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Home , Developmental biology, Embryology

Developmental and Human BM Carlson, University of Michigan, Golden Valley, MN, USA

ã 2015 Elsevier Inc. All rights reserved.

Introduction 1 References 2

Introduction

Developmental biology and human embryology have followed converging courses since the early 1990s. Human embryology is an ancient discipline, with roots going back to the time of and some of the great medieval anatomists. produced an extensive series of drawings of aspects of human embryology (O‘Malley and Saunders, 1982). Following the first description of human ovarian follicles by de Graaf in 1672, Hamm and Leeuwenhoek observed human spermatozoa through one of their early microscopes in 1677. With the formulation of the theory by Schleiden and Schwann in 1839 and improvements in microscopes and preparatory techniques, the first flowering of human embryology occurred in the late 19th century. The production of serially sectioned and the formation of three-dimensional(3D; England, 1990) reconstructions of entire embryos or individual systems based on wax plates resulted in the first understanding of the detailed of human embryos at various stages of their . A greater understanding of the developmental anatomy of human embryos continued through the early 1900s, but knowledge of the details of early human embryology were hampered by the lack of specimens. It was not until the decades of the 1930s and 1940s, when Hertig and Rock and other investigators made a concerted effort to obtain early (less than 3-week) human embryos. Many of these studies were reported in the series Carnegie Contributions to Embryology, and the results of many descriptive studies of early human embryological development were codified into the detailed of Early Human Development by O’Rahilly and Mu¨ller (O’Rahilly and Mu¨ller, 1987). Several more recent publications and Websites provide detailed contemporary information. A photographic atlas by England (1990) and a scanning atlas by Steding (2009) describe in detail the structure of the overall body and major organs at most stages of . The Virtual Human (www.virtualhumanembryo.lsuhsc.edu), a project headed by Raymond Gasser, is a catalogue of thousands of digital images and 3D reconstructed images prepared from the serial microscopic sections from which the Carnegie stages of human embryos was created. Another online source of accurate material for the non- is the Endowment for Human Development (www.ehd.org), which contains descriptions of all the major stages of pregnancy and also includes intrauterine photographs of living human embryos. For an integrated textbook of and human embryology, see Carlson (2014). Although not named as such, developmental biology had its roots in the late nineteenth century, when investigators began to use experimental techniques to ferret out secrets of embryonic development that were inaccessible to purely descriptive techniques. One of the first and most important experiments was conducted by (1892), who asked the question, ‘If one separates a two-celled embryo into two individual cells, will these cells () develop into half-embryos or whole embryos?’ He found that each cell produced an entire complete embryo and through this experiment introduced the concept of embryonic regulation, i.e. many embryos or parts of embryos are capable of restoring ‘wholeness’ if portions have been removed. Interestingly, after more than a century we still do not understand the basis for embryonic regulation. Nevertheless, this phenomenon represents an important property of early human embryos and serves as the basis for identical twinning. Developmental biology was not consolidated as a field until the mid-1950s (Willier et al., 1955). It now represents a great variety of approaches to the understanding of the mechanisms of both and development and . These approaches include structural, biochemical and molecular description, experimental surgery (deletion and transplantation pro- cedures), genetic studies (both descriptive and manipulative), a wide variety of imaging techniques and . A molecular understanding of the mechanisms underlying the development of played a critical role in our understand- ing of developmental mechanisms in . Developmental biology and human embryology followed largely parallel pathways until the late 1980s. Several technical developments promoted a convergence of the two fields. One was the rise of , which allowed analysis of minute amounts of – even on microscope slides. Through in situ hybridization, monoclonal antibodies and other techniques it became possible to localize key molecules that were already known to play key roles in guiding the development of organs in experimental . These techniques were then applied to material from human embryos. Even more important was the rise of the mouse as the favored subject for studies in developmental . Through the use of transgenic animals, knock-outs and other techniques that altered the genetic material, it became possible to identify specific molecules that are key elements in the chain that leads to the normal development of normal structures. Because such experiments are not normally possible in human embryos, results from experiments on mice are often extrapolated to the human embryo (Epstein et al., 2004). Such experimen- tation not only leads to a deeper understanding about the molecular controls of normal development, but they also provide important clues about disturbed mechanisms that result in the formation of congenital malformations.

Reference Module in Biomedical Research http://dx.doi.org/10.1016/B978-0-12-801238-3.07822-3 1 2 Developmental Biology and Human Embryology

The study of human birth defects – the science of – has had a long history. For millenia, the birth of a malformed has attracted attention, often as a bad omen. During the flowering of descriptive human embryology , especially in Germany, produced exhaustive descriptions of the anatomy of malformed . It took many decades before even the barest understanding of mechanisms underlying birth defects was obtained. It wasn’t until the thalidomide disaster of the early 1960s, the ingestion of which resulted in infants being born with tremendously shortened arms and legs, that teratology emerged as a discipline of its own. For several decades, teratological investigations were biochemically or pharmacologically based, but now the ability to manipulate in mice and other animals has provided new insights into the mechanisms underlying many birth defects. Identifying environmental causes of congenital malformations and how environmental influences interact with expression is an important component of teratology.

References

Carlson BM (2014) Human embryology and developmental biology, 5th edn. Philadelphia, PA: Elsevier Saunders. Driesch H (1892) The potency of the first two cells in development. Experimental production of partial and double formations (in German). English translation in Willier BH, and Oppenheimer JM (1964) Foundations of Experimental Embryology, 2nd edn. New York, NY: Hafner Press, pp. 38–50. England MA (1990) Color atlas of before birth. Chicago: Year Book Medical Publishers, Inc. Epstein CJ, Erickson RP, and Wynshaw-Boris A (2004) Inborn errors of development. New York, NY: Oxford University Press. O‘Malley CD and Saunders JBdCM (1982) Leonardo da Vinci on the . New York, NY: Gramercy Books. O’Rahilly R and Mu¨ller F (1987) Developmental stages in human embryos. Washington, DC: Carnegie Institution of Washington Publication 637. Steding G (2009) The anatomy of the human embryo. Basel: Karger. Willier BH, Weiss PA, and Hamburger V (1955) Analysis of development. Philadelphia, PA: W.B. Saunders Company.