Histology of the Eye Flashcards
Name the 3 concentric layers of the eye.
➤ outer protective fibrous layer (tunica fibrosa) consisting of the sclera and the cornea.
➤ highly vascularized middle layer (tunica vascularis/the uvea/uveal tract) consisting of the choroid, ciliary body and iris.
➤ inner neural layer, the retina consisting of an outer pigment epithelium and an inner retina proper.
➤ NB: Note that the terms outer (external) and inner (internal) refer to the gross structure of the eye. Inner denotes a structure closer to the centre of the globe, while outer means closer to the surface of the eyeball.
[Diagram: Tunics of the Eyeball]
[Histological Slides: Tunics of the Eyeball]
Describe the histology of the sclera.
- The external surface of the sclera (the episclera) is connected by a loose system of thin collagen fibers to the Tenon’s capsule.
Between Tenon’s capsule and the sclera is Tenon’s space, a loose space in which the eyeball can make rotating movements. - The sclera proper/scleral stroma consists of:
~ tough, dense irregular connective tissue [collagen type I] made up mainly of flat collagen bundles intersecting in various directions while remaining parallel
~ a moderate amount of ground substance
~ a few fibroblasts - Between the sclera and the choroid is the suprachoroidal lamina/lamina fusca, a thin layer of loose connective tissue rich in melanocytes, fibroblasts, and elastic fibers. It is relatively avascular.
[Histological slide 1] [Histological slide 2]
[Diagram: Histology of the Sclera]
The cornea is the colourless and transparent anterior one sixth of the fibrous layer. Review the 5 histological layers of the cornea.
- Corneal epithelium: stratified squamous non-keratinised epithelium.
-
Bowman’s membrane (anterior limiting membrane): this thick homogenous layer consists of collagen fibres crossing at random, a condensation of the intercellular substance, and no cells, contributing greatly to the stability and strength of the cornea.
It may act as a barrier to spread of infections. - Corneal stroma (substantia propria): made up of about 200–250 lamellae of fine collagen fibres, which cross each other at right angles to form corneal spaces. Flattened fibroblasts are located between the lamellae. The transparency of cornea is due to precise lattice arrangement of its lamellae embedded in the ground substance (orthogonal).
- Descemet’s membrane: thick homogenous structure composed of fine collagenous filaments organised in 3-D network. (It forms the basal lamina of the endothelial cells.)
- Corneal endothelium: simple squamous epithelium. It facilitates metabolic exchange between cornea and aqueous humor.
- [Histological slide: Cornea]
Note:
The cornea is also supplied with sensory nerve fibers. Remind yourself of the corneal reflex.
Point out noteworthy features of the basal part and surface of the cornea.
- In the basal part of the corneal epithelium there are numerous mitotic cells responsible for the cornea’s remarkable regenerative capacity (every 7 days).
- The surface corneal cells show microvilli protruding into the space filled by the precorneal tear film.
State the specializations of the cornea that enable its transparency.
- Regularity of its tissue components which minimizes the scattering of light:
- outer and inner surfaces of corneal epithelium are smooth
- collagen fibers of the substantia propria are arranged into uniform layers with parallel fibers within each layer
- the water content of the ground substance is carefully regulated, to maintain uniform spacing among collagen fibers - The tissue elements of the cornea are specialised to minimize the scattering of light. The surfaces of these elements can scatter light when irregularly arranged. Such scattering (together with absorption of light by pigments such as melanin and hemoglobin) prevents light from passing freely through most tissues.
- The corneal endothelium actively pump ions and water from the corneal ground substance into the aqueous humour to prevent excess water from causing opacity.
(a) The limbus is highly vascularised, and its blood vessels assume an important role in ____________________.
(b) In the stromal layer (in the region of the limbus) there are irregular endothelium-lined channels forming a trabecular meshwork, which merge to form ____________________.
(a) corneal inflammatory processes
(b) Schlemm’s canal
Note:
At the limbus,
✔️ Bowman’s membrane ends abruptly.
