Week 10: The Lids Flashcards
What is the embryology of the eyelids?
- Derived from surface ectoderm
- Appear as two circular folds around 2 months
- Folds meet and fuse about half a month later
- At 5 months (post fertilisation) keratinisation of surface epithelial cells initiate the eventual separation of the lids
- Lids are open by 8 months gestation
- Human babies are born with their eyes open
What are the functions of eyelids?
Functions of the Eyelids • Protection (protect from debris) • Contributes to tear production • Spreads tears across the cornea (when blink) • Aids in tear drainage
- The opening between the lids forms the palpebral fissure or aperture
- The fissure is closed
- Medially (nasal) at the inner canthus
- Laterally (temporal) at the outer canthus
What is the inner canthus? (And the caruncle)
• nasal side of the eye
• Forms a small triangular space that contains
> The caruncle – reddish mass of tissue
> A fold of the conjunctiva – the plica semilunaris
• Often obscured in infants by an epicanthal fold of the upper eyelid
• Epicanthal fold remains in some ethnicities
• The palpebral position changes throughout life
What happens to lids as age?
As we age both lids move down
What are the number of rows of eyelashes and total number?
- 2 or 3 rows of lashes
- 100-150 on upper lid
- 50-75 on the lower lid
What glands are associated eyelashes?
- Glands of Moll, modified apocrine (sweat glands) next to the base of the eyelashes
- Glands of Zoe’s sebaceous glands, produce an oil/waxy substance
-glands of Moll and Zies secrete sebum that keeps lashes supple
Where are the lacrimal puncta?
At the nasal end of the lids
- one on the upper lid and one on the lower lid
- involved in tear drainage
- (look like little holes)
Where are the orifices of the tarsal glands visible?
Visible near the lid edge (little holes that are opening to mibomium glands)
What is the structure of the eyelid?
- Skin (front)
• Musculature
• Fibrous layer (including tarsal glands)
• Palpebral conjunctiva (back)
What type of skin cells are the lids made off?
> Skin on the eye lid is the thinnest in the human body > (0.05mm) - Epidermis • Basal layer • Columnar cells • Keratinocytes develop here • Keratin is produced as cells mature • Outermost layers are dead cells and cell remains rich in keratin a hard hydrophobic protein
What is the dermis?
- Layer below the epidermis
- Thin compared to other locations in the body
- Composed of dense irregular collagenous tissue
- Blends into the underlying areolar tissue
- There is no underlying lipid layer here – unlike most other locations in the body (layer of fat)
What are the 3 muscles surrounding the lids? (Musculature)
- Orbicularis oculi
- Levator palpebrae superioris
- Tarsal muscle
What is the orbicularis oculi? What nerve innervation?
- Striated voluntary muscle
- Concentrically orientated
- Has a sphincter like action
- Holds the lid against the eye and assists in spreading tears
- Contraction causes the eyelids to close
-Innervated by the 7th cranial nerve
• Orbital part
• Used in forced lid closure
- Palpebral part
- Used in reflex or spontaneous blinking
What is the Levator palpebrae superioris?
- Striated voluntary muscle
- Only found in the upper lid
- Becomes a tendon as it enters the lid
- Involved in raising the upper lid
- Innervated by the 3rd cranial nerve
- Ptosis caused by paralysis of this muscle
What is the tarsal muscle?
Tarsal Muscle
• Smooth muscle
• Sympathetic innervation
• Lines the levator muscle and helps aids its action
• Also associated with the wide-eyed expression of fear
