Week 8: The Uvea 1 Flashcards
What are the structures of the Uvea?
The iris, ciliary body and choroid
What’s the uvea?
-> The vascular layer of the eye, between the sclera and retina
Made up of blood vessels in a loose network of connective tissue and lots of pigment cells.
-Blood vessels provide circulation to the eye (direct or through uvea tract)
-pigment prevents unwanted light entering the eye (only through the pupil) and reduces light reflections in the eye
What is the iris?
- a think contractile pigmented diaphragm with a central aperture (the pupil) which forms the anterior and posterior chambers.
- controls the amount of light reaching the retina, depth of focus and recuses aberrations.
- attached to the ciliary body at the iris root
- is suspended in the aqueous humour between the cornea and the lens (forms the filtration angle with the cornea)
What are the iris dimensions?
> Diameter = 13mm
Thickness = 0. 1 mm at iris root (@ Ciliary Body)
0.6 mm at the thickest point (the collarette)
The collarette divides the anterior surface into two zones, the pupillary and ciliary regions.
What is the pupil?
-Not located centrally in the iris, is displaced slightly down and in (towards nasal side)
> Diameter varies from 1 mm to 8 mm
> Dilation of the pupil = mydriasis, Constriction = miosis
> Width of the ciliary region alters the most, the pupillary zone remains almost constant (when pupil changes in size, changes size of iris) (change of length of iris/size of pupil nearly all carried out by ciliary region)
What does iris colour depend on?
> Depends on the pigmentation of the anterior surface
More pigmentation = browner iris and more the underlying structure is hidden
Little or no pigment = blue, as scattering of light in the stroma (seen also at birth) Pigment hasn’t developed.
-Iris colour is inherited, brown being dominant and blue recessive.
Large variations between the L + R eyes can produce heterochromia (different coloured irises)
What are the three main structures in the iris?
- anterior border layer
- stroma
- epithelium
What is the anterior border layer?
What are Fuch’s crypts?
> The anterior surface is bare (no epithelium), dense covering of:
•Melanocytes (contain granules of the pigment melanin)
•Fibroblasts and collagen fibres
Surface not entirely flat…
The connective tissue forms bands that enclose oval crypts (Fuch’s crypts)
Crypts allow aqueous humour to access the tissues (stroma) of the iris
Fuch’s crypts most noticeable at the collarette
What is the stroma?
> Made of collagenous; connective tissue containing fibroblasts (cells that produce fibrous substances) and pigment cells in a mucopolysaccharide matrix. (Scaffolding)
Iris blood vessels and sphincter muscle are contained within the stroma (one of set of muscles that control pupil size and size of iris).
What is the sphincter pupillae muscle?
is a band of smooth muscle
> 0.75 - 1mm wide
> 0. 1mm thick
> Runs parallel to the pupil margin.
> In extreme miosis the whole muscle can be reduced to 15% of original length.
> Like drawing string on a drawstring bag, as S reduces length, pupil reduces size..
What is the epithelium?
> A double epithelial layer
From the pupil margin to the peripheral edge of the iris sphincter both epithelial layers are deeply pigmented
Beyond the sphincter, the deeper layer carries contractile processes (up to 70mm long) which are radially directed and form the dilator pupillae muscle (2nd muscle).
> The anterior epithelial cells have dilator processes that form the basal part of the cell, and an apical part that contains the nucleus and pigment granules.
The posterior epithelial cells (35-50mm high) form the posterior pigment epithelium, and are densely packed with melanin.
The entire posterior surface appears black prohibiting the passage of light
What is the deeply pigmented epithelium?
> The deeply pigmented epithelium extends beyond the tip of the iris to form the pupillary ruff
This is visible around the pupil, particularly in those with a pale iris
What is the blood supply to the back of the eye?
-Radial blood vessels have a spiral course enabling movement (grow longer and shorter) during mydriasis and miosis. (Like a telephone cord, spring extends and contract)
> Arteries are derived from the major arterial circle (MAC) situated in the ciliary body, and run through the iris to anastomose (join again) in the minor iridic circle at the level of the collarette.
> From here, capillary loops pass towards the pupil margin.
> Venous drainage follows approximately the same path back to the ciliary plexus, and there to the anterior ciliary veins or vortex veins (@ back of eye, 4).
These blood vessels also serve as a route of exchange for the metabolites and waste products of the aqueous humour.
What is the nerve supply of the eye?
> Sensory nerves are only found in the iris root
Motor nerves are autonomic sympathetic and parasympathetic.
The sphincter has principally parasympathetic innervation (miosis) from the parasympathetic root of the oculomotor (IIIrd cranial) nerve.
The dilator has sympathetic innervation (mydriasis) Sympathetic fibres come from cells situated in the superior cervical ganglion
