The eyeball is a globe-like structure constituting the organ of sight. The inside of the eyeball can be divided into two segments: the anterior segment, which includes the cornea, iris, ciliary body, lens, anterior chamber, and posterior chamber, and the posterior segment, which includes the vitreous body, retina, choroid, sclera, optic nerve head. The anterior chamber is located between the cornea and the iris1,2. The iris is a circular structure with an opening in the center called the pupil. At the junction of the cornea and the iris, there is the drainage angle, where a specialized system of trabeculae drains the aqueous humor from the eyeball to the venous system. The posterior chamber is bound by the iris, the ciliary body, and the lens, as well as the suspensory ligaments that hold the lens in place. Behind the lens, the vitreous body is located, which is the largest structure of the eyeball. The vitreous body adheres to the retina and stabilizes its position. The retina is surrounded by the choroid and the sclera1. The anatomy of the eyeball is summarized in Figure 1.
The eyeball wall constitutes three layers. The outermost layer is the sclera, which protects the eyeball from injury and maintains its shape. At the anterior pole of the eyeball, the sclera turns into a transparent cornea. Under the sclera, there is a vascular structure called the uvea, whose primary function is to nourish the structures of the eye. The uvea forms the choroid, and within the anterior pole, the ciliary body and the iris1. The innermost layer is the retina, which is a highly specialized structure of the eye, made up of neurons, glial cells, and pigment epithelial cells. The retinal neurons include the photoreceptor cells (rods and cones), horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells (RGCs). These cells are organized into complex layers, and their role is to receive and process light stimuli2,3. The glial cells of the retina comprise the Müller cells, astrocytes, and microglia, which are present in all layers of the retina. The many functions of glial cells include the nutrition of neurons, support of the blood-retina barrier, structural support of neurons to maintain the layered structure of the retina, regulation of neurons' metabolism, secretion of biologically active proteins regulating functioning, growth, and survival of neurons, and regulating the immunological processes within the retina4. The pigment epithelial cells make up the outermost layer of the retina and act as a barrier between the choroid and photoreceptors. These cells regulate the bidirectional transport of waste products and nutrients and also protect the retina from excessive light-induced damage by absorbing light energy and neutralizing light-generated reactive oxygen species5.
The retina can be divided into ten layers: (i) the retinal pigment epithelium, (ii) the photoreceptor segment layer (rods and cones), (iii) external limiting membrane (ELM), (iv) outer nuclear layer (ONL), (v) outer plexiform layer (OPL), (vi) inner nuclear layer (INL), (vii) inner plexiform layer (IPL), (viii) ganglion cell layer (GCL), (ix) retinal nerve fiber layer (RNFL), and (x) internal limiting membrane (ILM)2. The nucleated layers of the retina include the ONL, INL, and GCL, while the layers with synaptic connections between cells include the OPL and IPL6. The nuclei of rods and cones form the ONL, and their axons extend into the OPL, where they connect to the dendrites of bipolar and horizontal cells. Within the INL, the horizontal, bipolar, and amacrine cell nuclei are located. Within the IPL, axon terminals of bipolar and amacrine cells synapse with dendrites of RGCs. Within the GCL, the RGCs make up most of the cell bodies, but also some dislocated amacrine cells can be found there. The axons of RGCs form the RNFL and, further - the optic nerve7,8,9.
Many ophthalmic diseases, such as neurodegenerative disorders of the retina, lead to irreversible and incurable blindness10. Vision is considered one of the most important senses11, and permanent blindness significantly reduces the patient's quality of life. To further the understanding of many diseases' pathomechanisms, preclinical research using cell cultures or animal models is broadly performed. Preclinical studies using experimental animals provide an opportunity to assess the pathomechanisms underlying eye diseases and to develop new therapeutic strategies. Eye diseases can be modeled in both large animals (monkeys, cows, dogs, and cats) and small animals (rabbits, rats, mice, and zebrafish). The use of larger animals allows better access to the eye due to its size. Experiments performed with rodents, on the other hand, due to their relatively rapid reproduction, allow the breeding of inbred strains characterized by susceptibility to certain diseases12,13.
In animal models, enucleation is possible, which enables performing a detailed histopathological examination of the eyeball, with the assessment of minor pathologies that appear in particular structures of the eye during the development of the disease - an examination that can be very rarely performed among humans. Histopathological assessment is an extremely valuable tool while researching the pathomechanisms of ocular disorders. In the published literature, the descriptions of the methodology for histological examination of eyeballs are very limited, and step-by-step guides are lacking, which makes it difficult for beginners to recreate. This study aims to provide a simple and concise method of preparing histological slides and assessing the rat and mouse retina.