✔️ The limbus contains corneal stem cells: healing of corneal ulcers.
The aqueous humour is drained by the Schlemm’s canal into anterior ciliary veins. Blockage of the canal results in glaucoma.
Discuss the histology of the choroid.
✔️ highly vascularized
✔️ loose connective tissue rich in fibroblasts, macrophages, lymphocytes, mast cells, plasma cells, collagen fibres and elastic fibres
✔️ abundance of melanocytes hence its characteristic black color
✔️ The inner layer of the choroid is richer in small vessels than the outer layer, and is called the choriocapillary layer
✔️ Bruch’s membrane is a thin hyaline membrane that separates the choriocapillary layer from the retina, and extends from the optic papilla to the ora serrata. [The optic papilla is the region where the optic nerve enters the eyeball.]
✔️ The choroid is bound to the sclera by a loose layer of connective tissue rich in melanocytes called the suprachoroidal lamina.
Describe the basic histological structure of the ciliary body.
The basic histological structure of the ciliary body consists of loose connective tissue surrounding the ciliary muscle. The ciliary muscle consists of two bundles of smooth muscle fibres. One of these bundles has the function of stretching the choroid; another bundle, when contracted, relaxes the tension on the lens.
The surfaces of the ciliary body that face the vitreous body, posterior chamber and lens are covered by the anterior expansion of the retina. In this region, the retina consists of only 2 cell layers. State these layers.
- the layer directly adjacent to the ciliary body consists of simple columnar cells rich in melanin and corresponds to the forward projection of the pigment layer of the retina
- the second layer covers the first, and is derived from the sensory layer of the retina and consits of simple non-pigmented columnar epithelium
- [Histological Slides: Ciliary Body and Choroid]
- [Histological Slide: Ciliary Body Epithelium]
Histologically, the iris presents two basic layers: the ____(a)____ layer with pigmented fibrovascular tissue and ____(b)____ layers.
(a) stromal
(b) pigmented epithelial
[Histological Slide]
The smooth posterior surface of the iris is covered by two layers of epithelium which also cover the ciliary body and its processes.
(1) The inner epithelium is heavily pigmented with melanin granules, and is in contact with the ____(a)____ chamber. (b) What is the function of the heavy pigmentation?
(2) The outer epithelial cells have radially directed tongue like extensions of their basal region. They are filled with overlapping myofilaments, creating the ____(c)____ muscle of the iris.
(a) posterior
(b) The heavy pigmentation prevents the passage of light into the interior of the eye except through the pupil.
(c) dilator pupillae
[Histological slide: Iris]
Appreciate the sphincter and dilator pupillae muscles. Appreciate their locations within the iris and that the pupillary sphincter muscle is smooth muscle, while the pupillary dilator muscle is formed by contractile extensions of myoepithelial cells of the deep layer of the two-layered pigmented epithelial tissue.
Name and briefly discuss the three components of the lens.
(a) Lens capsule: homogenous, refractive, carbohydrate-rich capsule coating the outer surface of the epithelial cells. It is a very thick basement membrane and consists mainly of collagen type IV and a glycoprotein called laminin.
(b) Subcapsular epithelium: consists of a single layer of cuboidal epithelial cells present only on the anterior surface of the lens. This epithelial layer secretes the lens capsule
(c) Lens fibres: elongated, thin and flattened structures. They are highly differentiated cells derived from the cells of the subcapsular epithelium. The lens fibres eventually lose their nuclei and other organelles and become greatly elongated. These cells are filled with a group of proteins called crystallins.
[Histological Slide: Lens]
The lens is held in place by a radially oriented group of fibres known as the ____(a)____ that inserts onto the ____(b)____ on one side and onto the ____(c)____ on the other.
(a) zonule
(b) lens capsule
(c) ciliary body
[Histological Slide]
Name the glycosaminoglycan found in small quantities in the vitreous body.
hyaluronan/hyaluronic acid
Here are some slides on the histology of the eyelid. Click on Answer to view them.
[Virtual Microscope] [Slide 1] [Slide 2]
