Page 386 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

368 Veterinary Histology of Domestic Mammals and Birds

nerve fibre bundles, separating these into individual fibres. Internal structures of the bulb
VetBooks.ir Fibrous astrocytes located at the interface with the axon The interior of the bulb contains the lens and the vitreous
provide nutritional support.
body (corpus vitreum). In addition, three cavities can be
Species variation distinguished within the eyeball (Figures 16.11 and 16.13):
Birds: In most birds, all of the nerve fibres within each · anterior chamber (camera anterior bulbi) between
optic nerve decussate at the optic chiasm. Thus, a true the cornea and the anterior surface of the iris,
consensual pupillary light response does not occur. · posterior chamber (camera posterior bulbi)
Throughout its extracranial course, the optic nerve is between the posterior surface of the iris, the ciliary
surrounded by pia and dura mater. It incorporates very body, the zonular fibres and the lens and
little slack in most avian species, and none at all in birds · vitreous chamber (camera vitrea) posterior to the
with minimal eye movement (e.g. owls). lens, surrounded by the retina.
A further peculiarity of the avian optic nerve is the
presence of efferent nerve fibres originating from the LENS
isthmo-optic nucleus in the mesencephalon. These are The lens originates from the ectoderm. It arises embry-
responsible for further enhancing visual acuity. The ologically from the lens placode, separating from the
point at which the optic nerve leaves the retina is oval in surface epithelium as the lens vesicle. Elongated cell pro-
birds and is largely covered by the pecten oculi (Figures cesses extending from the posterior wall of the lens vesicle
16.31 and 16.32). The heavily vascularised connective fill the internal cavity and differentiate into hexagonal lens
tissue core of the pecten arises directly from the optic fibres.
nerve. Consequently, the ovoid area that represents the The lens is a transparent biconcave structure that is
equivalent of the optic nerve head in mammals is visible connected by zonular fibres to the ciliary body (Figures
ophthalmoscopically only as a narrow, whitish-yellow 16.11 and 16.13). The posterior surface, facing the vitre-
margin at the base of the pecten. According to ontogenic ous body, is usually more convex than the anterior surface.
studies, the pecten is derived from the retina. From its Thickening of the lens during accommodation occurs
origin, the pecten extends into the vitreous body. Three mainly through an increase in curvature of the anterior
structural forms of pecten have been described: pleated, surface. The lens is insensitive and avascular. It receives
vaned and conical. nutrition by diffusion from the aqueous humour. The lens
The pleated type is characterised by closely apposed has the following components:
vertical folds that are joined at their tip by a bridge,
the pons pectinis. This type is typical of carinate birds · lens capsule (capsula lentis),
(Figure 16.31). In the vaned type, 25–30 vertical vanes · lens epithelium (epithelium lentis) and
are connected to a central lamina. A vaned pecten is · lens fibres (fibrae lentis).
found in ratites, such as the ostrich, emu and nandu.
Only the kiwi is known to have the conical type, an The lens is enclosed in a flexible, refractive capsule that is
undivided structure devoid of folds or laminae, resem- formed from secretions of the lens epithelium. The lens
bling the conus papillaris of reptiles. capsule comprises a broad basal lamina (anterior thick-
Despite extensive investigation, the function of the ness 10–20 μm, posterior thickness 5 μm) composed of
pecten remains unclear. The many hypothesised roles a dense network of type IV collagen microfilaments and
include protection of certain regions of the retina from embedded glycoproteins. The zonular fibres insert on the
glare, reduction of scattered light, immune functions, periphery of the lens capsule, merging with filaments of
motion detection and sensing of magnetic fields to facil- the capsule. The capsule forms a semipermeable barrier
itate orientation. It is generally agreed that the pecten that is penetrable by metabolically active substances.
has a nutritional role, supplying the vitreous body and The lens epithelium lies on the anterior surface of
avascular retina, and that it contributes to intraocular the lens, beneath the lens capsule. It is simple cuboidal in
pressure and temperature regulation. This is supported type. The epithelial cells are the only cells of the lens that
by the characteristics of the endothelium of the capil- are capable of replication. Particularly at the lens equator
laries in the pecten, which indicate continuous, active (equator lentis), the cells divide and elongate, eventu-
trans-epithelial transport. The passage of substances ally differentiating into lens fibres (see below). The lens
from the capillaries to the vitreous body has been dem- enlarges throughout life, as new lens fibres are laid upon
onstrated using fluorescein angiography. Distribution of existing ones. The posterior surface of the lens consists of
these substances throughout the vitreous body is facili- lens fibres; there is no posterior epithelium.
tated by rhythmic contractions of the extrinsic muscle of The lens fibres, which form the bulk of the lens, are
the eye, which result in oscillatory eye movements and elongated prismatic cells (7–10 mm long, 5–12 μm wide,
corresponding passive movement of the pecten.

Vet Histology.indb 368 16/07/2019 15:07