The Origin of Developmental Biology BY Zeinab Salah Dalia Ahmed

The Origin of Developmental Biology BY Zeinab Salah Dalia Ahmed

Definition Developmental biology is the branch of life science, which deals with the study of the process by which organisms grow and develop and which the genes in the fertilized egg control cell behavior in the embryo and so determine its pattern, its form, and much of its behavior.

It is an integration of embryology ( the study of embryonic development) with cytology (the study of cellular structure and function) and later with genetics (the study of inheritance). 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.

Historical background Hippocrates in 5 th century BC ; He tried to explain development in terms of the principles of heat, wetness, and solidification. Aristotle; Established ‘embryology’ as an independent field. Defined epigenesis and preformation..

Preformation theory suggests that all structures exist from the very beginning, they just get larger. Epigenesis (~upon formation) is a theory of development that states that new structures arise by progressing through a number of different stages.

Marcello Malpighi in 17 th century : Italian embryologist. He provided a remarkably accurate description of the development of the chick embryo. He argued that at the very early stage the parts were so small that they can not observed even by his best microscope.

Mathais Schleiden and Theodor Schwann ( ) ; Developed the Cell Theory. This theory is one of the foundations of biology. The theory says that new cells are formed from other existing cells, and that the cell is a fundamental unit of structure, function and organization in all living organisms.

August Weismann (1834 – 1914) ; German evolutionary biologist. Weismann's theory of inheritance Proposed that the sperm and egg provided equal chromosomal contributions, both quantitatively and qualitatively, to the new organism. Moreover, he postulated that the chromosomes carried the inherited potentials of this new organism and hypothesized that only the germ cells contained all the inherited determinants.

Mosaic development depends upon specific determinants in the one-celled zygote that are not divided equally between the daughter cells (asymmetric division).

Wilhelm Roux (1888) ; German embryologist. He support weismann’s theory by experiments in frog embryos, he destroyed one of the two cells with hot needle and then result into well –formed half larva.

Early evidence for mosaic development He concluded that the frog embryo was a mosaic of self-differentiating parts and that it was likely that each cell received a specific set of determinants and differentiated accordingly.

Hans Driesch( 1892); Demonstrate regulative development which depends upon interactions between 'parts' of the developing embryo can result in causing different tissues to form (even if parts of the original embryo are removed). Driesch destroyed one cell of a sea urchin embryo at the two cell stage and a normal appearing but smaller sea urchin larvae resulted.

Sea Urchin Embryos show regulative Development

So is development mosaic or regulative? A. It is a combination of both

Hans Spemann and Hilde Mangold; Discovery of primary Embryonic Induction. Induction is a type of regulatory development. They proved the importance of induction and other cell-cell interactions in development by transplant experiment in amphibian embryos.

Spemann and Mangold transplanted dorsal lip of the blastopore to a different blastula and a new main body axis developed.

Gregor Mendel's; In 1900, the significance of Gregor Mendel's work on heredity was finally appreciated. Mendel was the first scientist to develop a method for predicting the outcome of inheritance patterns. He is the father of modern genetics for his study of the inheritance of traits in pea plants. Mendel showed that the inheritance of traits follows particular laws, which were later named after him.

Theodor Boveri; German biologist. Had major support for the chromosomal hypothesis of inheritance. One of the biologists who felt that the nucleus contained the instructions for development who in a paper published in 1902 demonstrated that normal development is dependent upon the normal combination of chromosomes..

Thomas Hunt Morgan American geneticist and embryologist. Noted in 1926 that the only way to get from genotype to phenotype is through developmental processes. Morgan redefined genetics as the science studying the transmission of traits, as opposed to embryology, the science studying the expression of traits.

He able to combine genetics with developmental biology and create the discipline of developmental genetics. Morgan had shown that nuclear chromosomes are responsible for the development of inherited characters. The final proof of the chromosome theory of inheritance resulted from research using the fruit fly, Drosophila melanogaster that was conducted in Thomas Hunt Morgan's laboratory at Columbia University.

Animal Development

Summarize the basic steps in animal develop starting gametogenesis and ending with death. GAMETOGENESIS –Specialization of ovum and sperm FERTILIZATION –Block of polyspermy

CLEAVAGE –Cell division without an increase in total volume MORULA –Solid mass of cells BLASTULATION –Hollow mass of cells

5 Processes of Development 1. Cleavage Division: No increase in cell mass 2. Pattern Formation: A/P(anterior head/ posterior tail) and D/V axes(Dorsal upper or back/Ventral lower or front): Coordinate system 3. Morphogenesis: take 3D form..

4. Cell Differentiation: cells become structurally and functionally different. 5. Growth: cell multiplication, increase in cell size, deposit extracellular material (bone, shell) growth can be morphogenetic

Developmental model organisms Vertebrates –Zebrafish Danio rerio –Frog Xenopus laevis, Xenopus tropicalis –Chicken Gallus gallus –Mouse Mus musculus Invertebrates –Roundworm Caenorhabditis elegans –Fruit fly Drosophila melanogaster

The developmental-genetic toolkit Consists of genes whose products control the development of a multicellular organism. The majority of toolkit genes are components of signaling pathways, and encode for the production of transcription factors, cell adhesion proteins, cell surface receptor proteins, and secreted morphogens.

The most important toolkit genes are those of the Hox gene cluster. Mutations in any one of these genes can lead to the growth of extra, typically non-functional body parts in invertebrates, for example aristapedia complex in Drosophila, this mutation is known as Antennapedia.Antennapedia The genes that regulate development are best understood in two species, the mouse and the fruitfly

HOMOLOGOUS HOX SEQUENCES:EXPRESSION

HOMOLOGOUS HOX GENES: DERIVATION MUTATIONS IN REGULATORY GENES CAN GIVE THE PROTEINS NEW PROPERTIES: UBX ACQUIRES THE ABILITY to REPRESS DISTAL-LESS in the INSECT CLADE

Evolutionary developmental biology “Evolution is a change in the genetic composition of populations. The study of the mechanisms of evolution falls within the province of population genetics.” Theodosius Dobzhansky. 1951

Evolutionary developmental biology The emergence of new phenotypes is made possible by changes in development. The regulatory genes that have long been thought to control the generation of novel structures are now being discovered

Evolutionary developmental biology, now often known as “evo-devo,” is the study of the relation between evolution and development. The relation between evolution and development has been the subject of research for many years, However, the subject has been transformed in recent years as the genes that control development have begun to be identified.

The Modern Era The Impact of Molecular and Cellular Biology The molecular biology revolution in the middle of the Twentieth Century provided the means to study the role of genes in development that Wilson and his contemporaries lacked. The key technological advance for the study of gene control of development was the ability to isolate and clone genes.

Much of the current research in developmental biology involves attempts to understand the cellular and intercellular events that signal the nucleus to express genes or initiate a sequence of gene expression.

There are two powerful techniques allow investigators to test directly the roles of specific genes during development:- Introduce cloned genes into embryos and assess their effects on development. Eliminate or "knock out" specific genes and determine the effects on development.

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References Developmental biology 6 th edition. Gilbert