ANA 305 Histology Of Special Sense Organs

Question 1

Discuss the organs of special sense

Answer 1

The organs of special sense are sophisticated sensory structures in which the specific neural receptors are incorporated in a non-neural structure which enhances and refines the reception of incoming stimuli.

• Information about the external world is conveyed to the central nervous system from sensory receptors.

Question 2

What are PHOTORECEPTORS?

Answer 2

• They are the sensors of the visual system that convert the capture of photons (light) into nerve signals in a process called phototransduction.

Question 3

Describe the eye and it’s function

Answer 3

The eye is a complex and highly
developed photosensitive organ.

Internally, the eye contains transparent
tissues that refract light to focus the image via a layer of photosensitive cells, and a system of neurons that collect, process, and transmit visual information to the brain.

The eyes are located in protective areas of the skull known as the orbits

In humans, the eye is recessed in a bony orbit and is connected to the brain by the optic nerve.

The eyeball protects and facilitates the function of the photoreceptive retina, the inner layer of the eyeball.

Question 4

Describe the chambers of the eye

Answer 4

Anterior chamber : which occupies the space between the cornea and the anterior surface of iris.

• Posterior chamber: occupies space between the posterior surface of iris, ciliary processes, zonular attachments and lens

• The vitreous chamber/cavity lies behind the lens and its zonular attachments and is surrounded by the retina. It is the largest of the three compartment.

Question 5

Tunics/ layers of the eye and what they consist of:

Answer 5

1. a tough external fibrous layer consisting of the sclera and the transparent cornea joined at the limbus
2. A middle vascular layer consisting of the choroid, ciliary body and iris.
3. An inner sensory layer, the retina, which consists of an outer pigmented epithelium and an inner retina proper.

Question 6

The eye communicates with cerebrum via?

Answer 6

It communicates with the cerebrum through the posterior optic nerve.

Question 7

Describe the lens

Answer 7

lens is a perfectly transparent biconvex structure held in place by a circular system of zonular fibers that attach it to the ciliary body and by close apposition to the posterior vitreous body.

Question 8

Where are the iris and pupil located?

Answer 8

Partly covering the anterior surface of the lens is an opaque pigmented extension of the middle layer called the iris, which surrounds a central opening, the pupil

Question 9

Describe the aqueous humor

Answer 9

Located in the anterior portion of the eye, the iris and lens are bathed in clear aqueous humor that fills both the anterior chamber between the cornea and iris and the posterior chamber between the iris and lens.

• Aqueous humor flows through the pupil that connects these two chambers

Question 10

Location of the posterior vitreous chamber and vitreous body

Answer 10

The posterior vitreous chamber, surrounded by the retina, lies behind the lens and its zonular fibers and contains a large gelatinous mass of transparent connective tissue called the vitreous body

Question 11

Describe the sclera

Answer 11

It is the posterior five-sixth (part) of the fibrous layer

• The sclera is a 1.0- to 0.5-mm (0.5mm) thick layer of collagen (type 1) and elastic fibers produced by fibroblasts.

• The inner side of the sclera faces the choroid, from which it is separated by a layer of loose connective tissue and an elastic tissue network known as the suprachoroid lamina (with less collagen, more fibroblast, elastic fibers and melanocytes).

Tendons of the extraocular muscles (6 extrinsic M) which move the eyes insert into the anterior region (outer surface) of the sclera.

Question 12

Describe the cornea

Answer 12

• In contrast to the sclera, the anterior one-
sixth of the eye—is the cornea
• The cornea is 0.8-1.1 mm thick
• Colourless, transparent and avascular
• Rich in nerve endings.
• The anterior surface of the cornea is always kept wet with a film of tears retained by microvilli of the apical epithelial cells.

