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Animal Development Mammalian FertilizationSperm binds to receptors in zona pellucida (extracellular matrix of egg)Acrosomal reaction: sperm releases hydrolytic enzymes to digest z.p. (Sea Urchins) Depolarization of membrane: prevent other sperm from binding = fast block to polyspermySperm + Egg FuseCortical reaction: sperm + egg fusion triggers release of Ca2+Ø cortical granules fuse with z.p. à z.p. hardens to form fertilization envelope = slow block to polyspermyCa2+ release also triggers activation of the egg Cleavage: rapid mitotic cell division• Zygote cytoplasm partitioned into smaller cells (blastomeres)• Solid ball of cells = morula• Blastula (hollow ball of cells) filled with fluid (blastocoel)§ Blastocyst (human) Gastrulation: rearrange cells to form 3-layered embryo w/primitive gut
Organogenesis: development of 3 germ layers into organs• Notochord – stiff dorsal skeletal rod, forms from mesoderm• Neural plate à neural tubeà brain and spinal cord• Neurulation – forms hollow dorsal nerve chord• Somites – blocks of mesoderm arranged along notochord; sign of segmentation Amniotic embryos (reptiles, birds, mammals)• Develop in fluid-filled sac w/in a shell or uterus• Amnion: fluid protects embryo – prevent dehydration, cushions mechanical shock• Yolk : nutrients in egg• Mammalian eggs: little stored food Patterns of development• Cytoplasmic determinants: chemical signals such as mRNAs and transcription factors, influence pattern of cleavage• Induction: interaction among cells that influences their fate, cause changes in gene expression• Totipotent cells: capable of developing into all the different cell typesØ all cells of mammalian embryos are totipotent until the 16-cell stage
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