ANATOMY OF CILIARY BODY

In this article we discuss in detail about the anatomy of the ciliary body. In our previous post we studied in detail about the anatomy of iris. If you are an audio visual learner you can visit my video on the anatomy of ciliary body here. Also you may refer to the text below as a read along notes for the same.

To study about intermediate Uveitis; Check this post 

What is Ciliary Body ?

  • Ciliary Body is the middle part of the uveal tract between the iris and the choroid.
  • More specifically it the forward continuation of the choroid at the ora serrata.
diagram depicting the location of ciliary body as a middle part of the uveal tract

Sides of the Ciliary Body

  • The Ciliary body is triangular in cross section .It has a anterior surface , an outer side and an inner side.
  1. ANTERIOR SIDE : Forms part of the angle. In middle it is attached to the iris.This side is also known as the “face” of the ciliary body.
  2. OUTER SIDE : Lies against the sclera with intervening suprachoroidal space
  3. INNER SIDE : Divided into two parts – PARS PLICATA and PARS PLANA
diagram showing the triangular cross section of the ciliary body

Parts of Ciliary Body

  • The inner surface of the ciliary body is divided into two parts:
  • PARS PLICATA: Anterior Part of ciliary body which has finger like projections is known as the pars plicata. 
  • The finger like projections are known as the ciliary processes and they have zonules attached to them.
  • Pars plicata is highly vascular
  • PARS PLANA : The posterior part of the ciliary body is known as the pars plana.
  • It is relatively smoother and relatively avascular .

IMPORTANT MEASUREMENT  OF CILIARY BODY 

  • PARS PLICATA – 2-2.5 mm wide
  • PARS PLANA – 5 mm wide temporally, 3 mm wide nasally
image showing important dimensions of the ciliary body

CLINICAL NUGGET : PARS PLANA

  • PARS PLANA is relatively avascular.
  • Therefore this route is utilised to access the vitreous cavity in pars plana vitrectomy  and while giving intravitreal injections.

How to find pars plana?

  • Pars Plana is situated 4 mm from limbus in phakic individuals
  • Distance of pars plana is 3.5 mm from the limbus in pseudophakic/aphakic individuals.
image showing pars plana entry site into the vitreous cavity is 4mm from limbus in phakic eye.

Layers Of Ciliary Body

The ciliary body consists of 5 layers .From outside to inside :-

  1. Supraciliary Lamina
  2. Stroma Of Ciliary Body
  3. Layer Of Pigmented Epithelium
  4. Layers Of Non Pigmented Epithelium
  5. Internal Limiting Membrane
image showing 5 layers of ciliary body

Supraciliary Lamina

  • Supraciliary lamina is formed by the outermost condensed part of the stroma of the ciliary body
  • Anteriorly continuous with the anterior limiting membrane of the iris.
  • Posteriorly it is continuous with the suprachoroidal lamina of the choroid.

Ciliary Body Stroma

Ciliary Body Muscle: Ciliaris

  • Ciliary muscle is a non striated/ involuntary muscle of the ciliary body.
  • It forms the bulk of the ciliary body stroma.
    In cross section it is triangular in shape.
  • It has Three types of fibres:-
  1. LONGITUDINAL / MERIDIONAL MUSCLE FIBERS: These muscle fibres run parallel to the sclera and are attached to the scleral spur.
  2. CIRCULAR MUSCLE FIBERS : These are located anterior most and are form a circular band around the lens similar to the sphincter pupillae in iris.
  3. RADIAL/ OBLIQUE MUSCLE FIBERS: These fibres are located in between the longitudinal and circular fibres are are directed radially .

NERVE SUPPLY:

  • The ciliary muscle receives parasympathetic innervation from the third nerve postganglionic fibers.
  • The postganglionic fibers of the ciliary ganglion form short ciliary nerves that supply the ciliaris muscle and the sphincter pupillae. 
image showing muscle fibres of ciliaris muscle

ACTIONS OF CILIARY MUSCLE

ACCOMMODATION

  • Contraction of the ciliary muscle causes dcecrease in the distance between the ciliary processes and the lens .
  • This leads to slackening of the zonules.
  • This leads to an increase in the anterior curvature of the lens leading to ACCOMMODATION
  • Circular muscles mainly help in accommodation.

AQUEOUS DRAINAGE 

  • The longitudinal muscle fibers due to attachment to scleral spur, pulls the spur backwards on contraction leading to opening up of the trabecular meshwork.
  • This aids in drainage of the aqueous.
image depicting role of circular muscle fibres in accommodation
image depicting teh role of longitudnal muscle fibres in drainage of aqeuous humor. The longitudnal muscle fibres of the ciliary body are attached to the scleral spur

Vascular Stroma Of Ciliary Body

  • The vessels and capillary network form the vascular stromal component of ciliary stroma.
  • It consists of the major arterial circle.
  • Is situated just anterior to the circular muscle of ciliary body.
  • The major Arterial circle is formed by the long posterior ciliary arteries and the anterior ciliary arteries .
  • The circle then sends radial branches to the iris stroma.
  • The vascular stroma structure of the ciliary processes is discussed below .