Question 13

A section of cornea is composed of five distinct layers which are:

Answer 13

• 1. The corneal epithelium (external stratified squamous epithelium)
• 2. The membrane of Bowman (anterior limiting membrane): which is the basement membrane of the external stratified epithelium
• 3. The thick stroma or substantia propria.
• 4. The membrane of Descemet ( posterior limiting membrane ); which is the basement membrane of the endothelium:
• 5. The corneal endothelium (inner simple squamous endothelium)

Question 14

Describe the corneal epithelium of the cornea

Answer 14

The corneal epithelium is stratified squamous and consists of five to seven layers of cells.
– Cells of the outer surface have microvilli and all cells are connected to one another by desmosomes.
– The cytoplasm contains cytokeratin associated with desmosomes.
• The epithelium of the cornea is very sensitive because of large number of free nerve endings and has a remarkable wound healing capacity.
• At the limbus, the corneoscleral junction, the corneal epithelium is continuous with that of the conjunctiva.

Question 15

Describe Bowman’s layer of the cornea

Answer 15

• 2. Bowman’s layer is 6 to 9 μm thick, consists of type I collagen fibrils, and lacks elastic fibers.
• This layer is transparent and does not have regenerative capacity.
• Bowman’s layer is the anteriormost part of the corneal stroma
• Bowman’s layer represents a protective barrier to trauma and bacterial invasion

Question 16

Describe the stroma of the cornea

Answer 16

3. The highly transparent stroma or substantia propria represents about 90% of the thickness of the cornea.
   • Bundles of types I and V collagen form thin layers regularly arranged in successive plane (highly resistant to deformations and trauma).
   • Fibers and layers are separated by an extracellular matrix rich in proteoglycans containing chondroitin and keratan sulfate.
   • Nerves in transit to the corneal epithelium are found in the corneal stroma

Question 17

Describe Descemet’s membrane of the cornea

Answer 17

4. Descemet’s membrane, one of the thickest basement membranes in the body (5 to 10 μm thick), is produced by the corneal endothelium and contains type VII collagen, which forms a hexagonal array of fibers

Question 18

Describe The corneal endothelium of the cornea

Answer 18

5.The corneal endothelium lines the posterior surface of Descemet’s membrane and faces the anterior chamber of the eye.

•It consists of a single layer of squamous epithelial cells, with impermeable intercellular spaces preventing influx of aqueous humor into the corneal stroma.

The structural and functional integrity of the corneal endothelium is vital to the maintenance of corneal transparency

• The cornea is one of the few organs that can be transplanted without a risk of being rejected by the host’s immune system.

• This success can be attributed to the lack of corneal blood and lymphatic vessels.

Question 19

Describe the choroid of the middle layer

Answer 19

Choroid
• The choroid is a highly vascular tunic in the posterior two-thirds of the eye, with loose, well-vascularized connective tissue rich in collagen and elastic fibers, fibroblasts, melanocytes, macrophages, lymphocytes, mast cells, and plasma cells.

• The abundant melanocytes give the layer its characteristic black color and block light from entering the eye except through the pupil.

Question 20

The choroid consists of three layers which are:

Answer 20

1. Bruch’s membrane
2. The choriocapillaris
3. The choroidal stroma

Question 21

Describe Bruch’s membrane

Answer 21

1. Bruch’s membrane, the innermost component of the choroid, consists of a network of collagen and elastic fibers and basal lamina material.
   • Basal laminae derive from the pigmented epithelium of the retina and the endothelia of the underlying fenestrated capillaries.

Question 22

Describe the choriocapillaris

Answer 22

2. The choriocapillaris contains fenestrated capillaries that supply oxygen and nutrients to the outer layers of the retina and the fovea.

Question 23

Describe the choroidal stroma

Answer 23

3. The choroidal stroma consists of large arteries and veins surrounded by collagen and elastic fibers, fibroblasts, a few smooth muscle cells, neurons of the autonomic nervous system, and melanocytes

Question 24

Describe the Aqueous humor

Answer 24

Aqueous humor is a continuously flowing liquid that carries metabolites to and from cells and helps maintain an optimal microenvironment within the anterior cavity of the eye. Epithelial cells covering the ciliary body secrete the aqueous into the posterior chamber of the anterior cavity (1), from which it flows past the lens and through the pupil into the anterior chamber of that cavity.
(2). The fluid then drains into the iridocorneal angle and is removed at the scleral venous sinus
(3), which is continuous with veins in the sclera

Question 25

Describe the ciliary body and what it is made up of

Answer 25

The ciliary body is anterior to the ora serrata and represents the ventral projection of both the choroid and the retina.
It is made up of two components: (1) the uveal portion and
(2) the neuroepithelial portion.

