Embryology Lecture Notes PDF

Lecture (1) Embryology Embryology Definition 1- Embryology is the study of the origin and development of an organism 2- Embryology is the study of an animal’s development from the fertilized egg to the formation of all major organ systems. 3-But in its broadest sense it deals with the complex change...

Lecture (1) Embryology Embryology Definition 1- Embryology is the study of the origin and development of an organism 2- Embryology is the study of an animal’s development from the fertilized egg to the formation of all major organ systems. 3-But in its broadest sense it deals with the complex changes which an individual organism undergoes in its life cycle from fertilization to death. Historical of Embryology 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 Animalium". Theories of Embryology 1-Preformation Theory Biologists of the seventeenth and eighteenth centuries believed in preformation, the concept that gametes contain miniaturized versions of all of the elements present in the adult. Some biologists thought that the ovum contained miniature of the adult, which was in some way stimulated to growth by the seminal fluid. Some other biologist believed that preformed miniature organism presented in the head of the sperm. 1 2-Epigenesis Theory In the mid-eighteenth century, the theory of epigenesis maintained that the egg contains the material from which the embryo is gradually built. This theory assumes that embryonic development and differentiation originate in a homogeneous mass of living matter in which there is no preformed materials, neither tissue nor organs. 4-Cell theory After the formation of the cell theory, embryological research entered an entirely new avenue. It was discovered that the ovum was a single cell and that fertilization consisted of the union of the ovum and spermatozoon in the formation of the zygote. 3- Recapitulation Theory: This theory was postulated by Ernst Haeckel (1868) and Muller (1864). It is based on the contribution of evolutionary theory. This theory states that higher animals in their development passes through stages which are similar to the adult stages of lower animals which were their ancestors. In other words the ancestral characters reappear in the developmental stages of an individual. Thus the recapitulation theory states that the embryonic stages of a higher animal resemble the adult stages of its ancestor. 2  Branches of Embryology 1-Descriptive embryology Describes developmental stages of an animal with direct observation and description. (Development of a new animal begins with a sperm fertilizing an egg.) 2-Experimental embryology Experiments are used for studying the developmental. 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. 3-Teratology It is a branch of embryology concerned with the study of malformations. During this century, great effort is being spent to identify and eliminate genetic and environmental factors that cause congenital defects. 4-Comparative Embryology The embryological development of different animals are studied and compared. In recent years, comparative embryology has undergone a resurgence through the investigations of taxonomists, who have recognized that valuable clues to taxonomic relationships among species can be found by studying their embryonic development. 5-Chemical Embryology Provided descriptive information about chemical and physiological events in the embryo. More recent biochemical and molecular studies are understanding of the manner in which different components of embryos interact and how the basic body pattern of the embryo is laid down. 6- Developmental Biology It includes not only embryonic but also development postnatal processes such as normal and neoplastic growth, metamorphosis, regeneration and tissue repair. 3 7- Reproductive biology Involving techniques of Transport of gametes, fertilization and contraception in both humans and domestic animals, early embryonic development, and implantation of the mammalian embryo The value of Embryology 1- From embryology we can learn in one short, story how each individual grows into an adult. 2- Much of the new technology is based on the results of laboratory researches. 3- Recent techniques make possible the diagnosis and/or treatment of genetic diseases and birth defects before a baby is born. 4- Study of the gene action by transfer the genes from one species into the egg of another species. 5- Examination of a small amount of the amniontic fluid that surrounds an embryo makes it possible to determine the sex of a baby before it is born and to detect the presence of genetic conditions that could lead to a defective child. 6-The " test-tube baby ( IVF) in vitro fertilization and , has allowed some childless couples to have children from their own genetic heritage. The Normal Sequence of Events in Embryology 1- Gametogenesis 2- Fertilization 3- Cleavage and blastula formation 4- Gastrulation 5- Organgenesis 4