Anatomy and Physiology of the Uveal Tract: Maintaining the Eye’s Internal Balance

Abstract

The uveal tract, comprising the iris, ciliary body, and choroid, is pivotal in maintaining the eye’s internal balance. The iris, with its pigmentation, regulates light entry through pupil control and contributes to eye color variation. The ciliary body, positioned behind the iris, facilitates accommodation by adjusting the lens for focusing at various distances and produces aqueous humor, crucial for intraocular pressure maintenance. The choroid, a vascular layer at the back of the eye, nourishes the retina. Together, these components uphold eye shape, control light entry, and support retinal function. Understanding uveal anatomy and physiology is fundamental to comprehending ocular mechanisms, encompassing both structural integrity and functional dynamics. This book chapter provides a knowledge of uveal anatomy and physiology.

1. Introduction

The uveal tract, encompassing the iris, ciliary body, and choroid, is integral to maintaining the delicate internal equilibrium of the eye. These components work synergistically to regulate light entry, support retinal function, and sustain intraocular pressure. Understanding the intricate anatomy and physiological interactions within the uveal tract is essential for comprehending the complexities of ocular health and vision. In this chapter, we delve into the detailed anatomy and dynamic physiology of the uveal tract, shedding light on its crucial role in preserving the eye’s internal balance and functionality (Figure 1) [1].

Figure 1.

The uveal tract consists of three parts: Iris, ciliary body and choroid. Adapted from the public domain, https://www.nih.gov/news-events/news-releases/eyes-cjd-patients-show-evidence-prions. Credit: Ryan Kissinger, NIAID.

The ciliary body produces aqueous humor, a clear fluid that fills the front part of the eye and helps to maintain the eye’s shape and pressure [2]. The choroid is a layer of blood vessels that provides oxygen and nutrients to the retina [3]. Together, these structures help to maintain the eye’s overall health and function (Figure 2) [4].

Figure 2.

Functions of iris (anterior most part of uveal tract). Created with the help of BioRender.com.

1.1 Iris

The iris is the colored part of the eye that surrounds the pupil, which controls the amount of light that enters the eye. It is responsible for regulating the size of the pupil and the amount of light that enters the eye. In humans, the iris is typically brown, blue, green, or hazel in color [3, 5]. The iris also contains a ring-shaped muscle called the sphincter pupillae, which constricts the pupil in response to bright light, and a muscle called the dilator pupillae, which dilates the pupil in dim light. Some eye diseases and conditions can affect the iris, such as iritis (inflammation of the iris) and heterochromia (difference in color between the irises) (Figure 3) [6].

Figure 3.

Parts of human eye image. Created with the help of BioRender.com.

(Anterior most part of uveal tract)

Thin circular disc.

Average diameter 12 mm.

Thickness 0.5 mm.

Center aperture 3–4 mm known as Pupil.

Pupil regulates the amount of light entering the eye [2].

1.1.1 Macroscopic appearance

1.1.1.1 Anterior surface

• Divided into ciliary and pupiallary

• Zone by a zigzag line collarette

1.1.1.2 Ciliary zone

• Radial streaks

• Straight when pupil dilated

• Wavy when pupil constricted

• Due to underlying radial vessels

1.1.1.3 Crypts

• Depressions where superficial layer of iris is missing

• Two rows Peripheral Iris root

• Central near Collarette

1.1.1.4 Contraction furrows

• Faint lines concentric to Collarette more marked on outer part of ciliary zone

• More prominent when pupil dilates.

1.1.1.5 Pupillary zone

• 1.6 mm wide

• Between collarette and pigmented papillary frill.

• Relatively smooth and flat

1.1.1.6 Pupillary frill

• Fringe of black pigment at Pupillary margin

• Due to slight extension of posterior pigment epithelium of iris round the edge of pupil

1.1.1.7 Posterior surface

• Dark brown or black

• Broken by furrows only

• Those near papillary margin are known as Schwalbe’s contraction folds

• Those which run downwards between the ciliary processes are known as Schwalbe’s structural folds besides several circular furrows [7].

