The study, identification, and treatment of ocular surface disease (OSD), dry eye, and tear film dysfunction is an area of high interest. Optometrists and ophthalmologists alike are now sensitive and critical evaluators of the ocular surface. They observe and characterize the tear film and ocular surface in its normal, stable, and homeostatic state. When it is out of balance due to presumed chronic and dynamic inflammation, it is dysfunctional.
They have become experts of observation, data collectors, and keepers of tear film metrics. Surface staining, gland imaging, measures of tear film and blink dynamics, and tear film quantity and content are now routine tests. Practitioners have learned to examine eyelids and eyelid margins, the quality and caliber of the eyelashes, and to search for overpopulations of eyelid commensals that may precipitate inflammatory reactions.
What is currently hidden, however, are the niches in which the stem cells of the ocular surface reside. In these protected areas are the progenitor cells of tissue rejuvenation and repair. Pluripotency (the capacity to differentiate into multiple lineages) and proliferation are two characteristic attributes of stem cells. How do the known stressors of the ocular surface impact these populations of cells? In what way, if any, does the therapy employed (both mechanical and chemical) to treat the signs and symptoms of OSD alter the function or cycling of stem cells?
Stem Cells of Interest in OSD
The Cornea Stem cells in the corneal epithelium are located in the basal layer of the limbal region at the corneal periphery, called the palisades of Vogt. Limbal epithelial stem cell deficiency is pathological, either partially or completely, and is caused by mechanical injury or chemical and thermal burns, is associated with aniridia, or can be acquired as in Stevens-Johnson syndrome (Ricardo et al, 2013).
The Conjunctiva Conjunctival stem cells can differentiate into either mucin-producing goblet cells or epithelial cells. The dividing basal cells migrate from the bulbar conjunctiva to the corneal surface and differentiate (Dhamodaran et al, 2014). The stem cells residing in the fornical niche can differentiate into epithelial cells as well as into goblet cells, as shown in clonal culture assays. This provides strong evidence that the stem cell population for conjunctiva renewal is in the fornix region (Dhamodaran et al, 2014).
The Eyelash Follicle Eyelashes are an integral part of the lid margin anatomy, contributing to the overall homeostasis of the ocular surface. The existence of hair cycles implies that stem cells must be cyclically activated and that the hair melanin unit has to be renewed. Two distinct epithelial stem cell reservoirs, located in the upper and lower thirds of the anagen hair follicle outer root sheath, have been identified; anagen is the active growth phase of hair follicles during which the root of the hair is dividing rapidly, adding to the hair shaft (Bernard, 2003). Studies have demonstrated that the lash follicle is sensitive to many external environmental factors, a variety of systemic/topical medications, and cosmetics (Aumond and Bitton, 2018).
Remember that a lot of regenerative magic occurs to maintain a healthy, functional ocular surface. Consider how a patient may be taxing these most-important cells of regeneration in your dry eye disease workup. CLS
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