Welcome to the wonderful world of histology. Histology is how we study the human body on a cellular level.
We are continuing with our special senses and this week we are taking a ‘look’ at vision!!
The eye itself is composed of three layers. Working from the outside to in, the first layer is the corneoscleral coat which contains the cornea and sclera. The cornea is the transparent part of the eye that covers the iris and the pupil and allows light to enter the inside. It has an outer layer of thin non-keratinised stratified squamous epithelium sitting on a layer of collagen called Bowman’s membrane. This has layers of collagen fibrils with corneal fibroblasts between these layers. On the inside, there is a layer of endothelium sitting on a layer of meshwork collagen called Descemet’s membrane. The sclera is composed of collagen and elastic fibres which define the global shape of the eye.
Next up is the uveal tract which is the vascular layer and contains the iris and ciliary body as well as the choroid coat. The iris is made up of loose connective tissue with a pigmented epithelium on the posterior. It contains melanocytes. A large number of melanocytes produce brown eyes whereas few produce blue eyes. The iris contains smooth muscle that can constrict the pupil.
The ciliary body contains smooth muscle that, during 'near vision', releases tension on the zonular fibres allowing the lens to round up due to its natural elasticity.
Last but not least, is the retina which contains layers of nerve cells, support cells and a layer of pigmented epithelium. The retina contains about 130,000,000 rods and 65,000,000 cones. These are the photoreceptors, the cells responsible for sight. Cones allow us to see red, blue or green light.
The eye can be divided into two chambers. The anterior chamber is in front of the lens and contains the aqueous humor. Excess fluid from the anterior chamber is drained via the canal of Schlemm into the blood vessels. Behind this is the posterior chamber which contains the vitreous humor which is very gelatinous and gives the eye its round shape and structure. With the Murder of Anton Orlov, you have the opportunity to marvel at the eye up close and personal and get to dissect an eye specimen to see these chambers!