HöRSTADIUS, SVEN OTTO

(b. Stockholm, Sweden, 18 February 1898; d. Uppsala, Sweden, 16 June 1996),

developmental biology, experimental embryology.

Hörstadius was one of the leading experimental embryologists during several decades around the mid-twentieth century. His most important work was on the determination and differentiation of the sea urchin embryo, a topic he studied for half a century. Early work inspired by his teacher John Runnström’s double gradient theory showed that gradients of animalness (ectodermal determination) and vegetalness (endodermal determination) existed in the 16- and 32-cell embryos. Hörstadius became famous for his elegant extirpation and transplantation experiments using glass needles, and for his skill in performing microsurgery on the minute embryos of echinoderms. He also made important contributions to the study of cranial neural crest development in the Mexican axolotl, in collaboration with his student Sven Sell-man. Hörstadius obtained several honorary doctorates, for example, in the United Kingdom (Cambridge and Bristol) and in France (Sorbonne), and was elected a member of many academies and learned societies, including the Royal Society in London and the Vatican Academy of Sciences.

Short Biography . Sven Hörstadius was born into an upper-class Stockholm family. His father Wilhelm was a judge of appeal, and the family spent their summers in a summer house in the Stockholm Archipelago. Sven showed a keen interest in bird watching already as a schoolboy at the Northern Latin School in Stockholm, and was to remain active as an amateur ornithologist all his life. He went to college in Stockholm and rapidly came to know a zoologist, John Runnström, who was to have a formative influence on his scientific interests and career. Hörstadius obtained a PhD degree in 1928, and married Greta (born Kjellström, 1903–1987). She also had a natural science background and became an important collaborator and assistant to Sven. They published together on protein digestion in gastropods. The marriage produced two children, Göran, who was a physician at Uppsala University Hospital, and Dagmar, who went into banking and married Sven Ågren. Sven Hörstadius remained active as a scientist well into his eighties. The death of his wife in 1987 was difficult to handle, but he remained in good physical shape well into his nineties. In the last decade of his long life, however, his memory often failed him. He died in a nursing home at the age of 98.

The First Experiments . Sven Hörstadius was introduced to experimental embryology as an undergraduate by John Runnström, a young zoologist who would later occupy the chair in zoology at Stockholm University. Runnström and his students did their experimental work at Kristineberg marine station on the Swedish west coast. Kristineberg had been founded by, and was run by, the Royal Swedish Academy of Sciences in Stockholm, and had laboratories where the Runnström group could perform their experiments. Already as an undergraduate, Hörstadius made an important investigation of factors regulating the ripening of the eggs of a polychaete worm, Pomatoceros triqueter. He experimented with factors such as temperature, alkalinity, osmotic pressure, and ion concentration, and noted their effects on egg ripening. This led to his first international publication (in 1923) and to a smaller follow-up paper published the following year in a Swedish journal.

Echinoderm Experimental Embryology . Kristineberg was not suitable for experimental work on embryology in the winter, and the north Atlantic echinoderm species did not spawn year-round. Therefore Hörstadius now started a long series of successful stays at the Stazione Zoologica in Naples, Italy. The Stazione became his main winter research station, although he sometimes also visited other marine biology research stations, such as Roscoff in France and Plymouth in England. He first visited Naples in 1922, and then again in 1924. Here he came to work almost exclusively with echinoderms. Much later, when Hörstadius summed up his research on echinoderm experimental embryology in a classic book, he listed the reasons why these animals became so important as experimental objects:

(a) artificial fertilization is readily achieved and hence desired stages of development can be obtained at any time; (b) the larvae are transparent, as are also the eggs of some species, and thus allow microscopical studies of the living material to be made; (c) eggs can be obtained in large quantities, which facilitates physiological and biochemical investigations; (d) the regular type of cleavage makes possible work with fragments of known size and origin; (e) the egg axis is recognizable by the characteristic 16-cell stage …; (f) furthermore, ripe ova and sperm are available for long periods from several species, so that by choosing suitable marine stations it is possible to work on the development of sea-urchins at all times of the year. (p. 2)

