Development beyond the gastrula stage and digestive oraganogenesis in the apple-snail Pomacea canaliculata (Architaenioglossa, Ampullariidae)

E KOCH.

Tucumán

Workshop; POMACEA; 2010

Development of Pomacea canaliculata from the gastrula stage until the first day after hatching is described. Trochophore embryos are developed after gastrulation, showing the prototroch as a crown of ciliated orange-brownish cells. However, no true veliger embryos are formed, since the prototroch does not go beyond the formation of a double row of ciliated cells. Therefore doesn´t fully develop into a velum. The stomodaeum (foregut) can be recognized only as a funnel which opens dorsally into the midgut while the radular sac will develop at its ventral side later (about 24 hours later). The mouth is covered by a cephalic ectodermal flap, whose anterior section originates both eyes and tentacles, while the posterior one originates the right and left nuchal lobes. Both the midgut and hindgut show the characteristic reddish color of the eggs indicating that the albumen that flows through the foregut is concentrated there. The rudiment of the mantle cavity forms as an ectodermal depression, by the time the radular sac becomes visible, as a clear zone close to the anus. At this stage the midgut becomes full of the pink-reddish albumen, which is stored into a central archenteron’s lake, from where it is accumulated into the giant cells, which developed until being 100 µm high, forming the midgut wall. An incipient sulcus is being formed at the front of the embryo that will separate the head from the foot. At this time the midgut is pear-shaped and shows the albumen concentrating giant cells forming most of the wall of the midgut cavity with no infoldings. Nevertheless, a zone of smaller albumen-loaded cells appears at the posteroventral and right part of the midgut which will originate the adult stomach. Later on the midgut is differentiated into a larger anterior lobe and a smaller posterior lobe which is being included into the growing shell. The foregut/midgut connection is now occurring at the anteroventral aspect of the midgut. The colored hindgut is bent and the anus is now opening into the mantle cavity. In the context of digestive organogenesis the anterior lobe is reduced to a narrow club-like structure, while the posterior lobe is a hollow structure lined by giant cells and divides into two communicating lobes which will originate the “digestive” or “midgut” gland: (1) the right one occupying the shell apex, and (2) the left one extending dorsally to the right of the posterior kidney, and ventrally to cover the visceral aspect of the stomach. The closed space delimited by the posterior kidney (dorsally), the posterior wall of the mantle cavity (anteriorly) and the two parts of the left midgut lobe (ventrally and to the right), constitutes the posterior renal chamber, where the coiled part of the intestine is developing. By the time the embryos are about to hatch giant cells become gradually replaced by two new epithelial cell types which are similar to those found in the adult midgut gland: the pre-columnar and the pre-pyramidal cells. Pre-columnar cells have inconspicuous basal nuclei and are crowned by stereocilia, between which small endocytic vesicles are formed. Pre-pyramidal cells have large nuclei with 2-3 nucleoli and show a striking development of the rough endoplasmic reticulum. The genesis of the three cell lineages (giant, pre-columnar and pre-pyramidal cells) is hypothetically attributed to epithelial streaks that occur at both sides of the midgut since early stages of development.