ANATOMY OF CHOROID.pptx

The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye. It contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye (at 0.2 mm), while in the outlying areas it narrows to 0.1 mm. The choroid provides oxygen and nourishment to the outer layers of the retin

1. ANATOMY OF CHOROID

2. EMBRYOLOGY The loose mesenchyme surrounding developing eye differentiates into an inner layer comparable with the pia mater of the brain and an outer layer comparable with the dura mater The inner layer forms a highly vascularized pigmented layer known as the CHOROID The outer layer develops into the sclera and is continuous with the dura mater around the optic nerve

3. THE CHOROID Most posterior part of the uveal tract- homologue of the pia arachnoid, which vascularizes the brain The choroid maintains but does not vascularize the outer retinal layers Extends from the optic nerve to the ora serrata, the scalloped peripheral margin of the retina Consists largely of vessels, and compared with cavernous tissue

4. THE CHOROID The thickness of the choroid estimated about 100micrometer anteriorly and 200micrometer posteriorly, with greatest thickness over the macula Central thickness in life estimated by ultrasound to be 500-1000micrometer Thinner in high myopia, congenital and chronic glaucoma

5. THE CHOROID The choroid is firmly attached to the margin of the optic nerve and loosely at points where vessels and nerves enter it Its attachment to the sclera is strongest behind the equator The inner surface of the choroid is formed by Bruch’s membrane If the retina, including the pigment epithelium, is stripped away, this presents a smooth, brown, glistening and transparent aspect

6. THE CHOROID On separating choroid from sclera, the outer surface appears roughened This is partly because the deep layer of the suprachoroidal lamina remains with the choroid, and partly because the outer surface contains many interwoven vessels The lamina suprachoroidea is interposed between choroid and sclera

7. THE LAMINA SUPRACHOROIDEA 10-34micrometer in thickness Anteriorly, it is continuous with supraciliary lamina of the ciliary body Its closely packed lamellae adjoin potential spaces, which become evident when the suprachoroid is pathologically distended by serous fluid or blood Seen to interlace at acute angles The lamellae consist of a delicate mesh of collagen fibres and run from the sclera anteriorly to the choroid

8. THE LAMINA SUPRACHOROIDEA They are shorter posteriorly, where they are more adherent to each other and to the sclera- hence, detachments of the choroid take place anteriorly and rarely pass behind the equator However, ultrasound examination can reveal suprachoroidal effusions posteriorly in the absence of ballooning The suprachoroidal space is traversed by the long and short ciliary arteries and nerves which supply the uveal tract

9. MELANOCYTES Spread out in the suprachoroid in the plane of the choroidal surface, interweave between lamellae and form an interlacing network with fibrocytes Their nuclei are flat and oval with long axes parallel to the choroidal curve Melanocytes also described around the optic nerve (Fuchs) When choroid and sclera are separated, part of the suprachoroid adheres to sclera and part to choroid, which accounts for shaggy appearance of the external choroidal surface

10. THE MUSCULO-ELASTIC SYSTEM The whole vascular system of the choroid, excluding the choriocapillaris, is surrounded by an elastic network which extends from Bruch’s membrane to the adventitia of the suprachoroid and partly into the elastica of the overlying sclera This elastic network is best recognized in tangential sections of the uvea and can be demonstrated with special stains Anteriorly, this fine elastic net extends into the connective tissue stroma of the ciliary body, including Bruch’s membrane of the pars plana

11. THE MUSCULO-ELASTIC SYSTEM Into this net, there inserts the posterior elastic tendon of the ciliary muscle With contraction of the ciliary muscle during accommodation, the elastic tendon of the muscle, like the elastic net of the choroid, is stretched The recoil of the elastica provides the energy required for disaccommodation These smooth muscle cells transmit the forces of accommodation to the elastic network of the choroid

12. THE MUSCULO-ELASTIC SYSTEM The vessels together with the Musculo-elastic system, Could provide a compliant pillow for the neighbouring retina Could protect the retinal ganglion cells, and the photoreceptor outer segments, from deformation and damage during eye movement or eye rubbing Also maintain the choroidal blood flow

