The Choroid
The choroid is the most vascular structure in the body and serves the most metabolically active tissue, the retina. As such the choroid is an integral portion of the visual system and highly prone to alteration and atrophy in highly myopic eyes. The chapt
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The Choroid Richard F. Spaide
The choroid is situated between the sclera and Bruch’s membrane and most of its substance is occupied by blood vessels; more than 70 % of all of the blood flow to the eye goes to the choroid [1]. The photoreceptors have the highest rate of oxygen use per unit weight of tissue in the body [2], and nearly all of that is accounted for by the mitochondria of the inner segments. The retinal circulation, which is about 5 % of the blood flow to the eye, supplies the inner retina, but the choroid supplies the oxygen used by the outer retina, including the inner segments. The choroid is the only source for the avascular fovea. The choroid has additional functions including acting as a heat sink [3], absorbing stray light, participating in immune response and host defense [4], and is an integral part in the process of emmetropization [5]. Although the ocular manifestations of high myopia have become apparent over the past centuries, appreciation of the abnormalities within the choroid has occurred only recently. High myopia is associated with profound changes in the choroid that are important in the pathogenesis of many important visually significant abnormalities. Advances in imaging have greatly increased our ability to visualize the choroid, providing an opportunity to better understand the choroid in health and disease.
the outer layer, the retinal pigment epithelium (RPE). The inferior portion of the cup initially has a gap that forms the choroidal fissure, which allows access for the hyaloid artery to enter the eye. Eventually the gap closes. The uvea develops from the mesoderm and migrating neuroectoderm that surround the optic cup. Mesodermal cells start to differentiate into vessels around the same time that the RPE appears. The choriocapillaris starts to form at about the fifth to sixth week. The basal lamina of the RPE and of the choriocapillaris defines the boundaries of the developing Bruch’s membrane by week 6 [6]. The choriocapillaris becomes organized with luminal networks well before the rest of the choroidal vasculature develops. The posterior ciliary arteries enter the choroid during the eighth week of gestation, but it takes until week 22 before arteries and the veins become mature. Melanocyte precursors migrate into the uveal primordia from the neural crest at the end of the first month but start differentiating at the seventh month. The pigmentation of the choroid begins at the optic nerve and extends anteriorly to the ora serrata. This process is complete by about 9 months [7]. The sclera is derived from mesenchymal condensation starting anteriorly and completing posteriorly by week 12.
9.1.2
9.1 9.1.1
The Embryology and Anatomy of the Choroid Embryology
Each optic vesicle forms as outpouching of the forebrain. This vesicle invaginates to form a double-walled optic cup. The inner layer of the cup is destined to form the retina and
R.F. Spaide, MD Vitreous, Retina, Macula Consultants of New York, 460 Park Ave, 5th Floor, New York, NY 10021, USA e-mail: rickspaide@gma
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