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How do humans go from one cell, too small to see with the naked eye, to the more than 30 trillion cells that make up a human body (Bianconi et al. 2013; Sender, Fuchs, and Milo 2016)? From the earliest step, the fusion of a single egg and sperm, through the first eight weeks after fertilization, human embryo development requires many complex processes that we are only beginning to understand. Our understanding is largely limited because our knowledge of human development comes from a few human embryo specimens. With this limited number of embryos, scientists cannot examine human embryo development with the same reproducibility or detail as they do model organisms, such as frogs and mice.
From animal models, scientists have discovered that embryo development is a dynamic process. There are complex cell movements and repeated interactions between cells that are required for normal development. Experiments increasingly require examination of live embryos to understand
these complex interactions. Unfortunately, we have limited understanding of how these
processes occur in the human embryo. This problem is compounded by the unique geometry of the early human embryo, which makes direct comparison between
animal models and humans challenging. Investigating human development teaches us about ourselves and improves our understanding of the genetic and environmental causes of health conditions like infertility and congenital diseases. The embryonic period from conception to eight weeks of development is a period when the basic plan of embryo organization is laid down. The foundation for future organ development is established and elaborated. Many congenital birth defects and later-manifesting diseases have their roots in events that occur during embryogenesis (the first eight weeks of development). Furthermore, it is estimated that a third of human conceptions result in spontaneous abortion (Schoenwolf et al. 2015). About half of these have clear chromosomal abnormalities, but the causes are mostly unknown for the other half (Nagaoka, Hassold, and Hunt 2012; Hassold et al. 1996). Thus, understanding embryogenesis will have long-reaching implications for our comprehension of the genetic and environmental causes of human disease.
The goal of this paper is to outline scientists’ current knowledge of human embryo development. We review the use of model organisms to understand vertebrate development and the methods that developmental biologists use in these models. Investigation with these model organisms seeks to improve our understanding of human development by capitalizing on the similarities among animals. However, these techniques can be limited by the differences between organisms. To highlight the complexity of embryo development, we provide an overview of the development of the central nervous system (CNS), especially brain and spinal cord development. We also describe features of embryos at different stages during embryogenesis. In the end, we hope to highlight what we know, how we know it, and what is still unknown about early human embryo development during the first eight weeks after fertilization.