. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. Figure 33. Mesoglea (arrow) of the hilayer stage stained with an anti-laminin monoclonal antibody, MG52. E, ectoderm; EN, endo- derm. Bar = 50 ^m. translucent. The layer becomes two to three times thicker, with the ectodermal cells changing from squa- mous to columnar in shape. The increased width and the absence of the opaque nurse cells render the layer translucent. This change may occur primarily because of the rapid accumulation of cells of the interstitial cell lineage among the cells of the outer layer during the bi- l
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. Figure 33. Mesoglea (arrow) of the hilayer stage stained with an anti-laminin monoclonal antibody, MG52. E, ectoderm; EN, endo- derm. Bar = 50 ^m. translucent. The layer becomes two to three times thicker, with the ectodermal cells changing from squa- mous to columnar in shape. The increased width and the absence of the opaque nurse cells render the layer translucent. This change may occur primarily because of the rapid accumulation of cells of the interstitial cell lineage among the cells of the outer layer during the bi- layer stage. Since the surface area of this layer is constant, the increasing mass of the interstitial cells may force the epithelial cells to undergo the observed shape change. At this point the outer layer is very similar to the adult ecto- derm. At the bilayer stage when the ectoderm is mostly com- plete, the definitive endoderm is nonexistent. The cells that will form the endoderm are still a disorganized mass in the interior of the embryo. Between the bilayer stage and hatching 2 days later, many of these cells begin to align themselves on the overlying ectoderm, changing in shape from roughly spherical to columnar (Figs. 43 and 44). This alignment occurs independently in several places, with the enlarging patches merging and fusing into a complete endodermal layer. These cells still con- tain large numbers of nurse cells and will do so for several days after hatching. The role of the nurse cells is unclear, although the most probable explanation is that they pro- vide a source of nutrients. The last step in the development of the two epithelial layers is the formation of the mesoglea between them (Figs. 43 and 44). This occurs after the alignment of the endodermal cells with the overlying ectodermal cells. Ev- idence in adult animals indicates that both layers are in- volved in the formation of the mesoglea (Epp ct a/., 1986; Sarras ct al., 1993). Most likely the same proc
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
