Pecten oculi
The pecten or pecten oculi is a comb-like structure of blood vessels belonging to the choroid in the eye of a bird. 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.[1]
Histologically, it contains 3 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.[2][1]
The pecten is believed to both nourish the retina and control the pH of the vitreous body.[3] 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.[4] It is present in all birds, and homologous to the conus papillaris, which occurs in some reptiles.[5]
In the vertebrate eye, there are blood vessels in front of the retina, partially obscuring the image. The pecten helps to solve this problem by greatly reducing the number of blood vessels in the retina and leading to the extremely sharp eyesight of birds such as hawks.[6] 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.[7]
History
[edit]It was first noted by Nicolas Steno in 1673 in developing chicken embryo, though it was an incorrect description and interpretation.[8] It was then correctly described and interpreted by Perrault (1676)[9][10] whose observation was elaborated by Petit (1735)[11][2].
Comparison
[edit]
The structure varies across bird species. The conical type is only reported in the brown kiwi (Apteryx mantelli); the vaned type is present in ostriches (Struthio camelus) and rheas (Rhea americana). It is pleated in most other birds, including the Neognathae birds and the cassowary.[1][12][13] See Plate XII at page 411 for examples.[1]
The conical type looks similar to the conus papillaris, and is a simple cone rising up from the base on a circular optic disc. It has no folds. It is trumpet-shaped and heavily brown-black in color. It almost touches the lens.
The vaned type looks like a thin sheet rising up from the base over an oval-shaped optic disc. There are 25-30 thin folds extending out from the sheet. The folds are roughly trapezoidal, short on the top and long on the base. See Figure 507 for an example.[1]
The pleated type looks like an accordion. The base is longer than the top. There is usually a ridge at the top called the "bridge", which keeps the accordion shorter at the top. If the pectan is cut off from the retina at the base, then its bridge is cut off, then it can be flattened to a flat sheet. Owls, Podargus, and Haliaetus albicilla do not have the bridge. In the Alcedo atthis japonica, the crest of each ridge contains 1 to 3 membranous extensions, which resembles the vaned type.[14]
The pectan tend to be larger and have more folds in diurnal birds than nocturnal birds. The number of pleats varies between 5 and 30. In predators the folds are thicker but fewer (13 to 17). Sea-birds and shore-birds tend to have fewer pleats (<= 12).
In the owl Bubo virginianus, it projects out into the vitreous cavity 5-6 mm, whereas in the dove Leucosarcia picata, it reaches almost as far anteriorly as the equatorial lens.[15]
More than 30 functions have been proposed for the pectan. Of these 8 are reviewed in [1].
See also
[edit]- Conus papillaris, a similar structure found in reptiles
References
[edit]- ^ a b c d e f Duke-Elder, Stewart (1958). System of ophthalmology. Vol. 1. St. Louis: Mosby. pp. 410–417. OL 14044452M.
- ^ a b Seaman, Arlene R.; Storm, Harriet (1963-04-01). "A correlated light and electron microscope study on the pecten oculi of the domestic fowl (Gallus domesticus)". Experimental Eye Research. 2 (2): 163–IN26. doi:10.1016/S0014-4835(63)80009-3. ISSN 0014-4835.
- ^ Kiama, S. G.; Maina, J. N.; Bhattacharjee, J.; Weyrauch, K. D. (2001). "Functional morphology of the pecten oculi in the nocturnal spotted eagle owl (Bubo bubo africanus), and the diurnal black kite (Milvus migrans) and domestic fowl (Gallus gallus var. domesticus): a comparative study". Journal of Zoology. 254 (4): 521–528. doi:10.1017/s0952836901001029.
- ^ Bawa, S R; YashRoy, R C (1972). "Effect of dark and light adaptation on the retina and pecten of chicken". Experimental Eye Research. 13 (1): 92–97. doi:10.1016/0014-4835(72)90129-7. PMID 5060117.
- ^ Bonney, Rick; Rohrbaugh, Jr., Ronald (2004), Handbook of Bird Biology (2nd ed.), Princeton, NJ: Princeton University Press, ISBN 0-938027-62-X
- ^ Ainsworth, Claire; Le Page, Michael (August 11, 2007), "Evolution's greatest mistakes" (PDF), New Scientist, 195 (195.2616): 36–39, doi:10.1016/S0262-4079(07)62033-8, retrieved 7 November 2016
- ^ Bawa, S R; YashRoy, R C (1974). "Structure and function of vulture pecten". Cells Tissues Organs. 89 (3): 473–480. doi:10.1159/000144308. PMID 4428954.
- ^ Steno, Nicolas (1673) [1673]. Bartholin, Thomae (ed.). "In ovo & pullo obervationes" [Observations on Egg and Chick]. Acta Medica & Philosophica Hafniensia. 2 (Article No. XXXIV): 81–92.
Hiatus in tunica oculi nigra clausus, nisi qva opticum nervum contingebar. Optici nervi filamenta nigra, qvae per vitreum pergunt crystallinum.
[The gap in the black tunica of the eye was closed except at the place where it touched the optic nerve. The filaments which this nerve sends to the lens were black.] - ^ Perrault, M. (1676). "Description anatomique de deux Cocqs Indien, and Description anatomique de trois aigles". Mémoires de l'Academie royale des Sciences (in French). III: 292–300.
- ^ Perrault, M. (1676). "Description anatomique de deux Cooqs Indien, and Description anatomique de trois aigles". Mémoires de l'Academie royale des Sciences (in French). III: 303–308.
- ^ Petit, M. (1735). "Description anatomique de l'Oeil der coq-d'Inde". Histoire de l'Academie royale des Sciences (Paris) (in French): 123.
- ^ Kiama, S G; Bhattacharjee, J; Maina, J N; Weyrauch, K D (December 1994), "A scanning electron microscope study of the pecten oculi of the black kite (Milvus migrans): possible involvement of melanosomes in protecting the pecten against damage by ultraviolet light", Journal of Anatomy, 185 (Pt 3): 637–642, PMC 1166670, PMID 7649799
- ^ Dayan, Mustafa Orhun; Ozaydın, Tugba (2013). "A comparative morphometrical study of the pecten oculi in different avian species". TheScientificWorldJournal. 2013: 968652. doi:10.1155/2013/968652. ISSN 1537-744X. PMC 3674711. PMID 23818837.
- ^ Meyer, David B. (1977), Crescitelli, Frederick (ed.), "The Avian Eye and its Adaptations", The Visual System in Vertebrates, Handbook of Sensory Physiology, vol. 7 / 5, Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 549–611, doi:10.1007/978-3-642-66468-7_10, ISBN 978-3-642-66470-0, retrieved 2025-06-24
- ^ Ringvold, Amund. "The function of pecten oculi. Conus papillaris in reptiles and its analogue pecten oculi in birds evolved in tandem with increasing uric acid in serum." Int J Pure Appl Zool 10 (2022): 136-149.