Question 26

Describe the uveal portion of the ciliary body

Answer 26

The uveal portion of the ciliary body includes:

The continuation of the outer layer of the choroid, known as the supraciliaris.

The ciliary muscle, a ring of smooth muscle tissue that, when contracted, reduces the length of the circular suspensory ligaments of the lens; this is known as the ciliary zonule.

A layer of fenestrated capillaries supplying blood to the ciliary muscle.

Question 27

Describe the neuroepithelial portion

Answer 27

The neuroepithelial portion contributes the two layers of the ciliary epithelium:

• An outer pigmented epithelial layer, continuous with the retinal pigmented epithelium.

• The pigmented epithelial layer is supported by a basal lamina continuous with Bruch’s membrane.

• An inner nonpigmented epithelial layer, which is continuous with the sensory retina.

Question 28

Describe the iris

Answer 28

The iris is a continuation of the ciliary body and is located in front of the lens.

• At this position, it forms a gate for the flow of aqueous humor between the anterior and posterior chambers of the eye and also controls the amount of light entering the eye.

Question 29

What are the two components of the iris

Answer 29

(1) the anterior uveal or stromal face
(2) the posterior neuroepithelial surface

Question 30

Describe the anterior (outer) uveal face

Answer 30

The anterior (outer) uveal face is of mesenchymal origin and has an irregular surface.
• It is formed by fibroblasts and pigmented melanocytes embedded in an extracellular matrix.
• The number of pigmented melanocytes determines the color of the iris.
• In albinos, the iris appears pink due to the abundant blood vessels.

Question 31

Describe the posterior (inner) neuroepithelial surface

Answer 31

The posterior (inner) neuroepithelial surface consists of two layers of pigmented epithelium.
•
The outer layer is a continuation of the pigmented layer of the ciliary epithelium which consists of myoepithelial cells that become the dilator pupillae muscle.
Note: The smooth muscle of the sphincter pupillae is located in the iris stroma around the pupil.

Question 32

What is the retina?

Answer 32

•It is the innermost layer of the eye.
– light-sensitive layer of tissue of the eye
– is derives from the neuroectoderm and it
represents an extension of the brain.

Question 33

two major layers of the retina

Answer 33

❖ The inner one, the neural retina
❖ The outer pigmented layer (of Retina)

Question 34

What is the optic papilla?

Answer 34

The optic disk includes the optic papilla, formed by protruding nerve fibers passing from the retina into the optic nerve.
• The optic papilla lacks photoreceptors and represents the blind spot of the retina.
Note: The fovea centralis is the area of sharpest vision

Question 35

Describe the the neural retina

Answer 35

The inner one, the neural retina, which contains the neurons and
photoreceptors (inner sensory retina).
✓ The neural retina consists of several layers of neurons interconnected by synapses, and is supported by an outer layer of pigmented epithelial cells.
• Note: The sensory retina spans from the margin of the optic disk posteriorly to the ciliary epithelium anteriorly.

Question 36

Describe The outer pigmented layer (of Retina

Answer 36

The outer pigmented layer (of Retina) is an epithelium resting on Bruch’s membrane just inside the choroid (nonsensory retinal pigmented epithelium)
This pigmented layer is cuboidal epithelium that lines the ciliary body and posterior iris.

Question 37

What are the ten layers of retinal
histologically?