13. VESSELS AND NERVES The suprachoroid contains the long and short posterior ciliary arteries and nerves, both myelinated and non-myelinated They divide into progressively smaller branches which supply the choroid

14. THE CHOROID Composed almost entirely of vessels Classically, three superimposed strata Outer layer of large vessels (Haller’s layer) Middle layer of medium-sized vessels occupying the choroidal stroma (Sattler’s layer) Internal layer of capillary vessels ( Choriocapillaries )

15. THE CHOROID The choriocapillaris is fed by arterioles derived from the short posterior ciliary arteries These arterioles do not pass directly into the choriocapillaris, but create a second capillary layer which shows few fenestrations and is covered by pericytes on its scleral side Internal to this is a non-cellular connective tissue possessing two basal laminae, two layers of collagen and a single layer of elastin, termed the membrane of Bruch

16. THE CHOROID Bruch’s membrane is intimately bound to the choriocapillaris and to the retinal pigment epithelium so this latter layer remains attached to the choroid in clinical retinal detachment Three major choroidal layers the stromal layer (layer of large and medium vessels) the choriocapillaris (layer of capillaries) Bruch’s membrane (non-cellular layer)

17. STROMAL LAYER Substantia propria contains vessels, nerves, cells and connective tissue The stromal cells include melanocytes, fibrocytes, macrophages, mast cells and plasma cells

18. MELANOCYTES Characterize the stroma and impart its brown colour Form an almost continuous layer in the outer choroid spreading in the plane of the choroid Cell numbers vary regionally with age, race and general pigmentation Most numerous around the optic disc, less so in the periphery and in the inner choroid Melanocyte nuclei are round, oval and show an even chromatin dispersal and no nucleolus

19. MELANOSOMES Pigment granules are fine, 0.3-0.4micrometer wide, oval in shape, yellowish to dark brown in colour , always smaller than that of the retinal pigment epithelium Occupies 70% of the cytoplasm

20. FIBROCYTES Fibrocytic processes intermingle with those of the melanocytes More dense in the outer choroid, and more numerous in males The collagen framework of the stroma is loose and randomly oriented The collagen encircles vessels to provide an adventitia The elastin in flat ribbons upto 13micrometer in length

21. CHORIOCAPILLARIS Consists of a rich capillary network which receives most of its blood from the medium and large vessels of the stroma Nourishes the pigment epithelium and the outer layers of sensory retina These capillaries are fenestrated, have a wide limb and consist of endothelial cells joined together by zonulae occludentes The endothelial cells are lined by a basement membrane which contains pericytes on the outer side of the capillaries

22. INNERVATION OF THE CHOROID In the choroidal stroma, each nerve bundle contains 50-100 axons which may lose their myelin sheaths as they enter the choroid but retain their schwann cells Postganglionic fibres arising from the ciliary ganglion remain myelinated Ganglion cells 40micrometer in diameter, appear in this layer Axons make contact with and indent the ganglion cells, and exhibit synaptic vesicles

23. INNERVATION OF THE CHOROID The vessels of the suprachoroid and stroma show dense parasympathetic and sympathetic innervation Sympathetic adrenergic fibres arise from the cervical sympathetic chain, have a vasoconstrictor action The parasympathetic innervation of the choroid is from the facial nerve and pterygopalatine ganglion , and from the oculomotor nerve via the ciliary ganglion and short ciliary nerves

24. INNERVATION OF THE CHOROID The perivascular and ganglionic neural plexuses appear to serve a vasodilator role in the choroid, perhaps adjusting blood flow in response to reduction in arterial blood pressure or protecting the retina from thermal damage associated with light exposure Submacular location of the intrinsic choroidal ganglionic plexus may afford additional protection from light damage at a site where light is focused and the photoreceptors and RPE are most susceptible Certainly Parva demonstrated a rise in blood flow in the choroid in response to light of high intensity falling on the retina

