Retina: Structure and 10 Layers in Histology

General characteristics of the retina

The retina is the inner photosensitive tunic of the eye. The ora serrata (ora serrata) divides it into two parts: the optic part (posterior, optically active) and the blind part (covering the ciliary body and the posterior surface of the iris).

On the posterior surface, two functionally significant zones are distinguished: the optic disc (blind spot)—the exit site of the optic nerve, devoid of photoreceptors; and the macula (fovea), with the maximum concentration of cones, providing the highest visual acuity.

The retina is an example of an inverted organ: photoreceptors are located in the outer layers, so light passes through the entire thickness of the retina before reaching the rods and cones. Studying the retina is important not only for ophthalmologists: it is often called a "nervous system extended to the periphery," and the condition of the optic disc can be used to assess, for example, brain edema.

Cellular composition of the retina

The retina includes the following cell types:

• Neurosensory (photoreceptor) cells—rods and cones;
• Bipolar (associative) cells;
• Ganglion cells;
• Horizontal and amacrine cells;
• Neuroglia—astrocytes and Müller cells;
• Pigment epithelium (of neural origin).

Characteristics of the main cells

Neurosensory cells are elongated bipolar neurons. Their peripheral processes (rods and cones) consist of outer and inner segments connected by a cilium and form the photosensory layer.

Bipolar cells contact photoreceptor axons with their dendrites and transmit the signal to ganglion and amacrine cells via their axons. At this level, the principle of convergence is implemented: several rods converge on a single bipolar cell (approximately 3:1), meaning the number of optic nerve fibers is significantly smaller than the number of photoreceptors.

Ganglion cells receive signals from bipolar neurons; their axons gather in the optic disc region to form the nervus opticus.

Horizontal and amacrine cells are located at various levels of the retina and regulate image contrast by inhibiting or enhancing the activity of neighboring neurons.

Neuroglia is represented by astrocytes, which form the blood-retinal barrier, and large process-bearing Müller cells, which permeate almost the entire thickness of the retina and form the outer and inner limiting glial membranes.

Pigment epithelium is the outermost layer. The cell bodies and their processes—in which rods and cones are embedded—contain melanosomes. In bright light, melanin granules migrate into the processes, absorbing part of the spectrum: sensitivity decreases while resolving power increases. In the dark, melanin withdraws into the cell body—sensitivity rises, but clarity drops.

Ten layers of the retina

The layers are listed from outside to inside (outer—facing the choroid, inner—facing the vitreous body):

1. Pigmented layer—pigment epithelium;
2. Layer of rods and cones—outer segments of photoreceptors;
3. Outer limiting glial membrane—formed by Müller cells;
4. Outer nuclear layer—nuclei of neurosensory cells;
5. Outer plexiform layer—photoreceptor axons and contacts with bipolar cells;
6. Inner nuclear layer—nuclei of bipolar, horizontal, amacrine, and Müller cells;
7. Inner plexiform layer—processes of bipolar cells and their synapses with ganglion cells;
8. Ganglion cell layer—bodies of ganglion cells;
9. Nerve fiber layer—axons of ganglion cells forming the optic nerve;
10. Inner limiting membrane — separates the retina from the vitreous body.

Logic of memorizing the layers

To avoid rote memorization of the list, it is convenient to rely on the principle of alternation of nuclei and processes of the three neuron types: photoreceptors, bipolar cells, and ganglion cells.

• Externally lies the pigmented layer, adjacent to it the layer of rods and cones and the outer glial membrane—this is the "support block" formed by Müller cells and the pigment epithelium;
• Next, the "nuclei - processes" pair repeats three times: photoreceptor nuclei → their axons (outer plexiform layer); bipolar nuclei → their axons (inner plexiform layer); ganglion cell bodies → their axons (nerve fiber layer);
• The structure is completed by the inner limiting glial membrane, formed by the endfeet of Müller cells.

This pattern—two "glial" frameworks and three pairs of "nuclei + processes"—allows you to logically reconstruct all ten layers.

Key points

• The retina is divided into optic and blind parts separated by the ora serrata;
• The organ is inverted: light passes through the entire thickness to reach the photoreceptors;
• It includes photoreceptor, bipolar, ganglion, horizontal, and amacrine cells, neuroglia, and pigment epithelium;
• The principle of convergence reduces the number of optic nerve fibers relative to the number of rods and cones;
• Müller cells form the outer and inner limiting glial membranes;
• The pigment epithelium regulates light flux by moving melanin granules;
• The 10 layers are easy to memorize through the alternation of nuclear and plexiform layers of the three neurons;
• Ganglion cell axons form the nervus opticus in the region of the optic disc.