Historical Roots of Developmental Biology
Who laid the foundations of developmental biology, and why should we care?
The multidisciplinary approach to the study of development first arose before the turn of the Twentieth Century as an integration of embryology (initially the descriptive study of embryonic development) with cytology (the study of cellular structure and function) and later with genetics (the study of inheritance). The leading cytologists of that time (primarily E.B. Wilson at Columbia University in New York City) recognized that development of the embryo is a manifestation of changes in individual cells and that an understanding of the fundamental principles of development would come from studying cellular structure and function.
Wilson recognized that the characteristics of an organism gradually emerge by utilization of the inherited information that is located on the chromosomes. Therefore, it was important to comprehend the nature of that information and how it is utilized during development. However, in the absence of concrete evidence, there was a great deal of rampant speculation as to how the chromosomes participate in development. The German embryologist Wilhelm Roux was the source of much of this speculation.
Roux believed that the fertilized egg receives substances that represent different characteristics of the organism, which - as cell division occurs - become linearly aligned on the chromosomes and are subsequently distributed unequally to daughter cells. This "qualitative division" fixes the fate of the cells and their descendants because some of the determinants are lost to a cell at each division.
Roux (1888) appeared to have confirmed his theories by an experiment he conducted on frog eggs.
(See Browder et al., 1991, Fig. 1.1A; Gilbert, Fig. 15.2; Kalthoff, Fig. 6.7; Shostak, 1991, Figs. 12.38, 12.39; Wolpert et al., Fig. 1.8.)
Another German embryologist, Hans Driesch (1892), approached the problem differently with sea urchin embryos. Instead of destroying one of the cells of the two-celled embryo, he separated the cells from one-another and found that isolated cells at the four-cell stage also develop normally. Thus, Driesch concluded that each cell retains all the developmental potential of the zygote.
The conflict between these two opposing views of development has been settled in favor of Driesch's interpretation by numerous cell separation experiments.
(See Browder et al., 1991, Fig. 1.2B.)
The experiment conducted by Roux illustrates the importance of proper experimental design. Roux had introduced an artifact into his experiment by allowing the damaged half of the embryo to remain attached to the uninjured half, interfering with its development.
Roux had done something that nobody had done before: He manipulated embryos and observed the effects of these manipulations on them. For this reason, many embryologists consider him to be the "Father of Experimental Embryology."
The Role of the Hereditary Material in Development
Although equal distribution of hereditary information to all cells had been established in the late 1800's, its role in development remained an enigma. There were two key contributions at the dawn of the Twentieth Century that provided the impetus for additional progress.
(See Browder et al., 1991, Fig. 1.3.)
Boveri's demonstration that each chromosome has qualitatively unique effects on development did not convince everybody that the hereditary factors are on the chromosomes. That conclusion required both cytological and genetic evidence. The final proof of the chromosome theory of inheritance resulted from research using the fruit fly, Drosophila melanogaster that was conducted in Thomas Hunt Morgan's laboratory at Columbia University. The use of Drosophila enabled Morgan and his colleagues to demonstrate conclusively that genes are located on chromosomes and to establish within a decade the basic principles of transmission genetics.
Morgan's student, H.J. Muller, demonstrated that ionizing radiation could be used to induce mutations in Drosophila. By inducing mutations, Muller, Morgan and a growing number of Drosophila geneticists acquired a vast library of mutations, which have been perpetuated and studied by generations of geneticists. As we shall discuss, many of these mutations have profound effects on the development of Drosophila. The detection of a mutant gene affecting development indicates that the normal allele of the gene affects that developmental process. Many of these genes that affect Drosophila development are also represented in the genomes of higher organisms.
One of the most important concepts to emerge from Wilson's writing was the concept that genes function at the cellular level to cause development of the organism. However, major questions remained unresolved, such as:
Wilson proposed that the specific role of the nucleus is the regulation of cellular metabolism, which was beyond the level of understanding of scientists of that time. Further progress required an understanding of the nature of the gene and how genes function to influence cell biochemistry. The study of the roles of genes in development waned, and an era had ended.
Mystery of the Century
Embryologists became disillusioned with genetics and preoccupied themselves with describing development and experimenting on the interactions and rearrangements of the cells and tissues that form embryos. A remarkable series of experiments conducted by the German embryologist Hans Spemann and his student Hilde Mangold in 1924 fired the imagination of embryologists and spawned a new era of embryological investigation that was truly the heyday of embryology.
Spemann and Mangold demonstrated that the transplantation of a small amount of tissue from the dorsal side of an early embryo of Triturus (tailed amphibian) could induce host tissue to form a secondary embryonic axis, including neural tissue. Spemann concluded that the behavior of an implanted region (the dorsal lip of the blastopore) reflects its normal function in inducing the primary axis and designated it as the embryonic organizer.
A vast amount of experimental data was collected, but the essential nature of the organizer and the process of embryonic induction remained a mystery until recent years. Current progress in understanding embryonic induction has come from research on a class of protein molecules called growth factors, which were discovered by Rita LeviMontalcini and Viktor Hamburger in the 1950s. A large number of growth factors have now been discovered that play important roles in a variety of developmental processes, including embryonic induction.
These discoveries have fostered intense investigation into how cellcell interactions are involved in organizing the embryonic body plan, and embryonic induction has re-emerged as one of the most active topics of investigation in developmental biology.
Biographical Information on key individuals
Embryonic induction is discussed in depth in the following Dynamic Development modules:
Browder, L.W., C. A. Erickson and W.R. Jeffery. 1991. Developmental Biology. Third Edition. Saunders College Publishing. Philadelphia.
Driesch H. 1892. Entwicklungsmechanisme Studien. I. Der Werth der beiden ersten Furchungszellen in der Echinodermentwicklung. Experimentelle Erzeugen von Theil und Doppelbildung. Zeitschrift für wissenschaftliche Zoologie, 53: 160-178; 183-184.
Gilbert, S.F. 1997. Developmental Biology. Fifth Edition. Sinauer.
Roux, W. 1888. Beiträge zur Entwicklungsmechanik des Embryo. Ueber die künstliche Hervorbringung halber Embryonen durch Zerstörung einer der beiden ersten Furchungskugelin, sowie über die Nachentwicklung (Postgeneration) der fehlenden Köperhälfte. Virchows Arch. Pathol. Anat. Physiol., 114: 113-153; 289-291.
Shostak, S. 1991. Embryology. An Introduction to Developmental Biology. HarperCollins. New York.
Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and Meyerowitz, E. 1998. Principles of Development. Current Biology. London.
|This material may be reproduced for educational purposes only provided
credit is given to the original source.|
Leon Browder & Laurie Iten (Ed.) Dynamic Development
Last revised Wednesday, June 10, 1998