A BRIEF HISTORY OF EMBRYOLOGY
The process of progressing from a single cell through the period of establishing organ primordia (the fi rst 8 weeks of human development) is called the period of embryogenesis (sometimes called the period of organogenesis); the period from that point on until birth is called the fetal period, a time when differentiation continues while the fetus grows and gains weight. Scientifi c approaches to study embryology have progressed over hundreds of years. Not surprisingly,
anatomical approaches dominated early investigations. Observations were made, and these became more sophisticated with advances in optical equipment and dissection techniques. Comparative and evolutionary studies were part of this equation as scientists made comparisons among species and so began to understand the progression of developmental phenomena. Also investigated were offspring with birth defects, and these were compared to organisms with
normal developmental patterns. The study of the embryological origins and causes for these birth defects was called teratology.
In the 20th century, the fi eld of experimental embryology blossomed. Numerous experim entswere devised to trace cells during development to determine their cell lineages. These approaches included observations of transparent embryos from tunicates that contained pigmented cells that could be visualized through a microscope. Later, vital dyes were used to stain living cells to follow their fates. Still later in the 1960s, radioactive labels and autoradiographic techniques were employed. One of the fi rst genetic markers also arose about this time with the creation of chickquail chimeras. In this approach, quail cells, which have a unique pattern to their heterochromatin distribution around the nucleolus, were grafted into chick embryos at early stages of development.
Later, host embryos were examined histologically, and the fates of the quail cells were determined. Permutations of this approach included development of antibodies specifi c to quail cell antigens that greatly assisted in the identifi cation of these cells. Monitoring cell fates with these and other techniques provides valuable information about the origins of different organs and tissues.Grafting experiments also provided the fi rst insights into signaling between tissues. Examples of such experiments included grafting the primitive node from its normal position on the body axis to another and showing that this structure
could induce a second body axis. In another example, employing developing limb buds, it was shown that if a piece of tissue from the posterior axial border of one limb was grafted to the anterior border of a second limb, then digits on the host limb would be duplicated as the mirrorimage of each other. This posterior signaling region was called the zone of polarizing activity (ZPA), and it is now known that the signaling molecule is sonic hedgehog (SHH).About this same time (1961), the science of teratology became prominent because of the drug thalidomide that was given as an antinauseant and sedative to pregnant women. Unfortunately, the drug caused birth defects, including unique abnormalities of the limbs in which one or more limbs was absent (amelia) or was lacking the long bones such that only a hand or foot was attached to the torso (phocomelia). The association between the drug and birth defects was recognized independently by two clinicians,W. Lenz and W. McBride and showed that the conceptus was vulnerable to maternal factors that crossed the placenta. Soon, numerous animal models demonstrating an association between environmental factors, drugs, and genes provided further insights between developmental events and the origin of birth defects. Today, molecular approaches have been added to the list of experimental paradigms used to studynormal and abnormal development. Numerous means of identifying cells using reporter genes,fl uorescent probes, and other marking techniques have improved our ability to map cell fates. Using other techniques to alter gene expression, such as knockout, knock-in, and antisense technologies has created new ways to produce abnormal development and allowed the study of a single gene’s function in specifi c tissues. Thus, the advent of molecular biology has advanced the fi eld of embryology to the next level, and as we decipher the roles of individual genes and their interplay with environmental factors, our understanding of normal and abnormal developmental processes progresses.
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