Conjunctiva Flashcards
What is the conjunctiva?
thin transclucent mucous membrane that derives its name from the fact that it attaches the eyeball to the lids
What are the 2 broad microscopic parts of the conjunctiva?
superficial conjunctival epithelium
overlies loose connective tissue stroma
What 2 structures is the conjunctival epithelium continuous with?
corneal epithelium at the limbus, and with the skin at the mucocutaneous junction on the lid margin
What is the conjunctiva reflected onto from the anterior portion of the sclera?
onto the tarsal surface of the eeylids at the superior and inferior fornices
What is the conjunctival sac?
a potential sac formed when the eyelids are closed
What is the volume of the conjunctival sac?
7 μL
Why do eye drops from commercial dispesers overflow unless the lower lid is held away from the globe?
because the volume of the sac when the eye is closed is 7 μL, and the eye drops have volume 50-70 ml
What are 2 functions of the conjunctiva?
- production of mucous component of tear film
- variety of immunological defence mechanisms that protect the ocular surface from infectio
What are 3 main regions into which the conjunctiva can be divided for descriptive purposes?
- palpebral conjunctiva
- forniceal conjunctiva
- bulbar conjunctiva
Where is the palpebral conjunctiva found?
lines the inner surfaces of the eyelids
What is the palpebral conjunctiva bound to?
the tarsal plate
What is the subepithelial connective tissue stroma of the palpebral stroma like?
subepithelial connective tissue stroma thin in this region
What is the conjunctival epithelium of the palpebral conjunctiva continuous with and via which structure?
with the lining of the inferior meatus of the nasal cavity - via the lacrimal puncta
What is the clinical link between the connection of palpebral conjunctiva epithelium and the lining of the inferior meatus of the nasal cavity?
infection spreads between these two sites
What structure is important in trapping and removing foreign particles and debris on the ocular surface, and where is it located?
subtarsal sulcus, close to the lid margin
Under what situations might accumulations of immune cells in the palpebral conjunctiva become enlarged?
various allergic conditions
What is common procedure in examination of the eye for signs of allergy?
examination of the everted lid
What are 2 major types of abnormal accumulations of immune cells which may occur in allergy?
- follicles - lymphocytes
- papillae - inflammatory cellular infiltrates
What are follicles in the context of allergic responses in the conjunctiva?
similar to mucosal-associated lymphoid follicles elsewhere, consist primarily of lymphocytes
What are papillae in the context of allergic responses in the conjunctiva?
focal aggregates of chronic inflammatory cellular infiltrates and accompanying vascular changes
What are 2 situations that follicles and papillae of the conjunctiva are commonly associated with?
- allergic conditions
- irritation of ocular surface e.g. in contact lens wear
What are the superior and inferior fornices continuous with?
continuous at the medial and lateral canthi, forming a circular cul de sac
What structures empty into the superolateral fornix?
the ducts of the main lacrimal gland and the bulk of accesssory lacrimal glands
Why does the forniceal conjunctiva move slightly with the eye during contraction of certain muscles?
it is loosely attached to the fascial sheaths of levator palpebrae superioris and rectus muscles - moves with the eye during contraction of these muscles
What colour is bulbar conjunctiva?
normally translucent, white sclera is visible through it
What parts of the eyeball are clothed by the bulbar conjunctiva?
anterior part of the eyeball, including extraocular muscle insertions and Tenon’s capulse
How does how tightly the bulbar conjunctiva is bound to the globe change in different areas?
near the limbus it is tightly bound to the globe, but further from the limbus there is a loose episcleral tissue layer within which lies the pericorneal vascular plexus
What lies within the loose episcleral tissue layer of the bulbar conjunctiva further from the limbus?
pericorneal vascular plexus
What can cause the pericorneal vascular plexus vessels to become dilated and conspicuous?
due to physical and inflammatory stimuli
What are the 2 specialisations of the conjunctiva in the medial fornix?
