2020–2021 BCSC Basic and Clinical Science Course™
2 Fundamentals and Principles of Ophthalmology
Part II: Embryology
Chapter 4: Ocular Development
Development of the Extraocular Muscles, Adnexa, and Orbit
The upper eyelid first develops at 4–5 weeks of gestation as a proliferation of surface ectoderm in the region of the future outer canthus. During the second month, both the upper and lower eyelids become discernible as undifferentiated skinfolds that surround mesenchyme of neural crest origin (see Figs 4-6E, F and 4-12). Later, mesodermal mesenchyme infiltrates the eyelids and differentiates into the palpebral musculature. The eyelid folds grow toward each other as well as laterally. Starting near the inner canthus, the margins of the folds fuse between weeks 8 and 10 of gestation. As the folds adhere to each other, development of the cilia and glands begins. The orbicularis muscle condenses in the fold in week 12. The eyelid adhesions begin to gradually break down late in the fifth month (Fig 4-17), coincident with the secretion of sebum from the sebaceous glands and cornification of the surface epithelium.
The lacrimal gland begins to develop between the sixth and seventh weeks of gestation. Solid cords of epithelial cells proliferate from the basal cell layer of the conjunctiva in the temporal region of the fornix. Neural crest–derived mesenchymal cells aggregate at the tips of the cords and differentiate into acini. At approximately 3 months, ducts of the gland form by vacuolation of the cord cells and the development of lumina. Lacrimal gland (reflex) tear production does not begin until 20 or more days after birth. Therefore, newborn infants cry without tears.
Figure 4-17 Development of the eyelids. A, During the seventh week, the upper and lower eyelid folds grow over the developing eye. B, Eyelid folds fuse during weeks 8 to 10; fusion starts along the nasal margin. C, Subsequently, cilia and glandular structures develop. D, From the fifth to seventh months, the eyelids gradually separate.
(Illustration by Paul Schiffmacher.)
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.