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Melanosomes of retinal pigment epithelium--distribution, shape, and acid phosphatase activity
1Department of Ophthalmology, School of Medicine, Kyungpook National University, Taegu, Korea.
Abstract:
The distribution and shape of melanosomes of the retinal pigment epithelium (RPE), and acid phosphatase activity in melanosomes were studied in rabbits. The rabbit eyes were observed using electron microscopy and enzyme cytochemical electron microscopy. The majority of melanosomes were located near the apical region of the RPE. Melanosomes in the RPE were classified as two shapes, elliptical and spherical or oval. Elliptical melanosomes were located parallel to the apical process and spherical or oval melanosomes were arranged vertically or obliquely to the apical process. We think that the distribution and shape of melanosomes contributes to the effective absorption and blocking of light coming from all directions. Almost all of the mature and immature melanosomes we identified showed positive in acid phosphatase reaction, indicating that melanosomes are commonly incorporated into the lysosomal system of the RPE. However, a few melanosomes showed negative in acid phosphatase reaction, suggesting that some melanosomes are stable and inert. The observed premelanosome showed negative reaction. Two types of melanosome-related complex granules were identified; melanosomes with a cortex of enzyme-reactive material (melanolysosome) and melanosomes with a cortex of lipofuscin (melanolipofuscin). These findings indicate tha a relationship between melanosomes and the lysosomal system of the RPE exists, and suggest that melanosomes may undergo modification or degradation in the cytoplasm. Also, the observation of a premelanosome and the positive acid phosphatase activity in mature and immature melanosomes indicates that melanosomes of the RPE may continue to be synthesized at a low rate in adult eyes.
Insights
Rabbit retinal pigment epithelium (RPE) melanosomes exhibit distinct shapes and distributions, contributing to light absorption. Acid phosphatase activity suggests melanosomes interact with the lysosomal system, with some appearing inert.
Area of Science:
- Ophthalmology
- Cell Biology
- Biochemistry
Background:
- The retinal pigment epithelium (RPE) plays a crucial role in vision by absorbing scattered light.
- Melanosomes, the pigment granules within RPE cells, are responsible for light absorption and scattering.
- Understanding melanosome morphology and function is vital for comprehending RPE physiology.
Purpose of the Study:
- To investigate the distribution and morphology of melanosomes in rabbit RPE.
- To analyze the acid phosphatase activity within RPE melanosomes.
- To explore the relationship between melanosomes and the lysosomal system in RPE cells.
Main Methods:
- Electron microscopy was employed to examine melanosome distribution and shape.
- Enzyme cytochemical electron microscopy was used to assess acid phosphatase activity.
- Rabbit eyes were the biological model for these investigations.
Main Results:
- Melanosomes were predominantly found in the apical region of the RPE.
- Two main melanosome shapes were identified: elliptical and spherical/oval, with distinct orientations.
- Positive acid phosphatase activity was observed in most mature and immature melanosomes, indicating lysosomal involvement, while premelanosomes were negative.
- Two types of complex granules, melanolysosomes and melanolipofuscins, were identified.
- Some melanosomes showed negative acid phosphatase activity, suggesting inertness.
Conclusions:
- Melanosome distribution and shape likely optimize light absorption and blocking in the RPE.
- The presence of acid phosphatase activity links melanosomes to the RPE's lysosomal system, suggesting degradation or modification.
- The identification of premelanosomes and acid phosphatase activity in developing melanosomes suggests ongoing synthesis in adult RPE.