1.2 Microscopic structure of iris

The iris is made up of two main layers: the anterior part of iris is stroma and the posterior part is iris pigment epithelium. The pigmented epithelium is a thin layer of cells that contains melanin, the pigment that gives the iris its color. The stroma is the thicker, more complex layer of the iris that gives it its shape. It is made up of fibroblasts, a type of cell that produces collagen and other fibers, as well as blood vessels and nerves [8].

The stroma of the iris also contains muscle fibers that make up the sphincter pupillae and dilator pupillae, which control the size of the pupil [9]. The iris also contains the crypts of the iris, which are small, deep recesses in the stroma that are thought to help protect the eye from injury by diverting debris away from the pupil. Overall, the iris is a complex and important part of the eye that plays a crucial role in regulating the amount of light that enters the eye and helping the eye to see clearly [1, 10].

Layers:

1. Anterior limiting layer

2. Stroma

3. Anterior pigmented epithelial layer

4. Posterior pigmented epithelial layer

5. Anterior limiting layer

   • Condensation of anterior part of stroma

   • Deficient at crypts, thin at contraction furrows

   • The type and density of pigment in this layer gives definitive color to iris

     • Blue iris—this layer is thin and has few pigmented cells

     • Brown Iris—thick and doubly pigmented [9]

6. Stroma

   • Main bulk of iris loose connective tissue

   • Contains

     • Pupil constrictor muscle

     • Pupil dilator muscle

     • Vessels nerves and pigment cells

     • Other cells lymphocytes, fibroblasts, macrophages etc.

1.2.1 Pupil constrictor muscle

• 1 mm broad band in pupillary part

• Flat circular band of plain muscle fibers

• Nerve supply parasympathetic fibers through third nerve

• Constrics the pupil

1.2.2 Pupil dilator muscle

• Present in posterior part of stroma of ciliary zone

• Nerve supply cervical sympathetic

• Dilates the pupil

1.2.3 Blood vessels

• Main bulk of stroma

• Branches from major arterial circle

• Responsible for radial streaks on anterior surface

• Straight when pupil constricts

• Wavy when pupil dilates

• Pigment cells melanocytes—branched} Clump cells—circular} both contain melanin [9].

1. Anterior epithelial layer

   • Gives rise to dilator pupillae

2. Posterior pigmented epithelial layer

   • It forms papillary frill at pupillary margin

   • Pigment cells are columnar type contain dark brown pigment granules.

1.3 Ciliary body

The ciliary body is a ring-shaped structure located in the eye, between the iris and the choroid. It plays an important role in maintaining the shape of the eye’s lens. The ciliary body also contains the ciliary muscle, which is responsible for changing the shape of the lens in order to adjust the eye’s focal length. This process is called accommodation and allows the eye to focus on objects at different distances. The ciliary body also produces aqueous humor, a clear fluid that nourishes the eye and helps to maintain its shape [11].

Ciliary body begins at ora serrata when choroid and retina ends (Figure 4).

Figure 4.

Anterior cross section of the human eye (ciliary body). Created with the help of BioRender.com.

Easily recognized by its black color (Whereas choroid is brown).

Symmetrical girdle, narrow above and nasally and broader below and temporally.

Triangular Apex continuous with choroid.

Base toward cornea.

Externally sclera.

Internally vitreous body.

Smooth near ora pars plana.

Finger like processes near base (anteriorly).

1.4 Microscopic structure

The ciliary body is a ring-shaped structure located behind the iris in the eye. It is composed of both muscle and glandular tissue. The ciliary muscle is responsible for controlling the shape of the lens, which helps to focus light on the retina. The ciliary body also contains the ciliary epithelium, which produces aqueous humor, a clear fluid that fills the front chamber of the eye and helps to nourish and maintain the shape of the lens and cornea [12]. Following are the main points to be remembered while studying the microscopic structure.

