General Principles
Embryogenesis can be thought of as a series of steps that build on one another; each step creates a ripple effect on all subsequent steps. The steps are regulated by genetic programs that are activated in specific cell types and in a specific order. These genetic programs consist of cascades of genes that are expressed in response to external cues. Often, the same genes participate in different cascades and play different roles in different contexts.
For example, gene products that activate transcription in a particular program may repress transcription in the context of another program, depending on the position of the program within the overall developmental cascade. The cascades are regulated by diffusible ligands (growth factors and hormones) that create overlapping zones of concentration gradients that allow cells to triangulate their position within the developing embryo and determine which program to activate. Misactivation of genetic cascades, whether the result of gene mutations, oocyte abnormalities, or exposure to teratogens, causes embryologic abnormalities that, in the most severe cases, are embryonic lethal or, in less severe cases, give rise to congenital abnormalities.
During gastrulation (development from a single-layered blastula to a multilayered gastrula), 3 germ layers form in all animal embryos: ectoderm (superficial layer of cells), mesoderm (middle layer), and endoderm (inner layer) (Figs 4-1, 4-2). In addition, vertebrate embryos have an ectomesenchymal cell population that arises from neuroectoderm at the dorsal edge of the neural tube. These cells, known as neural crest cells, are transient migratory stem cells that can form tissues with ectodermal and mesodermal characteristics (Fig 4-3). There are several types of neural crest cells, depending on their location and subsequent contributions. Ocular structures are derived from cranial neural crest cells, which are referred to as neural crest cells in this chapter.
The eye and orbital tissues develop from ectoderm, mesoderm, and neural crest cells, with the neural crest cells making a particularly large contribution. In addition, neural crest cells make key contributions to facial, dental, and cranial structures (Fig 4-4). For this reason, syndromes that arise from neural crest maldevelopment (eg, Goldenhar syndrome) often involve the eye as well as facial, dental, and calvarial abnormalities.
Following gastrulation, the ectoderm separates into surface ectoderm and neuroectoderm. Each makes a key contribution to development of the eye (Fig 4-5, Table 4-1).
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Billon N, Iannarelli P, Monteiro MC, et al. The generation of adipocytes by the neural crest. Development. 2007;134(12):2283–2292.
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Foster CS, Sainz de la Maza M, Tauber J. The Sclera. New York: Springer Science + Business Media LLC; 2012.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.