Choroid to Lens Fibres

Lens Structure

• The main constituent of the lens capsule is the basement membrane.
• Tight junctions attach the lens epithelial cells to the underlying cortical fibers.
• The embryonic primary lens fibers form in the posterior part of the lens.

Choroid Layer

• The choroid layer is absent in pigs, squirrels, rodents, rabbits, kangaroos, llamas, and many nonhuman primates.
• In primates, the irregular and serrated junction where the anterior margin of the choroid joins the ciliary body is called the ora serrata.

Retina Nutrition

• The main source of nutrition for the outer layers of the retina is the choroid.

Tapetum Structure

• In tapetal cells on the scleral side of the tapetum, organelles are limited to the perinuclear in small numbers.
• In ungulates, the main component of the fibrous tapetum is collagen.
• In carnivores, the blood vessels that penetrate the tapetum are choroidal vessels.
• In birds, the tapetum lucidum is located in the sclera.

Focusing Mechanism

• The lens is responsible for focusing light onto the retina.

Bruch's Membrane

• Some animals have a trilaminate structure of Bruch's membrane.

Accommodation Mechanism

• Some species have no known accommodative mechanism.

Lens Refractive Power

• The refractive power of the lens in humans is approximately 15-20 diopters.

Choroidal Nerve Fibers

• Predominantly, myelinated nerve fibers are associated with the vascular system in the choroid.

Tapetum Function

• The tapetum lucidum functions to reflect light back onto the retina, increasing the amount of light available for photoreception.

Tapetum Types

• Some animals have a tapetum fibrosum in their tapetal layer.

Tapetal Cells

• The most striking ultrastructural feature of tapetal cells in dogs and cats is the presence of ribosomes.

Avian Choroid

• The avian choroid contains a layer of nonvascular smooth muscle cells.
• The function of the band or layer of smooth muscle within the suprachoroidea is to regulate the choroidal blood flow.

Retinal Oxygenation

• The major source of oxygen and nutrients for the retina is the choroidal blood flow.
• The avian choroid is distinguished from the mammalian choroid by the presence of nonvascular smooth muscle cells.