Hörstadius pointed out repeatedly that in order to even begin to investigate the processes and mechanisms underlying normal development experimentally, one first needs to have a detailed description of cell division, cell movements, and cell fate. In his 1973 book he wrote, “A prerequisite for experimental work is a thorough knowledge of the normal development” (p. 10). Although Hörstadius is known as the great experimentalist, he also made several important descriptive studies of the development of different echinoderm species. A method to keep track of cell divisions and cell movements was to use vital dyes such as Nile blue sulfate to mark selected cells at an early stage. The dye was kept in small blocks of agar-agar, which were put into contact with the target cell or cells. Hörstadius used vital dye staining to determine the fate at gastrulation of ventral cells (see Figure 1), and also in his later studies of neural crest fate (see below). Fate mapping is important for understanding the cellular origin and morphogenesis of the different organs in an organism. By staining the entire vegetal (lower) half of the echinoderm embryo, Hörstadius showed that about a third of the ectoderm (the outer layer of the early embryo, which gives rise to the skin and the nervous system) was derived from the

macromeres (the large cells on the underside). When staining only the most ventral cells, veg₂, and micromeres (the small cells on top), Hörstadius could show that their descendants were found in the archenteron (the primitive gut) and in primary mesenchyme. Finally, and most elegantly, he stained single cells at the 64-cell stage using a micropipette. Staining one veg₂-cell resulted in stained cells only in the archenteron (Figure 1[a]₁–[a]₄) whereas marking both a veg₁- and a veg₂-cell showed that the descendants of the veg₁-cell contribute to the presumptive ectoderm (Figure 1[b]₁–[b]₄). At the time, this was probably the most detailed fate-mapping study ever undertaken, and served as the basis for Hörstadius’s experimental work. Knowing the normal fate of cells makes it possible to interpret experiments in which normal development is altered.

During his first stays at the Stazione Zoologica in Naples, Hörstadius did a number of elegant experiments on determination and induction, the processes by which cells are instructed to develop into a specific cell type. In his first experiments on starfish larvae he used relatively advanced stages such as late gastrulae and did his experiments using glass needles and simple microscopes. He could show that isolated vegetal halves were able to develop into normal-looking older larvae, so-called plutei, while isolated animal (upper) halves were not. Contrary to earlier beliefs, he found that all cells of the early stages had the same potency: isolated vegetal halves of such embryos developed into small but quite normal larvae while animal halves did not (see Figure 2). The logical experiment was then to test how different combinations of cell layers from the 16-cell stage might influence development. The main finding was that even animal halves of the larva developed into normal plutei if they were given influences from the vegetal part, for instance by transplanted micromere cells from the very bottom of the morulae. Other experiments, in which he delayed the normal cleavage timetable, showed that differentiation of the embryo is independent of the cleavage scheme. Most of this work was published as his doctoral thesis (1928).

In 1929, Hörstadius’s teacher Runnström published an idea he had developed over several years, the double-gradient theory. According to this theory, the animal and vegetal poles of the embryo each set up a gradient of a morphogen, and the cells differentiate according to the levels of these two morphogens that they encounter. Hörstadius was the experimentalist who could put this theory to the test. Already his thesis work had pointed in this direction, and he now followed it up in a range of different experimental setups (see Figure 2).

Another question hotly disputed at the time was whether combining the cytoplasm from one species and the nucleus from another could lead to a zygote that developed normally. This was an important part of the question whether development is determined by the cytoplasm or by the nucleus. Hörstadius published a preliminary report in 1932 and later (1936) a large paper in which he showed convincingly that enucleated eggs from one species with an injected sperm from another could develop to a certain degree, and that the nucleus determines the species-specific development.

