Developmental Biology

Definition:

• Developmental biology is the study of the process by which organisms grow and develop.

• Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis," which is the process that gives rise to tissues, organs and anatomy.

• Embryology is a subfield, the study of organisms between the one-cell stage (generally, the zygote) and the end of the embryonic stage.

• Embryology and developmental biology today deal with the various steps necessary for the correct and complete formation of the body of a living organism.

The term “development” is used with two different meanings:

• (1) The processes that are involved in the transformation of the fertilized egg into a new adult individual(Developmental Biology=Ontogenetic Development).

• (2) Gradual historical transformation of the forms of life(Evolution=Phylogenetic Development)

In this course( Developmental Biology ) we are going to deal only with development of multicellular organisms (Metazoa)

Reproduction is one of the most important character of the Metazoa in order to preserve their origins by production a new individuals. Reproduction in Metazoa could be by two methods:

(1) Sexual reproduction: The most widespread form of ontogenetic development in multicellular animals.In this type of reproduction new individuals are produced by special generative cells (Gametes).

(2) Asexual reproduction: reproduction that occurs without the union of male and female gametes.In asexual reproduction, the offspring is a clone of the parent and therefore results in low genetic variation in the species as a whole.

Forms of asexual reproduction:

1. Fission

2. Budding

3. Fragmentation followed by regeneration

4. Gemmules: a reproductive structure in some sponges that remains dormant through the winter and later develops into a new individual.

5. Parthenogenesis

Phases Of Ontogenetic Development

• (1) Formation of Gametes (Gametogenesis)

• A- Formation of sperms (Spermatogenesis )

• Note: study male reproductive system

• B- Formation of Ova ( Oogenesis )

• Note: Study Female reproductive system

• (2) Fertilization: Involves several steps

• A- Gametes come close to each other

• B- Sperms get in contact with the egg

• C. Penetration

• D. Union between the two pronuclei

• E. Activation of egg by the sperm and start development(cleavage)

• (3) Cleavage : The fertilized egg (Zygote) begins with a new type of division called “Mitotic division”

• (4) Gastrulation : During this phase many important things will be created:

• A- Germ layers formation( ectoderm, mesoderm and endoderm)

• B- Gastrocoel(archentron) formation which will be the primitive gut

• C- Blastopore formation, which will be anus or mouth, it depends

on the type of the organism.

(5) Organogenesis: formation of organs from the germ layers.

(6) Growth and histological differentiation: organs grow and histologically differentiated.

Historical Background

• 1) Aristotle 340 B.C, had described the development of chick embryo.• 2) 17th and 18th centuries, appearance of “Preformation theory”, : which• Says that the parts of embryo are already present in the egg, but they are

emaginated to be transparent, folded together and very small. • When the embryo begins to form, these parts, unfolded, grow and stretch

them selves and become more readily visible. • 3) In 1740: spermatozoa and ova are discovered. Scientists are divided into

two groups:• a. Ovists: believe that, the embryo is derived only from the egg.• b. Animaliculists: believe that, the embryo is derived only from the sperm.•

• 4) In 1745: Discovery of parthenogenesis by Bonnel.• 5) In 1759: Wolf C.F, proposed the theory of “Epigenesis”.• Wolf couldn’t see any part of the future embryo in the egg, but only the

material of which the embryo is built.• 6) In 1828: K.E.Baer wrote every thing discovered at that time in

embryology an a book named “Animal Embryology”• He put Baer’s Law : The more general features that are common to all

members of a group of animals are developed earlier than • the special features, which distinguish the various members of the

group.• Example : Notochord and spinal cord (in vertebrates) developed earlier

than hair, feather, scales,……..

• 7) 1850 Wilhelm Roux tested the ability of first two cleavage cells to give each an embryo.

• 8) 1903 Hans Spemann proved that separation of the first two cleavage cells give two embryos.

• 9) 1919 Thomas Organ proved that the genes are arranged in a linear

• fashion in the chromosomes of cells.

• 10) 1953 Watson& Crick, discovered the double helical structure of the DNA.

• 11) Invention of the electron microscope and development of many defined chemical methods like, chromatography, centrifugation,

• electrophoresis etc…

• 12) 1978 first baby borne by test tube baby method.

• 13) 1993 PCR technique was invented by Kary Mullis.