Morphological and Functional Organization of the Human Retina

The retina is the most internal of the three sheets that coat the posterior wall of the eye. This sheet extends from the optical nerve to the pupil-iris border, and it can be divided into an optical part, dedicated to the function of vision, and a blind part, in conjunction with the border of the pupil. The two parts are separated by the ora serrata. The retina is structurally divided into two shares: one outer and one inner.

The outer share of the retina consists of the retinal pigment epithelium (RPE), which lies on the basal membrane and the Bruch's membrane. The RPE with the basal membrane, Bruch's membrane, and the inner choroid form a single functional complex named the choroid/Bruch's membrane/basal membrane/RPE complex, or chorio-retina (Streiff and Faggioni, 1971). The integrity of this complex maintains the integrity of the retinal photoreceptors, and therefore the integrity of the whole retina.

The inner share of the retina or neurosensorial retina represents the real nervous part of the eye bulb. It is of encephalic derivation and, therefore, derives directly from the central nervous system. It consists of three consecutive neurons, identifiable as the rods and cones (first neurons), bipolar cells (second neurons), and the ganglion cells (third neurons). In reality, the retinal cells and their processes are assigned into 10 layers (see Figure 67.1-67.6):

• Retinal pigmented epithelium (RPE)

• Layer of the acromeres of the photoreceptors

• Outer limiting membrane (OLM), which is not a real membrane but a small zone with numerous zonulae adherens between the Muller cells and the photoreceptors

• Outer nuclear layer (ONL), formed by the bodies of the photoreceptor cells

• Outer plexiform layer (OPL), formed by the synapses between the photoreceptors, the bipolar cells, and the horizontal cells

• Inner nuclear layer (INL), which includes the cell bodies of five cell types: horizontal, bipolar, amacrin, interplexiform, and the Muller cells

• Inner plexiform layer (IPL), formed by the synapses between the bipolar cells, the amacrin cells, and the ganglion cells

• Ganglion cell layer (CGL), which also contains some amacrin cells

Figure 67.1 Light microscopic image of a normal human retina. The RPE is detached from the other retinal layers (it is not contained in this image on the left side). P = photoreceptors; B = bipolar cells; G = ganglion cells (magnification 1600x).

• Nervous fiber layer (NFL), which contains the axons of the ganglion cells

• Inner limiting membrane (ILM), formed by the terminal processes of the Muller cells

The neurosensorial retina, therefore, is formed by a large variety of cells described in the next sections.

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