The embryological and fetal morphogenesis of the eyelids and lacrimal drainage
system is multifactorial and complex. This protracted process begins in the 5th
week of prenatal life and involves a successive series of subtle and tightly
regulated morphogenetic events. Major milestones of the embryological and fetal
development of the eyelids include the beginning of eyelid formation during the
6th week, eyelid fusion by the 8th week, and the development of eyelid
structures beginning in the 9th week (immediately following eyelid fusion), with
progression until eyelid separation by the 24th week. After eyelid separation,
the eyelids begin to assume their newly developed shape. Around the 32nd week,
eyelids are almost fully developed and fully separated but still visibly closed.
Key development steps of the lacrimal drainage system include formation of the
lacrimal lamina (the primordium of the future lacrimal system) and the lacrimal
cord (the primordium of the lacrimal canaliculi) in the 7th week, with
canalization starting from the 10th week. During the 10th week, the excretory
lacrimal system displays a lumen with a true lacrimal duct that can be
distinguished. The epithelium of the lacrimal canaliculi is in contact with the
palpebral conjunctival epithelium. The two epithelia form a continuous
epithelial lamina. The caudal extreme of the lacrimal duct and the inferior
meatal lamina join and the latter begins to cavitate. Understanding this
multidimensional process of development in prenatal life, as well as identifying
and linking signaling cascades and regulatory genes to existing diseases, may
pave the way for developing minimally invasive interventions and scar reducing
surgical methods, controlling the spread of malignancies, and the use of
progenitor/stem cell and even regenerative therapy.