Eyelids: Function and Embryology

Importance of Eyelids

• Eyelids shield eyes from foreign objects, dust, harmful substances
• Lubricate eyes through tear spreading
• Blink reflex clears debris and refreshes tear film
• Regulate light entering the eye
• Influence facial appearance and emotional expression

Embryology

• Eyelids are derived from surface ectoderm
• Upper eyelids formed from the frontonasal process
• Lower eyelids formed from the maxillary process
• Development timeline:
  • Appearance of eyelid fold marks: 6 to 7 weeks of gestation
  • Eyelid fusion: 8 to 10 weeks of gestation
  • Development of eyelid structures: 3 to 4 months of gestation
  • Eyelid reopening: 5 to 6 months of gestation

Clinical Correlation (Embryological)

• Congenital Ectropion: eyelid turns outward
• Ankyloblepheron: eyelids are fused together
• Lid Coloboma: eyelid defect or gap
• Cryptophthalmos: complete or partial eyelid fusion
• Congenital Entropion: eyelid turns inward

Clinical Correlation (Other)

• Blepherophimosis: narrow palpebral fissure
• Epiblepheron: fold of skin at the eyelid margin
• Epicanthus: vertical fold of skin at the inner corner of the eye

Anatomy

• Upper eyelid:
  • Extends from eyebrow to free margin
  • Anatomical portions:
    • Orbital portion (upper part close to the eyebrow)
    • Tarsal portion (central part supported by the tarsal plate)
    • Marginal portion (lower part near the lashes)
  • Superior Palpebral Sulcus: groove that separates the orbital and tarsal portions
• Lower eyelid:
  • Extends smoothly into the cheek
  • Anatomical portions:
    • Orbital portion (upper part closer to the eye socket)
    • Tarsal portion (middle region supported by the tarsal plate)
    • Ciliary margin (lower part near the eyelashes)
  • Inferior Palpebral Sulcus: faint groove separating orbital and tarsal portions

Palpebral Aperture (Eyelid Opening)

• Diameter:
  • At birth:
    • Horizontal: 18-21 mm
    • Vertical: 8 mm
  • Adult:
    • Horizontal: 28-30 mm
    • Vertical: 9-11 mm

Lid Margin

• Marked by eyelashes and tarsal plate
• Contains:
  • Meibomian glands (sebaceous glands) opening along the margin
  • Ciliary glands opening near eyelashes
  • Openings for the glands of Zeis and Moll

Clinical Correlation (Lid Margin)

• Cyst of Moll: cyst formed from a sweat gland
• Cyst of Zeis: cyst formed from a sebaceous gland
• Trichiasis: misdirected eyelashes growing inward
• Distichiasis: abnormal growth of eyelashes from the meibomian glands

Layers of Eyelids

• From outermost to innermost:
  • Skin and subcutaneous areolar tissue
  • Muscles of protraction (orbicularis oculi)
  • Orbital septum (connective tissue barrier)
  • Orbital fat
  • Muscles of retraction (levator palpebrae superioris, superior tarsal muscle)
  • Tarsus (connective tissue plate)
  • Conjunctiva

Conjunctiva

• Innermost layer of the eyelid
• Transparent vascularized membrane
• Lines the posterior surface of the eyelids (palpebral conjunctiva)
• Connects to the conjunctiva of the eye (bulbar conjunctiva)

This quiz explores the significance of eyelids in eye protection and lubrication, as well as their embryological development. It covers the anatomy, timeline of eyelid formation during gestation, and associated clinical conditions. Test your knowledge on both the functional and developmental aspects of eyelids.