The role of the melanin synthesis pathway in human development - RESEARCH SEMINAR - NOTES

Conference 2 Pigmentation and the Visual Brain

Research Question: How is melanin connected to the visual system?
- Cortex Areas:
  - Parietal Cortex: Higher order integration, recognition, and memorization of objects and forms.
  - Occipital Cortex: Primary integration pathway for the perception of shape, color, and motion.
  - Temporal Cortex: Higher order integration for precise localization of animate or inanimate objects.

Where Does It All Go Wrong?:
- ↓Melanin impacts various regions: Iris, Fundus, Skin, Hair.
- Abnormalities:
  - Foveal Hypoplasia
  - High Refractive Errors
  - Nystagmus
  - Strabismus
  - Miswiring relating to Retinal ganglion cells.
- Potential Causes:
  - Genetics: Oculocutaneous albinism (OCA), Optic atrophy (OA), Hermansky-Pudlak syndrome (HPS), and other mutations.
  - Melanosome Abnormalities: Maturation, Enzymes, and Internal pH.

Key Features of Retinal Development in Albinism:
- Comparison of normal and albino retinal development.
- Focus on:
  - Foveal Development
  - Retinal Ganglion Cell Development

Layers and Functionality:
- Light Detection:
  - Photoreceptors: Rods and Cones.
- Layers of Retina:
  - Outer Nuclear Layer
  - Outer Plexiform Layer
  - Bipolar Layer: Processing signals with cells including:
    - Bipolar Cells
    - Horizontal Cells
  - Inner Nuclear Layer and Amacrine Cells
  - Inner Plexiform Layer: Output through Ganglion Cells to the brain via the optic nerve.

Research Technique:
- Hand-held Optical Coherence Tomography (OCT) used to measure retinal development from birth.
Observations:
- At Birth: Inner layers invade across the fovea, while outer layers are still developing.
- Development occurs from birth to 12 years of age gestationally.

Significance of Foveal Abnormalities:
- Comparative study of inner layers present in Albinism versus Controls.

Spectrum of Foveal Abnormalities:
- Cone photoreceptors lack specialisation at the fovea - appear shorter and less developed.
- Inner layers not adequately displaced at the fovea, resembling immature foveal structures.

Connections in the Brain:
- Nasal retina fibers cross to contralateral side.
- Temporal axons remain uncrossed, leading to distinct representation of visual fields in each hemisphere.

ABCDEF...: Representation of connections in Albinism compared to normal individuals, likely creating a atypical display in visual processing.

Neurogenesis Delay in Albinism:
- Transcription factors Zic2 and Islet2 play roles in retinal development classifications, contributing to misrouting in retina.

Two Main Features Identified:
- Foveal Development
- Retinal Ganglion Cell Development
- Overall pattern indicates delayed development in the retina.

Melanin products in melanosomes within the retinal pigment epithelium are essential for proper rod and cone responses.

Genetic Mutations Identified:
- OCA classification genes: TYR, OCA2, TYRP1, etc. Each with specific chromosome locations and mutation impacts on albinism.

Melanosome pH and Enzyme Activity:
- Importance of pH levels on enzyme activity and the overall production of melanin in ocular cells.

L-Dopa Reduction: Significant decrease during key retinal developmental periods in albinism, affecting retinal cell maturation and pigmentation response.

Growing body of evidence highlighting independence of certain visual processing systems from melanin production.

Unknown Functions: Many pigmentation genes lack defined roles, complicating the understanding of melanin production pathways.

OCA4 and FHONDA Types:
- Regulation of melanosome stability and effects on retinal development linked to specific protein expressions affecting cell fate.

Research articles that expand upon the theories and findings discussed related to retinal development and albinism.

Acknowledgment: Thank you from the Ulverscroft Eye Unit at the University of Leicester.