Exploring the Historical Foundations of Developmental Biology
Delve into the historical background of developmental biology starting from ancient Greek scholars like Hippocrates and Aristotle to key figures like William Harvey and their contributions to the field. Learn about the evolution of theories such as epigenesis and preformation, shaping the study of embryology and developmental processes over the centuries.
- Developmental Biology
- Historical Background
- Embryology
- Scientific Contributions
- Evolution of Theories
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Teacher: Dr Gouri Course: Zoology Hons. 6thsem Paper: Developmental Biology Unit 1 Introduction
Unit 1 - Introduction Definition of Developmental Biology - Holistic study approach including embryonic development, and all post natal processes such as normal growth, metamorphosis, regeneration and tissue repair at different levels of complexity ranging from molecular to the organism level. Historically, Developmental Biology was known as Embryology. Definition of Embryology study of animal development that exits between fertilization and birth. But, we know that Development does not stop at birth or even at adulthood and organism never stop developing. So now, the term Embryology has been replaced by DevelopmentalBiology . Ontogeny Process by which zygote is transformed into multicellular and well organized being. Embryogenesis development of an individual from a fertilized egg or zygote. Blastogenesis- development of a new organism from an asexual body like a bud, fragment of body, spore etc.
Historical Background 1. Hippocrates in Greece in 5th Century BC gave the first scientific approach to study development. 2. Aristotle (384 to 322 BC) Greek Philosopher- first embryologist known to history. He cracked open a hen egg on each successive day of its 3 week incubation period to observe thin band cells giving rise to an entire bird. He observed formation of major organs. He tried to address how different parts of the embryo are formed. He had two views : first that all the organs in the embryo are preformed from very beginning which got bigger during development (Preformation) and the other that new structures arise progressively which he termed Epigenesis . Aristotle favored epigenesis which was later found correct. Thus Aristotle established embryology as a new branch of science and he is known as Founder of Embryology. 3. Galen (130-200 AD) studied about structures of relatively big features but the minute dimensions of early embryos could not be done until 17th century, when the microscope were available to study early stages of embryos. 4. William Harvey (1951) concluded that all animals originated from eggs and discarded the idea of spontaneous generation of animals from mud. He first observed the blastoderm of chick embryo and described chick development. He supported epigenesis. He first noticed the blood islands formation and suggested that amniotic fluid might function as shock absorber for embryos.
5. Marcello Malpighi (1672) described different stages in the development of chick as seen with simple lens. Italian embryologist Marcello Malpighi followed Preformation idea and remained convinced against his own accurate observations of development of chick embryo, that a small embryo was already present from the beginning. 6. Regnier de Graaf (1672) described ovarian follicles, the mature follicles were named after him as Graafian follicles. EPIGENESIS AND PREFORMATION During 17th and 18th century Aristotle s ideas influenced European thoughts which was then the cradle of embryology. Although Aristotle favored epigenesis was championed by preformation at this time. With contemporaneous religious background of belief in divine creation of the world and all living beings, many researchers were not convinced with the idea of epigenesis and supported preformation. Most of them believed that all embryos had existed from the beginning of world and that the first embryo of a species must contain all future embryos. Epigenesis was supported by Aristotle and Harvey; Preformation was supported by Malpighi, and others of Spermist and Ovist group. But Preformation was unable to explain variations between the generations of organisms.
7. Preformation/ Conservation Theory it states that the prefigured miniature human being is preformed (homunculus) either in the ovum or in the sperm. Thus two distinct schools of preformists thoughts emerged: A. Animalculists or Spermists: Antony Von Leeuwenhoek (1632-1723) and Nicolaas Hartsoeker (1656-1725) believed that preformed miniature organism was present in the head of the sperm which was merely nourished in egg. Fig: Homunculus in human sperm B. Ovists: Marcello Malpighi (1628-1694), Jan Swammerdam (1637-1680) and Lazzaro Spallanzani (1729-1799) argued that the ovum contained preformed miniature adult which is stimulated to grow by the seminal fluid. C.Bonnet in 1745 discovered that the eggs of some insects can develop parthenogenetically (without participation of sperm), and the Ovists almost won. However both schools agreed that miniature body in each egg or sperm must inturn enclose the successive miniatures of all future generations. 8. Encasement/ Emboitment Theory given by C. Bonnet in 1770, states female encapsulates the preformed germ cells of future generations one inside the other. This theory was discarded when importance of both egg and sperm was explained by CF Wolff.
