Light and electron microscopic studies on the retina of the booted eagle (Aquila pennata)

Abstract

The retinal structure of the booted eagle (Aquila pennata) was investigated using light and electron microscopy. Particular attention is paid to the main ultrastructural features of the receptor cells. This study reveals six distinct varieties of cones. Unequal double cones differ in shape, structure, and length and are comprised by principal long and accessory short members. Principal member contains a green oil droplet and accessory member contains a paraboloid and a pale green droplet. Four types of single cones are distinguished on the basis of their morphology and oil droplets: red, green, blue, and ultraviolet. Cones outnumber rods in all regions. Two types of horizontal cells and several morphological types of amacrine cells are abundant. A large number of bipolar cells are divided into long longitudinal rows by Müller cell processes, a prominent feature of this retina. These processes extend through the external limiting membrane to reach the ellipsoid region of the cones. Moreover, thick processes divide the inner nuclear and plexiform layers and surround the myelinated ganglion cell axons at fairly regular intervals. In the ganglion cell layer and optic nerve fibre layer, abundant oligodendrocytes are present, close to the myelinated axons. The morphological characteristics of this retina indicate that A. pennata have good colour discrimination, a complex visual processing to mediate contrast and motion and an elevated acuity in areas of high cell densities.