The lens is a transparent crystalline structure in the eye which makes up 1/3 of the refractive power of the eye. It is able to change shape to help refract light onto the retina. In this article, we will describe the detailed anatomy of the lens.
The human lens is a biconvex structure responsible for transmitting light to the retina. In order to do this, it contains one of the highest concentrations of proteins of any tissue. It is able to change shape to allow focusing at objects at different focus lengths.
The process of the lens changing shape to focus on an object is called accommodation.
<aside> 💡 The lens contributes 1/3 of the refractive power of the eye, contributing to about 18 D of refractive power. The remaining 2/3 of the refractive power of the eye is attributed to the cornea.
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The mean power of the lens in newborns is 45 D and it decreases as the individual gets older to around 25 D at the age of 6 years. In adults, the lens contributes around 18 D of refractive power.
The lens has a biconvex structure, with a diameter of around 10 mm and an axial length of around 4 mm. These figures can vary between individuals. The size and shape of the lens can change due to accommodation.
The lens is composed of epithelial cells and fibres. The fibres of the lens are packed tightly to allow it to become a transparent structure in order to allow the transmission of light. The lens contains high concentrations of solluble structural proteins called crystallins. As much as 60% of the total mass of the lens is made up of proteins.
There are three layers to the lens: the innermost layer is called the nuclear, followed by the cortex and finally the capsule.
The fibres of the lens are arranged tightly to maintain the transparency of the lens.
Diagram showing the structure of the lens. Image courtesy of BMJ Open Ophth.
The lens capsule functions as a barrier to diffusion and contains components such as type IV collagen and laminin, amongst others. The capsular filaments are thickest at the equator of the lens, where the zonular fibres insert.
The epithelial cells of the lens are connected via gap junctions in order to allow the exchange of metabolites and ions. The epithelial cells in the anterior part of the lens contains cytoskeletal proteins to help stabilise the cells during accommodation.
The nucleus of the lens is made up of fibre cells. These cells form concentric shells around the fetal nucleus. They are arranged in an ordered fashion to allow light transmission and to maintain the transparency of the lens.
The adult human lens is an avascular and non-innervated structure. Its nutrient supply depends on the aqueous humour and vitreous body. However, during embryological development, the lens is supplied by the hyaloid artery.