Selection of Appropriate Animal Models in Ocular Research: Ocular Anatomy and Physiology of Common Animal Models - Ocular Pharmacology and Toxicology - page 14

Table 4
Average blink rates of normal eyes (modified from Venzina) ( 9 )

Animal

Non-
human
primate

Guinea
pig ( 95 )

Mice Rat Rabbit Cat Dog Pig

Blink
rates

1 per
5 min

1 per
5 min

1 per 10
to 1 per
4 min

1 per
6 min

1 per
18 s

1 per
10-20 s

1 per
20-30 s

1 per 6 s

The normal blink rate of animals varies tremendously also (Table 4 )
[ 9 ]. Knowledge of the normal blink rate is important when accessing
ocular irritation of drugs where an increased blink rate, especially if
prolonged, is a parameter suggesting increased irritation.

Components of tear film are quite varied and depend on whether or
not the eye is inflamed and if the animal has a harderian gland. In
general, the tear film is composed of lipids (usually secreted by
eyelid meibomian glands), mucins (usually secreted by conjunctival
goblet cells and in some animals the orbital lacrimal gland), and an
aqueous component (secreted mostly by the lacrimal gland and
accessory glands in the conjunctiva). Table 5 lists the common
tear film components in the normal eye of the various animal
models. In a recent study evaluating the meibomian gland lipids
of rabbits, dogs, and mice, it was determined that mouse and dog
meibomian gland lipids were closer biochemically to humans than
the rabbit [ 8 ].

3.2.3 Tear Film

The cornea of most laboratory animals consists of the corneal
epithelium externally, the corneal stroma, Descemet's membrane,
and corneal endothelial cells [ 10 , 11 ]. Descemet's membrane is the
basement membrane for the endothelial cells and becomes thicker
with age as it is continuously produced. The corneal stroma consists
primarily of collagen fibrils, keratocytes, nerves, and glycosamino-
glycans. Corneal collagen fibrils exist in broad belts called lamellae
that run approximately parallel to the corneal surface. Corneal
thickness, as measured by ultrasonic pachymetry, confocal micros-
copy, or OCT, and endothelial cell counts are listed in Table 6 . The
peripheral cornea is thicker on average than the central cornea and
corneal thickness increases gradually with age [ 12 , 13 ].

3.2.4 Cornea

The components of aqueous humor also vary among animal species
and differ greatly if there is active inflation. Table 7 lists the com-
mon tear film components in the normal eye of the various animal
models.

3.2.5 Anterior Chamber/
Aqueous Humor