What is the Difference Between Choroid and Sclera

The key difference between choroid and sclera is that the choroid is the middle layer of the eye that is located between the sclera and retina, while the sclera is the outermost layer that covers the eye.

The eye is a round ball-shaped organ with a slight bulge at the front. It is made up of three main layers. These layers normally lie flat against each other. They also form the eyeball. These layers include the sclera (the outermost layer), the middle layer, and the retina (the inner layer). Furthermore, the retina consists of two more layers called sensory retinal and retinal pigment epithelium (RPE). Therefore, the choroid and sclera are the two main layers of the eye.

1. Overview and Key Difference
2. What is Choroid
3. What is Sclera
4. Similarities – Choroid and Sclera
5. Choroid vs Sclera in Tabular Form
6. Summary – Choroid vs Sclera

What is Choroid?

Choroid is the middle layer of the eye located between the sclera and retina. It is a vascular layer filled with blood vessels. In addition to blood vessels, choroid contains connective tissue, as well. It is also known as choroidea or choroid coat. Choroid is a part of the uvea, which is also a vascular tissue of the eye. Choroid brings nutrients and oxygen to the outer layers of the retina.

The human choroid is the thickest and has a diameter of 0.2 mm. Moreover, in the outline areas, it narrows to 0.1mm. Together with the ciliary body and iris, the choroid forms the uveal tract. The structure of the choroid is normally further divided into four layers: Haller’s layer (outermost layer and layer containing larger diameter blood vessels), Sattler’s layer (layer containing medium diameter blood vessels), choriocapillaris (layer containing capillaries), and Bruch’s membrane. Furthermore, the choroid is the most common site for metastasis in the eye due to its vascular supply. This area is highly affected by metastatic breast and lung cancers. This area is also affected by primary tumors such as uveal melanoma.

What is Sclera?

Sclera is the outermost layer that covers the eye. It is also known as the scleral layer. Sclera is made up of dense connective tissue and contains a higher amount of collagen. It is an opaque, fibrous, protective outer layer containing collagen and elastic fibers.

In humans and some vertebrates, the whole sclera is white. But in most mammals, the sclera matches the colour of the iris. In children, the sclera is thinner and shows some of the underlying pigments. Sclera appears slightly blue in children. However, in adults, it appears slightly darker due to fatty deposits. Moreover, people with darker skin have naturally darkened sclera due to melanin pigmentation.

What are the Similarities Between Choroid and Sclera?

Choroid and sclera are the two main layers of the eye.
Both layers have connective tissue.
Both layers have a protective function.
They are extremely important for vision.

What is the Difference Between Choroid and Sclera?

Choroid is the middle layer of the eye situated between the sclera and the retina, while the sclera is the outermost layer that covers the eye. Thus, this is the key difference between choroid and sclera. Furthermore, choroid contains blood vessels, capillaries, and connective tissue. On the other hand, the sclera contains dense connective tissue made up of collagen and elastic fibers.

The below infographic presents the differences between choroid and sclera in tabular form for side-by-side comparison.

Summary – Choroid vs Sclera

Choroid and sclera are the two main layers of the eye. Both layers contain connective tissue and are extremely important for eye function. Choroid is the middle layer of the eye that is located between the sclera and retina. On the other hand, sclera is the outermost layer that covers the eye. Moreover, the choroid provides oxygen and nutrients to the retina. On the other hand, the sclera mainly plays a protective role. So, this is the key difference between choroid and sclera.

Reference:

1. “Choroid.” An Overview | ScienceDirect Topics.
2. “Sclera: Definition, Anatomy & Function.” Cleveland Clinic.

Image Courtesy:

1. “Blausen 0388 EyeAnatomy 01” By “Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia