Schematic diagram of the human eye Hyaloid canal (Cloquet's canal and Stilling's canal) is a small transparent canal running through the vitreous body from the optic nerve disc to the lens. It is formed by an invagination of the hyaloid membrane, which encloses the vitreous body. In the fetus, the hyaloid canal contains a prolongation of the central artery of the retina, the hyaloid artery, which supplies blood to the developing lens. Once the lens is fully developed the hyaloid artery retracts and the hyaloid canal contains lymph.
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Its proximal portion remains as the central artery of the retina. Regression of the hyaloid artery leaves a clear central zone through the vitreous humor, called the hyaloid canal or Cloquet's canal. Cloquet's canal is named after the French physician Jules Germain Cloquet (1790–1883) who first described it. Occasionally the artery may not fully regress, resulting in the condition persistent hyaloid artery.
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The hyaloid canal appears to have no function in the adult eye, though its remnant structure can be seen. Contrary to initial belief, the hyaloid canal does not facilitate changes in the volume of the lens. The lens volume changes by less than 1% over its range of accommodation. Furthermore, lymph, being liquid, is incompressible, so even if the volume of the lens did change, the hyaloid canal could not compensate for it.
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The vitreous membrane (or hyaloid membrane or vitreous cortex) is a layer of collagen separating the vitreous humour from the rest of the eye. At least two parts have been identified anatomically. The posterior hyaloid membrane separates the rear of the vitreous from the retina. It is a false anatomical membrane.
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The hyaloid fossa is a depression on the anterior surface of the vitreous body in which lies the crystalline lens. The name hyaloid fossa is synonymous with patellar fossa. Lens is lined by capsular epithelium. Accumulation of fluid—blood or exudates—can occur in the potential space in which it creates between the vitreous and the lens, called the retrolenticular space.
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Philadelphia: Butterworth Heinemann, p. 107–8 By nine weeks into human development, the lens is surrounded and nourished by a net of vessels, the tunica vasculosa lentis, which is derived from the hyaloid artery. Beginning in the fourth month of development, the hyaloid artery and its related vasculature begin to atrophy and completely disappear by birth.The Eye: Basic Sciences in Practice, p.
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M P Snead, D R J Snead, A J Richards, J B Harrison, A V Poulson, A H C Morris, R M Sheard, J D Scott; Clinical, histological and ultrastructural studies of the posterior hyaloid membrane; "Eye", July 2002, Volume 16, Number 4, Pages 447-453. The anterior hyaloid membrane separates the front of the vitreous from the lens. Andres Bernal, Jean-Marie Parel, Fabrice Manns; Evidence for posterior zonular fiber attachment on the anterior hyaloid membrane; "Investigative Ophthalmology and Visual Science" 2006, 47, 4708-4713. Bernal et al. describe it "as a delicate structure in the form of a thin layer that runs from the pars plana to the posterior lens, where it shares its attachment with the posterior zonule via Weigert’s ligament, also known as Egger’s line".
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104, In the postnatal eye, Cloquet's canal marks the former location of the hyaloid artery. After regression of the hyaloid artery, the lens receives all its nourishment from the aqueous humor. Nutrients diffuse in and waste diffuses out through a constant flow of fluid from the anterior/posterior poles of the lens and out of the equatorial regions, a dynamic that is maintained by the Na+/K+-ATPase pumps located in the equatorially positioned cells of the lens epithelium. Glucose is the primary energy source for the lens.
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The hyaloid artery is a branch of the ophthalmic artery, which is itself a branch of the internal carotid artery. It is contained within the optic stalk of the eye and extends from the optic disc through the vitreous humor to the lens. Usually fully regressed before birth, its purpose is to supply nutrients to the developing lens in the growing fetus. During the tenth week of development in humans (time varies depending on species), the lens grows independent of a blood supply and the hyaloid artery usually regresses.
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During fetal development vascular lens capsule (tunica vasculosa lentis) develop from the mesenchyme that surrounds the lens. It receives arterial blood supply from the hyaloid artery. This blood supply slowly regress and vascular capsule disappear before birth. The disappearance of the anterior vascular capsule of the lens is useful in estimating the gestational age.
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More commonly, small remnants of the artery may remain. Free remnants can sometimes be seen as "floaters". An anterior remnant of the hyaloid artery can be seen in some people as Mittendorf's dot, a small pinpoint-like scar on the posterior surface of the lens.Lee Ann Remington, Clinical Anatomy of the Visual System, 2005 p.
