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The role of cellular and extracellular factors during mouth formation in embryos of the starfish Pisaster ochraceus Abed, Mona


The morphological changes in arrangement of both cellular and extracellular material (ECM) during mouth formation in embryos of the asteroid Pisaster ochraceus, have been studied using LM, TEM, and SEM. In early gastrula, the ECM consists of small vesicles and amorphous material of an intermediate staining density, and "beaded strands" consisting of scattered intermediately stained fialments associated with densely stained granules. Initially, the ECM is found in the blastocoel immediately adjacent to the ectoderm and endoderm cells. In later stages, it extends further into the blastocoel, until it bridges the gap between the ectoderm and endoderm. As this occurs, less amorphous material is seen and increasing numbers of "beaded strands" are present throughout the region occupied by the ECM. These are particularly evident and appear better organized in the quadrant of the embryo in which the mouth will form. Between 59 and 90 hrs after fertilization, several events occur in rapid succession. The first involves the migration of cells from the endodermal epithelium at the tip of the archenteron into the blastocoel to form mesenchyme cells. Their departure leaves a hole in the endodermal epithelium of the archenteron tip which is covered by the endodermal basal lamina. Shortly after this, the presumptive stomodeal cells send filamentous and conical cellular processes into the blastocoel. The endodermal basal lamina covering the hole extends as a blister into the blastocoel. At the same time the archenteron has become bent toward the presumptive stomodeal region. In the blastocoel, mesenchyme cells are enmeshed in "beaded strands" located in the presumptive mouth region. At this stage, the strands are highly branched, and tend to be radially arranged, and have become almost completely encrusted with densely stained material.- Filopodia of the mesenchyme cells connect with the filamentous ectodermal processes described above. The cell bodies of the mesenchyme cells appear to be connected with either the basal lamina blister itself, or with scattered endoderm cells located within the blister, through small gaps in it. At a slightly later stage, the basal lamina blister is seen to be in contact with the conical ectodermal processes. Still later, a complete tube of basal lamina has formed between the ectoderm and endoderm. Initially, only scattered cells are associated with the surface of the tube. Later, the tube is occupied by endodermal cells and invaginated ectodermal cells forming the stomodeum . For approximately 24 hrs after it is formed, the mouth is plugged with an oral plate consisting of both ectodermal and endodermal cells. These cells eventually round up, loose their connection with their neighbors, and are lost to complete the formation of the mouth. The above observations suggest that the ECM components are secreted into the blastocoel by both the ectoderm and endoderm cells. The components then appear to undergo a self-assembly into a filamentous meshwork. This meshwork appears to form a scaffold in which the mesenchyme cells migrate. The change in orientation of the fibers in the meshwork and the increase in densely stained material associated with it after mesenchyme cells migrate through it, suggests that they may be responsible, at least in part, for these changes. Mesenchyme cells tend to be localized to the quadrant of the embryo in which the mouth will form, before and during mouth formation. This is also the region in which the ECM is highly organized. The increased organization of the ECM may guide and concentrate the mesenchyme cells to the presumptive mouth region where they form associations with both the filamentous processes of the stomodeal ectoderm and the blister of basal lamina, either directly or via connections through the basal lamina to cells within it. Once this contact has been made, contractile filaments, known to be located in the filopodia, could serve to pull the basal lamina blister across the blastocoel to make contact with the conical ectodermal spikes. This appears to be followed by fusion of the blister of the basal lamina with that of the stomodeum forming the endodermal tube. Once formed, the endodermal tube appears to provide a framework for the organization of both the ectodermal and endodermal cells which will form the mouth.

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