Selection of Appropriate Animal Models in Ocular Research: Ocular Anatomy and Physiology of Common Animal Models

An eye on the dog as the scientist's best friend for translational research in ophthalmology: Focus on the ocular surface

“…Four types of lacrimal glands can be distinguished in mammals: (i) the orbital lacrimal gland (glandulae lacrimales superior), located in the dorsolateral orbit just caudal to the orbital rim, with secretory ducts that open into the upper conjunctival fornix (humans, dogs, rabbits); (ii) the gland of the third eyelid, located in the ventromedial orbit at the base of the nictitating membrane with secretory ducts that open into the nictitans' bulbar conjunctiva (dogs); (iii) the infraorbital gland (glandulae lacrimales inferior), located either intraorbital and ventromedial to the globe (rabbits) or extraorbital and caudal to the globe (rodents), with a single secretory duct that opens into the lower conjunctival fornix; and (iv) the Harderian gland, or Harder's gland, extending from the base of the third eyelid into the caudal orbit, with secretory ducts opening at the nictitating membrane (rabbits and rodents). 11,13 The histomorphology of lacrimal glands varies with age and sex of the individual. 87 In dogs, an orbital lacrimal gland and gland of the third eyelid contribute to 60%-70% and 30%-40% of the overall tear secretion, respectively.…”

“…15 However, the Harderian gland present in rabbits and rodents is a unique anatomical feature that has important repercussions for comparative studies; in fact, the gland's lipid secretions in the tear film have profound effects on the ocular surface physiology (e.g., tear composition, tear film dynamics, blink rate) and pharmacology of topically applied medications (see Sections 2.2 and 2.3). Lacrimal gland, Third eyelid gland 11,12 Lacrimal gland, Third eyelid gland, Infraorbital (intraorbital), Accessory glands of Wolfring 11,[13][14][15] Infraorbital (extraorbital) 11,13,14 Infraorbital (extraorbital) 11,13,14 Lacrimal glands volume (cm 3 Absent 11,19 Present 11,14 Present 11,14 Present 11,14 Nasolacrimal drainage apparatus Two puncta/canaliculi, no flexure 20,21 Two puncta/ canaliculi, 1 dorsal flexure [19][20][21] 18.9-34.1 26,27 10-16 22,23,28 3.7-5 b,29 90-137 55,60,62 159-170 60,62 Corneal epithelial thickness (µm)…”

“…16.2-33.1 71,72 1.3 71 1.1 73 25-40 74 6 1.7-3.0 84,85 1.1-1.8 24,85 0.0053 11,24 0.013-0.054 11,24 Abbreviation: The morphology of the canine lacrimal drainage system is remarkably similar to that of humans, except for a longer nasolacrimal duct (notably in long-nosed dogs), and the presence of accessory duct openings into the nasal cavity. 92 In both species, tear drainage begins with the lower and upper nasolacrimal puncta and canaliculi in the medial canthus, joining into a lacrimal sac in the bony lacrimal fossa, and extending into the nasolacrimal duct that runs through an osseous channel towards the nasal cavity.…”

“…The counterpart in humans is the plica semilunaris, a vestigial remnant in the form of a crescent-like conjunctival fold in the medial canthus. 11,40 Despite gross differences, both structures have important physio-morphological similarities such as the presence of goblet cells and lymphoid follicles, contributing to the lubrication and immune protection of the ocular surface. 40 Nonetheless, the presence of a third eyelid should be considered in comparative studies as it could impact ocular examinations (e.g., third eyelid protrusion from ocular irritation) or ocular drug delivery (e.g., altered retention time of a contact lens), 96 among others.…”

“…The anatomy of the cornea is unique to each species with important differences in corneal dimensions and ultrastructural features (e.g., thickness, collagen arrangement, nerve supply). 11,22,24,37,60,65,104,105 First, the cornea is generally larger in dogs and rabbits compared to humans, while the dimensions are much smaller in mice and rats. 23,[51][52][53][54][55][56][57][58] As such, the relative amount of cornea and conjunctiva exposed on the ocular surface varies among species, an anatomical fact that has important implications in ocular pharmacology and other research fields; for instance, the surface area ratio of conjunctiva to cornea is two times smaller in rabbits (8.6-8.9) than humans (17.2), a finding that could largely explain species differences in drug penetration into the anterior chamber.…”