بسم الله الرحمن الرحيم

.Zoo. Zooعلم األجنة التجريبيعلم األجنة التجريبي(424)(424)

Principle Principle Experimental EmbryologyExperimental Embryology

تاريخية ولمحة تاريخية مقدمة ولمحة مقدمة

Introduction and Historical reviewIntroduction and Historical review

Principal of Experimental Embryology :Principal of Experimental Embryology :Introduction and historical reviewIntroduction and historical review

• The embryology science reported by some verses of The embryology science reported by some verses of the Quran in particular with regard to human creation the Quran in particular with regard to human creation ""so let man see, create, creation of water flush from between so let man see, create, creation of water flush from between back bone and ribs,(altraeb)back bone and ribs,(altraeb)........ فلينظر اإلنسان مما فلينظر اإلنسان مما خلق،خلق من ماء دافق يخرج من بين الصلب والترائبخلق،خلق من ماء دافق يخرج من بين الصلب والترائب" " Yes it is God sinah in this life,

• All multicellular organisms arose from the mating between male and female gametes (fertilization)

• Note the embryos in different developmental stages from gamete to cleavage (from Zygote to blastula stage).

• Then Gastrulation stage, the formation of the three different embryonic layers, ectoderm,mesoderm and endoderm(gastrula).

• Later the organogensis stage: from these 3 embryonic layers were all the organs of the embryo will form.

• Finally the grow and increase in size until the exit of the embryo for life (hatching or birth).

Introduction :The development of multicellular organisms from a single cell—the fertilized egg—is a brilliant thinking . During embryonic development, the egg divides to give rise to many millions of cells ,which form structures as complex and varied as eyes, arms, heart, and brain. This amazing achievement raises many questions. -How do the cells arising from division of the fertilized egg become different from each other? -How do they become organized into structures such as limbs and brains?

-What controls the behavior of individual cells so that such highly

organized patterns emerge? -How are the organizing principles of development embedded within the egg, and in particular within the genetic material, DNA?

-Much of the excitement in developmental biology today comes from our growing understanding of how genes direct these developmental processes, and genetic control is one of the main themes of this course.

-Several of factors are involved in controlling development, but we will focus only on those that have key roles and illustrate general principles.

-The development of an embryo from the fertilized egg is known as embryogenesis. One of its first tasks is to lay down the overall body plan of the organism, and we shall see that different organisms solve this fundamental problem in several ways.

Branches of EmbryologyBranches of EmbryologyEmbryology has many sub-branches in it. They are:-

• 1- Descriptive Embryology1- Descriptive Embryology

• It is applied to the method of study concerned with the direct observation and description of embryological development. Embryology in ancient times started as a branch of study based on the direct observation and description by scientists like Aristotle (340 B.C.), Fabricius (1537 – 1619), Harvery (1578 – 1657) and so on.

• 2- Comparative embryology2- Comparative embryology

• It is the embryological development of different animals and studied and compared. Comparative embryology throws much light on the understanding of evolution and phylogenetic significance. It also give some ideas on the developmental stages of certain animals in whose case the study of development is different.

• 3- Experimental Embryology3- Experimental Embryology• In it is experiments are used for studying the

developmental stages. It helps to understand the fundamental developmental mechanisms. In experimental embryology the various parts of developing embryo are removed, transplanted, parts exchanged or the environmental conditions altered. This helps to understand induction, gradient system, etc. Experimental embryology is also called casual embryology of analytical embryology. Roux (1850 – 1924) is the pioneer in the field of experimental embryology.

4- Chemical Embryology4- Chemical EmbryologyThe developmental stages are studied by biochemical and physiological techniques. It is also called physiological or biochemical embryology. Needham (1931) is the pioneer in this field.5- Teratology5- TeratologyIt is a branch of embryology concerned with the study of malformations or abmormal development.6- Developmental Biology6- Developmental BiologyIt includes not only embryonic development but also postnatal processes such as normal and neoplastic growth, metamorphosis, regeneration and tissue repair in both animal and plant .

Historical and background of Exp. EmbryologyHistorical and background of Exp. Embryology

Aristotle (384 – 322 B.C Aristotle (384 – 322 B.C ): Before the 17th century embryological knowledge was based on the writings of Aristotle and Galen. Embryology as a branch of biology was initiated by the famous Greek philosopher Aristotle (384 – 322 B.C.).

He was the first embryologist to describe the development and reproduction of many kinds of organisms in his book entitled "Degeneratione AnimaliumDegeneratione Animalium". He believed firmly that the complex adult organism develops from a simple formless beginning.

Thus he laid the foundation for the basic principles of epigenesist, a theory postulated after 2000 years. For this Aristotle is honored as the father of embryology. Aristotle has written that the male contributes the semen and the female contributes the contamenia. The semen is secreted by testis and it is hot and active. The catamenia is carried through very fine vessels into the uterus. It is equivalent to the menstrual blood. He had no knowledge of ovary. He thought that the female reproductive system is formed of uterus only and not of anything else.

