Pale and dark bipolar cells in the chicken retina

Ultrastructurally, two different bipolar cell types can be distinguished in the retina of the chick embryo: one pale or electron-lucent and the other dark or electron-dense. The different electron density was seen over the whole cell, from its enclave in the outer limiting membrane to its termination in the inner plexiform layer. These observations prompted us to study the content and cytoplasmic variations of both cell types. The pale bipolar cell has a higher vacuole, vesicle and endoplasmic reticulum content and a lower number of microtubules and glycogen than the dark bipolar cell. The presence of these two cell types is probably due to a characteristic physiologic state, and the difference between the pale and dark bipolar cells can be attributed to the diverse number of gap unions which they establish with A II amacrine cells.