Figure 33.9 The Dorsal Lip Induces Embryonic Organization
The findings from Hans Spemann’s early experiments in which he constricted fertilized salamander eggs with a baby’s hair led to the hypothesis that dividing cells in the early embryo each receive a different suite of cytoplasmic factors and that these factors lead to different developmental cell fates. The results from various transplantation experiments supported this idea and led Spemann and his student, Hilde Mangold, to expand upon his original hypothesis and propose that some cells can induce other cells to follow a specific developmental path. Continuing with transplantation experiments, Spemann and Mangold transplanted early gastrula dorsal lip material into the presumptive ventral epidermis of a host gastrula. The transplanted dorsal lip induced a second site of gastrulation in the host embryo, subsequent development of dorsal structures, and the formation of a complete secondary embryo attached to the first embryo. Today, we know that the formation of the secondary embryo from the ventral cells of the host arose as a result of signaling molecules present in the tissue commonly referred to as Spemann’s organizer. Recent studies have identified a number of factors that play a role in the inductive properties of the organizer. The precise mechanism by which the organizer region forms and directs the formation of the animal body plan, however, is the subject of on-going research.
Original Paper
Spemann, H., and H. Mangold. 1924. Über induktion von Embryonalagen durch Implantation Artfremder Organisatoren. Roux' Arch. Entw. Mech. 100: 599–638. Viktor Hamburger’s translation first appeared in Foundations of Experimental Embryology (B.H. Willier and J.M. Oppenheimer, eds.), Prentice Hall, Inc., Englewood Cliffs, N.J., pp. 146-184, 1964.
Links
Sander, K., and P. E. Faessler. 2001. Introducing the Spemann–Mangold organizer: experiments and insights that generated a key concept in developmental biology. International Journal of Developmental Biology 45: 1–11.
http://www.researchgate.net/publication/12041884_Introducing_the_Spemann-Mangold_organizer_experiments_and_insights_that_generated_a_key_concept_in_developmental_biology
Fässler, P. E. 1996. Hans Spemann (1869–1941) and the Freiburg School of Embryology. International Journal of Developmental Biology 40: 49–57.
http://www.ijdb.ehu.es/web/paper.php?doi=8735910
Dr. Hans Spemann: Nobel Prize in Physiology or Medicine 1935
http://nobelprize.org/nobel_prizes/medicine/laureates/1935/index.html
Nature: Milestone 1 (1924): Organizing principles
http://www.nature.com/milestones/development/milestones/full/milestone1.html
Nature: Milestone 19 (1989): Chasing the elusive inducer
http://www.nature.com/milestones/development/milestones/full/milestone19.html
NCBI: Developmental Biology: Axis Formation in Amphibians: The Phenomenon of the Organizer
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A2295
DevBio: A Companion to Developmental Biology: A Selective History of Induction II
http://10e.devbio.com/article.php?id=114
Kimball’s Biology Pages: Organizing the Embryo: The Central Nervous System
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Spemann.html
R&D Systems: ADMP: A Ventralizing BMP in the Dorsal Embryo
http://www.rndsystems.com/cb_detail_objectname_SP06_admp.aspx
Figure. 33.10 Differentiation Can Be Due to Inhibition of Transcription Factors
It was thought that the organizer cells actively induced cells to become neutral, with epidermal being the default state. However, results in Grunz and Tacke (1989) suggested that this was assumption was incorrect. However, it was not until the work of Wilson and Hemmati-Brivanlou that it was demonstrated that inhibitory factors are required to prevent ectodermal cells from differentiating into neural cells; that is, a neural cell fate was the default, rather than an epidermal fate. Their experiment involved using disassociated cells of Xenopus (African frog) animal caps from late-stage blastulas. Some of the cells were incubated with bone morphogenesis protein 4 (BMP4), whereas others were incubated without this protein. After an incubation period of 4 hours, the cells incubated in the absence of BMP4 expressed the neural marker NCAM, whereas the cells incubated with BMP4 expressed epidermal keratin. In an additional experiment, cells were injected with RNA of the dominant-negative activin receptor, and some of these injected cells were then incubated with BMP4. When only the dominant-negative receptor was present, neural NCAM was expressed. Similarly, neural NCAM was expressed when both the dominant-negative receptor and BMP4 were present. Taken together, these data definitely show that the embryonic ectodermal cells only become epidermal in the presence of BMP4. When activity of BMP4 is inhibited by the dominant-negative activin receptor, then cells develop as neural, the default state of the amphibian dorsal ectoderm.
Original Paper
Wilson, P. A., and A. Hemmati-Brivanlou. 1995. Induction of epidermis and inhibition of neural fate by BMP-4. Nature 376: 331–333.
http://www.ncbi.nlm.nih.gov/pubmed/7630398
See also an excellent review article:
Hemmati-Brivanlou, A., and D. A. Melton. 1997. Vertebrate Embryonic Cells Will Become Nerve Cells Unless Told Otherwise. Cell 88: 13–17.
http://www.sciencedirect.com/science/article/pii/S009286740081853X
Links
Grunz, H., and L. Tacke. 1989. Neural differentiation of Xenopus laevis ectoderm takes place after disaggregation and delayed reaggregation without inducer. Cell Differentiation and Development 28, 211–218.
http://dx.doi.org/10.1016/0922-3371(89)90006-3
Nature: Milestone 19 (1989): Chasing the elusive inducer
http://www.nature.com/milestones/development/milestones/full/milestone19.html
Kimball’s Biology Pages: Organizing the Embryo: The Central Nervous System
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Spemann.html
NCBI: Developmental Biology: Axis Formation in Amphibians: The Phenomenon of the Organizer
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A2295
Rockefeller University: Laboratory of Molecular Embryology: Brivanlou Lab
http://xenopus.rockefeller.edu/