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Transcript
Gametogenesis, Fertilization and Blastula Formation Spermatogenesis The production of mature sperm cells in adult males Continuous process in adult males; can make 100’s of millions of sperm per day For each cell that enters meiosis, 4 sperm cells are produced It is continuous for the reproductive life of a male and occurs in an uninterrupted sequence Sperm structure Head: contains the haploid nucleus and an acrosome—vesicle containing enzymes Middle piece: contains a long, spiral shaped mitochondria which uses fructose as an energy source Tail: flagellum for movement Oogenesis The development of ova (mature, unfertilized egg cells) Begins in the embryo when primordial germ cells undergo mitosis to produce diploid oogonia Each oogonium develops into a primary oocyte by the time of birth resulting in all potential ova being present in the ovaries at birth Between birth and puberty they enlarge and the surrounding follicles grow; they replicate their DNA and enter prophase 1 where they stay until activated by hormones After puberty, during each cycle, hormones stimulate the oocyte to complete meiosis 1 which is then released from the follicle; if a sperm cell penetrates the membrane meiosis 2 occurs to complete oogenesis Is under the control of many hormones Egg structure Egg cell: contains a nucleus and lots of cytoplasm and organelles; has a heterogeneous distribution of mRNA, proteins, and other chemicals; surrounded by the plasma membrane and 2 other layers Vitelline layer: contains species specific receptor sites Jelly coat: outermost layer Fertilization 1. Sperm encounters the jelly coat and the acrosome releases enzymes that digest a cavity into the jelly coat (acrosomal reaction) 2. Recognition proteins on the sperm bind with the receptor site on the egg 3. The plasma membranes of the sperm and egg fuse 4. The cell becomes activated which prevents polyspermy (fast block) 5. The sperm nucleus enters the egg cell 6. The fertilization membrane forms (slow block) through the cortical reaction which causes the vitelline layer to swell, separate from the plasma membrane and harden 7. Sperm and egg nuclei fuse creating a zygote Phase 1: cleavage—rapid succession of cell divisions that produce a ball of cells (the blastula) from the zygote Cells replicate DNA and divide rapidly but no protein synthesis takes place Different regions of the zygote can divide at different rates Nutrients from the egg cytoplasm (yolk) nourish the zygote The cell division organizes the developing embryo into developmental regions The morula (solid ball of cells) gets rearranged into the blastula (hollow ball of cells) with a fluid filled center (blastocoel) The yolk gradient is a key factor in determining the polarity of the blastula: the most yolk forms the vegetal pole, the least forms the animal pole (forms the anterior of most animals) Phase 2: Gastrulation: sorts the cells of the blastula into distinct germ layers Phase 3: organogenesis: formation of organs from the germ layers