Eyelashes grow in imperfect rows of five to six in the upper lid and three to four in the lower lid. There mean number is 90 to 160 in the upper lid and 75 to 80 in the lower lid; their length 8 to 12 mm in the upper lid, 6 to 8mm in the lower lid.
The skin of the eyelids is the thinnest of the body (< 1 mm). The nasal portion of the eyelid skin has finer hairs and more sebaceous glands than the temporal aspect, making this skin smoother and oilier.
Apocrine sweat glands of Moll are coiled glands in the dermis that empty via a ductile into the hair follicle. Sebaceous glands at the root of the hair follicle are holocrine glands that shed the entire epithelial cell along with secretory products of complex oils, fatty acids, wax, and cholesterol esters that is sebum. A large sebaceous gland is associated with each hair follicle and empties its secretions directly into the follicle. Additional small sebaceous glands of Zeis are present between lash follicles and dischreg their contents directly onto the skin surface. Its main purpose is to make the skin and hair waterproof and to protect them from drying out.
It is thought that overpopulation of Demodex mites precipitates pathological changes of the eyelids/eyelashes. These changes are consequences of blockage of the follicles and tubules of sebaceous glands by the mites and by reactive hyperkeratinization; epithelial hyperplasia from micro-abrasions caused by the mite’s claws; the mites acting as bacterial vectors; the host’s inflammatory reaction to the presence of parasite’s chitin as a foreign body; and stimulation of the host’s humoral responses and cell-mediated immunological reactions in response to the mites and their waste products.3 We can appreciate that maintaining the proper balance of these acarids is critical to overall normal lid/ocular surface function.4
Related: How ODs can do better with dry eye
Staphylococcal blepharitis is believed to be associated with staphylococcal bacteria on the ocular surface. However, the mechanism by which the bacteria cause symptoms of blepharitis is not fully understood.5
Early in our lives, bacteria from the environment colonize our conjunctiva, corneal surface, and associated tissues, including the eyelid and lacrimal systems. It is estimated that more than 200 species of bacteria commonly inhabit the human conjunctival mucosa.6