Cont .Historical and background of EmbryologyCont .Historical and background of Embryology

Wilhelm Roux (1888)and Hans Driesch (1892) both In separate experiments, Wilhelm Roux (1888) and Hans Driesch (1892) set out to determine whether epigenesis or preformation was correct. Both allowed a fertilized egg to divide to the two-cell stage. Roux, using amphibian embryos (frogs, toads, salamanders), killed one of the two cells with a hot needle. Driesch, using echinoderm embryos (sea stars, sea urchins, sea cucumbers), completely separated the divided cells. An entire animal developing from a single cell would support epigenesis. A portion of the animal developing would favor preformation. What was the result? Interestingly, Roux described the formation of a half embryo that he called a “hemiembryo” (figure a), and Driesch found that each cell retained the potential to develop into an entire organism (figure b).  

August Weismann 1880s August Weismann 1880s propose that the germ plasma theory: The egg and sperm contribute chromosomes equally to the zygote (fertilized egg). The chromosomes are carriers of the hereditary potentials, and the germ cells (gametes) of the embryo are the only ones to carry the complete set of hereditary potentials (nuclear determinants), whereas each somatic (body) cell type contains only part of these potentials required for the specific cell type.

• Oscar Hertwig (1872)Oscar Hertwig (1872)• He repeat the Wilhelm's experiments, but he separate

the 2 cell frog embryo from them and improved that each cell had a potential to grow, but the embryo result is half size than normal embryo. This experiment is considered the one of explanations of twins formation result from one egg.

• Theodor Boveri (1902)Theodor Boveri (1902)• He studied the effect of nucleus and cytoplasm on the

development of the sea urchin fertilized ovum by study of controls and filaments formation during different division stages. Also, he tries to insemination of specific species ovum with another species sperm, and he found that the chromosomes are different in quantity and numerically.

•

Thomas Hunt Morgan (1866 – 1945)Thomas Hunt Morgan (1866 – 1945)• He is the first scientist that suggested that the embryonic

development is controlled by expressions of different genes, and he awarded Nobel prize for discovered the mutations in Drosophila and their role in understand the role of genes in heredity.

• Otto Warburg (1908 – 1910)Otto Warburg (1908 – 1910)• He studied the respiratory mechanism in sea urchin

embryo cells, and he considers the first scientist who definition oxygen consumes in respiratory of ovum and embryonic cells. Their study resulted in know that the fertilization make to increase the cellular reparatory rate of fertilized ovum. So, he discovered the cytochrome Oxidase and he awarded the Nobel prize on this discovery at 1931.

Ernest E. Just (1883 – 1941)Ernest E. Just (1883 – 1941)• He descriptive the changes occurred around the oocytes during the

fertilization in sea urchin and he studied the parthenogenesis to understand the relationship between penetrate of sperm and the first division of Nematoda oocytes.

• Viktor Hamburger (1954)Viktor Hamburger (1954)• The discovery of chick embryo's neural sector. Hamburger

proposed an antiapoptotic action of the hypothetical target-derived trophic signals, identified as nerve growth-stimulating factor, later named nerve growth factor (NGF).

• Ross Granville Harrison (1870 – 1975) Ross Granville Harrison (1870 – 1975) • He successfully cultured frog neuroblasts in a lymph medium and

thereby took the first step toward current research on precursor and stem cells. He was considered for a Nobel prize for his work on nerve-cell outgrowth, which helped form the modern functional understanding of the nervous system, and he contributed to surgical tissue transplant technique.

• Hans Spemann and Mangold (1924)Hans Spemann and Mangold (1924)• Hilde Mangold’s experiment involved taking a piece of

the lip of the blastopore of the gastrula stage of the amphibian embryo and grafting it to the wall (flank) of another gastrula at a site distant from the host blastopore, and thereby induced a secondary axis of polarity in the host that eventually developed into a secondary embryo.

• In 1935 Spemann received the Nobel Prize for Medicine or Physiology as the first embryologist.

• Jean Brachet (1988 – 1909)Jean Brachet (1988 – 1909)• In 1933 Brachet was able to show that DNA was found in

chromosomes and that RNA was present in the cytoplasm of all cells. His work showed that RNA plays an active role in protein synthesis. Brachet also carried out pioneering work in the field of cell differentiation. Brachet later demonstrated papers that differentiation is preceded by the formation of new ribosomes and accompanied by the release from the nucleus of a wave of new messenger RNA.

• Robert Briggs (1983 – 1911)Robert Briggs (1983 – 1911) and Thomas Joseph Kingand Thomas Joseph King• He was a scientist who in 1952, together with Thomas Joseph

King, cloned a frog by nuclear transfer of embryonic cells. • The same technique, using somatic cells, was later used to

create Dolly the Sheep by Willmut and Kith Kamble 1997 . Their experiment was the first successful nuclear transplantation performed in metazoans.