Pecten oculi

The pecten or pecten oculi (Latin for "comb of the eye") is a comb-like structure of blood vessels belonging to the choroid in the eye of a bird, and no other species. It is a non-sensory, pigmented structure that projects freely into the vitreous humor from the point where the optic nerve enters the eyeball, and undulates with movements of the vitreous humor. It almost entirely covers up the optic disc.

Histologically, it contains three types of tissues: a plexus of modified blood vessels, darkly pigmented cells interdigitated between the blood vessels, and supporting tissue. The supporting tissue is glial and syncytium, and derived from the optic disc. There are no muscle, nerve fibers, or sensory tissue. The arterial blood is supplied by a branch of the hyaloid artery emerging from the optic disc entirely separate from the choroidal circulation. The artery runs along the base of the pecten and sends ascending branches to each of the folds. Melanin is abundant at the apical and peripheral pecten, produced by pleomorphic melanocytes that form incomplete sheaths along the plexus of capillaries.

More than 30 functions have been proposed for the pectan, which are reviewed in and. The most commonly accepted theory is that it provides nutrition to the retina, and control the pH of the vitreous body. High levels of alkaline phosphatase and carbonic anhydrase activity in the pecten oculi have been linked to the transport of nutrient molecules from the highly vascularized choroid into vitreous and retinal cells, thus nourishing the eye. Saccadic eye movements caused the pecten to oscillate, fanning the liquid in the vitreous. This suggests that saccade and the pecten co-evolved to diffuse metabolites such as oxygen and glucose out from the pecten.

In the vertebrate eye, there are blood vessels in front of the retina, partially obscuring the image. In most avians, the retina is completely free of blood vessels and leading to the extremely sharp eyesight of birds such as hawks. The retina is supplied instead by the choroid and the pecten. The pigmentation of the pecten is believed to protect the blood vessels against damage from ultraviolet light. Stray light absorption by melanin granules of pecten oculi is also considered to give rise to small increments in temperature of pecten and eye; this may offer increased metabolic rate to optimize eye physiology in low temperatures at high-altitude flights.