Hörstadius stayed in Runnström’s research group throughout the 1930s. Runnström was an ambitious leader and needed more modern labs for his group. Government money was hard to come by, but he had contacts with the Swedish businessman Axel Wenner-Gren, and managed to get him interested in supporting his work. Wenner-Gren was the founder of the Electrolux Company, and in the 1930s one of the most influential Swedish industrialists. This contact resulted in the founding of the Wenner-Gren Institute of Experimental Biology in Stockholm, inaugurated in 1939. This institute became important for Swedish physiology, and formed a strong school in developmental biology. Among its members were Per-Erik Lindahl, Tore Hultin, Tryggve Gustafsson, and Björn Afzelius, in addition to Runnström and Hörstadius. The constellation Wenner-Gren Institute–Kristineberg Marine Station has been invaluable for strengthening Swedish developmental biology. Hörstadius became a zoology professor at Uppsala University in 1942, but kept his links with the Wenner-Gren Institute. In collaboration with Runnström and other members of the institute, Hörstadius did important studies of the effects of animalizing and vegetalizing substances on the development of sea urchin larvae. In animalized embryos all cells develop as if they were derived from its animal (upper) part, whereas in vegetalized embryos, all cells differentate into cells of the gut, which normally arise only from the vegetal (lower) part of the embryo. Substances investigated included amino acids

(1957) and sulfate (1964), but also the vegetalizing effects of dinitrophenol and animalization by trypsin and ficin were documented. Animalized larvae develop an enlarged apical tuft of stereocilia but no intestine, while vegetalized larvae develop an intestine but no ciliary tuft. Together with the biochemist Lars Josefsson in Copenhagen, Hörstadius analyzed the morphogenetic effects of fractions extracted from lyophilized sea urchin eggs (1967). Some of these fractions, which were never clearly defined, were very potent animalizers (1972).

Cranial Neural Crest Development . Toward the end of the 1930s Hörstadius started to work on a new experimental organism and in a new research area: head, in particular cranial neural crest, development in the Mexican axolotl. Head development was a classical subject in morphology and developmental biology, and at the time the origin of cartilage, bone, teeth, and other structures in the head region was a subject receiving renewed scientific attention. Earlier studies had indicated that most of the cranium was derived from the neural crest, but details were largely unknown. The neural crest is an embryonic structure in vertebrates, which develops when the central nervous system is formed at neurulation. Neural crest cells briefly form a crestlike structure on top of the neural tube, and then migrate away to form a large array of different cell types in the embryo, including skeletal, pigment, and nerve cells. There may be more than one explanation for why Hörstadius started to work on the neural crest. He now had a family, and although sea urchin studies could be combined with family life during the summer months, when the Kristineberg station was the ideal place to bring the family, long winter and spring stays in Naples were difficult to combine both with academic teaching and with family life. Carl-Olof Jacobson (2000a, b), however, writes that another factor might have been even more important. In order to be able to obtain a professorship at Swedish universities, it was important to not be seen as too much of a specialist. Usually, candidates who came across as narrowly focused specialists were not chosen, because as a teacher, a zoology professor had to cover a very large area including both invertebrate and vertebrate zoology. Hörstadius might have reasoned that if his skill in invertebrate cell and developmental biology could be supplemented with vertebrate morphology and embryology, his chances of obtaining a chair in zoology would be much better.

Hörstadius had also visited Ross G. Harrison’s laboratory at Yale University and the Woods Hole Oceano-graphic Institute, in Massachusetts, in 1936. From Harrison he learned how to perform microsurgery on amphibian larvae. In the 1930s Harrison was occupied with a series of neural crest experiments and Hörstadius became interested in this remarkable embryonic structure and in the ongoing research on what tissues and organs the neural crest actually gave rise to. He immediately started what would become a thorough analysis of the skeleton-forming capacity of the cranial neural crest in the amphibian genera Ambystoma and Triturus. These studies were done in collaboration with Sven Sellman, a graduate student from the school of dentistry in Stockholm. Sellman had started an investigation of tooth development in Triturus, and the common interest in neural crest biology led to a very fruitful cooperation.

