Lecture – Eyelid and Conjunctiva PDF

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Lecture – Eyelid and ConjunctivaOcular adnexa – collective term for ocular appendages: o Eyelids o Conjunctiva o Lacrimal system These play a vital role in the formation of the pre-ocular tear film and serve to protect the eye against injury and antigenic challenge.Eyelids o Act as occluders – shiel...

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Lecture – Eyelid and Conjunctiva Ocular adnexa – collective term for ocular appendages: o Eyelids o Conjunctiva o Lacrimal system These play a vital role in the formation of the pre-ocular tear film and serve to protect the eye against injury and antigenic challenge. Eyelids o Act as occluders – shield eyes from excessive light and through reflex closure – protection against injury. o Form a pre-corneal tear film through blinking as well as tear drainage. Gross anatomy  Eyelids – joined at canthi – when the eye is open, an elliptical space, the palpebral fissure is formed between the lid margins.  Upper lid – covers 2/3 of the cornea and the lower lid is level with the inferior corneal limbus.  Eyelid margins are about 2.5mm thick from front to back.  Posterior ¼ consists of conjunctival mucosa and anterior ¾ is skin – junction between the two is the muco-cutaneous junction.  The Meibomian (tarsal) gland duct orifices emerge just anterior to the muco-cutaneous junction. – about 30-40 glands open onto the upper margin and 20-40 onto the lower.  Lid eversion – yellowish Meibomian acini are visible as yellow clusters through the tarsal conjunctiva.  2-3 rows of eyelashes arise from the anterior border of the lid margins – receive a rich sensory nerve supply – sensitivity provides an effective alerting mechanism. Muscles of the eyelids  Orbicularis oculi  Sphincter muscle (innervated by facial VII nerve) – divided into palpebral and orbital.

 Palpebral part divided into marginal, pre-tarsal and pre-septal parts. It produces the reflex of closure of the lids during blinking. o Marginal (pars ciliary) – known as Riolan’s muscle – responsible for maintaining the apposition of the lid to the cornea during the lid closure. o Third part – pars lacrimalis – encloses the canaliculi and provides attachments to the lacrimal sac.  Orbital part – lies outside the palpebral – the contraction of this division produces the forcible closure of the lids that occurs in sneezing or in response to painful stimuli.  Levator palebrae superioris  Responsible for elevating the upper lid during the blinking and maintain an open palpebral aperture.  Arises from lesser wing of the sphenoid and terminates anteriorly as a tendon (aponeurosis). Supplied by nerve III.  Orbital septum – extends from tarsal plates to orbital rim, connective tissue sheet, retains orbital contents. Laxity in age leads to herniation of orbital fat. Sagittal section through the orbit showing the levator palebrae superioris. t= tarsal plate, a= aponeurosis, s= septum, tm= superior tarsal muscle

 Superior and inferior tarsal muscles  Smooth muscles arise from lower border of the levator in the upper eyelid and the inferior rectus in the lower lid and insert into the orbital margins of the tarsal plates.  Superior – assists the levator in maintaining the width of the palpebral aperture.  Mild degree of ptosis – results from damage to its sympathetic nerve supply (Horner’s syndrome). In spontaneous and reflex blinks the levator is inhibited prior to orbicularis contraction as the lid closes. Conversely, in lid opening the orbicularis relaxes as the levator contracts .

Microscopic anatomy  Sagittal section – 6 layers resolved:  Skin - thin and very elastic – continuous with the palpebral conjunctiva at the lid margin.  Sub-cutaneous CT – composed of loose areolar tissue – contains small hair follicles and related glands.  Muscle layer – consists of striated muscle fibres of the orbicularis oculi – arranged in fascicles and separated by CT.  Loose sub-muscular CT layer – lies between the orbicularis and the tarsal plate – contains major nerves and blood vessels of the lid.  Tarsal plate – dense fibrous CT and embedded Meibomian (tarsal) glands – provide the supportive framework of the lids and determine the shape.  Palpebral conjunctiva – firmly attached to the underlying tarsal plate – rich blood supply, provides oxygenation of the cornea during eye closure. Glands 1. Meibomian gland  Elongated tubuloacinar sebaceous glands – lies within tarsal plates of both lids.  Ducts are vertically orientated – with multiple secretory acini – opening laterally onto each gland – that discharges its contents onto the lid margin – spreads across the tear film.  Fewer and shorter in the lower lid. 2. Glands of Zeis and Moll  Ciliary glands – found in association with eyelash follicles.  Zeis – uniobular sebaceous glands – open directly into the follicle – function of the oily secretion is to lubricate the lashes to prevent them from drying out and becoming brittle.  Moll – specialised apocrine glands – consisting of coiled secretory portion and an unbranched duct. Blood and nerve supply  Oculomotor and facial nerves innervate the levator palpebrae and orbicularis oculi muscles respectively. Sensory supple of upper lid = branches of ophthalmic nerve and lower lid = maxillary nerve.  Arterial supply = ophthalmic, lacrimal and facial arteries.

