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The Egg By the next day, continued cleavage has produced a hollow ball of thousands The frog egg is a huge cell; its volume is over 1.6 million times larger than a it. into a tadpole containing millions of cells but containing the same volume of During this entire process of cells called the blastula. A fluid-filled cavity, the blastocoel, forms within normal frog cell. During embryonic development, the egg will be converted material. The upper hemisphere of the egg — the animal pole — is dark. of the blastula are so small, the blastula looks just like the original egg to the unaided eye. The lower hemisphere — the vegetal pole — is light. When deposited in the water and ready for fertilization, the been run by gene products (mRNA and proteins) deposited by the mother when she formed the egg. Fertilization Meiosis II is completed. The cytoplasm of the egg rotates about 30 degrees relative to the poles. In some amphibians (including Xenopus), this is revealed by the Gastrulation The start of gastrulation is marked by the pushing inward ("invagination") of cells in the region of the embryo once occupied by the middle of the gray crescent. This produces: appearance of a light-colored band, the gray crescent. The gray crescent forms opposite the point where the sperm entered. The haploid sperm and egg nuclei fuse to form the diploid zygote As gastrulation continues, three distinct "germ layers" are formed: nucleus. Fertilization 1. a cluster of cells that develops into the Spemann organizer (named remarkable inductive properties). This becomes the notochord ventral (V) surfaces; its anterior (A) and posterior (P); its left and an opening (the blastopore) that will be the future anus after one of the German embryologists who discovered its It foretells the future pattern of the animal: its dorsal (D) and right sides. Not until the blastula contains some 4,000 cells is there any transcription of zygote genes. All of the activities up to now have haploid egg is at metaphase of meiosis II Entrance of the sperm initiates a sequence of events: there has been no growth of the embryo. In fact, because the cells ectoderm mesoderm endoderm Frogs practice external fertilization, meaning the male fertilizes eggs outside the female’s body. Some frog eggs after fertilization develop light-gray bands around them. The band expands on the opposite of the egg where the sperm cell entered. The way the band fans out determines in which direction the frog will develop inside the egg. During fertilization, the sperm and egg nucleus fuse into a diploid zygote nucleus. Cleavage The zygote nucleus undergoes a series of mitoses, with the resulting daughter nuclei becoming partitioned off, by cytokinesis, in separate, and ever-smaller, cells. The first cleavage occurs shortly after the zygote nucleus forms. Some are listed in the table. Germ-layer origin of various body tissues Ectoderm Mesoderm Endoderm skin notochord inner lining of gut, liver, pancreas brain muscles inner lining of lungs spinal cord blood inner lining of bladder all other neurons bone sense receptors thyroid and parathyroid glands sex organs thymus The Spemann organizer (mostly mesoderm) will: A furrow appears that runs longitudinally through the poles of the egg, passing through the point at which the sperm entered and bisecting the gray crescent. Each of these will have special roles to play in building the complete animal. This divides the egg into two halves forming the 2-cell stage 4 8 16 32 cell stage However, as cleavage continues, the cells in the animal pole begin dividing more rapidly than those in the vegetal pole and thus become smaller and more numerous. develop into the notochord, which is the precursor of the backbone; induce the ectoderm lying above it to begin to form neural tissue instead of skin. o This ectoderm grows up into two longitudinal folds, forming the neural folds stage. o In time the lips of the folds fuse to form the neural tube. o The neural tube eventually develops into the brain and spinal cord.