Ontogeny of the visual system in the cuttlefish, Sepiella japonica. I. Morphological differentiation of the visual cell

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Abstract
To provide morphological bases for functional studies, the differentiation of photoreceptors in the retina of a cuttlefish, Sepiella japonica, was examined by electron microscopy. Development of the retina occurs in four phases. In phase 1 (stages 21–23), the retinal epithelium consists of uniform columnar cells. In phase 2 (stages 24–29), the epithelial cells are classified into light and dark cells on the basis of the electron-density of their cytoplasm. The perikarya of the light cells are located in the distal half of the epithelium, each extending proximally (i.e., sclerad) a thin foot, which protruded out of the epithelium through a narrow break in the basal lamina. The nuclei of the dark cells are distributed in the proximal half of the epithelium. Both cell types have a short cilium in the distal end. In phase 3 (stages 30–35), the light and the dark cells morphologically differentiate into receptor and supporting cells, respectively. The distal end of the light (receptor) cell begins to form an apical process, which gives off short irregular microvilli from its lateral surface. Beneath the basal lamina the feet of the receptor cells swell and the nuclei migrate into them through narrow breaks in the basal lamina. Thus, at the end of phase 3, each receptor cell becomes divided into an outer and inner segment. In phase 4 (stages 36–40), the receptor cells complete the specific structure as photoreceptive cells. The microvilli arising from the apical processes increase in length, density, and regularity to be organized into rhabdomes. The lamellated membranes of the myeloid bodies appear first in the outer segment and later increase in number in the inner segment. Morphometric data on the rhabdomeric microvilli in the adult and embryonic retinas are presented.