Chapter 5. Fundamentals of human embryology
Embryology (from the Greek embryon for embryo, logos for doctrine) is a science about the patterns of embryo development.
Medical embryology studies the patterns of development of human embryos. Particular attention is paid to embryonic sources and natural processes of tissue development, metabolic and functional features of the mother-placenta-fetus system, critical periods of human development. All this is of great importance for medical practice.
Knowledge of human embryology is necessary for all physicians, especially those employed in the field of obstetrics and pediatrics. It helps in making the diagnosis in case of disturbances in the mother-fetus system, identifying the causes of deformities and diseases of children after birth.
Currently, knowledge of human embryology is helpful in disclosing and eliminating the causes of infertility, performing fetal organ transplantation, developing and using contraceptives. Particular emphasis is made on issues of oocyte cultivation, in vitro fertilization and implantation of embryos into the uterus.
The process of human embryonic development is the result of a long evolution and, to a certain extent, it reflects the features of the development of other representatives of the animal world. Therefore, some early stages of human development are very similar to embryogenesis of lower-organized chordates.
Human embryogenesis is a part of ontogenesis, which includes the following main stages: 1 — fertilization and formation of a zygote; 2 — cleavage and formation of blastula (blastocyst); 3 — gastrulation when the blastula is reorganized into a multilayered structure; 4 — histogenesis and organogenesis of embryonic and extraembryonic organs; 5 — systemogenesis.
Embryogenesis is closely related to progenesis and early postembryonic period. Thus, tissue development begins in the embryonic period (embryonic histogenesis) and continues after the baby is born (postembryonic histogenesis).