Surface Structures of the Lens
Externally, the organ is covered by a transparent capsule. Histologically, this structure represents a highly modified basement membrane formed from the epithelium of the primary lens vesicle. Just like any hollow organ with an epithelial lining, this basement membrane lies strictly external to the cell layer. The primary mechanical function of the capsule is to serve as a strong attachment site for the fibers of the suspensory ligament of the lens (zonule of Zinn).
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 because, during embryonic development, the cells of the posterior wall elongate significantly and are consumed in the formation of the primary lens fibers.
Interestingly, to nourish the actively growing primordium during the prenatal period, a temporary vascular tunic forms over the capsule. However, by mid-gestation, it completely regresses (disappears), which is a mandatory condition for ensuring the absolute transparency of the lens.
Histogenesis: Formation of the Lens Substance
All internal substance of the lens is derived from epithelial tissue. The differentiation of cells into transparent prisms (lens fibers) occurs in three consecutive "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 generated. They gradually elongate and 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 lower intensity. The source is the cells of the equatorial zone of the lens. They slowly differentiate, layering externally to form the cortex.
Finished lens fibers are tightly linked together via finger-like cytoplasmic projections (interdigitations). In the most superficial layers of the cortex, fibers may still retain remnants of cell nuclei.
Age-Related Changes
Throughout human 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 marked difficulty in focusing vision on near 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.