The fate of cranial neural crest cells needed to be mapped, and Hörstadius and Sellman used basically the same technique—vital staining—that had proven so useful in Hörstadius’s echinoderm work. Alternating red and blue markers (Nile blue sulfate and neutral red, respectively) were applied to different portions of the neural folds, and the migration of the neural crest cells between the epidermis and the mesoderm could be followed. The main problem was that the stains became more and more diluted with each cell division. In order to learn more about the properties of the neural crest of the head, a vast number of extirpation and transplantation experiments were performed. In this way, they could elucidate both the migration and the normal fate of cranial neural crest cells, as well as the inductive influences from neighboring tissues such as the underlying mesoderm (which gives rise to the axial skeleton and striated muscles) and endoderm (which produces the cells of the digestive tract). Hörstadius and Sellman especially emphasized the role of the endoderm, whereas the overlying ectoderm seemed to have no effect. From 2000 onward the role of the endoderm has received renewed attention.

Other questions addressed in the paper include the importance of the presumptive forebrain for the differentiation of the trabeculae (parts of the neurocranium, which underlie the brain), and the clear evidence that part of the trabeculae, the “ectomesodemal trabeculae” are neural crest derived. Also the neural crest material normally destined to be integrated into the trabeculae was shown to not have the potential to differentiate into visceral arch skeleton, and vice versa. In addition, it was shown that both the neural crest closest to the oral region and the trunk crest do not form cartilage. Finally, new and important observations concerning the development of the ear, teeth, and eyes were also reported.

Another researcher interested in the neural crest, Professor (later Sir) Gavin de Beer, head of the Department of Embryology at University College London, was very impressed by the large (170-page) paper published by Hörstadius and Sellman in 1946. He invited Hörstadius to give a series of lectures on the neural crest in London in the fall of 1947. In his lectures Hörstadius told the full story of how our knowledge of the neural crest has developed, starting with Wilhelm His’s first description from 1868 of the neural crest in the chick embryo, and summarizing later work until the mid-1940s. The lectures were later (1950) published as a book, The Neural Crest, by Oxford University Press. This book has become a classic and has been reprinted a few times. In 1988 it appeared in facsimile together with a modern treatment of the same topic by Brian K. Hall. Hörstadius always argued that he was an experimentalist and not a theoretician. And perhaps this is exactly why he was able to make such a long-lived summary of a research area. He never went further in his analyses than the results of the experiments allowed him.

Sven Hörstadius the Teacher . Hörstadius gave a lecture series for many years on developmental biology. It ran over two semesters and his lectures are reported to be clear and didactically excellent. Jacobson, who worked together with Hörstadius in undergraduate courses, has given the only eyewitness report:

[Hörstadius] was a brilliant lecturer and showman, and he loved to perform. It is a pity that his two-semester lecture course on embryology and its research history was never published as a book, even though his lively way of demonstrating form-shaping movements with his whole body would not have appeared in a written version. Especially instructive and entertaining was his elegant way of demonstrating embryogenesis with hand movements, while his face was radiating enthusiasm. (2000b, p. 253)

The only complaint about Hörstadius’s lectures noted by Jacobson is that “his wish to be clear in his lectures often led him to oversimplify a scientific problem. As his assistant I often overheard his lectures, and in my mind he too often presented a plausible theory as an established fact” (2000b). Another of Hörstadius’s former graduate students, Göran Gezelius, also reports that Hörstadius tended to gloss over problems in his lectures. When asked about this, Gezelius said, “There were never any unsolved problems,” whether this was apparent to the undergraduate students is another matter altogether.

Statesman of Science, Bird-watcher, and Environmentalist . Progressively, from the 1930s onward, Hörstadius became more and more involved in academic leadership. When still in Stockholm, he worked as head of department, and in Uppsala, where he got his chair of zoology in 1942, he had to deal with teaching and administration more than before. His zoology professorship made Hörstadius responsible for the teaching of ecology, a new branch of science, both in undergraduate courses and in graduate school. For several years he served as dean of the Faculty of Natural Sciences, and he also organized large conferences. He was a member of the Swedish Natural Science Research Council and the Royal Swedish Academy of Sciences. Hörstadius also took on a number of international responsibilities. He was chairman of the International Union of Biological Sciences for a considerable time and he also served as president of the International Council of Scientific Unions. Hörstadius became a respected statesman of science.

