Formation of the ectodermal organs during the metamorphosis and definitive organogenesis in the holothurian Apostichopus japonicus

Abstract

The fine structure of the ectodermal organs (epidermis and nervous system) in late doliolaria, pentactula, and 1-month-old juveniles of the holothurian Apostichopus japonicus was studied. The definitive structure of the epidermis is shown to start forming prior to metamorphosis. The cell types of late doliolaria epidermis are the same as those in adult holothurian epidermis, consisting of support, secretory, and granular cells. The rivet-like structures are also present already at this stage of development. The ciliary rings of late doliolaria represent ribbon-like aggregations of tall ciliary cells. The destruction of ciliary rings is due to the programmed death of ciliary cells, which are then utilized by mesenchymal cells. Changes to the epidermis during metamorphosis are linked to the sinking of the cells into the underlying extracellular matrix, resulting in the formation of the typical sunken epidermis of the body and appendages. The nervous system in late doliolaria consists of a nerve ring, five radial nerve cords, and tentacle and tube foot nerves. The nerve ring and the radial nerve cords represent epithelial tubes. First hyponeural cells appear in the mid-ventral nerve cord during the late doliolaria stage. In the rest of the nerve cords, the hyponeural part begins developing only after the settlement. Functioning of the adult nervous system in holothurians begins only after metamorphosis, following the complete loss of provisory organs and the switch to benthic life.