Cl05 - MCDB 4650 DEVELOPMENTAL BIOLOGY CLASS NOTES Class 5...

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MCDB 4650 DEVELOPMENTAL BIOLOGY CLASS NOTES Class 5 Fertilization and early embryonic cleavage Reading : Chp 7: 175-180, 205-206--skim rest of chapter if you want; cleavage: Chp 8: 211-215, 243- 246; Chp 10:291-295. You may also read about cleavage in chick and mammals if you like (Chp 11: 336-337; 348-349). Learning goals : Be able to: Describe, in a general way, the process of fertilization and the different features of sperm and oocyte. Explain how the orientation of cleavage planes is determined. Explain the consequences of cleavage planes on establishment of cell fate. Describe the onset and consequences of the "mid-blastula transition", and compare its importance in different organisms. Fertilization Fertilization involves some spectacular cell biology, but in terms of development it simply provides the stimulus to start the process. It serves three major developmental functions: bringing in the paternal genome, bringing in a new centrosome, and re-starting the cell cycle in the arrested oocyte. Contact of sperm with egg initiates a sequence of pre-programmed responses of the two gametes to each other. These result in: invasion of the egg by the sperm, transmembrane signals that block further sperm entry to prevent polysomy and activate the egg's metabolism, triggering of lipid and macromolecular synthesis in preparation for cleavage, re-initiation of the cell cycle, and finally fusion of sperm and egg nuclei to produce a diploid zygote with a combination of maternal and paternal genetic information. Interestingly, in almost all animals, the process of ooctye meiosis is not complete when the sperm enters the egg (7.5). In amphibians, fish, and most mammals, the oocytes is arrested in metaphase of meiosis II. Accordingly, the oocyte nucleus still has two copies of each chromosome when the sperm nucleus penetrates the egg! The oocyte has to complete meiosis, shunting off those chromosomes into the last polar body (7.32) before fusing with the sperm nucleus. The fertilized egg then goes immediately into mitosis--the chromosomal material from the egg and sperm replicate separately, and then all the chromosomes line up along the metaphase plate, and go on to complete the first mitotic division of the embryo. Finally, at the now 2-cell stage, each cell has an integrated complete set of information from each parent, and development can proceed. Cleavage Cleavage is the first stage of embryogenesis. The processes that occur during cleavage accomplish several things: 1) partition the contents of the zygote into many cells (called blastomeres), with no increase in total volume; 2) begin to establish different cell identities and separate certain cells from each other; 3) form a hollow ball (or disc) of cells called the blastula, and 4) shift control of development from maternally derived mRNA's and proteins to embryonically encoded gene products. Also during cleavage, the axes of the embryo (A/P, D/V, L/R) become established; we will discuss this process in more detail later. Embryos of different species differ in the
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Cl05 - MCDB 4650 DEVELOPMENTAL BIOLOGY CLASS NOTES Class 5...

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