In addition to his academic interests, Hörstadius continued to be interested in bird watching throughout his life. For a while, he was president of the Swedish Ornithological Society. Other hobbies included cross-country skiing and walking tours, and he was an excellent nature photographer. Hörstadius loved to travel, and was a member of the Swedish Tourist Association. Like many biologists, he developed an interest in nature conservation, and was a member of the Swedish Association for Nature Preservation. Hörstadius had considerable social talents, and Jacobson reported that “one of his most cherished accomplishments was to be President of the Uppsala Student Choir called Allmänna Sången.… The duties there and in the Students’ Association of Folk Dancers suited his vivid, witty and light personality” (2000b, p. 254).

The Legacy of Sven Hörstadius . Hörstadius had very few graduate students who actually worked on topics close to his own research. Gezelius did a doctoral thesis in 1974 on the effects of sulfate and other ions on echinoderm development, but then became head of the Klubban marine station and did little further research. Jacobson worked on the development of the central nervous system in the Mexican axolotl, a topic that fascinated Hörstadius and which he realized he would not be able to work on himself. Jacobson became a zoology professor in Uppsala, and some of his students, for example, Ted Ebendal and Jan Löfberg, continued the experimental embryology tradition, but with an increasingly molecular emphasis. The transition into the twenty-first century brought an end to the era of experimental embryology in the Hörstadius tradition in Uppsala. Jacobson’s student Löfberg retired from his professorship, and his student Lennart Olsson took up a professorship in Jena, Germany. Olsson’s research group in Jena continues the fate-mapping and experimental embryology tradition, but with an evolutionary component largely absent from Hörstadius’s work. The founder of Entwicklungsmechanik, the experimental embryologist Wilhelm Roux, was born in Jena, so in a way history has come full circle, back to Jena, and the glorious era of experimental embryology started by Runnström and his student Hörstadius when they both worked in Stockholm in the first part of the twentieth century has come to an end, having been replaced by a booming field of model systems developmental biology with medical applications, as well as by a growing interest in the old question of the role of developmental biology for a deeper understanding of evolution.

BIBLIOGRAPHY

Only the most important papers and books by Hörstadius are listed. For a complete bibliography of his scientific works, see Olsson (2000). In 1981, Hörstadius collected reprints from both his scientific works and his writings (mostly in Swedish) on nature photography, nature conservation, and popular science (mainly ornithology). He also made a complete bibliography of his works until then. Hörstadius had the papers and the bibliography bound in five volumes. Only two copies were made: one for the university library in Uppsala, and one for the zoology department in the same city, where he had worked since 1942. Remarkably, no major work about Hörstadius exists, not even a book-length biography. The only sources are two short overviews written by Carl-Olof Jacobson, one of Hörstadius’s graduate students, and a few obituaries. As a consequence, this account relies very heavily on Jacobson’s papers, and on conversations with him and with Göran Gezelius.

WORKS BY HÖRSTADIUS

“Physiologische Untersuchungen über die Eireifung bei Pomatoceros triqueter L.” Archiv für mikroskopische Anatomie und Entwicklungsmechanik 98 (1923): 1–9. This paper and the next are based on Hörstadius’s undergraduate research on factors regulating the ripening of the eggs of a polychaete worm.

“Weitere Studien über die Physiologie der Eireifung bei Pomatoceros triqueter L.” Arkiv für Zoologi 16, no. 11 (1924): 1–4.

“Über die Determination des Keimes bei Echinodermen.” Acta Zoologica (Stockholm) 9 (1928): 1–191. Hörstadius’s doctoral thesis; here he continues investigations of determination and induction in echinoderm embryos, and establishes his experimental techniques of extirpation and recombination of different parts of the embryos using glass needles.

“Über die Determination im Verlaufe der Eiachse bei Seeigeln.” Pubblicazioni della Stazione zoologica di Napoli 14 (1935): 251–479. A paper the size of a monograph in which many of Hörstadius’s classical experiments of determination in sea urchin embryos are detailed and illustrated.