1.4.1 Supra ciliary lamina

• Outermost condensed part of stroma,

• Consists of collagen fibers (pigmented)

• Posteriorly continuous with suprachoroidal lamina

• Anteriorly continuous with anterior limiting membrane of iris

1.4.2 Stroma of ciliary body

• Consists of collagen and fibrblasts

• Contains

1.4.2.1 Ciliary muscle

Non striated, occupies outer part of ciliary body [13].

1.4.2.1.1 Three parts

• Longitudinal fibers—Origin sclera spur insertion suprachoroidal lamina

• Circular fibers—Present anteriorly and in inner portion and run parallel to limbus (Lies nearest to lens)

• Radial fibers—Obliquely placed fibers which continuous with circular fibers.

• Action—To slacken the suspensory ligament of lens and thus help in accommodation.

1.4.2.2 Vascular stroma of ciliary body

• Major arterial circle lies just anterior to circular muscle fibers

• Formed by anastomosis between long posterior ciliary arteries and anterior ciliary arteries

• Send branches to iris and ciliary body

2. Layer of pigmented epithelium

• Forward continuation of retinal pigment epithelium continues as anterior pigment epithelium of iris

3. Layer of non pigmented epithelium

• Forward continuation of sensory retina which stops at ora serrata. Continues as posterior pigment epithelium of iris

4. Internal limiting membranes

• Lines non pigmented epithelium and is forward continuation of internal limiting membrane of retina [14].

4.1 Ciliary processes

• Site of aqueous production

• Whitish finger like projection from pars plicata of ciliary body

• Number 70–80 Size 2 × 0.5 mm

4.2 Choroid

The choroid is a layer of tissue located in the eye between the retina and the sclera (the white of the eye). It contains blood vessels that provide oxygen and nutrients to the retina, as well as pigment cells that help to absorb light and reduce glare. Damage or disease of the choroid can lead to vision problems (Figure 5).

• Posterior part of vascular coat

• Extends from optic disc to ora

• Inner surface smooth, brown lies in contact with pigment epithelium of retina

• Outer surface rough in contact with sclera thicker

• Posteriorly (0.22) compared to Anteriorly (0.1)

Figure 5.

Choroid. Created with the help of BioRender.com.

Microscopic Structure Explained with flow chart (See Figure 6).

1. Supra choroidal lamina thin membrane of condensed collagen continuous anteriorly with supraciliary lamina.

   Space between this and sclera-supra choroidal space contains long ciliary and short posterior ciliary arteries.

2. Strom mainly formed by vessels arranged in two layers

   Outer large Haller’s layer.

   Inner medium Sattler’s layer.

   Also cells pigment cells, macrophages mast cells plasma cells

3. Choriocapillaris: Rich capillary network, Gets most of the blood from medium and large vessels from stroma. Nourishes pigment epithelium and outer layers of sensory retina.

4. Basal lamina (Bruch’s membrane)

   Multi-layered structure lies between choriocapillaris and pigment epithelium of retina becomes thickened with age and produces hyaline excrescence known as drusens.

Figure 6.

Microscopic structure of the choroid.

5. Conclusion

The present chapter was written to understand the detailed anatomy and the Microscopic examination of the eye’s Uveal region along with its structure and functions. This chapter will be good direction for the Graduate, Master and clinical optometry course students and will be a good note and reading material during the examinations and interview preparation for competitive exams as well as jobs.

Acknowledgments

I would like to thanks my PhD Guide Sr. Professor A.K. Khurana, Ex. PGI Rohtak for motivating us for to write the book chapter, for his continuous motivation for writing this book chapter, Authors are also thankful to Galgotias University for providing the basic amenities and affiliation. We are also thankful to the authors and the public domains whose images are used and the link are pasted just below the image.

We are also thankful to the Ken Pekoc (NIH/NIAID) for giving us permission for the use of image in our book chapter.

Notes

Figure 1 is adapted from the (NIH/NIAID) with author’s permission, Figure 2, Figure 3, Figure 4 Figure 5 and Figure 6 created with the help of biorender.com. Mr. Vamsi Kumar Attuluri created all the images for this book chapter.