Iris Flashcards
What is the iris?
a thin, heavily pigmented, contractile circular disc analogous to the diaphragm of a camera
Where is the anatomical location of the iris?
suspended in the frontal or coronal plane anterior to the lens and ciliary body
What substance surrounds the iris?
aqueous humour
What structures are separated by the iris?
anterior and posterior chambers
What communication connects the anterior and posterior chambers of the eye?
the pupil - opening in the iris
What is the anatomical location of the pupil?
lies slightly inferonasal to the centre of the iris
What are the boundaries of the iris?
- it is attached by its root at the angle (iridocorneal) of the anterior chamber where it merges with the ciliary body and trabecular meshwork
- the free edge is known as the pupillary margin
What is the diameter of the iris?
12mm
What is the circumference of the iris?
37mm
What is the general shape of the iris?
cone shaped with the pupil margin positioned more anteriorly than the root
Where does the pupil margin rest?
on the lens
What would happen if the pupil margin did not rest on the lens e.g. in aphakic patients?
the iris would become tremulous (iridonesis)
What is meant by aphakia?
without a lens
What is the function of the pupil and what is this function dependent on?
size of the pupil regulates the amount of light entering the eye
this is dependent on the state of contraction of the intrinsic pupillary muscles, the dilator and sphincter pupillae
What are the 2 key types of muscles involved in regulating the size of the pupil and therefore the amount of light entering the eye?
the intrinsic pupillary muscles: dilator and sphincter pupillae
By how much may the pupil diameter vary?
1 to 8 mm
Is a slight degree of asymmetry between pupil size normal between individuals?
yes - may be slight degree of asymmetry between right and left
What 2 parts of the iris are thin?
pupil margin
iris root
What is a consequence of the pupil margin and iris root being thin parts of the iris?
they are more susceptible to tearing in contusion injuries (iridodialysis)
What is the definition of iridodialysis?
separation of the iris from its attachment to the ciliary body, commonly occurs in penetrating or blunt trauma
What are the two zones into which the anterior surface of the iris is divided?
- pupil zone
- ciliary zone
What structure divided the anterior iris into pupillary and ciliary zones?
the collarette, a thickened region
What features characterise the anterior surface of the iris? 2 things
- radial streaks
- contraction furrows
How do radial streaks on the anterior surface of the iris differ when the pupil is contracted vs dilated?
radial streaks are straight when pupil is contracted
wavy when pupil is dilated
When are contraction furrows on the anterior surface of the iris more noticeable?
in dilated irides
How does the visible appeares of dark irides vs blue irides differ? What causes this?
the surface of the iris appears smooth in dark irides in which the intrastromal melanocytes are heavily pigmented
and more irregular in blue irides which have a less heavily pigmented stroma
What causes the blue appearances of some irides?
abosprtion of long wavelengths and reflectance of shorter wavelenghts, especially by the collagenous stroma
What are the 4 microscopic layers of the iris?
- anterior border layer
- stroma
- dilator pupillae muscle
- posterior pigment epithelium
What is the anterior border layer made up of?
modified stroma consisting of a dense collection of fibroblasts, melanocytes and a few interspersed collagen fibres
(not epithelial cells)
What is the consequence of the anterior border layer of the iris being deficient in some areas?
the iris stroma is in free communication with the aqueous humour in the anterior chamber
What are large deficiencies in the anterior border layer of the iris macroscopically evident as?
crypts
What forms macroscopic naevi in the iris?
aggregates of heavily pigmented melanocytes in the anterior border layer of the iris
Why are melanocytes in the uveal tract unlike skin melanocytes?
in the uveal tract, melanocytes reside in connective tissue and do not constantly shed melanin into adjacent cells (the basis of skin pigmentation) and iris colour is a reflection of both the density of melanocytes, their location in the iris and their melanin content
How does iris colour relate to melanocytes in the anterior border layer?
iris colour is a reflection of both the density of melanocytes, their location in the iris and their melanin content
Label the following on the iris image: limbus, root, pupil, pigmented pupil margin, pupillary zone, collarete, collagenous trabecula, crypts.
Label the following on the diagram: radial folds, collarette, crypts, pupillary margins, melanocyte, blood vessel, stroma, anterior iris epithelium, dilator pupillae muscle, posterior iris pigment epithelium