Anatomy of vertebrates’ eyeball :
The eye of vertebrates is formed by three layers, or tunics. Each of
these three layers contributes with parts that have structural / nutritive
functions and parts that form the optic and photoreceptive apparatus of
the eye. From the outside to the inside of the eyeball the three tunics are
the:1. Fibrous tunic, which forms a capsule enclosing and protecting the
other components of the eye. It is subdivided into the sclera, with
primarily structural functions, and the cornea, which is part of the
optic apparatus.
2. Vascular tunic, which forms the choroid, ciliary body and iris. This
tunic is also called the uveal tract. The choroid has primarily nutritive
functions. The ciliary body generates the aqueous humor of the eye,
but the ciliary muscle also functions in the optic apparatus. The iris is
part of the optic apparatus in which it functions a contractile
diaphragm, i.e. the aperture of the eye.
3. Neural tunic consists of the retina. It includes two portions;
The neural retina is the photoreceptive, imaging processing tissue.
The pigmented epithelium lies behind the neural retina, it also
extends forward to line the hidden side of the iris.
The retina proper forms the photoreceptive layer of the eye. As a
double-layered epithelium, the retina also covers the ciliary process and
the posterior surface of the iris, where it has both nutritive and structural
functions.
 The lens is a specialized epithelial structure, suspended behind the
pupil.
 The anterior chamber, a space filled with aqueous humor, lies between
the iris and the cornea.
 The posterior chamber lies behind the iris.
Histological criteria of vertebrates’ retina :
The retina is a thin layer of neural cells that lines the back of the
eyeball of vertebrates. In vertebrate embryonic development, the retina
and the optic nerve originate as outgrowths of the developing brain.
Hence, the retina is part of the central nervous system .
The retina is a complex, layered structure with several layers of
neurons interconnected by synapses. The only neurons that are directly
sensitive to light are the photoreceptor cells. These are mainly of two
types: the rods and cones. Rods function mainly in dim light and provide
black-and-white vision, while cones support daytime vision and the
perception of color. A third, much rarer type of photoreceptor, the
photosensitive ganglion cell, is important for reflexive responses to bright
daylight.
The vertebrate retina has ten distinct layers (figure 1). From
innermost to outermost, they include:
1. Inner limiting membrane - Müller cell footplates.
2. Nerve fiber layer.
3. Ganglion cell layer - Layer that contains nuclei of ganglion cells and
gives rise to optic nerve fibers.
4. Inner plexiform layer.
5. Inner nuclear layer contains bipolar cells.
6. Outer plexiform layer - In the macular region, this is known as the
Fiber layer of Henle.
7. Outer nuclear layer.
8. External limiting membrane - Layer that separates the inner segment
portions of the photoreceptors from their cell nuclei.
9. Photoreceptor layer - Rods / Cones.
10. Retinal pigment epithelium.
1
2
3
4
5
6
7
8
9
10
Figure (1): Vertebrates' retina under L.M. (400X) shows the ten histological layers.
Of these the four main layers of the ten, from outside in: pigment
epithelium, the photoreceptor layer, bipolar cells, and finally, the
ganglion cell layer. Additional structures, not directly associated with
vision, are found as outgrowths of the retina in some vertebrate groups. In
birds, the pecten is a vascular structure of complex shape that projects
from the retina into the vitreous humor; it supplies oxygen and nutrients
to the eye, and may also aid in vision. Reptiles have a similar, but much
simpler, structure, referred to as the papillary cone .
The complexity of the retina becomes simplified by appreciation of
the fact that the retina is only three neurons deep. The first neuron, the
photoreceptors (i.e. rods and cones), accounts for the outer layers, with
each rod and cone having a sensory end organ (layer 9) lying outermost
against pigment epithelium (layer 10), a nucleus (in layer 7), and an inner
terminal fiber passing to the outer plexiform layer (6). Layer 8, the
external limiting membrane, is formed by processes of supporting
(Müller) cells. The outer plexiform layer (6) is the site of junction
(synapse) between the first and second neurons. The intermediate, or
second, neurons have nuclei in the inner nuclear layer (5), together with
nuclei of other cells (association and supporting), and, in turn, the inner
plexiform layer (4) is the site of junction between the second and third
neurons. Cell bodies of these third neurons, the ganglion cells, lie in layer
(3) together with neuroglial, and their central processes (axons) pass to
layer (2) as optic nerve fibers, all of which pass to the optic papilla and
hence to the optic nerve. These fibers are unmyelinated but acquire a
myelin sheath as they exit through the cribriform plate at the optic
papilla. The internal limiting membrane (layer 1) separates the retina
from the vitreous body. It should be recognized that the retina is gray
matter of the central nervous system, while the optic nerve is white
matter.
In the center of the retina is the optic disc, a circular to oval white
area measuring about 2 x 1.5 mm across. From the center of the optic
nerve radiate the major blood vessels of the retina. Approximately 17
degrees (4.5-5 mm), or two and half disc diameters lateral to the disc, can
be seen the slightly oval-shaped, blood vessel-free reddish spot, the
fovea, which is at the center of the area known as the macula by
ophthalmologists.
A circular field of approximately 6 mm around the fovea is
considered the central retina while beyond this is peripheral retina
stretching to the ora serrata, 21 mm from the center of the optic disc. The
total retina is a circular disc of approximately 42 mm diameter.
The retina is approximately 0.5 mm thick and lines the back of the
eye. The optic nerve contains the ganglion cell axons running to the brain
and, additionally, incoming blood vessels that open into the retina to
vascularize the retinal layers and neurons). A radial section of a portion
of the retina reveals that the ganglion cells (the output neurons of the
retina) lie innermost in the retina closest to the lens and front of the eye,
and the photosensors (the rods and cones) lie outermost in the retina
against the pigment epithelium and choroid. Light must, therefore, travel
through the thickness of the retina before striking and activating the rods
and cones. Subsequently the absorption of photons by the visual pigment
of the photoreceptors is translated into first a biochemical message and
then an electrical message that can stimulate all the succeeding neurons
of the retina. The retinal message concerning the photic input and some
preliminary organization of the visual image into several forms of
sensation are transmitted to the brain from the spiking discharge pattern
of the ganglion cells.