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Floaters are generally harmless, but the sudden onset of recurring floaters may signify a posterior vitreous detachment or other diseases of the eye. Posterior vitreous detachment: Once liquid vitreous enters the sub-hyaloid space between the vitreous cortex and the retina, it may strip the vitreous cortex off the retina with each eye movement (see Saccade).
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Spontaneous venous pulsations are present in about 80 percent of patients with ODD, but absent in cases of true disc edema. Other causes of disc elevation clinicians must exclude may be: hyaloid traction, epipapillary glial tissue, myelinated nerve fibres, scleral infiltration, vitreopapillary traction and high hyperopia. Disorders associated with disc elevation include: Alagille syndrome, Down syndrome, Kenny-Caffey syndrome, Leber Hereditary Optic Neuropathy and linear nevus sebaceous syndrome.
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Normal-tension glaucoma, which comprises one-third of POAG, is also associated with genetic mutations (including OPA1 and OPTN genes). Various rare congenital/genetic eye malformations are associated with glaucoma. Occasionally, failure of the normal third-trimester gestational atrophy of the hyaloid canal and the tunica vasculosa lentis is associated with other anomalies. Angle closure-induced ocular hypertension and glaucomatous optic neuropathy may also occur with these anomalies, and has been modelled in mice.
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The vitreous humor is a transparent, colorless, gelatinous mass that fills the space in the eye between the lens and the retina. It is surrounded by a layer of collagen called the vitreous membrane (or hyaloid membrane or vitreous cortex) separating it from the rest of the eye. It makes up four-fifths of the volume of the eyeball. The vitreous humour is fluid-like near the centre, and gel-like near the edges.
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A posterior vitreous detachment (PVD) is a condition of the eye in which the vitreous membrane separates from the retina. It refers to the separation of the posterior hyaloid membrane from the retina anywhere posterior to the vitreous base (a 3–4 mm wide attachment to the ora serrata). The condition is common for older adults; over 75% of those over the age of 65 develop it. Although less common among people in their 40s or 50s, the condition is not rare for those individuals.
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Confusion, decreased level of consciousness or coma may be present, as may neck stiffness and other signs of meningism. Neck stiffness usually presents six hours after initial onset of SAH. Isolated dilation of a pupil and loss of the pupillary light reflex may reflect brain herniation as a result of rising intracranial pressure (pressure inside the skull). Intraocular hemorrhage (bleeding into the eyeball) may occur in response to the raised pressure: subhyaloid hemorrhage (bleeding under the hyaloid membrane, which envelops the vitreous body of the eye) and vitreous hemorrhage may be visible on fundoscopy.
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The zonule of Zinn is split into two layers: a thin layer, which lines the hyaloid fossa, and a thicker layer, which is a collection of zonular fibers. Together, the fibers are known as the suspensory ligament of the lens. The zonules are about 1–2 μm in diameter. The zonules attach to the lens capsule 2mm anterior and 1 mm posterior to the equator, and arise from the pars plana region of the ciliary epithelium and pass forward closely related to the lateral surfaces of the ciliary process of the pars plicata.
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Persistent hyperplastic primary vitreous (PHPV), also known as persistent fetal vasculature (PFV), is a rare congenital developmental anomaly of the eye that results following failure of the embryological, primary vitreous and hyaloid vasculature to regress. It can be present in three forms: purely anterior (persistent tunica vasculosa lentis and persistent posterior fetal fibrovascular sheath of the lens), purely posterior (falciform retinal septum and ablatio falcicormis congenita) and a combination of both. Most examples of PHPV are unilateral and non-hereditary. When bilateral, PHPV may follow an autosomal recessive or autosomal dominant inheritance pattern.
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The white fluffy material is seen in many tissues both ocular and extraocular, such as in the anterior chamber structures, trabecular meshwork, central disc, zonular fibres, anterior hyaloid membrane, pupillary and anterior iris, trabecula, and occasionally the cornea. The flakes are widespread. One report suggested that the granular flakes were from abnormalities of the basement membrane in epithelial cells, and that they were distributed widely throughout the body and not just within structures of the eye. There is some research suggesting that the material may be produced in the iris pigment epithelium, ciliary epithelium, or the peripheral anterior lens epithelium.
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