Choanocytes of all sponges fixed 5-10 min after being fed were already filled with phagosomes containing either beads or bacteria, suggesting that uptake occurs within only a few minutes of particles entering the animal.
Most particles were found in association with the cell surface, lodged between neighboring choanocytes, rather than on the collar microvilli (Fig.
A total of 30 lateral and 12 apical extensions were counted in 44 images; 9 extensions were directed into the chamber, while 33 contacted particles that were between choanocytes.
The validity of the varied mechanisms of particle uptake we introduced experimentally is substantiated by examples of natural uptake of intact or broken diatoms (2-3 [micro]m) by five separate choanocytes (Fig.
Examination of a large number of electron micrographs revealed that the flagellated cells become the three principal cell types of juvenile sponges--that is, pinacocytes, scleroblasts, and choanocytes.
The choanocytes of juvenile sponges contain autophagosomes whose contents have been almost entirely digested.
As the gastral cavity develops, the choanocytes are flattened except at their thick central region.
In amphiblastulae, another larval type of calcareous sponges, the flagellated cells become scleroblasts and choanocytes but not pinacocytes (Amano and Hon, 1993).
We suggest that such amoeboid cells are precursor cells of choanocytes.
Minute ellipsoid granules were found in choanocytes that were still in the course of differentiation about 36 h after settlement.
In calcareous sponges, the consensus is that the choanocytes of juveniles derive from larval flagellated cells (Duboscq and Tuzet, 1937; Amano and Hori, 1993).
In cases where a supply of larval flagellated cells is not available, choanocytes can derive from archeocytes.