A Very Brief and Incomplete History of Developmental Biology

The past is full of ideas about how organisms developed and where they came from.

These range from pure magic

To cultural myths

To flights of fantasy

And, in recent times,

To observational and experimentally based inquiry

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.

One important thing to keep in mind,

Information and mechanisms at the cellular and/or molecular

levels are needed to accomplish development.

Development DOES NOT happen by magic.

HISTORY

Who was the first embryologist?

Aristotle - 400 B.C.

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

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.

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

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.

Mid 17th century - 18th century

Epigenesis vs Preformation - a matter for debate

Spermists vs ovists

Jan Swammerdam ~ 1672

Marcello Malpighi ~ 1672

Nicholas Malebranche ~1673

Nicolas Hartsoeker ~ 1694

Charles Bonnet ~ 1762

Preformationists

William Harvey ~ 1651

Rene Descartes ~ 1664

Pierre Maupertuis ~ 1745

Epigeneticists

Jan Swammerdam, ~1672

17th century Dutch microscopist

Debunked “Spontaneous generation” using meticulous dissections and careful experimentation.

Preformationists

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.

SPONTANEOUS GENERATION

LOUISE PASTEUR’S EXPERIMENT

Jan Swammerdam

17th century Dutch microscopist

Debunked “Spontaneous generation” using meticulous dissections and careful experimentation.

Preformationists

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.

Marcelo Malpighi, ~1672Professor 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 skinMalpighian tubules in insects

Marcello Malpighi - did not believe what was right before his eyes when examining chicken development with the microscope.

Preformationists

Nicholas Malebranche, ~1673

A priest and philosopher

Most original and influential of the Cartesian philosophers

Preformationists

The Cartesians tried to develop a comprehensive science of nature and to resolve the problems about mind-body interaction.

Nicolas Hartsoeker, ~1694

Dutch mathematician and physicist

Invented the screw-barrel microscope

Co-discoverer of sperm.

Preformationists

The homunculus

Nicolas Hartsoeker (1694) - a spermist

Spermatozoan

Preformationists

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.”

PreformationistsCharles Bonnet, ~1762

Epigeneticists

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

William Harvey, ~1651

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.”

Rene Descartes, ~1664

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.

Epigeneticists

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).