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- Human development is a continuous process that begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male.
- Cell division, cell migration, programmed cell death (apoptosis), differentiation, growth, and cell rearrangement transform the fertilized oocyte, a highly specialized, totipotent cell, a zygote, into a multicellular human being.
- Most changes occur during the embryonic and fetal periods; however, important changes also occur during later periods of development.
- The field of study which includes investigations of the molecular, cellular, and structural factors contributing to the formation of an organism is called embryology.
- It is a branch of science that is related to the formation, growth, and development of an embryo.
- It mostly deals with the prenatal stage of development beginning from the formation of gametes, fertilization, the formation of a zygote, development of embryo and fetus to the birth of a new individual.
Image Source: University College London
History of Embryology
Early scientists and philosophers were aware of sperm as soon as the microscope was invented. However, there have been competing theories in early embryology.
Theory of Epigenesis
- Aristotle first proposed the correct mechanism for the development of an embryo, without having a microscope to observe his theory.
- He suggested that animals form through the process of epigenesis, in which a single cell divides and differentiates into the many tissues and organs of an animal.
Theory of Preformation
- The theory of preformation gained much traction before the invention of microscopes and more advanced imaging techniques.
- This idea suggested that the embryo was contained, small but fully formed, inside the sperm.
- The theory also suggested women were simply vessels to carry the growing child, and that girls came from the left testicle, while boys came from the right.
The Discovery of Ovum
- It wasn’t until 1827 that clear evidence was obtained that female mammals also produce a sex cell, the ovum.
- The discovery of a female sex cell directly contradicted many aspects of the preformation theory and led to wider acceptance of the epigenesis theory.
The Germ Layer Theory
- Karl Ernst von Baer, discoverer of the ovum, and Heinz Christian Pander then proposed the germ layer theory.
- It postulates that a single cell becomes separate layers of cells as the early organism divides. These germ layers then give rise to the rest of the organism by growing and folding into organs, vessels, and other complex tissues and the cells within differentiating accordingly.
- The discovery and understanding of DNA led to a more comprehensive understanding of how sperm and egg become a zygote.
- The development of ultrasound greatly increased the understanding of fetus development in humans.
- Many studies were done on simple organisms to understand basic embryology. The flatworm was cultured intensively, as it reproduces sexually and the cells are large enough to watch develop under a good microscope. The fruit fly was also observed extensively, for similar reasons.
- Studying a polychaete worm, E.B. Wilson developed a coding process to label and understand the movements and divisions of cells during embryogenesis.
- While the exact process changes depending on the species, this method greatly expedited the understanding of embryology and led to medical and evolutionary science breakthroughs.
Fields of Embryology
- Descriptive embryology: This field of embryology associated with the morphological description of different embryonic stages in the ontogenetic development of individuals of different species.
- Comparative embryology: the study of how anatomy changes during the development of different organisms.
- Evolutionary embryology: the study of how changes in development may cause evolutionary changes and of how an organism’s ancestry may constrain the types of changes that are possible.
- Experimental embryology: It involves all those studies that attempt to understand the various fundamental mechanism in the development of different animals, like fertilization, cleavage, gastrulation, embryonic induction, determination, and differentiation.
- Behavioral embryology: the study of the early development of the nervous system and behavior with a view toward understanding how the formative periods of neural and behavioral development affect later stages of neurobehavioral ontogeny.
- Chemical embryology: The branch of embryology includes all those studies which employ various biochemical, biophysical and physiological techniques for understanding embryological events at a molecular level.
- Teratology: Teratology is the division of embryology and pathology that deals with abnormal development (birth defects). This branch of embryology is concerned with various genetic and/or environmental factors that disturb normal development and produce birth defects.
Scope and Careers in Embryology
An embryologist is a scientist who studies embryology.
- Embryologists most commonly work with physicians to assist their patients with reproductive health issues and clinical research.
- Clinical embryologists are responsible for retrieving eggs, assisting with in vitro fertilization, fertility specialist, maintaining clinical records and running tests on eggs.
- Many professions employ knowledge of embryology in their practices.
- Many pharmaceutical companies develop drugs for both fertility and sterility, and the processes of embryology are key to these efforts.
- Scientists developing insecticides, or ways to deal with other pests, often turn to embryology to battle the reproductive cycles of the organisms.
- Others use embryology for the advantage of a species, like the scientists trying to repopulate endangered species.
Embryology is the basis for understanding the intimate relation between structures in different organ systems, such as the nervous system and muscle, and is primordial for understanding disorders of development that in the human may present as one of the congenital myopathies.
- Bridges the gap between prenatal development and obstetrics, perinatal medicine, pediatrics, and clinical anatomy
- Develops knowledge concerning the beginnings of life and the changes occurring during prenatal development
- Builds an understanding of the causes of variations in human structure
- Illuminates clinically oriented anatomy and explains how normal and abnormal relations develop
- Provide knowledge essential for creating health care strategies for better reproductive outcomes
- A better understanding of embryology results in new techniques for prenatal diagnoses and treatments, therapeutic procedures to circumvent problems with infertility, and mechanisms to prevent birth defects, the leading cause of infant mortality.
- Supports the research and application of stem cells for the treatment of certain chronic diseases
- Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R., Francis-West, P.H. & Philippa H. (2015). Larsen’s human embryology (5th ed.). New York; Edinburgh: Churchill Livingstone.
- Sadler, T. W., & Langman, J. (2004). Langman’s medical embryology. Philadelphia, Pa: Lippincott Williams & Wilkins.
- Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2008). The developing human: Clinically oriented embryology. Philadelphia, PA: Saunders/Elsevier.
- Gilbert, S. F. (2000). Developmental biology. Sunderland, Mass: Sinauer Associates.