Avaliação descritiva ultrassonográfica e histológica de olhos de Baleias-jubarte (Megaptera novaeangliae).

Abstract

The anatomy of the cetacean eye differs from terrestrial mammals and has specific adaptations for the aquatic environment, such as the ability to adjust to the properties and challenges of the aquatic environment. They have remarkable adaptations, such as a thicker sclera and cornea, high vascularization, and well-developed extraocular muscles. The veterinary ophthalmology of marine mammals is an emerging and little-explored area, with studies concentrated on pinnipeds and odontocetes and few studies on free living mysticetes. The aim of this study was to analyze and describe the anatomical structures of the eyes of Humpback whales (Megaptera novaeangliae) and possible pathological processes, relating the dimensions of the ophthalmic structures to the age group, sex and total length of the animal, using ultrasound and histology. To this end, 21 eyes were used, from 19 individuals that stranded alive and died, between the municipalities of Linhares/ES and Salvador/BA, between 2001 and 2019. Seventeen (17) individuals were classified as code 2 (fresh carcasses, with no signs of decomposition) and two individuals as code 3 (carcasses in a moderate stage of decomposition), with a predominant age group of young (n=15), followed by juveniles (n=3) and a single adult specimen. About sex, there were 12 males and five females among the individuals with a determined sex (n=17). The ultrasound findings show that the cornea of these animals is thinner in the center of the eye and thicker in the periphery, with an oval-shaped lens. Part of the animals (n=14) showed an intraocular filamentary structure, which originates near the optic nerve and extends to the lens. Among the alterations found ultrasonographically, there was a prevalence of retinal detachment (n=13) and corneal alterations (n=10), with irregularities and thickening and, histologically, a predominance of circulatory alterations (n=6), with the hypothesis that the circulatory alterations found are due to systemic alterations that occurred during the stranding of these animals. Statistically, a positive correlation was found between the size of the animal and some variables, such as eyeball width (r=0.021), lens thickness (r=0.010) and lens diameter (r=0.020). Significant differences in means were found when comparing the age 23 range of the animals, between calves and juveniles, in the variables eyeball width (ρ=0.019), corneal thickness in the center (ρ=0.012), lens diameter (ρ=0.018) and lens thickness (ρ=0.028). There were no differences in the variables analyzed (eyeball width, corneal thickness at the center and extremities, corneal thickness at the center and extremities, scleral cartilage thickness at both extremities, anterior chamber thickness, lens thickness and diameter, vitreous, axial eyeball length, posterior chamber thickness, ciliary body thickness and optic nerve diameter) in relation to the sex of the individual. Histologically, the cornea is divided into five layers, with the epithelial layer divided into 13 layers of cells, separated by Bowman's membrane, and the iris has a poorly developed dilator muscle and a more developed sphincter muscle. The retina is made up of ten photosensitive layers, with the presence of cones and rods and has a structure made up of loose connective tissue, interspersed with blood vessels, which originates in the region of the optic nerve and extends throughout the vitreous chamber, up to the lens and has an ophthalmic network that forms a tangle of arterial and venous vessels, dispersed in abundant connective tissue and the ciliary muscle is vestigial. The intraocular structure, histologically, originates in the central region of the optic nerve and extends throughout the vitreous chamber, until it attaches to the lens and is formed by loose connective tissue, interspersed with blood vessels. Due to the size of humpback whale eyes, the study showed that linear transducers with a range between 7 and 12 MHz and convex transducers with a range between 2 and 5.5 MHz were more suitable for visualizing the structures. The sonographic and histological findings corroborate those described in the literature for other mysticetes in terms of the morphology of most of the structures. The filamentous structure found has not been described in any other cetacean species and its function remains unknown, but it may be a lens support tissue, since it appears to stretch and retract, or it may be a hyperplastic persistence of the primitive vitreous, a congenital anomaly reported in other animal species, although in this study there was a high prevalence of this structure. Ophthalmology in marine animals is still little explored, especially when it comes to mysticetes, and new studies are needed to clarify the functions of ocular structures, generating information that can help veterinarians during the stranding of large cetaceans and generate knowledge for conservation actions.