The lobster optic lamina: I General organization

Summary The lamina is 150-220 µ thick and has five layers: (a) ganglion cells (2nd-order neurons); (b) a thick-layered glial sheath; (c) more ganglion cells of the same kind; (d) columnar structures called optic cartridges, where retinula fibres terminate in inflated bags which are penetrated by numerous spines of the ganglion cell axons; and (e) a lower multilamellate layer of glial cells among which are neurosecretory cells with short centrifugal axons. Horizontal nerve fibres run along the lamina in the columnar region, where there is a I:I relation between ommatidia and cartridges. Retinula fibre terminals are recognized by numerous vesicles and by large pale mitochondria, ganglion cell spines by small dark mitochondria and lack of vesicles. Ganglion cell axons have neurotubules whereas transverse fibres do not. The latter have both synaptic and other vesicles. Some of the horizontal fibres are secretory in appearance.