Manatee Histology
An Introduction to Mammalian Eyes and Associated Structures
Written by Dr. Don Samuelson and edited by Kelin Maciejewski
Throughout centuries of evolution, the eye has endured little change in both its design and development. The intensity and availability of light in an animal’s environment is a major factor in determining whether a vertebrate is diurnal, nocturnal, or arrhythmic.
Rods and cones are located inside of the eye’s retina. These two cells determine visual acuity either at night or during the day. Rods located in the retina allow animals to see at night, while cones allow animals to see during the day. The retinas of diurnal animals consist mainly of cones which allow the animal to see during the day and cause “poor” night vision. Diurnal animals include the domesticated pig, squirrels, many birds, the marmots, and non- human primates. There is also evidence that the manatee is diurnal due to lack of the tapetum “eye shine structure”. In addition, manatees see in dichromatic color ranges, allowing them to see in the blue and possibly yellow to green color range. Being dichromatic allows manatees to see with more color capability in the water which suggests that the manatee is most active during the day. Nocturnal animals possess “good” vision with dim amounts of light usually during dusk, twilight, and dawn. Some nocturnal mammals include bats, reptiles, rodents, ungulates, and carnivores. Lastly, arrhythmic animals are both active during the day and night. Domesticated animals are usually classified in this category. In addition, feeding factors (i.e. prey or predator) and visual activities have caused further evolution of the eye.
The principle means by which most animals are made aware of their surroundings and changes in these surroundings is the reflection or emission of light toward them by external objects and the reception of this light by special organs which we term photoreceptors. The more complicated of these photoreceptors are called eyes. Of course, our eyes do not see; the “seeing” is actually a function of higher centers located in the brain.
The well-worn comparison of the eye to a camera is helpful. The exterior of the camera, the “box”, is comparable to the fibrous tunic of the eye. The light sensitive component of a camera is the film; in the eye is the retina. The focusing lens in a camera is similar to the focusing power of the cornea and the crystalline lens. The eyelids act like a camera shutter, while the pupil is a diaphragm aperture. The heavily pigmented uvea is similar to the black interior of the camera which reduces light scatter.
The eye is composed of three basic layers or coats. The outer coat is the fibrous tunic and is further divided into the cornea and sclera. The fibrous tunic gives the eye a constant shape and form which is imperative for a functional visual system. In addition, the anterior portion of the fibrous tunic, the cornea, is transparent, enabling light to pass through, and shaped in a manner that makes it a powerful lens which refracts light rays centrally towards the visual axis of the eye.
The second and middle layer is the uvea. The uvea is further divided into the choroid, the ciliary body, and the iris. The choroid, located in the posterior one-half of the eye is found between the outer sclera and the retina centrally. The basic functions of the choroid are to provide nourishment to the highly metabolic retina and to modify internal light reflection and scatter, as it is either heavily pigmented or reflective.
The ciliary body is involved in both the production and outflow of aqueous humor, a fluid which flows through the anterior segment. Aqueous humor is secreted from ciliary body processes, heavily pigmented central extensions of the ciliary body. Aqueous leaves the eye through the iridocorneal angle, a portion of which is of ciliary body origin. The ciliary body and its processes provide a base on which lenticular zonules are attached. These zonules are fine fibrous bands which attach to the outer portions of the lens and hold it in place. Contractions of ciliary body muscle alter the tension of these zonules and are able to change the shape of the lens due to the inherent elasticity of the lens capsule. This alters the degree to which light is refracted, termed “accommodation”. Thus, the lens acts as a fine focusing mechanism, while the cornea serves as the most powerful fixed “lens” of the visual system.
The most anterior portion of the vascular tunic, the iris, extends form the ciliary body centrally just anterior to the surface of the lens. The iris is heavily pigmented and contains muscles which change the shape of the iris and the central “hole” within the iris, the pupil. In this manner, the iris is able to control the amount of light that enters the posterior segment to stimulate the retina.
The third and most central layer is the nervous coat which is made up of retina and associated optic nerve. Briefly, the retina contains light sensitive cells (photoreceptors) which, after a series of intermediate modifying processes transmit impulses to the brain via the optic nerve.
Two additional ocular systems previously mentioned are:
1) the intraocular fluids; and
2) the crystalline lens.
The intraocular fluids include the vitreous and aqueous humors. The fibrous tunic is normally kept distended to the portion of rigidity by the pressure of fluid secreted within the eye. This fluid, the aqueous humor, is continuously produced by ciliary body processes at a slow rate and drains out of the eye into the bloodstream by a complex arrangement which is so regulated that intraocular pressure normally remains around 18 to 30 mmHg. Overproduction of aqueous humor or reduction of aqueous outflow will result in increasing intraocular pressure (IOP). This condition is known as glaucoma and, if unrelieved, results in damage to the optic nerve and a permanent loss of vision. Aqueous humor provides vital nutrients to the avascular lens and cornea and also assists in removing metabolic waste products.
The greater portion of the intraocular fluids, occupying the large chamber in the back of the eye, is rendered gelatinous by the addition of proteins secreted during development of the retina. The mass of jelled fluid aqueous is called the vitreous humor. It is relatively permanent and is fixed in the volume in the adult eye. The vitreous humor helps support and distends the globe and also provides an optically clear medium through which light may pass essentially unaltered.
There is no compelling reason the material which fills the intraocular spaces should be of two consistencies- liquid anteriorly and semisolid posteriorly. However, in the evolutionary predecessors of the fishes, the material near the cornea was fluid so that the lens could be moved in accommodating the focus of the eye to different distances, and the lens would have dropped back into the globe if there was only liquid behind it. In higher vertebrates, the lens is not changed in position but slightly altered in shape, and is held firmly in place by lenticular zonules which the lower fishes lacked; but the differentiation in consistency of the intraocular media has persisted.
The crystalline lens is a transparent, avascular, nonpigmented, flattened spheroidal structure lying behind the iris and held in place by lenticular zonules, as previously mentioned. The lens of the visual system, the crystalline lens, may be thought of as a fine tuning lens which changes focus to enable a clear visual image at a variety of distances. The inherent hardness of the lens of most domestic animals and the lack of a highly developed ciliary body musculature results in a weak power of accommodation in most animals.
Image Courtesy of Jennifer McGee