The choroid is the vascular layer of the eye located between the retina and sclera. It develops from mesenchyme surrounding the eye. The choroid consists of three layers - an outer layer of large vessels, a middle layer of medium vessels, and an inner layer of densely packed capillaries. It receives its blood supply from the short and long posterior ciliary arteries and is innervated by both the sympathetic and parasympathetic nervous systems to regulate blood flow. The choroid plays an important role in nourishing the outer retina and maintaining a smooth interface for retinal attachment.

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
i. Outer layer of large vessels (Haller’s layer)
ii. Middle layer of medium-sized vessels
occupying the choroidal stroma (Sattler’s layer)
iii. 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
Water and Ions Nutrients
Plasma-borne protein
molecules
BRUCH’S MEMBRANE

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/