Embryology
The nasolacrimal drainage system serves as a conduit for tear flow from the external eye to the nasal cavity. It consists of the puncta, canaliculi, lacrimal sac, and nasolacrimal duct (see the image below).
Overview of the nasolacrimal apparatus.
At 32 days’ embryonic gestation, the maxillary and frontonasal prominences appear, and as these processes enlarge, a groove forms between them (see the image below). Ectoderm from the floor of the groove becomes entrapped between the processes and detaches from the surface ectoderm as a cord of epithelium. At the same time, cords of epithelium invaginate at the upper and lower lid margins, eventually forming the canaliculi. These epithelial cords fuse to form the nasolacrimal drainage system.
Maxillary process grows medially and overrides the paraxial mesoderm of the nasolacrimal process.
Canalization of the epithelial cords occurs simultaneously throughout their entire length, beginning at 4 months’ gestation. Remnants of epithelium within the cords form inconsistent, valvelike folds. A membranous covering consisting of conjunctival and canalicular epithelium remains over the puncta (see the image below), and a covering consisting of nasal and nasolacrimal epithelium remains over the nasolacrimal duct outlet (ie, the membrane of Hasner). Punctal membranes open at full term; however, the membrane of Hasner remains imperforate in up to 70% newborns. This usually opens within the first month but may remain imperforate for a longer time, resulting in epiphora and/or mucopurulent discharge.
Membrane at the puncta consisting of conjunctiva and canalicular epithelium. Membrane at the distal end of the nasolacrimal duct is adhesion between nasal mucosa and epithelium lining the nasolacrimal duct.
Rapid growth of the maxillary bone in relation to the frontal bone results in greater lateral stretching of the inferior canaliculus with a subsequent lateral position of the inferior punctum with respect to the superior punctum. Epithelium at the site of the future lacrimal sac is initially thicker, and canalization in this area is more extensive.