Conjunctiva

o Thin transparent mucous membrane – extends from the eyelid margins anteriorly, providing a lining to the lids – covers the sclera up to its junction with the cornea. o Forms a sac that opens anteriorly through the palpebral fissure. o Divided into 5 regions: o Marginal – extends from a line immediately posterior to the openings of the tarsal glands and passes around the eyelid margin – continues on the inner surface of the lid as far as the sub-tarsal fold. o Tarsal – highly vascular – firmly attached to the underlying tarsal plate. o Orbital o Bulbar – here the conjunctiva is freely movable – due to its loose attachment to Tenon’s capsule (fascial sheath of the globe) o Limbal – rich blood supply. Microscopic anatomy  2 distinct layers are visible:  Epithelium – contains a variable number of goblet cells. o Marginal zone – epithelium is stratified and squamous with few goblet cells. o Tarus – epithelium thins to 2-3 layers of cuboidal cells with scattered goblet cells. o Orbital – epithelium is slightly thicker (2-4 cells) with numerous goblet cells. o The number of goblet cells decline over the bulbar and reaching the limbus, the epithelium is striated squamous, with sparse goblet cells. o Goblet cells – provide the mucous component of the tear film – arise in the basal cell layers of the epithelium.  Stroma – 2 distinct layers: o Superficial adenoid layer – contains lymphatic vessels and numerous lymphocytes. o Deeper fibrous layer – thicker and contains the majority of nerves and blood vessels . o Mast cells – found in high concentrations throughout the conjunctival stroma – central role in pathogenesis of ocular allergy.

Lacrimal System o Comprises of the main and accessory lacrimal glands, the drainage system – consisting of the puncta, canaliculi, lacrimal sac and the nasolacrimal duct. Lacrimal gland  Main – provider of the aqueous component of the tears. o Located in a shallow depression of the frontal bone behind the superolateral orbital rim. o Split by aponeurosis into – upper larger orbital lobe and lower palpebral lobe. Microscopic anatomy  Tubulo-acinar – secretory units (acini) contain secretory cells (presence of secretory granules) surrounded by myoepithelial cells (dendritic cells – associated with the perimeter of acini and ducts – play a role in expulsion of tears from the gland)  Ducts consist of a single layer of cuboidal cells – lack secretory granules. Ductal cells modify the composition of tear fluid by selecting electrolytes. Blood and nerve supply – arterial supply provided by the lacrimal artery – enters the posterior border of the gland. Accessory lacrimal gland  Include glands of Wolfring and Krause – found within conjunctival stroma.  Contribute to 5-10% of aqueous tear volume.  True acini are absent and glands consist of elongated tubules – connect with ducts – open into conjunctival surface. Main function – secrete proteins, electrolytes and water onto the ocular surface and ocular secretory immune system (first line of defence against microbial infection) – mediated through secretory IgA.  Lacrimal gland – main source of tear IgA and the gland contains large number of plasma cells.

Lacrimal drainage system  Constant turnover of tears and tear elimination – matched by tear production.  Majority of tears are eliminated via the nasolacrimal drainage system.  Tears collect at the medial canthal – drain into the puncta of the upper and lower lids.  Puncta directed towards the surface of the eye to receive tear fluid.  From each punctum – canaliculus passes first vertically for about 2mm and then turns sharply to run medially for about 8mm.  Canaliculi converge towards the lacrimal sac – forming a common canaliculus before entry.  Lacrimal sac occupies a fossa formed by the maxillary and lacrimal bones. From here tears drain into the nasolacrimal duct – passes into an opening in the nose beneath the inferior nasal turbinate – fold of mucosa observed at the termination of the duct.  Process of tear drainage is an active process mediated by the contraction of the orbicularis during blinking.  Tears enter canaliculi by capillary action – early part of blink, the puncta are occluded. As the orbicularis further contracts, the canaliculi and lacrimal sac are compressed, forcing fluid into the nose. Pre-ocular tear film  Tear film – complex fluid that covers the exposed parts of the ocular surface mediated by the eyelid margins.  Regarded as a trilaminar structure, with a superficial lipid layer (secreted by the meibomain glands) that overlies an aqueous phase (derived from the main and accessory lacrimal glands) and an inner mucinous layer (produced mainly by conjunctival goblet cells). Functions: o Optical – smoothing the irregularities of the corneal epithelium – tear film creates an even surface if good optical quality – reformed by each blink. Air-tear – provides 2/3 of its total refractive power.

o Metabolic support – constant turnover provides a mechanism for the removal of metabolic waste. Gaseous exchange takes place across the tear interface. o Protective – defence against microbial colonisation – washing action of the tear film reduces the microbial adhesion to the ocular surface. o Lubricative – smoothing the passage of the lids over the corneal surface and preventing the transmission of damaging shearing forces. Sources and composition  Tears – combine the products of several glands (main and accessory lacrimal gland. Meibomian gland and conjunctival goblet cells) – can be considered as a watery secretion containing electrolytes and proteins with less amounts of lipid and mucin.  Electrolytes – contribute to tear osmolality.  Proteins – originate from lacrimal gland, ocular surface epithelia and conjunctival blood vessels – major lacrimal proteins include: secretory IgA, lysozyme, lactoferrin and lipocalin (contribute to tear stability). IgA – role in the defence by the ocular surface against microbial infection by preventing bacterial and viral adhesion and inactivating bacterial toxins.  Mucins – derives from conjunctival goblet cells and the glycocalyx of the ocular surface epithelium - has a lubricative role and a hydrophilic ocular surface. o Major role in stabilising and spreading the tear film and provide protection against desiccation and microbial invasion. o 16 human mucin (MUC) genes have been identified: MUC1, MUC4 and MUC16 – on the stratified epithelium of the cornea and conjunctiva – form the glycocalyx of apical microvilli. MUC5AC – gel-forming – produced by conjunctival goblet cells.  Lipids – from the Meibomian gland embedded within the tarsal plates of each lid – delivered directly as clear oil onto the lid

margins – spread over the tear film from the inner edge of the lid margins with each blink. Lipid layer = 100nm thickness. o Provides a hydrophobic barrier at the lid margin to prevent overspill of tears and also retard tear evaporation. The total tear volume is approx. 7µl and is distributed within the peroneal tear film (1-2 µl), and along the upper and lower marginal tear strips (5-6µl)....