Some special embryological words: Learn to talk like a pro. Polar bodies: Small cells formed on oocytes by the two meiotic divisions. Animal pole: The place on an embryo surface where the polar bodies formed. (In most species, yolk is less concentrated here. Frog eggs rotate with animal pole upward.) I know people who think that yolk density and distribution is what defines the animal pole, but they are wrong. It's the location of the polar bodies, and the oocyte nucleus) Vegetal pole: The side of an oocyte or early embryo 180 degrees opposite the animal pole. Blastomere: Any cell of a cleavage-stage or blastula-stage embryo (Why not just call them early embryonic cells? It's traditional.) Blastula: A stage of development of a sea urchin or an amphibian in which a hollow cavity has formed inside the embryo (with no connection to the outside) Blastocoel: The hollow (water filled) cavity inside a blastula. Gastrulation: Active movement of cells from the surface into the interior of a blastula. "Early gastrula": An embryo in which gastrulation has just begun.      Not many cells have moved into the interior, yet. Guess what is meant by "mid-gastrula" and"late-gastrula" stages. Invagination: inward folding inward of part of a epithelial sheet of cells (Caused by contraction of the concave ends of the cells; & there are other theories) Sea urchin gastrulation occurs by invagination. Human lungs form by invagination of part of the archenteron. Archenteron: The future digestive tract, formed during gastrulation. Blastopore: The opening into the archenteron. In sea urchin embryos, the blastopore forms at the vegetal pole. In frog and salamander embryos, the blastopore forms below the equator, on the side of the embryo that will become the posterior. Stomodeum: A small invagination that becomes the mouth. Connects to the archenteron. In sea urchin gastrulas, it forms near the animal pole. Ingression: Movement of epithelial cells out of an epithelium, and change to becoming mesenchymal. Partly caused by weakening of cell-cell adhesion, and increased adhesion to extracellular fibers on the side of the epithelium toward which the cells move. For example, the primary and secondary mesenchyme cells of sea urchin embryos are formed by ingression. In bird, reptile & mammal embryos, gastrulation is by ingression instead of by invagination. Involution: A variation of invagination (in frog & salamander gastrulation) in which epithelial cells "roll" into the interior of the embryo. The blastopore is a curved slit, when formed this way. As this slit elongates, its ends curve downward, and eventually meet each other, forming a circle ("yolk plug stage"). Epithelial fusion Some examples are the fusion of the archenteron with the stomodeum and the fusion of one neural fold with the other neural fold. Cavitation (Of masses of mesenchymal cells, to form epithelial tubes, or sheets), Reorientation of cells to form an epithelial sheet surrounding a water-filled cavity. The blastocoel is formed by cavitation. Teleost fish form their neural tube by cavitation, but other vertebrates form their neural tubes by folding.

 

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