25. THE CHOROIDAL ARTERIAL SUPPLY The anterior choroid is supplied by recurrent and perforating branches of the anterior ciliary arteries and branches of the ciliary intramuscular artery The short posterior ciliary arteries supply most of the choroid The short ciliary arteries, after giving branches to the sclera, pierce it in the region temporal to the optic nerve and overlying the macula The space around the vessels contains loose tissue which is a prolongation of the suprachoroid

26. THE CHOROIDAL ARTERIAL SUPPLY The short posterior ciliary arteries are formed by the second and third- order divisions of the posterior ciliary arteries, close to the optic nerve head The smaller, paraoptic arteries supply the peripapillary choroid and a vertical trapezoid strip of choroid above and below the optic nerve head through branches of the anastomotic circle of Zinn and Haller The circle also supplies the retrolaminar part of the optic nerve

27. THE CHOROIDAL ARTERIAL SUPPLY In the absence of this circular anastomosis, its place is taken by small branches of the paraoptic short ciliary arteries, which lie within the sclera and supply portions of the optic nerve head In its complete form, the circle of Haller and Zinn is an intrascleral anastomosis between branches of the medial and lateral paraoptic short posterior ciliary arteries

28. THE CHOROIDAL ARTERIAL SUPPLY The branches of the circle of Haller and Zinn are Recurrent pial branches: four to seven from each segment; Small branches are given off to the retrolaminar nerve Recurrent choroidal branches supply the immediate peripapillary choroid, laminar and retrolaminar regions of the optic nerve head Arteriolo -arteriolar anastomosis occur between the components of the circle and the pial and recurrent choroidal arteries

29. THE CHOROIDAL ARTERIAL SUPPLY They bifurcate dichotomously and eventually divide into the choriocapillaris, the capillary bed of the choroid extending from optic disc margin to ora serrata Branches from the deep surface of the short posterior ciliary arteries, lying in the outer (Haller’s layer), give rise to the choroidal arteries of the intermediate layer (of Sattler) The short posterior ciliary arteries supply the posterior choroid up to the equator The temporal long posterior ciliary artery supplies a wedge-shaped sector of choroid

30. THE CHOROIDAL ARTERIAL SUPPLY The anterior part of the choroid is supplied by the recurrent ciliary arteries which arise in the ciliary body from the circulus iridis major and from the long posterior and anterior ciliary arteries before they join the muscular circle The choroidal arterioles appear anatomically to be end arterioles, they are not completely so in the functional sense, because choroidovascular occlusions frequently recover over a days

31. ANATOMICAL AND FUNCTIONAL VASCULAR UNITS Each terminal arteriole appeared to supply an independent segment, or lobule of choriocapillaris comprising a central feeding arteriole, the capillary bed and a series of peripheral draining venules Such lobules may be termed ‘ arteriocentric ’ Peripheral to the disc (beyond 3mm) and to the macula (beyond 2mm) and excluding the region between the disc and macula, the choroid has a regular, lobulated pattern

32. ANATOMICAL AND FUNCTIONAL VASCULAR UNITS A lobular pattern is absent in the peripapillary or submacular regions, where the wide-bore capillaries are interconnected in a rich honeycomb pattern Anteriorly, the lobules increase in size, and towards the ora serrata they are radially elongated In the posterior choroid, the arterioles and venules enter and leave the lobules at right angles to the plane of choriocapillaris The diameter of the arterioles entering the choroid at right angles ranges from 1 to 70micrometer, while the veins are 22-90 micrometer

33. ANATOMICAL AND FUNCTIONAL VASCULAR UNITS The submacular choroid is fed by 8-16 precapillary arterioles, which show frequent interarteriolar anastomosis Arterial endothelial nuclei form spindle-shaped impressions along the axis of the vessel, while venous nuclei produce round impressions which are randomly disposed along the vessel

34. ANATOMICAL AND FUNCTIONAL VASCULAR UNITS The deficient anastomosis between each zone creates vascular watersheds, regarded as important determinants of the shape and location of occlusive events in the choroid and at the optic nerve head

