Embryology of Eyelid Development

Development of the eyelids follows 5 stages. During the first stage, 6–8 weeks after fertilization, mesodermal proliferations with an ectodermal covering appear above and below the developing eye. In the second stage these eyelid folds approach each other and are fused from week 8. Behind the fused lids is a closed conjunctival sac. Following the fusion specialized structures develop in the eyelids (third stage, 8 weeks to 7 months). These are the characteristic anatomical structures such as orbicularis oculi, tarsal plates, Meibomian glands, eyelashes and associated glands, attachments of levator palpebrae superioris and arterial arcades. The fourth stage, 5–7 months, shows separation of the eyelids while keratinization of the opposed eyelid surfaces takes place. During the fifth stage, 7 months to term, the eyelids obtain their final shape and smooth margins. Key events during eyelid development are fusion and separation of the eyelids. Eyelid closure protects the developing cornea and conjunctiva from renal excretions in the amnion fluid. Moreover, the closed lids provide structural support for the development of the intrinsic eyelid structures.