9. Epigenesis Theory- Kasper Friedrich Wolff carefully observed development of chick embryos and in 1767 postulated that embryonic parts develop from tissues that have no counterpart in the adult organism. The different organs present in adult, develop anew in each embryo by progressive growth and differentaition called epigenesis. Wolff proposed a force vis essentialis acting to create an organism anew in each generation. This force was supposed to be analogus to gravity or magnetism to organize the embryonic development. 10. Bildungstrieb theory- proposed by German philosopher Immanuel Kant (1724- 1804) and Johann Friedrich Blumenbach (1752-1840). It postulated experimental existence of a mechanical, goal-directed force called Bildungstrieb (developmental force). This predetermined force drives development and is inherent part of the embryo. This tried to bring in reconciliation between preformationism and epigenesis, wherein epigenetic development is directed by preformed instructions (in form of genes, untranslated mRNA etc) present in the fertilized egg. The preformation/epigenesis controversy continued through 18th century and was finally laid to rest by Cell Theory, developed between 1820-1880, which proposed that all living things including embryos were composed of cells. In 1840s, it was realized that development could not be based on preformation but must be epigenetic because during development many new cells are formed from the egg which itself is a single specialized cell.
11. Germplasm Theory Proposed by August Weismann in 1893 that the parental characters are passed to the offspring through the germ cells (egg and sperm) and not from the somatic cells. The germplasm forming the germ cells is very distinct from the somatoplasm of somatic cells that characteristics (mutations) acquired by the body somatic cells cannot be transmitted to the germline and thus cannot be inherited by the offspring. Adapted from Wolpert et al chapter1
12. Mosaic Theory originally the idea of Mosaic development was conceived by A. Weismaann in 1880s. He explained nucleus of the zygote contained a number of special factors or Determinants which were distributed unequally to the daughter cells after cleavage of the zygote and control the future development of these cells. Thus this type of model was termed mosaic as the egg is a mosaic of discrete localized determinants. The experimental evidence for the Mosaic development was given by Wilhelm Roux in 1888, who is known as Father of Experimental Embryology, performed experiments on frog s egg. After first cleavage of fertilized frog egg, Roux killed one of the two blastomeres with a hot needle and observed that the remaining live blastomere developed into a well formed half-larva. He concluded that the development of frog is based on a mosaic mechanism, the cells having their character and fate determined at each cleavage. This is also called as mosaic theory of development/ Determinate development. Adapted from Wolpert et al; chapter 1
jjj Adapted from Wolpert et al; chapter 1
13. Regulative theory- Proposed by Hans Driesch in 1891 after conducting experiments on the fertilized eggs of sea urchins as they could be easily explored and are transparent. The regulative theory is in contrast to the idea of mosaic theory of development. He observed that the blastomeres of sea urchin when separated from each other at two cell stage , developed into two complete larvae but of smaller size. Thus early in cleavage all the blastomeres have equal potencies to develop into whole embryo. He also concluded that the development is regulative due to conditional specification as the fate of the cells is determined by the interaction with its neighbouring cells. Thus the cells has the ability to develop according to change in conditions. Its is a feature of all the vertebrates and some invertebrates. This is called as regulative theory of development. In determinate development, structures are formed strictly according to a fixed genetic program, where as in Regulative development, one tissue affects the subsequent development of another by a process known as Induction. continued in next slide-
