Page 375 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Receptors and sense organs (organa sensuum) 357

opening, the pupil (Figure 16.11). The iris is an opaque population results in a dark brown iris (horse, ox). The light-
VetBooks.ir diaphragm that separates the aqueous chamber of the eye brown to yellowish iris colour seen in dogs, pigs and small
into an anterior chamber (camera anterior bulbi) and a ruminants is associated with a relative paucity of melanin
posterior chamber (camera posterior bulbi). The anterior granules. Ocular albinism results from a lack of pigmentation.
and posterior chambers communicate via the pupil. The posterior surface of the iris is lined by a double
The anterior surface of the iris forms the posterior layer of epithelium (pars iridica retinae) derived from
boundary of the anterior chamber. A continuous epi- the leaves of the anterior rim of the optic cup. The inner
thelium is lacking, the surface layer being comprised of leaf becomes the pigmented continuation of the non-
flattened fibroblasts, melanocytes and connective tissue pigmented epithelial layer of the ciliary body. This layer is
fibres of the iris stroma (Figure 16.17). Irregular grooves simple columnar. At the free edge of the iris, the inner
and crypts on the anterior surface result in an intimate layer of the embryonic optic cup reflects on itself to become
association between the aqueous humour and the stroma. the outer layer. This develops into a single layer of apically
The iris stroma consists of a loose mesh of collagen fibres pigmented epithelial cells with non-pigmented basal
in an amorphous matrix. It is highly vascularised and con- extensions containing filamentous contractile elements
tains smooth muscle cells, melanocytes and nerve fibres. (Figure 16.17). These pigmented myoepithelial cells con-
The collagen bundles are arranged in arcades, allow- stitute the m. dilatator pupillae. The pars iridica retinae is
ing them to adapt to the narrowing (miosis) and dilation part of the nonsensory retina. Only the pars optica retinae
(mydriasis) of the pupil. The extensive stromal vascu- develops into a multi-layered photosensitive organ.
lar supply provides nutritional and mechanical support.
Collagen fibres form cuffs around the vessel walls, prevent- Species variation
ing interruption of the microcirculation during contraction Horse and ruminants: Iridic granules (granula
and relaxation of the iris. iridica) are found at the dorsal and ventral pupillary
The iris stroma incorporates two smooth muscle ele- margin. These are formed by proliferation of the pig-
ments that regulate the size of the pupil: the m. sphincter mented epithelium and increased vascularisation of
pupillae and the m. dilatator pupillae (Figure 16.11). the iris stroma. Particularly in small ruminants, iridic
The circular fibres of the m. sphincter pupillae form a granules contain isolated cystic cavities. Iridic granules
ring at the free edge of the iris. In species with a non-circu- secrete aqueous humour.
lar pupil (cat, sheep, ox), these muscle fibres are reinforced Birds: The iris forms an ‘aperture ring’ that surrounds
peripherally by a sharply angled muscle fibre lattice. The the usually round, occasionally transversely ovoid,
interweaving of these muscle fibres produces a slit-like or pupil. As in mammals, it separates the anterior chamber
oval (transverse or longitudinal) pupil. The m. sphincter (between the iris and cornea) from the shallow poste-
pupillae receives parasympathetic (cholinergic) innerva- rior chamber (between the iris and the lens). The colour
tion. A loose association exists between the fibres of the of the iris varies with species and, in some cases, with
m. sphincter pupillae and m. dilatator pupillae. sex and age. In some sexually monomorphic species,
The m. dilatator pupillae (Figures 16.11 and 16.17) is such as cockatoos, iris colour may thus be utilised for
formed by myoepithelial cells that make up the anterior sex determination. Nutritional and seasonal factors can
layer of the posterior iris epithelium (pars iridica retinae; also influence the colour of the iris. A feature unique
see below). Contractile, non-pigmented processes project to pigeons is the presence of a tapetum lucidum iridis
radially from the basal portion of the cells. This layer is sup- consisting of reflective iridocytes.
plied by sympathetic (adrenergic) nerve fibres. Contraction Also particular to pigeons is the annulus iridis, a non-
of the m. dilatator pupillae results in dilation of the pupil. pigmented region with few blood vessels. This region
Pigmented cells found in the iris originate from the appears dark because of the underlying pigmented pos-
neural crest (stromal melanocytes) and neuroectoderm terior portion of the iris. Referred to by pigeon breeders
(pigmented iris epithelium; see below). Melanin protects as the ‘circle of correlation’, it has no significance with
the retina from excessive incident light and from scattered respect to vision or flying ability. In female blue-footed
light by acting as a neutral-density filter. The degree of pig- boobies, the portion of the iris adjacent to the pupil
mentation (number and size of melanosomes) determines contains dark pigment deposits. The resulting contrast
the colour of the iris. Iris colour is encoded in several with the yellow of the remainder of the iris makes the
nondominant genes, resulting in a range of hues. pupil appear deceptively large.
A blue iris indicates an absence of stromal melanocytes. The width of the iris, and thus the diameter of the
The blue colour arises from light falling on collagen fibres pupil, is controlled by the m. sphincter and m. dilatator
lying anterior to the pigmented layer of the epithelium. pupillae. In birds, these muscles are predominantly stri-
Condensation of the collagen fibres produces a blue-grey to ated, permitting faster adjustment to changes in light
grey tone (pig, goat). A thin stroma and large melanocyte exposure compared with mammals.

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