Limbus and aqueous outflow pathways Flashcards
In addition to being the border zone beteen the cornea and sclera, what are 4 functions of the limbus?
- nourishment of the peripheral cornea
- corneal wound healing
- immunosurveillance of the ocular surface and hypersensitivity responses
- contains the pathways of aqueous humour outflow and is thus involved in the control of intraocular pressure
What may surgical incisions be made at the limbus to access?
the anterior chamber for cataract and glaucoma surgery
What is the width of the limbus?
1.5-2.0 mm
What 2 structures are produced by the change in radius of curvature between the sclera and cornea?
- external scleral sulcus
- internal scleral sulcus
What anatomical feature serves to deepen the internal scleral sulcus?
the scleral spur
What 2 key structures are housed by the internal scleral sulcus?
- canal of Schlemm
- trabecular meshwork
What structures attach to the posterior aspect of the scleral spur?
longitudinal ciliary muscle fibres
What structures does the anterior scleral spur give rise to?
corneoscleral trabeculae
What are the 6 important transitions at the limbus?
- corneal lamellae regular –> scleral random array
- stratified squamous non-keratinised corneal epithelium –> conjunctival epithelium
- conjunctival epihtleium contains goblet cells and MHC class II+ CD11c+ dendritic (Langerhans) cells
- Loops (arcades) of conjunctival and lymphatic capillaries terminate at limbus
- Descemet’s membrae and Bowman’s layer of cornea terminate in this region
- loose conjunctival substantia propria tapers off at the limbus
What is the transition of the regularly arranged coreal lamellae at the limbus?
give way to random array of lamellae in the sclear
What is the shape of the corneal termination (where it becomes the sclera)?
V-shaped
What is the change in epithelium at the limbus?
the stratified squamous non-keratinised corneal epithelium with its parallel internal and external surfaces gives way to conjunctival epithelium, characterised by a folded basal suface and interdigitating subepithelial connective tissue (sometimes forming distinct papillae)
What are the characteristics of conjunctival epithelium?
folded basal suface and interdigitating subepithelial connective tissue (sometimes forming distinct papillae)
What are 2 types of cells present in the conjunctival epithelium not present in the corneal epithelium?
- Goblet cells
- Rich network of MHC class II+ CD11c+ dendritic (Langerhans) cells
What is the change in capillaries at the limbus?
loops (or arcades) of conjunctival apillaries and lymphatic capillaries terminate at the limbus
What are the conjunctival capillaries derived from?
anterior ciliary arteries
Why are the smaller capillaries in the conjunctiva particularly susceptible to the effects of vasoactive amines?
they are not under neuronal control
What are 3 examples of vasoactive amines that small conjunctival vessels are particularly susceptibe to, that are released by local immune cells?
- histamine
- leukotrienes
- prostaglandins
What are the features of the connective tissue of the conjunctiva that tapers off at the limbus and is absent in the cornea?
- loose subepithelial vascularised connective tissue aka substantia propria
- contains immunocompetent cell types (mast cells, plasma cells, lymphocytes)
What are 3 types of immunocompetent cell types that are present in the conjunctival subepithelial vascularised connective tissue?
- plasma cells
- lymphocytes
- mast cells
What is Dua’s layer?
recently described layer anterior to Desçemet’s membrane in the cornea
represents condnsation of stromal collagen fibres
How can Dua’s layer be identified?
by dissection of the cornea using a bubble of air during preparation of the cornea for corneal transplantation
What does Dua’s layer consist of?
condensation of the stromal collagen fibres
What is the anatomical (histological) definition of the limbus?
defined by a line that follows a V shaped transition of corneal lamellae to scleral lamellae
How do pathologists define the limbus?
block of tissue bordered anteriorly by a line passing through the termination of Schwalbe’s line and the junction of the conjunctival and corneal epithleium (corneolimbal junction) and posteriorly by a line from the scleral spur perpendicular to the tangent of the external surface
What is the surgical definition of the limbus?
surgeons cut close to the blue-grey transition zone seen on external examination and incisions made here will pass anterior to the trabecular meshwork and Schlemm’s canal
Where is the trabecular meshwork?
in the chamber or iridocorneal angle, partially nestled in the internal scleral sulcus
What is the trabecular meshwork?
complex wedge-shaped circumferential band of specialised, sponge-like, connective tissue
Where is the canal of Schlemm in relation to the trabecular meshwork?
the canal of Schlemm is on the otuer aspect of the trabecular meshwork
What forms the base on which the trabecular meshwork sits?
- formed posteriorly by the scleral spur, anterior face of the ciliary muscle and the iris root
What is another name for the canal of Schlemm?
sinus venosus sclerae
Where does the apex of the trabecular meshwork terminate?
anteriorly, at Schwalbe’s line and the adjacent inntermost corneal lamellae