Hans Spemann

The German experimental embryologist Hans Spemann (1869-1941) was awarded the Nobel Prize in Physiology or Medicine for his discovery of the organizer effect in embryonic development.

Hans Spemann, son of Wilhelm Spemann, a publisher, was born in Stuttgart on June 27, 1869. After a period in his father's business and military service, he became a medical student at the University of Heidelberg, spent a period at the University of Munich, and in 1894 transferred to Würzburg. There he abandoned medicine for science, studied under Theodor Boveri, who greatly influenced his future research, and graduated in 1895.

Spemann then began research in the Zoological Institute at Würzburg, where he became a lecturer in 1898. In 1908 he was appointed professor of zoology and comparative anatomy in the University of Rostock, and in 1914 associate director of the Kaiser Wilhelm Institute for Biology at Berlin-Dahlem. He was called to the chair of zoology in the University of Freiburg in Breisgau in 1919, from which post he retired in 1935. Spemann devoted his scientific career to the study of the causes that act on the cells of the earliest embryos, leading to their differentiation and specialization for different functions.

Foundation of Experimental Embryology

The science of experimental embryology (or developmental mechanics) was founded about 1890 by Wilhelm Roux and Hans Driesch. Roux destroyed one of the two blastomeres formed by the first division of a fertilized frog's egg. He found that the other blastomere continued to develop, but it formed half an embryo. Then Driesch separated the two blastomeres of a sea urchin's egg and removed one entirely. The remaining blastomere formed, not half an embryo, but a normal embryo of small size.

It was well known that the eyeball developed from the optic cup, a protuberance from the primitive brain, and that the lens arose in the epidermis overlying the optic cup. Why the epidermis thickened and became transparent at the appropriate point was unknown, and the question was whether there was some unknown connection between optic cup and potential lens. In 1901 Carl Herbst found that, in abnormal embryos showing a single (median) optic cup, only one lens developed and that at a point opposite the cup. This strongly favored an influence exerted by the cup on the overlying ectoderm.

Spemann's Classical Experiments

This paper by Herbst fired Spemann's enthusiasm, and in the same year he demonstrated that the epidermis showed no change if the eye rudiment was destroyed. He suggested that proof of the correlation could be obtained if the optic cup was brought into contact with a foreign part of the epidermis, either by transplanting the optic cup or the epidermis overlying it. W. H. Lewis performed these experiments satisfactorily in 1904. During the next 6 years Spemann published his experiments on the eye and also his technique and instruments for "microsurgery." It was shown that practically any part of the epidermis could form a lens if it was activated by some influence in the optic cup.

Spemann then experimented on the amphibian gastrula, the early embryo consisting of undifferentiated cells forming a hollow sphere, with a mouth (the blastopore) opening to the exterior. He frequently transplanted minute pieces of the gastrula from one area of its surface to another, and he always found—with one exception—that the transplants developed according to their new positions. The exception was a transplant from the upper lip of the blastopore, which in its new position developed into a small secondary embryo. In 1918 Spemann thought that the whole of the secondary embryo was formed by the implanted material, so that the ectoderm (upper layer) of the implant formed the medullary plate (subsequently forming the central nervous system), and the lower layer developed into the notochord and muscular system. He concluded therefore that the blastopore region was already differentiated at that stage, while all other cells in the gastrula were still undifferentiated.

Further work then led Spemann to think that possibly the primitive nervous system of the secondary embryo was formed by induction from the ectoderm of the host tissue. To decide the point it was necessary to distinguish implanted tissue from host tissue. Until then his transplants had been from one part to another of the same embryo, but in 1921 he decided that the answer lay in using two embryos, of the same age but of different species. In 1924 Spemann and Hilde Mangold published their results. For the implant and the host they used respectively gastrulas of the newts Triton cristatus (almost colorless) and Triton taeniatus (highly pigmented). Implant and host cells were thus easily distinguished. In innumerable experiments they found that the graft disappeared below the gastrula surface to form the mesodermal elements (notochord and muscles) of the secondary embryo. Above it the ectoderm of the host was induced to form, from host material, the neural tube of the secondary embryo.

Organizer Effect

From these "heteroplastic transplants" Spemann concluded that the upper lip of the blastopore, when brought into contact with other cells of the gastrula, could exert an influence to induce them to become differentiated to form a medullary plate. This influence he called an "organizer." In all vertebrate embryos it is the first step in the series of differentiations that result in the fully formed fetus. He therefore termed the influence of the blastopore lip the "primary organizer." The formation of the optic cup being a sequel to this action, he called the organizer in the optic cup a "secondary organizer." Further development is due to chains of induction by successive orders of organizers. Spemann believed that the action of the organizer was transferred by a chemical substance; but he, and other scientists such as Joseph Needham and C. H. Waddington, succeeded only partially in identifying it.

Spemann was awarded the Nobel Prize in 1935, and he received many other honors, including the title of Geheimrat (Privy Councilor). He died at Freiburg on Sept. 9, 1941.

Further Reading on Hans Spemann

There is a biography of Spemann in Nobel Lectures, Physiology or Medicine, 1922-1941 (1965), which also includes his Nobel Lecture. For his work see his Embryonic Development and Induction (1938); see also C. H. Waddington, The Nature of Life (1961), and J. Needham, Biochemistry and Morphogenesis (1942).

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