Cornea, Conjunctiva, Tear film

The cornea is the transparent window of tissue that covers the iris, (the coloured part of the eye) and pupil. It has two major functions: it forms a nice smooth surface for the refraction of light (the cornea is responsible for 2/3 of the focusing ability of the eye) and it protects the underlying ocular structures. Unlike most tissues in the body, the cornea contains no blood vessels to nourish it. Instead, the cornea receives its nourishment from the tears and from the aqueous humor that fills the chamber behind it. The cornea is extremely sensitive - there are more sensory nerve endings in the cornea than anywhere else in the body (that’s why it hurts so much if you happen to get poked in the eye!). The adult cornea is only about 0.5 millimeters thick (in the centre) and structurally can be divided in to 5 layers :epithelium, anterior limiting lamina (Bowman's layer), stroma, posterior limiting lamina (Descemet's membrane), endothelium.

The epithelium is the outer most layer. It occupies approximately 10% of the total corneal thickness and consists of 5-7 layers of squamous epithelial cells. It is a self-renewing tissue and complete turnover takes about 1 week. All of the cells in the epithelium are epithelial cells (although cells of the immune system called Langerhan's cells will occasionally be found) but they can be divided into three distinct morphological groups: basal, wing and superficial. The basal cells form the innermost layer. They are the least differentiated of the corneal epithelial cells and are able to divide. Above the single layer of basal cells, are 2-3 layers of wing cells. The wing cells are derived from the basal cells and ultimately they become the flattened superficial cells. The latter cells form the outer 2-3 layers of cells and ultimately are shed in to the tear film. Stem cells that are located in the limbus (region where clear cornea meets white sclera) give rise to new basal cells to ensure a steady supply of cells to replace those that are lost in to the tear film.

The anterior limiting lamina (Bowman's layer) is directly below the epithelium. It is a thin acellular (no cells) layer of the extracellular matrix protein collagen. It was originally named after Sir William Bowman (1816-1892) an English ophthalmologist, anatomist and physiologist. No one really knows the purpose of Bowman's layer, and in fact many species and people who have had photorefractive surgery (PRK) get along perfectly well without it.

The conjunctiva is a thin membrane that covers the sclera (white part of the eye) and the inside of the eyelids. It is composed of two layers, the epithelium and the stroma. The epithelium is the outermost layer and has 2-4 layers of epithelial cells. Interspursed among these cells are goblet cells. These secrete a substance called mucin that becomes part of the tear film. The stroma is a layer of fibrous tissue that contains blood vessels and cells such as mast cells. In allergic individuals on exposure to allergens such as pollen, the mast cells secrete chemicals that cause itchiness and tearing.

The tear film covers the outer surface of the eye. It is estimated to be about 7um thick (although there is still much debate about this) and has three main components (it was though that there were 3 distinct layers, but now it seems that the 3components are blended together rather than being separated). There is an oily lipid component which helps reduce the rate of evaporation of tears from the ocular surface. The constituents of this part are secreted by the tarsal (Meibomian) glands which are present in the eyelids. There is an aqueous component. The constituents of this layer are secreted by the lacrimal gland and accessory lacrimal glands. Finally there is a mucin component which helps spread the tears evenly over the surface of the eye. The constituents of this component are secreted by the goblet cells of the conjunctiva and the epithelial cells of both cornea and conjunctiva.