Layer Of Pigmented Epithelium

  • It is the pigmented epithelium of the ciliary body.
  • It is also known as the outer epithelium .
  • It is forward continuation of the retinal pigment epithelium of the retina
  • Continues anteriorly with the anterior epithelium of the iris
image showing anterior continuation of the pigmented epithelium of the ciliary body as the anterior epithelium of the iris and posterior continuation as retinal pigment epithelium

Layer Of Non Pigmented Epithelium

  • It is also called as the inner layer of epithelium
  • Forward continuation of the neurosensory retina
  • Continues anteriorly with the posterior pigmented epithelium of the iris.
  • There are tight junctions present within the cells of the posterior epithelium, which do not allow the passage of proteins into aqueous humor .
  • Therefor they form a part of the blood- Aqueous barrier 
non pigmented epithelium of ciliary body continues with neurosensory retina

Layer Of Internal Limiting Membrane

  • The layer of limiting membrane is the forward continuation of the internal limiting membrane of the retina
image depicting anterior and posterior continuation of the ciliary epithelium

Microscopic Anatomy Of The Ciliary Processes

  • The ciliary processes are fingers like projections from the pars plicata part of the ciliary body
  • About 70-80 in number
  • Each is about 2 mm in length.

ULTRASTRUCTURE OF CILIARY PROCESS 

  • Each ciliary process basically is composed of :-
    • THE CAPILLARY NETWORK
    • THE CILIARY STROMA
    • TWO LAYERS OF EPITHELIUM (pigmented and Non pigmented)
image showing the microscopic structure of the ciliary process

Capillary Network of Ciliary Process

  • Each process consists of the network of capillaries.
  • The arteriole enters each ciliary process, forms a capillary network and ends in a venule .
  • These arterioles come from the major arteriolar circle.
  • These fenestrations are covered by a diaphragm.
  • Therefore , these are sites of increased permeability.
  • Facilitates ultrafiltration of water and ions into the adjacent stroma
  • The fenestrations also allow larger plasma-derived proteins, such as myoglobulin or gammaglubulin, to enter the stroma,
  • This leads to a high oncotic pressure in the ciliary process stroma

Stroma of Ciliary Process

  • The ciliary process stroma is very thin.
  • It separates the network of vessels from the epithelial cells
  • It is composed of ground substance and collagen fibres and few cells
  • The ground substance contains mucopolysacchrides, protein, and a solute of plasma

Epithelial layers in Ciliary Process

SALIENT FEATURES OF CILIARY EPITHELIUM 

  • It is bilayered consisting Pigmented and Non pigmented Epithelium 
  • The epithelium is Neuro-ectodermal in origin since it takes origin from the anterior end of the optic cup embryologically .
  • The epithelium has all the features which are present in cells associated with energy dependent transport process. 
  • It means that the cells of the ciliary epithelium have sodium potassium ATPase pumps and also carbonic anhydrase enzyme.
  • Although the two layers are anatomically different they are electronically coupled due to the presence of gap junctions between them. These gap junctions allow free transport of ions and other molecules across the two layers.
  • It is because of this reason that both the epitheliums act as a syncytium.

Pigmented Ciliary Epithelium

  • Low cuboidal cells
  • Numerous cytoplasmic melanin granules
  • Separated from the stroma by an ATYPICAL basement membrane , thought to be a continuation of the bruchs membrane
 
 

Non Pigmented Ciliary Epithelium

  • Columnar cells
  • Numerous cytoplasmic mitochondria and endoplasmic reticulum
  • Separated from the aqueous by a basement membrane
  • Tight junctions on apical surfaces form the  blood aqueous barrier

Apex to Apex Arrangement of the Bilayer epithelium

  • The two layers of epithelium , pigmented and non pigmented epithelium are placed APEX TO APEX
  • It means that their apices face each other and the basal laminas are present on the either side.
  • The basement membrane of the pigmented epithelium is towards the ciliary stroma; and the basement membrane of the non pigmented epithelium is present towards the posterior chamber .
image showing apex to apex arrangement of the bilayer epithelium of the ciliary body

IMPORTANT CELLULAR JUNCTIONS BETWEEN THE CILIARY EPITHELIAL CELLS :-

GAP JUNCTIONS

  • Low resistance pathways between the Pigmented epithelial cells and non pigmented epithelial cells.
  • Facilitate transport of ions and other molecules from one cell to another
  • Two types of cells act a functional syncytium
  • According to research, ion transport between the epithelial cells across gap junctions is crucial for aqueous humour secretion.
  • It has been shown that in mice with a conditional inactivation of connexin leads to non formation of  gap junctions  and a substantial reduction in aqueous humor production.

ADHERENS AND DESMOSOMES

  • Adherens and desmosomes are structural supports between cell membranes.
  • These maintain structural integrity. 
  • The desmosomes are the strongest of cell adhesion molecules.

TIGHT JUNCTIONS

  • Tight junctions (zonulae occludentae) connect non-pigmented ciliary epithelial cells at their apical membranes and are regarded to be a key component of the blood-aqueous barrier.

CLINICAL NUGGET : BLOOD AQUEOUS BARRIER 

The structures forming the blood aqueous barriers are as follows : 

  • The tight junctions between the cells of the non pigmented epithelium
  • Tight junctions between the posterior iris pigment epithelium
  • Non fenestrated Iris Vessels.

According to the new concept, while plasma proteins that leak from the ciliary body’s capillaries cannot cross the blood-aqueous barrier, they can travel to the iris stroma and diffuse into the anterior chamber via the iris’s anterior surface, which is not protected by an epithelial layer.

iamge showimg the new concept about the blood aqueous barrier

Anatomy of Zonules

The zonules arise from the ciliary body and are inserted into the lens equator. For detailed anatomy of the zonules, refer to post on anatomy on lens.

Conclusion

So that was the detialed anatomy of the middle part of the uveal tract. The ciliary body is an important structure that is involved in the aqueous formation. More about aqueous formation will be discussed in the post on aqueous humour dynamics.

If you are an audio visual learner you can check out our video on the ciliary body anatomy here.

MORE INSIGHTFUL ARTICLES

2 thoughts on “Anatomy Of The Ciliary Body”

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top
Copy link
CopyCopied