35. ANATOMICAL AND FUNCTIONAL VASCULAR UNITS Occlusion of the posterior ciliary or short posterior ciliary arteries gives rise to triangular zones of ischaemia , lying above or below the disc Occlusion of choroidal arterioles produces small foci of ischaemia which give rise to pale lesions seen as ‘ Elschnig spots’ The medial and lateral ciliary arteries each supply, by their short posterior ciliary branches, the nasal and temporal halves of the choroid The lateral posterior ciliary artery may supply up to two-thirds of the choroid

36. INTERCAPILLARY SEPTA Between the capillary meshes are bundles of collagen fibres which form so called intercapillary septa Because these occupy spaces between capillaries, they are round, oval or square in tangential section posteriorly and form elongated fillets at the equator and anteriorly The septa reinforced by fibres from the collagenous zones of Bruch’s membrane

37. INTERCAPILLARY SEPTA The capillaries are thus held in a relatively rigid collagen framework which prevents their collapse As in the retina, there appears to be a continuous flow in the choroidal capillaries

38. STRUCTURE OF CHOROIDAL VESSELS The arteries have a muscular tunica media and an adventitia of fibrillar collagenous tissue containing thick elastic fibres The arterioles possess muscular fibrils with long processes which surround the vessels like tentacles of an octopus The vascular adventitia is more or less continuous with the choroidal adventitia The veins have a perivascular sheath, outside which there is an adventitia of connective tissue

39. CAPILLARIES Capillaries of the choriocapillaris are large in calibre , allowing several erythrocytes to pass along together They are tubes of endothelial cells, with pericytes disposed only on the scleral face of the capillaries The ratio of pericytes to endothelial cells is about 1:2 in the retinal capillaries, the ratio is 1:6 in the choriocapillaris Pericytes are contractile cells, which in other tissues regulate blood supply or serve a nutritive function

40. CAPILLARIES Their disposition in choroidal capillaries would suggest that contraction is unlikely to bring about regulation of flow; this is in keeping with the constant high flow in the choroid Pericytes are more numerous at the fovea The capillaries are fenestrated (60-80nm in diameter) and account for the permeability of the choroid to small molecules (sodium fluorescein and even proteins)

41. BASAL LAMINA Bruch’s membrane or lamina vitrae Is the innermost layer of choroid 2-4micrometer in thickness near the disc and tapers in the periphery to 1-2 micrometer Multilayered structure which lies between the choriocapillaris and pigment epithelium of the retina Extends from the margin of the optic disc to the ora serrata

42. BASAL LAMINA On electron microscopy, consist of five layers Basement membrane of the retinal pigment epithelium (0.3micrometer) An inner collagen layer (1micrometer) A middle elastic layer An outer collagen layer The basement membrane of the choriocapillaris (0.14micrometer)

43. INNER BASAL LAMINA Is a continuous layer in continuity with the basal lamina of the ciliary epithelium Synthesized by the retinal pigment epithelium Its fine filaments blend with fibres of the adjacent collagenous zone

44. INNER COLLAGENOUS ZONE Composed of interweaving collagen fibres , some in the plane of the layer and others traversing the elastic layer to reach the outer collagenous zone 1micrometer thick, but thicker towards the ora

45. ELASTIC ZONE Composed of rod-like fibres with a dense cortex and homogenous core In cross-section, it is interrupted irregularly by collagen fibres passing between the collagenous zones In flat section, there are inter-woven bands of elastic fibres of varying thickness

46. OUTER COLLAGENOUS ZONE Traversed by collagen fibres from the inner zone which then pass through interruptions in the outer basal lamina to join collagen fibres of the intercapillary septa and the supraciliary region, contributing to the collagenous investment of the capillaries Vesicles, linear structures and dense bodies occur predominantly in the inner collagenous layer

47. OUTER BASAL LAMINA Deep stratum of the lamina which invests capillaries in the choriocapillaris Not continuous across the outer aspect of Bruch’s membrane At the ora serrata, the inner basal lamina continues forward into the ciliary body, but the outer lamina becomes separated from it by a well-marked connective tissue layer between them

