Devlopmental biology

1. Unit 1 – Introduction

2. 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 So now, the term “Embryology” has been replaced
by “Developmental Biology”.
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.

3. 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.

4. 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.

5. 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.
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.
Fig: Homunculus in human
sperm

6. 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.

7. 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

8. 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

9. • jjj
Adapted from Wolpert et al; chapter 1

10. 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-

11. Adapted from Wolpert et al; chapter 1