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 Photoreceptors 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 from outer 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.