Lecture 3
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| Holoblastic | isolecithal and mesolecithal eggs
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| Radial | Echinoderms and amphibians (mesolecithal - displaced radial)
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| Spiral | Annelids, mulluscs, and flatworms
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| Bilateral | Tunicates
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| Rotational | Mammals and nematodes
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| Cleavage | rapid mitotic division after fertilization
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| Most mammalian species | DNA replication is very intense and therefore no transcription occurs.
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| Process controlled by | maternal effect factors (maternally produced mRNA’s and proteins)
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| During ______ there is an increase in cell number but there is no increase in cell size. Cleavage leads to the formation of a blastula. In a blastula blastomeres surround blastocoel. | cleavage
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| Holoblastic cleavage | Entire cytoplasm of Zygote is partitioned into cells. Seen in species with low to moderate yolk content
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| Meroblastic (incomplete) | telolecithal and centrolecithal eggs
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| Discoidal (telolecithal) | Fish, reptiles and birds
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| Superficial (centrolecithal) | Most insects
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| Meroblastic cleavage | incomplete cleavage
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| This occurs in both ____ and ____ eggs | telolecithal and centrolecithal
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| Radial cleavage | planes are parallel or right angles to the animal – vegetal axis. Produce animals that are radially symmetric i.e., sea urchins or frog. In frog site of sperm entry (180 opposite grey crescent) causes symmetry.
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| Frogs Cleavage - Radial | Blastula is formed at the 128 cell stage with the blastocoel in the animal hemisphere.
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| Sea Urchin Cleavage - Radial | Have a blastula at 128 cell stage
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| No blastocoel is produced | stereoblastula
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| Spiral cleavage occurs in | snails - Cleavage planes are at oblique angles to animal -vegetal axis and produce a spiral arrangement
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| Rotational cleavage occurs in | mammals - 1st Cleavage is Meridional
2nd Cleavage is Meridional in one cell and equatorial in the other
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| Fluid secreted by trophoblast causes blastocoel formation Entire structure is called the | blastocyst
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| The ____ is the mammalian equivalent of a blastula but it is different in that it has the internal cells. | blastocyst
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| Discoidal cleavage occurs in | birds. Birds have telolecithal eggs - Cleavage is restricted to small disc of yolk free cytoplasm (above the yolk)
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| Cleavage produces single layer of cells called | blastoderm
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| Over time cells in center are lost becoming 1 cell layer thick. This is called the ____ ____. Surrounding area (5-6 cells thick) is called the _____ ____ | area pellucida and area opaca
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| New cells produced from the area opaca and pellucida form lower layer called the | hypoblast
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| Upper layer is called the ___ and in between the two layers is the Blastocoel. | epiblast
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| Superficial Cleavage occurs in | Drosophila. Drosophila have centrolecithal eggs.
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| A ____ is a single cell that has multiple nuclei. | Syncytium
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| Cleavage is restricted to nuclei at surface of egg. In Drosophila there are 7 rounds of synchronous mitosis. Cytokinesis does not occur yielding a single cell with many nuclei (syncytium). | .....
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| Cellularization | is the process by which cell membranes form around the nuclei.
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