Ultrastructure of the epidermis and digestive tract inSebastes embryos,with special reference to the uptake of exogenous nutrients

Synopsis Ultrastructural features of the epidermis and rectum were studied inSebastes schlegeli andS. melanops during the late stages of embryonic development, to confirm uptake of maternal substances. Ruthenium red (RR) and horseradish peroxidase (HRP) were used at fixation and in live embryos, respectively. Epidermal tissue of embryos after developmental stage 24 comprised two squamous cell layers. The outer, thinner cells and their intercellular spaces were easily infiltrated with RR, but the inner cells had no RR deposition. The HRP was not incorporated into the epidermis except in a few outer cells, which had well-developed microvillous projections of cytoplasm. Sacciform cells, chloride cells, and mucous cells distributed in the inner layer but protruding to the epidermal surface had no intracellular RR and HRP depositions. The rectal cells of embryos at about developmental stage 28 had many globular inclusions containing electron-dense substances. The rectal cells were found to take up and digest HRP actively. It is suggested that the embryonic epidermis is structurally loose and takes up low weight molecules, while rectal cells, after the opening of the mouth, actively ingest exogenous, high weight molecules.