48. BASAL LAMINA Bruch’s membrane becomes thickened with increasing age(>40 years), focal aggregations of debris are observed immediately external to the retinal pigment epithelium and produces hyaline excresences known as drusens These should not be confused with the congenital glial inclusions found rarely within the papilla and termed drusen of the optic nerve head

49. CHOROIDAL DETACHMENT Suprachoroidal space is a potential space which becomes a true space when filled with blood or fluid Firmly attached at the four vortex veins to the sclera which gives it a typical quadrilobed appearance of a large choroidal detachment Approximately 10microlitre of fluid in the space to allow for the choroid to smoothly glide over the sclera during accommodation Though no true lymphatics exist within the eye, the scleral opening through which these perforating vessels and nerves pass, may serve as lymph-like spaces

50. CHOROIDAL DETACHMENT Choroid is elastic and ordinarily under tension This tension creates an inward force which reduces the pressure in suprachoroidal space by 2-3 mmHg compared to anterior chamber The lower pressure in this space pulls the choroid towards the sclera Imbalance between fluid production and reabsorption Can be hemodynamic due to increased transmural pressure ( eg. Globe hypotony) or as a result of an alteration in the permeability of blood vessels Hemorrhage into suprachoroidal space usually after trauma or post surgery

51. CHOROIDAL BLOOD FLOW (THERMOREGULATION OF THE RETINA) Extremely high blood flow of the choroid protects the retina from damage in extreme hot or cold temperatures or from the heat generated during exposure to bright lights By acting as a heat source in the cold or as a heat sink for exogenous thermal radiation The choroid acts as heat source for the retina under LOW ILLUMINATION, when heat is lost through the cooler anterior chamber

52. CHOROIDAL BLOOD FLOW (THERMOREGULATION OF THE RETINA) Conversely, when flow was occluded under HIGHER ILLUMINATION Increase in choroidal temperature Loss of flow to the choroid acting as a heat sink Although protection from these temperature changes occur passively, choroidal circulation does not autoregulate, they are mediated by reflexive increases in choroidal blood flow in response to a stimulus

53. AGE RELATED MACULAR DEGENERATION

54. AGE RELATED MACULAR DEGENERATION In normal process of aging, a thickening of Bruch’s membrane and build of materials in inner collagenous layer Decrease in water permeability

55. AGE RELATED MACULAR DEGENERATION Impaired diffusion across Bruch’s membrane result in impaired diffusion of waste products from RPE Impaired delivery of hormones and oxygen to the RPE Atrophy of the RPE and the Retina

56. AGE RELATED MACULAR DEGENERATION Other factors include Decrease in thickness of choriocapillaris and the capillary lumen diameters Decrease in choroidal blood flow RPE degeneration Atrophic(Dry): Degeneration of RPE and underlying choriocapillaris Exudative(Wet): Choroidal neo-vascularisation ( Hemorrhage and RD)

57. CHOROIDAL EFFUSION In normal eye, the suprachoroidal space is nonexistent because of close apposition of the choroid to the sclera In pathological conditions that disrupt the normal ocular fluid dynamics , fluid accumulates in this potential space Serous- transudation of serum into the suprachoroidal space Hemorrhagic- blood accumulation from rupture of choroidal vessels Any process that shifts flow from the choroidal capillaries into the interstitium leads to effusion(tissue edema)

58. CHOROIDAL EFFUSION A decrease in IOP allows fluid to accumulate in interstitial spaces While inflammation increases the permeability of the choroidal capillaries Precursor for suprachoroidal hemorrhage

59. CHOROIDAL METASTASIS Choroid is the most common site for metastasis in the eye due to its extensive vascular supply Origin- breast, lung, gastrointestinal and kidney cancers Bilateral choroidal metastasis are usually due to breast cancer, while unilateral due to lung cancer Should be differentiated from uveal melanoma (primary tumor arising from the choroid)

60. REFERENCES Wolff’s ANATOMY OF THE EYE AND ORBIT Zia Chaudhuri- Postgraduate Ophthalmology Comprehensive Ophthalmology- A K Khurana https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2913695/