- Semilunar fold (plica semilunaris)
- Caruncle (caruncula lacrimalis)
What is the plica semilunaris of the bulbar conjunctiva in the medial fornix likely homologous to?
nictitating membrane of lower mammals and many non-mammalian vertebrates
What structures histologically are present in the plica semilunaris of the medial fornix conjunctiva?
high vascular, rich in goblet cells and interstitial immunocompetent cells
What is likely the function of the semilunar fold, or plica semilunaris?
may be to facilitate lateral movement of the eye
What is the caruncula lacrimalis?
highly vascular nodule of modified skin in the medial corner of the eye containing large nests of accessory lacrimal and sebaceous glandular tissue
What are 3 examples of systemic disease evident in the conjunctiva?
- Sickle cell anaemia - comma sign
- Jaundice - scleral icterus
- Vitamin A deficieny - Bitot’s spots
What is the general histology of the conjunctival epithelium and how does this vary?
- varies in structure depending on location
- stratified squamous non-keratinising epithelium (close to the lid margin) to stratified columnar epithelium (bulbar)
How many layers of epithelial cells are there in conjunctival epithelium (give a range)?
between 2 and 7 layers of epithelial cells
What are the 3 tpes of cells into which conjunctival epithelial cells are arranged?
- basal
- intermediate
- superficial
Is there are prickle cell layer in conjunctival epithelium and what does this indicate?
no - fewer desmosomes between the conjunctival epithelial cells (compared with corneal epithelium)
What do the numerous cell types resident within conjunctival epithelium indicate?
its protective function
What are 4 of the cell types resident within the conjunctival epithelium?
- goblet cells
- melanocytes
- intraepithelial MHC class II-positive dendritic cells
- intraepithelial lymphocytes
Describe the features of the goblet cells which are found in conjunctival epithelium.
unicellular, mucus-secreting cells that vary in density in different regions of the conjunctiva
In which 2 areas of the conjunctiva are goblet cells most numerous in the epithelium?
- fornices
- plica semilunaris
What is the function of corneal epithelium goblet cells?
responsible for the secretion of the majority of conjunctival mucins
What causes the degree of melanisation of the conjunctival epithelium to vary?
dependent on race (but they are present in all eyes)
How are melanin pigments in the conjunctival epithelium produced?
melanosomes are synthesised within the melanocytes before exocytosis and subsequent uptake by surrounding epithelial cells as occurs in the epidermis
What is another name for intraepithelial MHC class II-positive dendritic cells in the conjunctiva and why?
Langerhans cells because of similar morphology to analogous dendritic cell populations in epidermis of skin
What is the function of intraepithelial MHC class II-positive dendritic cells in the conjunctival epithelium?
function as ‘sentinels’ on the ocular surface; responsible for trapping and internalising antigens and transporting these signals to either local lymph nodes (such as the preauricular lymph nodes) or conjunctival associated lymphoid tissue (CALT) or follicles
here they are capable of presenting antigens to naive T cells and inducing primary immune responses or driving antigen-specific B-cell maturation and immunoglobulin production
What are 3 places that intrapeithlial MHC class II-positive dendritic cells of the conjunctiva transport signals from internalised antigens to?
- Local lymph nodes e.g. preauricular nodes
- Conjunctival associated lymphoid tissue (CALT)
- Follicles
What do intraepithelial MHC class-II positive dendritic cells of the conjunctiva do once they have transported signals to lymph nodes/CALT/follicles?
present antigens to naive T cells and induce primary immune responses or drive antigen-specific B-cell maturation and immunoglobulin production
What can affect the number of intraepithelial lymphocytes in the conjunctival epithelium?
they are a feature of normal conjunctiva but increased numbers occur in inflammatory conditions and close to subepithelial lymphoid accumulations
What type of lymphocytes are most common as intraepithelial corneal lymphocytes?
CD3+ T cells
half are CD3+ CD4+ cells, half are CD8+ T cells with occasional CD19+ B cells being present
What affecst the ratios of intraepithelial lymphocytes in the conjunctiva?
variation in different regions of the conjunctiva
What are 2 other types of lymphocyte in the conjunctival epithelium according to recent data?