Answer 37

• Inner limiting layer
• Nerve fibre layer
• Ganglion cell layer
• Inner plexiform layer
• Inner nuclear layer
• Outer plexiform layer
• Outer nuclear layer
• Outer limiting membrane
• Rod and cone layer
• Retinal pigment epithelium

Question 38

Layers of the retina

Answer 38

1. Inner limiting layer (ILL)
2. Nerve fiber layer (NFL): contains the ganglionic cell axons that converge at the optic disc and form the optic nerve
3. Ganglionic cell layer (GCL): contains cell bodies of the ganglion cells and of somewhat variable thickness throughout the retina. It contains RGC
4. Inner plexiform layer (IPL): containing fibers and synapses of the ganglion cells and the bipolar neurons of the next layer
5. Inner nuclear layer (INL): with the cell bodies of several types of bipolar neurons which begin to integrate signals from the rod and cone cells.
6. Outer plexiform layer (OPL):
   contains fibers and synapses of bipolar neurons and rod and cone cells
7. Outer nuclear layer (ONL): with the cell bodies and nuclei of the photosensitive rod and cone cells
8. Outer limiting layer (OLL): which is a fine line formed by the junctional complexes holding the rod and cone cells to the intervening glia called Müller cells
9. Rod and cone cell layer (RCL): which contains the outer segments of these cells where the photoreceptors are located
10. Retinal Pigmented layer (RPL) which is not sensory, but has several supportive functions important for maintenance of the neural retina

Question 39

What are the cell layers of the retina

Answer 39

Four cell groups are found in the sensory retina: these are 4 neurons that conduct visual impulses to the visual cortex.

1. The photoreceptor neurons: these are specialized receptor neurons in the retina. It consists of rods and cones (the outer nuclear layer (ONL) )
2. The conducting neurons: connect the rods and cones to the ganglionic cells. It includes bipolar and ganglion cells (The inner nuclear layer (INL) and ganglionic cell layer(GCL))
3. The association neurons: horizontal and amacrine cells
   (The inner nuclear layer (INL)
4. The supporting neuroglial cells- Muller cells (Müller cells are critical for retinal function, providing neurotrophic substances, removing waste products, regulating ion and water homeostasis, regulating blood flow, and maintaining a blood-inner retina barrier. Müller cells also organize two boundaries that appear as very thin retinal “layers”) With their perikarya in the INL, Müller cells extend processes that span the entire thickness of the neural retina.,

Question 40

four types of glial cells in the retina

Answer 40

– Muller cells (the main glial cells),
– astrocytes,
– microglia and oligodendrocytes.
occasionally

Microglia cells are phagocytic and part of reticuloendothelial system and it enters the retina from circulation.

Question 41

Three major layers contain the nuclei of the interconnected neurons

Answer 41

1. Outer nuclear layer (ONL)
2. Inner nuclear layer (INL)
3. Ganglionic layer (GL)

Question 42

Describe the ONL

Answer 42

Near the pigmented epithelium, the outer nuclear layer (ONL) contains cell bodies of photoreceptors (the rod and cone cells).

• These cells, like the pigmented epithelial cells, receive O2 and nutrients by diffusion from the choroidocapillary lamina of the choroid.

Question 43

Describe the INL

Answer 43

The inner nuclear layer (INL) contains the nuclei of various neurons, notably the bipolar cells, amacrine cells, and horizontal cells, all of which make specific connections with other neurons and integrate signals from rods and cones over a wide area of the retina.

Question 44

Describe the GL

Answer 44

Near the vitreous, the ganglionic layer (GL) has neurons (ganglion cells) with much longer axons.
• These axons make up the nerve fiber layer (NFL) and converge to form the optic nerve which leaves the eye and passes to the brain.
• The GL is thickest near the central, macular region of the retina, but it thins peripherally to only one layer of cells

Question 45

Between the three layers with cell nuclei are two fibrous or “plexiform” regions containing only axons and dendrites
connected by synapses

Answer 45

• The outer plexiform layer (OPL) includes axons of the photoreceptors and dendrites of association neurons in the INL.
• The inner plexiform layer (IPL) consists of axons and dendrites connecting neurons of the INL with the ganglion cells