During development, the process by which a group of cells determines the direction of differentiation of neighboring cells is called induction. Spemann's classical transplantation experiment became a foundation of developmental biology.
Basic Knowledge
Induction (embryonic induction): A phenomenon in which signaling molecules secreted by one tissue (the inducing tissue) determine the direction of differentiation of an adjacent tissue (the responding tissue).
Spemann's organizer experiment (1924): When the dorsal lip of the blastopore from an early newt (tailed amphibian) gastrula was transplanted to another embryo, a second body axis (a secondary neural tube and secondary notochord) formed in the transplanted embryo, producing an embryo resembling a conjoined twin. The dorsal lip of the blastopore was named the organizer. Spemann and his student Mangold received the 1935 Nobel Prize in Physiology or Medicine (Mangold died before the award).
Molecular mechanism of neural induction: Ectoderm normally tends to differentiate into epidermis by default. However, proteins such as nodal, chordin, and noggin secreted by the organizer inhibit the BMP (bone morphogenetic protein) signal, inducing the ectoderm to differentiate into the neural plate (neural tube).
Neural tube formation (neurulation): The neural plate of the ectoderm folds to form the neural tube, which becomes the precursor of the brain and spinal cord. Failure of neural tube formation causes spina bifida (a congenital malformation).
Answer: (1) The transplanted dorsal lip of the blastopore functions as a secondary organizer, inducing the surrounding ectoderm at the transplantation site to form a neural plate and neural tube, and also forming a secondary notochord. As a result, an embryo resembling a conjoined twin with two body axes develops. (2) It showed that the transplanted tissue (the organizer) determines the fate of host cells through signals from other cells rather than autonomously. This provided evidence that cell differentiation is determined by interactions with nearby tissues rather than by cell autonomy. (3) Chordin binds to BMP and inhibits its signal, inducing ectodermal cells to differentiate into the neural plate rather than the epidermis.
- Induction = signals from one tissue determine the direction of differentiation of an adjacent tissue
- Organizer = dorsal lip of the blastopore, named by Spemann (1924)
- Spemann = 1935 Nobel Prize in Physiology or Medicine
- Organizer inducers = chordin and noggin inhibit BMP
- Neural tube = formed by folding of the neural plate of the ectoderm
- Spina bifida = congenital malformation caused by failure of neural tube formation
Practice
- Explain why the dorsal lip of the blastopore is called the organizer in Spemann's experiment.
- Describe how inhibition of BMP signaling promotes neural differentiation during neural induction.
- Name one disease caused by failure of neural tube formation and explain its relationship to prevention (folic acid intake).