Anatomy and Physiology of the Eye and Eyelashes
The anatomy and physiology domain checks whether you understand the structures you work around and the growth cycle that explains what you see at every appointment. Expect questions on lash follicle biology, eyelid layers and glands, tear film, and how each fact changes an application or aftercare decision.
The lash follicle and its growth cycle
Every eyelash grows from a follicle in the eyelid margin, and each follicle runs an independent cycle rather than shedding in unison. Anagen (active growth) lasts roughly 30 to 45 days. Catagen, the transitional phase, runs two to three weeks. Telogen, the resting phase, lasts roughly 100 days before the hair sheds and the cycle restarts. Lash length is therefore capped: upper lashes typically reach 8 to 12 mm, lower lashes 6 to 8 mm. Losing one to five lashes a day is normal. Each lash has a root and bulb below the margin and a shaft of cuticle, cortex and medulla above it; cuticle scales overlap from root toward tip, which is the surface adhesive must wrap.
Eyelid layers and glands
The eyelid is built in layers: skin, subcutaneous tissue, orbicularis oculi muscle, tarsal plate and conjunctiva. The tarsal plate holds the meibomian glands, roughly 25 to 40 in the upper lid and 20 to 30 in the lower, which secrete the lipid layer of the tear film. The levator palpebrae superioris lifts the upper lid. The tear film has three layers: lipid from meibomian glands, aqueous from the lacrimal gland, and mucin from conjunctival goblet cells. A blink spreads it. Anything that interrupts that film, including adhesive placed on skin or a fan brushing the cornea, produces the gritty, watery complaints clients report.
What the exam expects you to do with it
Placement distance, shedding explanations and contraindication screening all follow from these facts. Extensions attach 0.5 to 1 mm from the eyelid margin on the natural lash, never on skin, so the follicle and meibomian openings stay clear. A client with active blepharitis, dry eye disease or a chalazion is a referral, not an application.
Worked example
A client returns at three weeks saying "they are all falling out." You count 45 lashes lost from a 90-lash set. Working the numbers: 1 to 5 natural sheds a day times 21 days is 21 to 105 lashes, so 45 is unremarkable. Retention, isolation and adhesive cure must still be checked, but the arithmetic points to normal telogen shedding rather than product failure.
Traps that cost marks
Swapping meibomian gland counts between lids; treating lash growth as scalp-like, where anagen lasts years; saying lashes grow from pores; forgetting the lower lid has its own gland population; and assuming a shedding lash means the bond failed.