Updated: Nov 15, 2022
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.
💡 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.
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.
Layers of the lens
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.
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.
Nutrient and nerve supply
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.
Embryonic development of the lens
The lens develops from the optic vesicle during embryological development. The optical vesicle induces the epidermal ectoderm to form a lens placode, which gradually detaches from the epidermal ectoderm to form the lens vesicles.
Relations of the lens
The lens is located in the posterior chamber of the eye and supported by the vitreous body. There are fibres called zonular fibres which attach on the outer layer of the lens capsule on either end of the lens. These radiate into the ciliary body and suspend the lens in place.
Contraction of the ciliary muscle can allow the zonular fibres to stretch and relax, which in turn changes the shape of the lens. This process is called accommodation and it allows the eye to focus on objects at different distances.
When looking at distant objects, the ciliary muscles relax, the lens zonules become tighter and the lens becomes flatter. On the other hand, when looking at nearby objects, the ciliary muscles contract, the lens zonules relax and the lens becomes more convex in shape.
The lens continually grows throughout life by laying new cells over the existing cells. Over time, this can cause the lens to become stiff and hence the lens can lose its ability to accommodate over time. This process is called presbyopia.
💡 The loss in ability of the lens to accommodate as a person ages is called presbyopia.
Common pathologies involving the lens
Cataract: A cataract is an opacification of the normally transparent lens. This can lead to blurred vision
Ectopia lentis: the lens may become displaced from its normal location. This is known as ectopia lentis
Nuclear sclerosis: this is a normal age-related change which occurs in the centre of the lens
‘Lens Anatomy’. The Cataract Course, http://cataractcourse.com/lens-anatomy-and-development/lens-anatomy/. Accessed 29 May 2022.
Lens (Anatomy). https://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/l/Lens_%2528anatomy%2529.htm. Accessed 29 May 2022.
Ruan, Xiaoting, et al. ‘Structure of the Lens and Its Associations with the Visual Quality’. BMJ Open Ophthalmology, vol. 5, no. 1, Sept. 2020, p. e000459. bmjophth.bmj.com, https://doi.org/10.1136/bmjophth-2020-000459.