A Very Brief and Incomplete History of Developmental Biology

1. A Very Brief and Incomplete History of Developmental Biology

2. The past is full of ideas about how organisms developed and where they came from. These range from pure magic

3. To cultural myths

4. To flights of fantasy

5. And, in recent times, To observational and experimentally based inquiry

6. Questions? Where are the plans or instructions for development? How are they interpreted and used? Where are the raw materials for development? How did those raw materials get to the embryo? How are they used? How do different cells and tissues know what to become? How do cells migrate? How do they know where to go? How is neural circuitry established? How do axons find their targets? Etc., etc, etc.

7. One important thing to keep in mind, Development DOES NOT happen by magic. Information and mechanisms at the cellular and/or molecular levels are needed to accomplish development.

8. HISTORY Who was the first embryologist? Aristotle - 400 B.C.

9. Aristotle considered two basic developmental questions: Do all parts of a developing organism come into existence together and simply grow larger? or Is development a stepwise process characterized by progressive organization and an increase in complexity? Preformation versus Epigenesis

10. Preformation - The organism is preformed as a complete miniature structure in the sperm or the egg and simply grows larger as it develops. This means that the first reproducing human would have had to have all succeeding generations within itself. Sort of like Russian dolls.

11. Epigenesis - The organism develops in a stepwise fashion from an unorganized state.

12. Aristotle believed that the embryo was formed from the menstrual blood as a result of that blood’s interaction with a male factor, called the male dynamic, that was present in the semen. His observations supported the concept of epigenesis, though this term would not be used until the mid to late 17th century.

13. Mid 17th century - 18th century Epigenesis vs Preformation - a matter for debate Epigeneticists Preformationists William Harvey ~ 1651 Rene Descartes ~ 1664 Pierre Maupertuis ~ 1745 Jan Swammerdam ~ 1672 Marcello Malpighi ~ 1672 Nicholas Malebranche ~1673 Nicolas Hartsoeker ~ 1694 Charles Bonnet ~ 1762 Spermists vs ovists

14. Preformationists Jan Swammerdam, ~1672 17th century Dutch microscopist Debunked “Spontaneous generation” using meticulous dissections and careful experimentation.

15. SPONTANEOUS GENERATION Recipe for bees: Kill a young bull, and bury it in an upright position so that its horns protrude from the ground. After a month, a swarm of bees will fly out of the corpse. Jan Baptista van Helmont’s recipe for mice: Place a dirty shirt or some rags in an open pot or barrel containing a few grains of wheat or some wheat bran, and in 21 days, mice will appear. There will be adult males and females present, and they will be capable of mating and reproducing more mice. LOUISE PASTEUR’S EXPERIMENT

16. Preformationists Jan Swammerdam 17th century Dutch microscopist Debunked “Spontaneous generation” using meticulous dissections and careful experimentation. Thought to have originated the idea of Preformation Greatest contribution to science - demonstrated that in insect development, the same organism persists through various stages, i.e. larva, pupa, juvenile, adult.

17. Preformationists Marcelo Malpighi, ~1672 Professor of medicine and personal physician to Pope Innocent XII Early microscopist One of the first scientists to study structures such as the lungs, kidneys, spleen, brain, and skin Because of the importance of his early work a number of anatomical structures still bear his name - Malpighian corpucles (renal corpuscle) in kidney, Malpighian layer in epidermis of skin Malpighian tubules in insects Marcello Malpighi - did not believe what was right before his eyes when examining chicken development with the microscope.

18. Preformationists Nicholas Malebranche, ~1673 A priest and philosopher Most original and influential of the Cartesian philosophers The Cartesians tried to develop a comprehensive science of nature and to resolve the problems about mind-body interaction.

19. Preformationists Nicolas Hartsoeker, ~1694 Dutch mathematician and physicist Invented the screw-barrel microscope Co-discoverer of sperm.

20. Preformationists Nicolas Hartsoeker (1694) - a spermist Spermatozoan The homunculus

21. Preformationists Charles Bonnet, ~1762 Swiss lawyer, naturalist, philosopher Ovist - from studies of parthenogenesis in Daphnia - felt that the theory of preformation was “…one of the greatest triumphs of rational thought over sensual conviction.” In Philosophical Palingests, or Ideas on the Past and Future - argued that females carry within them all future generations in miniature form. Comment on the preformation paradox: “…it is always possible, by adding zeros, to crush the imagination under the weight of numbers.”

22. Epigeneticists William Harvey, ~1651 Physician to King Charles I of England In 1628 Harvey published An Anatomical Study of the Motion of the Heart and of the Blood in Animals which explained how blood was pumped from the heart throughout the body, then returned to the heart - recirculation of blood. Also published Essays on the Generation of Animals - considered the foundation for modern embryology

23. Epigeneticists Rene Descartes, ~1664 Philosopher, physicist, physiologist and mathematician – famous in all. Considered one of the most important and influential thinkers in human history. Cogito ergo sum - “I think, therefore I am.” “If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.”

24. Epigeneticists Pierre Maupertuis, ~1745 French mathematician and biologist In Essai de cosmologie - introduced the theory of survival of the fittest. Argued that preformation could not account for hybrids or “congenital monsters” Proposed that the embryo goes through a number of distinct developmental stages.

25. What’s the correct answer, epigenesis or preformation? Neither is totally correct as originally stated; however, there’s some truth to both, though not in the context of 17th and 18th century thought. Epigenesis - in that development does occur gradually in a stepwise progression moving from a single cell to a multicellular organism of increasing complexity (however, development does not start out from an unorganized state, the zygote is highly organized at the cellular and molecular level). Preformation - in that the instructions for development are present “preformed” in the zygote - genes (DNA).