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. In two cases the extensions were clearly formed by fusion of collar microvilli (e.g., Fig.
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.
All sizes and shapes of particles were ingested, and all choanocytes took up particles regardless of size or shape of collar microvilli.
Choanocytes are flattened, but their central part projects into a gastral cavity.
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. This conclusion is supported by the fact that these three cell types contain glutinous granules, which only the flagellated cells have in the free-swimming larva.
The choanocytes of juvenile sponges contain autophagosomes whose contents have been almost entirely digested.
Lastly, the cells of an inner cell mass differentiate to choanocytes. In the inner cell mass, a space develops de novo among the cells and becomes larger as metamorphosis progresses.
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).
Our conclusions in the present paper are not contrary to the concept that the archeocytes are multipotential and capable of differentiating into choanocytes. In cases where a supply of larval flagellated cells is not available, choanocytes can derive from archeocytes.