Ultrastructural study of the tadpole stage of the primitive marine nematode Enoplus demani (Enoplia : Enoplida)

The ultrastructure of the tadpole (11/2 fold) embryo of the free-living marine nematode Enoplus demani was studied. The cells of the tadpole embryo show no distinct cyto-differentiation. Cytoplasm contains mitochondria and is filled with yolk granules and lipid droplets; nuclei contain clumps of condensed chromatin. The cells at the different stages of the mitotic cycle occur frequently in all the primordial embryonic layers. The centrosomes of the mitotic spindles include centrioles. The cells of the epidermis and the intestine form distinct layers of polarized cells underlain by basal lamina and connected at the apical ends by adherent junctions. The apical membranes of epidermal cells are slightly undulated, occasional cisternae of RER and Golgi bodies occur in the apical cytoplasm. The epidermis of the head-end forms many small funnel-shaped invaginations, interpreted here as the result of morphogenetic cell movement associated with cephalic sensilla formation. There is no distinct lumen in the tadpole intestine, rare microvilli occur squeezed between the apical membranes of intestinal cells. Presumptive muscle cells have no myofilaments, but mitochondria are abundant at the future basal (contractile) part of the cells. The pair of primordial germ cells (PGC) are positioned in the blastocoel latero-ventrally to the mid-part of the intestine. These cells are about twice the diameter of other cells of the embryo. The cytoplasm of PGC does not differ from the cytoplasm of somatic cells of the embryo, and specific P-granules are absent. Data on cell structure of E. demani tadpole embryo are compared with data on cell differentiation in Caenorhabditis elegans.