At the End of the Lecture the Student Will Able To

Retina

Learning Objectives

At the end of the lecture the student will able to:

Describe the layers of retina.

Discuss the cells of each layer.

Describe the cone and rods sturcture.

RETINA

•The innermost coat of the Eye, also known as neural tunic

•Contains neural portion, which contain Photo-receptors cells called RODS &CONES

•Retina develops from Optic cup

•Is formed by :

•Outer pigment layer : develops from outer wall of optic cup

•Neural portion of retina develops from inner layer of cup called RETINA PROPER

•Cells composing, the retina constitute the highly differentiated extension of Brain

RETINA

Optic disc located on posterior wall of orbit ,is the exit site of Optic nerve, because it contains no Photo-receptors cells therefore called BLIND SPOT

Approximately 2.5 mm lateral to optic disc is a yellow pigmented zone in the retinal called MACULA LUTEA yellow spot

Located in the centre of this spot is an oval depression Fovea centralis-visual acuity is greatest

Is an specialized area of Retina containing only cones, which are packed tightly as the other layer of Retina are pushed aside

RETINA

As the distance from Fovea number of cones decreases, whereas the number of Rods increases.

The Portion of retina that function as Photo-recepters, lines the inner layer surface of Choroid layer from optic disc to ora serrata & is composed of ten (10) distinct layers

From outside adjacent to Choroids ,to inside, where they are continuous with optic nerve

Retina

The layers are as follows

1.Pigment Epithelium layer

2 layer of Rods &Cones

3 Outer limiting membrane

4 Outer nuclear layer

5 Outer Plexiform layer

6.Inner nuclear layer

7 Inner Plexiform layer

8 Ganglionic cell layer

9 Optic nerve fibre layer

10.Inner limiting layer

Retina

PIGMENT EPITHELIUM

Derived fromouter layer of optic nerve

Composed of Cuboidal to Columnar layer (14um-10um long) whose nuclei are basally located

Cells rest on Basal membrane (Bruch’s membrane located between & pigment cells ,is thrown into numerous folds

Desmosomes ,zonula occludens & zonula adherentsare present on lateral cell membrane –forming blood –retina barrier

Gap Junctions on lateral cell membrane permits inter-cellular communications

The cell Apices exhibit microvilli a sleeve like structure ,which merely surround the Photoreceptors -----a jolt may result in detachment which is a common cause of Partial blindness

Cytoplasm of this epithelium contains many microtubules ,abundant Golgi bodies numerous apical cytoplasmic processes

Functions of Pigment Epithelium

These cells absorb light after it has passed through & stimulated the Photoreceptors, thus preventing reflections from Tunics, which would impair Focus

Play an active role in vision by esterifying Vitamin A derivatives in their smooth endoplasmic reticulum

Nutrition of Photoreceptors, thus ensuring the normal turn over of these Lamellae

Layers of RODS & CONES

The optical portion of retina houses two distinct type of photo receptors cells RODS & CONES.

They are polarized cells whose apical portion known as Outer segment are specialized dendrite

The outer segment of both Rods & cones are surrounded

By pigmented epithelial cells & connected by a neck, contain modified cilium, abundant mitochondria, SER, RER, Golgi complex

The bases of Rods & Cones cells form synapses with indulging cells of Bipolar layer

It is estimated that they are approximately 10-120million rods & six million cones.

Outer segment of rods, its dendrite end present several hundred flattened membranous lamallae or sac oriented perpendicular to its long axis.

Each lamella represented an invagination of plasma lemma which is detached from cell surface, thus forming a disc

Each disc is composed two membranes seperated from each other by an an 8um

The membrane contains Rodopsin (visual purple) –a light sensitive pigment

New lamallae constantly formed & added to outer segment of Rod cells,the oldest disc at their tips are phagocytised & by cells of pigment epithelium

Inner segment of Rods

Has adiameter of 2um in diameter ,is highly longer 32um

Protein synthesis in inner part pass through neck for incorporation into new membranous sac

Photorecepter of Rods begin with absorption by Rhodopsin ,ehich comprises the trans-membranes protein

Absorption of light causes isomerization of retinal moiety which then dissociate from opsin ,t

CONES

They are activated in bright light and produce greater visual acuity than do Rods

Are elongated cells (60umx1.5um) & their structure is similar to that of Rods with few exceptions

Their apical terminal (outer segment) is shaped more like a cone than a rod

The discs of cons, although composed of lamellae of the plasma lemma, are attached to plasma membrane, unlike the lamellae of rods, which are separated from plasma membrane

Protein produced in inner segment of cones migrates diffusely throughout the entire outer segment, whereas in rods, the protein is concentrated in distal region of outer segment

Unlike rods, cones are sensitive to light

There are three types of cones, each containing a different variety of photo pigment iodopsin

Each variety of iodopsin has a maximal sensitivity to bright light and colors, so they provide the greatest visual acuity.

External Limiting Membrane

Is not a membrane

E/M have revealed thin layer to be a row of zonule adherent between Muller cells & photoreceptors

Distal to this microvilli of Muller cells project into the interstices between inner segment of rods & cones

Outer Nuclear Layer

Consists of zones mainly occupied by nuclei of rods & cones

The nuclei of rods are smaller, more rounded & more darkly stained than cone nuclei

Outer Plexiform Layer

Axodendrite synapse between photoreceptors cells and other neuronal cells- including bipolar, amacrine, horizontal & Muller cells are located in outer plexiform layer

The nuclei of bipolar, amacrine, horizontal & Muller cells compose of inner nuclear layer

Inner Plexiform layer

The process of amacrine, bipolar, and ganglion cells are intermingled in the inner plexiform layer. Axodendrite synapse between axons of Bipolar cells & dendrites of ganglion cells also are located here.

Ganglion Cell Layer

Cell bodies of multipolar neurons of ganglion cells layer

Axons of these neurons pass to nerve fibre layer

Hyper polarization of rods & cones activates the ganglion cells which then generate an action potential than is passed to amacrine & horizontal cells & eventually to the brain via visual relay system

Optic Nerve Fibre Layer

Optic nerve fibres are formed of unmylinated axons of ganglion cells.

A myelin sheath is added as the nerve pierces the Sclera

Inner Limiting Membrane

Basal lamina of Muller cells comprise the inner limiting membrane.