Page 372 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

354 Veterinary Histology of Domestic Mammals and Birds

SCLERA The anterior epithelium (epithelium anterius corneae) is
VetBooks.ir The sclera consists predominantly of interwoven layers non-keratinised stratified squamous in type. It is continu-
of collagen fibres. Most fibres are oriented parallel to the ous with the conjunctiva at the limbus. The surface of the
surface of the bulb. In some regions, elastic fibres are inter-
corneal epithelium is protected by the thin precorneal tear
spersed among the collagen fibre bundles. The shape of film, a complex fluid layer consisting of mucous, aqueous
this fibro-elastic layer is determined by intraocular pres- and lipid components.
sure, and by tension exerted by the ocular muscles. The subepithelial basement membrane (Bowman’s
The space between the collagen fibres contains scant membrane in primates, humans and birds) lies deep to the
quantities of ground substance, solitary fibrocytes and, in anterior epithelium. It is composed of delicate fibrils and
ruminants, numerous pigmented cells. Few blood vessels are dense ground substance. The membrane serves as a barrier
present in the sclera. The thickness of the sclera gradually against the entry of water into the corneal stroma.
increases from the equator of the bulb towards the poste- The stroma (substantia propria corneae) consists
rior pole. At the sieve-like area cribrosa sclerae, the sclera is of perpendicularly oriented collagen fibre lamellae (ca.
perforated by vessels and by nerve fibres that form the optic 10%) and largely watery ground substance (ca. 90%). In
nerve. The posterior portion of the sclera is surrounded by a addition, the cornea contains insoluble collagen, glycosa-
loose network of collagen fibres (lamina episcleralis). minoglycans and ions. Present between the fibrous layers
Anteriorly, the sclera is covered by the conjunctiva. At are flattened fibroblasts and a dense network of non-
the corneoscleral junction, the sclera thickens (particularly myelinated sensory and autonomic nerve fibres, most of
in carnivores) and is associated internally with a ring of which extend into the epithelium. The stroma is avascular.
fibro-elastic tissue, the annulus sclerae. The plexus veno- Nutritional support is provided to the cornea by diffusion
sus sclerae, located adjacent to the annulus sclerae, drains from peripherally located arteries, the precorneal tear film
the aqueous humour and thus participates in regulation of and from the aqueous humour.
intraocular pressure. Corneal transparency is dependent upon optimal
hydration of the stroma (72–78% water) and mainte-
Species variation nance of the regular arrangement of collagen fibrils. These
Birds: Embedded within the connective tissue of the factors minimise the scattering of incident light. Excessive
sclera is a lamina of hyaline cartilage (lamina carti- hydration of the cornea results in gaps between collagen
laginea sclerae) and an osseous scleral ring (annulus fibrils and loss of corneal transparency. This occurs post-
ossicularis sclerae). The cartilaginous lamina (Figures mortem and as a consequence of corneal damage. The
16.18 and 16.19) reinforces the posterior wall of the surface layers of the cornea and the anterior and posterior
bulb. It may become ossified near the point of exit epithelia also play an important role in maintaining the
of the optic nerve, forming the horseshoe-shaped os optical clarity of the cornea.
nervi optici. The osseous scleral ring confers mechani- The posterior limiting lamina (Descemet’s mem-
cal stability upon the concave annular portion of the brane) lies internally adjacent to the corneal stroma. It
eyeball. It also serves as a buttress during accommo- consists of a thick basement membrane incorporating
dation which, in contrast to mammals, involves active hexagonally interwoven microfilaments. Descemet’s
compression of the lens. The scleral ring consists of membrane contains collagen type VIII, which is unusual
10–18 (usually 15) individual ossicles (ossicula sclerae) in other basement membranes.
that overlap in a manner resembling fish scales. The simple squamous posterior epithelium (epithe-
lium posterius corneae) lines the posterior aspect of the
CORNEA cornea. It also serves as the endothelial lining of the ante-
Towards the anterior pole of the bulb, the sclera is contin- rior chamber (Figure 16.12). This single-cell layer aids in
ued by the transparent cornea. The corneoscleral junction maintaining corneal transparency by actively removing
is termed the limbus corneae. The bulk of the cornea is water from the corneal stroma (ATPase and carbonic
formed by lamellae of parallel collagen fibres (substantia anhydrase pumps). It also produces proteins used in the
propria corneae) (see below). formation of Descemet’s membrane.
The cornea consists of five layers (Figures 16.15 and 16.16):
Species variation
· anterior epithelium (epithelium anterius), Birds: Except in water birds and several diurnal birds of
· subepithelial basement membrane, prey, the avian cornea is relatively thin. Compared with
· stroma (substantia propria corneae), mammals, its relative diameter is also small. In contrast,
· posterior limiting lamina (Descemet’s membrane, the radius of curvature exhibits considerable species
lamina limitans posterior) and variation. In water birds, the cornea is comparatively flat
· posterior epithelium (epithelium posterius). whereas in owls it is strongly curved with a correspond-

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