“Studien über heterosperme Seeigelmerogone nebst Bemerkungen über einige Keimblattchimären.” Mémoires du Muséum National d’histoire Naturelle Belgique, series 2, 3 (1936): 801–880. Hörstadius showed in this paper that sperm nuclei from one species could determine the first steps of development in enucleated eggs from another species, and that the species-specific characters were determined by the nucleus rather than the cytoplasm.

With Sven Sellman. Experimentelle Untersuchungen über die Determination des Knorpeligen Kopfskelettes bei Urodelen. Nova acta Regiae societatis scientiarum upsaliensis series 4, vol. 13, no. 8. Uppsala, Sweden: Almqvist & Wiksells, 1946. The major paper emanating from Hörstadius’s study of cranial neural crest development in the Mexican axolotl.

The Neural Crest: Its Properties and Derivatives in the Light of Experimental Research. London: Oxford University Press, 1950. The classic book on the neural crest, based on lectures given by Hörstadius at University College London in 1947.

With Tryggve Gustafson. “Changes in the Determination of the Sea Urchin Egg Induced by Amino Acids.” Pubblicazioni della Stazione zoologica di Napoli 29 (1957): 407–424.

With John Runnström, J. Immers, and M. Fudge-Mastrangelo. “An Analysis of the Role of Sulfate in the Embryonic Differentiation of the Sea Urchin (Paracentrotus lividus).” Revue suisse de zoologie 71 (1964): 23–54. Two papers in which Hörstadius and collaborators investigate the impact of different substances on sea urchin development.

With Lars Josefsson and John Runnström. “Morphogenetic Agents from Unfertilized Eggs of the Sea Urchin Paracentrotus lividus.” Developmental Biology 16 (1967): 189–202.

With Lars Josefsson. “Morphogenetic Substances from Sea Urchin Eggs: Isolation of Animalizing Substances from Developing Eggs of Paracentrotus lividus.” Acta embryologiae experimentalis(1972): 7–25. Two papers with Lars Josefsson, where endogenous substances with vegetalizing and animalizing effects were isolated. Maybe Hörstadius’s last important experimental papers.

Experimental Embryology of Echinoderms. Oxford: Clarendon Press, 1973. His legacy in which he summarized fifty years of work on echinoderm development.

With Brian K. Hall. The Neural Crest: Including a Facsimile Reprint of The Neural Crest by Sven Hörstadius. New York: Oxford University Press, 1988. The latest reprinting of Hörstadius’s 1950 book on the neural crest, here preceded by a review on the same topic by the coauthor.

OTHER SOURCES

Jacobson, Carl-Olof. “Sven Hörstadius död.” Upsala Nya Tidning, 8 July 1996. Obituary in an Uppsala newspaper (in Swedish).

———. “Sven Hörstadius: The Man and His Work.” In Regulatory Processes in Development: The Legacy of Sven Hörstadius (1898–1996), edited by Lennart Olsson and Carl-Olof Jacobson. Wenner-Gren International Series, vol. 76. London: Portland Press, 2000a. A short overview of Hörstadius’s major scientific contributions, and a brief biography.

———. “Sven Otto Hörstadius, 18 February 1898–16 June 1996.” Biographical Memoirs of Fellows of the Royal Society 46 (2000b): 244–256. Covers the same ground as the biography by the same author in Regulatory Processes, but is somewhat longer and more detailed.

Olsson, Lennart. “In Memory: Sven Hörstadius 1898–1996.” SICB Newsletter, Fall 1996. A short obituary.

———. “The Scientific Publications of Sven Hörstadius—A Bibliography.” In Regulatory Processes in Development: The Legacy of Sven Hörstadius (1898–1996), edited by Lennart Olsson and Carl-Olof Jacobson. Wenner-Gren International Series, vol. 76. London: Portland Press, 2000. This bibliography covers Hörstadius’s scientific works only, and has 100 entries.

Lennart Olsson