Covering structures of the lens
Externally, the organ is covered by a transparent capsule. From a histological standpoint, this structure is a highly modified basement membrane formed from the epithelium of the primary lens vesicle. Like any hollow organ with an epithelial lining, this basement membrane is located strictly on the outside of the cell layer. The main mechanical function of the capsule is to serve as a firm attachment site for the fibers of the ciliary zonule (suspensory ligament of the lens).
Beneath the capsule lies the subcapsular epithelium. This is a single-layered sheet of cuboidal cells. A crucial topographical feature is that it lines only the anterior surface of the lens. A typical epithelium is absent on the posterior surface, as during embryonic development, the cells of the posterior wall of the vesicle elongate significantly and are consumed in the formation of primary lens fibers.
Interestingly, to nourish the actively growing primordium during the intrauterine period, a temporary vascular tunic forms over the capsule. However, by the middle of pregnancy, it completely regresses (disappears), which is a prerequisite for ensuring absolute transparency of the lens.
Histogenesis: formation of the lens substance
The entire internal substance of the lens is a derivative of epithelial tissue. The differentiation of cells into transparent prisms (lens fibers) occurs in three successive "waves":
- Embryonic wave.
The cells of the posterior surface of the lens vesicle elongate, completely losing their nuclei and cell organelles. Their cytoplasm fills with a specialized protein—crystallin. As a result, a dense central cluster is formed—the embryonic nucleus.
- Fetal wave.
Due to the high mitotic activity of the anterior epithelial cells, new cell masses are produced. They gradually elongate and effectively envelop the primary embryonic nucleus, forming the fetal nucleus around it.
- Postnatal (cortical) wave.
This stage starts after birth and continues throughout life, albeit with a low intensity. The source is the cells of the equatorial zone of the lens. They slowly differentiate, layering on the outside and forming the cortex.
The finished lens fibers are firmly interconnected by finger-like outgrowths of the plasmalemma (interdigitations). In the most superficial layers of the cortex, fibers may still retain remnants of cell nuclei.
Age-related changes
Throughout a person's life, the slow formation of new cortical fibers continues in the lens, accompanied by a gradual loss of water by the tissues. These two factors lead to progressive hardening of the organ and two main clinical manifestations:
- Presbyopia (age-related farsightedness). Due to significant hardening, the lens loses its natural elasticity. When the ciliary muscle contracts, it can no longer assume a convex shape. The main symptom of this condition is a pronounced difficulty in focusing vision on close objects.
- Cataract. With severe dehydration and age-related accumulation of altered, pigmented (yellow-brown) proteins in the fibers, the lens irreversibly loses its transparency. Opacification develops, characteristic of elderly individuals.
Briefly: answers to topical questions
How is the nutrition of the avascular lens ensured in an adult?
The nutrition of the avascular lens in an adult is provided by the aqueous humor.
During the intrauterine period, a special vascular tunic forms over the capsule, responsible for nourishing the organ. However, by the middle of intrauterine development, this tunic completely regresses (disappears), which is necessary to ensure the transparency of the optical medium. In the formed eye, the lens is part of the transparent light-refracting internal core of the eye, and the function of nourishing its avascular structures is taken over by the aqueous humor of the anterior and posterior chambers.
Three stages of lens substance development (from center to periphery): Embryonic nucleus (from the posterior wall) → Fetal nucleus (from the anterior wall) → Cortex (from the equator).
Class and exam questions
Why is the basement membrane of the lens located on the outside?
The lens capsule is a derivative of the basement membrane of the embryonic lens vesicle. Since the vesicle is lined with epithelium from the inside, its basement membrane naturally lies on the outer surface.
Where does the epithelium on the posterior surface of the lens disappear to?
During intrauterine development, the cells of the posterior wall of the epithelial vesicle elongate significantly, lose their nuclei, and transform into primary lens fibers, filling the internal cavity.
What cells do new fibers form from in an adult?
New lens fibers slowly form throughout life from dividing epithelial cells located in the equatorial region.
What is the morphological cause of presbyopia development?
Presbyopia occurs due to age-related hardening of the lens (resulting from water loss and the accumulation of new fiber mass), causing it to lose elasticity and fail to become convex when attempting near focusing.
What else to study on this topic
- Features of the ultrastructural organization of interdigitations between lens fibers.
- Biochemical properties and types of crystallin proteins.
- Mechanisms of regression of the embryonic vascular network of the lens.
- Dynamics of mitotic activity of the subcapsular epithelium.
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