- IL-22 producing γδ T cells
- IL-22 producing γδ - innate lymphoid cells
What can be found adjacent to the underlying connective tissue of the conjunctival epithelium?
irregular basal aspect
What are 2 aspects that the basal layer of conjunctival epithleium is sometimes described as having?
- looser lymphoid layer
- deeper fibrous layer
Where are distinct papillae of the conjunctival connective tissue stroma only found?
near the limbus
What are 4 examples of immunocompetent cells in the conjunctival subepithelial connective tissue?
- mast cells
- eosinophils
- plasma cells
- lymphocytes
In which eyes may there be diffuse or discrete aggregates of immunocompetent cells in the subepithelial connective tissue of the conjunctiva?
in some eyes, particularly of older individuals
What is the function of some follicles (aggregates of immunocompetent cells) in the conjunctival subepithelial connective tissue layer?
some contain pale germinal centres and represent the local mucosal lymphoid tissue (MALT) or CALT
How do the immunocompetent cells in the CALT differ from other MALT such as Peyer’s patches or tonsils?
in CALT there is no clear evidence of the specialised antigen-transporting intraepithelial M cells typically found in other similar MALT
What 2 things form the efferent arm of the immune system of the conjunctiva? What are their function?
- diffuse subepithelial aggregates
- intraepithelial lymphocytes
aid in the immunological protection of the ocular surface
What suggests that the lenticular lymphoid follicles and epithelial crypts in the tarso-orbital conjunctiva may approximate to the cornea during eye closure?
the topographical distribution suggests they approximate to the cornea during eye closure, and thus act as an ‘immunological cushion’
What else is contained within the connective tissue stroma of the conjunctiva, in addition to immune cells, and what is this similar to?
contains rich vascular network, similar to the eyelids
What is the blood supply of the conjunctiva? 2 sources
- loose connective tissue stroma contains a rich vascular network
- also receives blood from anterior ciliary arteries
What is the sensory nerve supply of the palpebral conjunctiva?
almost entirely from branches of ophthalmic division of trigeminal: supraorbital, supratrochlear, lacrimal nerves
What are 3 neurotransmitters contained in the nerves supplying the palpebral conjunctiva?
- substance P
- calcitonin generelated peptide (CGRP)
- gallanin
What is the nerve supply for the medial portion of the inferior forniceal and palpebral conjunctiva?
maxillary division of trigeminal (infraorbital nerve)
What is the nerve supply to the bulbar conjucntiva?
long ciliary nerves (branches of nasociliary nerves, branch of ophthalmic nerve V1)
In addition to sensory supply to the conjunctiva what are 2 other nervous supplies?
- parasympathetic nerves from pterygopalatine ganglion
- sympathetics
What is the pathway of parasympathetic supply to the conjunctiva?
via pterygopalatine ganglion
What are 2 neurotransmitters in the parasympathetic supply to the conjunctiva (via the pterygopalatine ganglion)?
- acetylcholine
- VIP
What are 2 neurotransmitters in the sympathetic supply to the conjunctiva?
- norepinephrine
- neuropeptide Y
What structure do the sympathetic fibres to the conjunctiva travel with?
ophthalmic artery
What is the distribution of sympathetic, parasympathetic and sensory nerve endings in the conjunctiva?
- parasympathetics and sympathetics identified around goblet cells
- sensory nerve endings only among stratified squamous epithelial cells
What are 3 types of glands within the conjunctiva?
- Goblet cells (unicellular mucous glands)
- Accessory lacrimal glands (Krause and Wolfring)
- Mucus-secreting: glands of Henle
What are the 2 types fo accessory lacrimal glands in the conjunctiva and where are they located?
- glands of Krause in upper fornices
- glands of Wolfring in upper border of tarsus
What is the name of the mucus secreting glands in the conjunctiva?
glands of Henle
What is the nervous supply to the accessory lacrimal glands (Krause and Wolfring)?
sympathetic stimulation
What is the function of the accessory lacrimal glands (Wolfring and Krause)?
baseline tear production
