiment proved cellular totipotency.
- Name the hormone that promotes frog metamorphosis and explain how its secretion is regulated.