Page 387 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Receptors and sense organs (organa sensuum) 369

2–4 μm thick). At the lens equator, the lens fibres contain tein within the lens from the immune system. Protein
VetBooks.ir a nucleus and numerous organelles. Towards the interior released as a consequence of lens trauma may be rec-
of the lens the nucleus is lost. The lens fibres form con-
ognised as foreign and can thus result in phacogenic
centric layers that become condensed towards the centre,
forming the lens nucleus. Individual lens fibres contact uveitis (endophthalmitis phacoanaphylactica). In con-
trast to mammals, the lens equator is closely associated
one another through zipper-like interdigitations. This con- with the ciliary processes, the tips of the processes
tributes to the plasticity of the lens, which is required for being tightly fused with the capsule. Fixation of the
accommodation. lens is supplemented by the ciliary zonule (zonula cili-
Lens fibres contain water (70%), membrane proteins, aris), composed of the zonular fibres (fibrae zonulares).
cytoskeletal proteins, enzymes, crystallins and electro- The intimate association of the lens with the ciliary
lytes. The shape of the lens is determined largely by its body and the relative pliability of the avian lens facili-
cytoskeletal elements. These include microfilaments such tate the process of active lenticular accommodation.
as fibronectin, vimentin and actin. Accordingly, the lens is relatively flexible in diving birds, in
The lens fibres traverse the lens equator from the ante- which lenticular accommodation predominates, and com-
rior to the posterior pole of the lens and vice versa. Where paratively hard in owls, which rely almost exclusively on
they meet at their end points, lens sutures (radii lentis) are corneal accommodation. Age-related loss of lens pliability,
formed. These join to form three-pointed lens stars. The seen in humans, dogs and cats, is not observed in birds.
orientation of the lens stars at the anterior and posterior
aspects of the lens is offset by 60°. VITREOUS BODY (CORPUS VITREUM)
Lens transparency is dependent upon the ordered struc- The vitreous body is bounded by the lens, the ciliary body
ture and metabolic integrity of the lens fibres. Changes in and the retina (Figure 16.11). It is tightly adherent to the
these factors result in lens opacity. ora serrata and the retinal vessels. The vitreous body is
composed of a transparent, colourless hydrogel (humor
Species variation
vitreus) comprising 99% water. The fluid phase contains
Birds: As in mammals, the lens is a transparent, biconvex dissolved hydrophilic glycosaminoglycans (particularly
structure of epithelial origin, positioned between the iris hyaluronic acid polymers) bound with water.
and the vitreous body (Figure 16.14). In altricial birds, the The solid phase includes a relatively small population of
lens is opaque during the nestling stage, only becoming cells, hyalocytes, that produce microfibrillar structural pro-
transparent after fledging. In diurnal birds, the lens is rela- teins and phagocytose free cells and fibril fragments. Type
tively flat, while in water birds and nocturnal species it is II collagen microfibrils form a scant structural framework.
spheroid. Beneath the lens capsule (see below) is a layer of Optimal hydration of these collagen molecules is facilitated
simple epithelium. Towards the lens equator, the epithe- by the hyaluronidase system. Superficially, the collagen
lial cells become elongated, giving rise to radially oriented fibrils condense to form the hyaloid membrane that lies
hexagonal prisms that combine to form the equatorial adjacent to the internal limiting lamina of the retina.
annular pad (pulvinus anularis lentis) (Figure 16.14).
The annular pad is a characteristic feature of the avian Species variation
eye. It is marked by a row of indentations that correspond Ox, pig and carnivore: A fluid-filled channel (canalis
in number with the ciliary processes to which they are hyaloideus), a remnant of the embryonic a. hyaloidea,
attached. The function of the annular pad is incompletely extends from the lens to the optic nerve head.
understood, although it is not considered to be part of the
optical system. Instead, it is presumed to play an impor- Ox, pig and sheep: The vitreous body is dense, giving
tant role in the speed of lenticular accommodation, it a firm consistency.
since the pad is particularly thick in fast fliers (especially Horse: The vitreous body is amorphous and has a low
diurnal birds of prey and pigeons) and less developed in optical density.
diving birds. It is notably narrow in psittacines.
The annular pad may also have a nutritional func- Carnivores: The vitreous has a dense core and a less
tion. Its cells secrete fluid, the aqua vesiculae lentis, structured outer portion.
into the cleft-shaped lens vesicle (vesicula lentis)
(Figure 16.14) between the pad and the central lens As well as serving as a refractive medium, the vitreous
core (corpus centrale lentis). The fluid is absorbed body participates in retinal metabolism and homeostasis.
by the core. Lens stars, as seen in mammals, are not By contributing to intraocular pressure, the vitreous body
present in birds. The lens capsule, a derivative of holds the retina in place against the retinal pigment epithe-
basal lamina, forms a semipermeable barrier through lium. A reduction in pressure can lead to detachment of
which nutrients can diffuse from the aqueous humour. the retina (posterior to the ora serrata, where the corpus
It also forms a permanent barrier separating the pro- vitreum is firmly attached to the ciliary epithelium).

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