The Pig Retina: Insights From Histological, Histochemical and Ultrastructural Studies

A detailed light and electron microscopic study of the retina was performed to provide comprehensive data on its different layers for the evaluation and conclusions concerning experimental observations, as pigs have been extensively used as experimental animals. The central retinas from 12 adult pigs (> 2 years) of local mixed breed were studied to explore the retina. The light and ultrastructural study revealed the complex laminar organisation and cellular composition of the retina. The retinal pigmented epithelium is the outermost layer and their nuclei are usually oval in shape, located basally, with their long axis parallel to the length of the retina. The outer and inner segments of photoreceptors showed an intense reaction to Alcian blue and colloidal iron for acidic mucopolysaccharides, while the inner segments of these cells showed a weak to moderate reaction to PAS. Outer segments of both rods and cones are covered by a plasma membrane, but during the renewal and shedding of discs, the plasma membrane loses its continuity towards the scleral side. The basal and vitreal parts of the outer segment were covered by cytoplasmic prolongations (calyces) of their inner segments. Mitochondria were numerous and densely packed in the ellipsoid of cones, whereas they were fewer and elongated in rods. A noteworthy finding in the present study is the presence of electron-dense rounded structures surrounded by mitochondria in the ellipsoid of cones. The outer limiting membrane was formed by the zona adherens between the cell membrane of the inner segment of the rods and cones and the fibres of the Müller cells, as well as between Müller cells. The axons of the rods terminated in dilated oval-shaped spherules, while cones formed pyramidal-shaped pedicles that formed synaptic complexes with bipolar and horizontal cells. The synaptic ribbon, an electron-dense linear structure and arciform density was observed within the rod spherules adjacent to the synaptic complex. The lateral surface of the spherules was in close contact with the same surface of the cones' pedicles, but no specific cell junctions were observed. The nuclei of bipolar cells were usually round and situated in the mid-portion of the inner nuclear layer, mostly containing electron-dense materials. The nuclear material of horizontal cells was usually euchromatic, with some electron-dense material also present. Few mitochondria, free ribosomes, smooth and rough endoplasmic reticulum and microtubules were observed in the perikaryon of the horizontal cells. A comprehensive light and ultrastructural examination of the pig retina revealed significant similarities to the human retina as reviewed, along with some new findings. The translation of scientific findings into actionable technologies for prevention and intervention requires suitable alternate models, and pigs may play an integral role by providing basic data for various researchers.