Choroid Anatomy & Histology Study Notes

Choroid Anatomy & Histology Study Notes

Key Concepts from the NBEO Outline on Choroid Anatomy

A. Choroid

  1. Extent

    • Extends from the ora serrata to the optic nerve.

  2. Thickness

    • Varies by age and location:

      • Thicker in younger individuals (200μm at birth).

      • Thins with age to about 80μm by age 90.

      • Thicker at the posterior pole (220μm) compared to the periphery (100μm).

      • Thicker in individuals with hyperopia and thinner in myopic individuals.

  3. Relationship to Lamina Fusca of Sclera

  4. Choriocapillaris

    • Ultrastructure and type of capillaries.

  5. Stroma

  6. Blood Supply

  7. Venous Drainage

  8. Innervation

  9. Bruch Membrane

    • Location and composition.

Structure of the Eye

  • The eye has three major tunics:

    1. Outermost Tunic - Fibrous Tunic

    • Composed of the Cornea, Limbus, and Sclera.

    1. Middle Tunic - Uveal/Vascular Tunic

    • Composed of the Choroid, Ciliary Body, and Iris.

    1. Innermost Tunic - Nervous Tunic

    • Consists of Sensory Retina and Retinal Pigmented Epithelium (RPE).

Functions of the Choroid

  • Nourishes the outer one-third of the retina.

  • Absorbs excess light that passes through the RPE.

  • Provides protective canal for vessels and nerves.

  • Stabilizes ocular temperature.

  • Contributes to Bruch’s membrane.

    • Highly vascularized and heavily pigmented; separates retinal and choroidal components.

Anatomical Information

  1. Location of Choroid

    • Lies between the retina (internal) and the sclera (external).

  2. Thickness Trends

    • Thickness generally decreases with age.

  3. Color

    • Dark color due to melanocytes, promotes light absorption.

Layers of the Choroid and Associated Structures

  • Choroidal Vascular Layers:

    • Choriocapillaris

    • Inner layer with small vessels.

    • Sattler’s Layer

    • Middle layer with medium-sized vessels.

    • Haller’s Layer

    • Outer layer with larger vessels.

Bruch's Membrane

  • Definition: A transition zone between the choroid and the retina composed of extracellular matrix materials contributed by both RPE and choroid.

  • Extends from the optic disc to the ora serrata; thickness varies (2-4μm at the optic disc to 1-2μm peripherally).

  • Functions:

    1. Physical barrier between retina and choroid.

    2. Molecular Regulation: Nutrient supply to and waste removal from retina, offering structural support to RPE.

Aging and Clinical Relevance

  • Normal Aging Changes:

    • Bruch’s membrane increases in thickness with age; decreased permeability affects metabolic exchange.

  • Clinical Connection:

    • Dry Age Related Macular Degeneration (AMD) associated with excessive deposits in Bruch’s membrane, impacting retinal function and nutrient exchange.

Choroidal Blood Supply

  • Arteries:

    1. Short Posterior Ciliary Arteries:

    • Branch from the ophthalmic artery, penetrates lamina fusca and contributes to choriocapillaris (15-20 branches).

    1. Long Posterior Ciliary Arteries:

    • One lateral and one medial branch, important for supplying the suprachoroidal space and overall choroid.

    1. Anterior Ciliary Arteries:

    • Branch off muscular arteries and loop inward to pierce the sclera and contribute to the choriocapillaris.

Nerve Innervation of the Choroid

  • Sympathetic:

    • Vasoconstriction → decrease choroidal blood flow.

  • Parasympathetic:

    • Vasodilation → increase choroidal blood flow.

  • Nerves travel with ciliary arteries near the optic nerve.

Venous Drainage

  • Vortex Veins:

    • Major drainage system of the choroid and anterior uvea, converging at the vortex ampulla before exiting the eye.

Summary of Bruch's Membrane Structure

  1. Consists of five layers:

    • Basement membrane of choriocapillaris

    • Inner collagenous layer

    • Elastic layer

    • Outer collagen membrane

    • Basement membrane of retinal pigment epithelium

  2. Importance in structural integrity between the retina and underlying tissues affects ocular health.

Reference Figures

  • Figures detailing choroidal blood supply, innervation, and structure of Bruch's membrane are important visual aids to understand the anatomy and function of these structures in the eye.