Cell surface changes during cleavage of newt eggs: scanning electron microscopic studies

Scanning electron microscopic studies were carried out on changes of the cell surface during the first cleavage of eggs of the newt, Cynopus (Triturus) pyrrhogaster. A dense population of elongated microvilli, which initially covered a double line indicating the initial furrow and stress marks running on each side of it, became located inside the pigmented area a little apart from its border with the unpigmented area when this borderline was established. This finding, together with data of carbon marking experiments, indicates that membrane growth occurs not only at the presumptive unpigmented surface, but also in a small portion of the presumptive pigmented area adjoining it. During cleavage within the vitelline membrane, a stable intercellular junction at the border of the pigmented and unpigmented surfaces was formed through the following successive processes: the appearance of individual lamellipodia on the borderline; contact of their apical filopodia with those of the opposite blastomere; contact of the lamellipodial bodies; and complete joining of the whole of the lamellipodia in contact. In cleavage of demembranated eggs, in which there is no chance of contact of lamellipodia of the two blastomeres, each lamellipodium persisted in an isolated state until the onset of the next division. In most cases, many strained or broken threads were seen across the space formed by opening of opposing blastomeres, at their peripheries. These threads were cytoplasmic in nature and showed cell contact. Similar threads were found stretched between blastomeres when the closed furrow was opened in medium containing EGTA.