Nasolacrimal epithelium

TTie ability of certain drugs to cross the ocular and nasolacrimal epithelium means that these drugs are to some extent absorbed systemically. The degree to which this occurs in the horse has not been investigated extensively. However, topical atropine has been recognized as causing transient ileus, particularly in foals, and has been implicated in a very small number of fatal colic cases in adult horses (Brooks 1999). Dexametha-sone has been detected in the serum of horses 24 h after cessation of administration of a 0.1% ointment three times a day (Speiss et al 1999) and trainers of competition horses should be made aware of the possibility of forensic detection of topically administered ophthalmic drugs. 

The synchronized movement of the eyelids spreads the precorneal tearfilm across the cornea and pushes it toward the nasolacrimal duct. Precorneal drainage is quite efficient. An aqueous instilled dose leaves the precorneal area within 5 min of instillation in humans. Most of the drug absorbed by transcorneal penetration, without retention modification, is spread across the cornea by the eyelids in the first minutes postdosing. In the precorneal space transcorneal penetration is limited by solution drainage, lacrimation and tear dilution, tear turnover, conjunctival absorption, and the corneal epithelium. Slowing down tear film turnover has well-established benefits to topical ocular drug delivery. 

Fig. 1 (A) Cross-sectional structure of the human nose. NV = nasal vestibule AT = atrium NP = nasopharynx IT = interior turbinate and orifice of the nasolacrimal duct MT = middle turbinate and orifices of frontal sinus, anterior ethmoidal sinuses, and maxillary sinus ST = superior turbinate and orifices of posterior ethmoidal sinuses hatched area, olfactory region. (B) Four major cell types in the nasal epithelium (a) non-ciliated columnar cell with microvilli (b) goblet cell with mucous granules and Golgi apparatus (c) basal cell and (d) ciliated columnar cell with many mitochondria in the apical part. (Reprinted from Ref. with permission from Elsevier.)...

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