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Development Animals – Embryonic Development Plants – Seed Germination Embryonic Development 1. 2. 3. 4. 5. 6. 7. Fertilization: fusion of 2nd oocyte & sperm Cleavage: rapid division w/o growth Morula: solid ball of cells Blastula: hollow sphere of cells Gastrula: germ layers form Extraembryonic membranes form Organogenesis: cell differentiation Fertilization • Recognition: receptor binding between head of sperm & zona pellucida (oocyte) • Penetration: plasma membranes fuse, sperm nucleus enters oocyte • Fertilization membrane: forms to block entry of any other sperm • Meiosis II of oocyte: triggered by sperm nucleus, polar body discharged • Fusion of nuclei: sperm & ovum nuclei fuse to form zygote nucleus, DNA replication (creates sister chromatids) Cleavage • Rapid division without growth to create blastomeres • Each blastomere contains less cytoplasm than original zygote • Embryo polarity: animal pole (top) & vegetal pole (bottom), more yolk at bottom (yolk = heavier) Morula & Blastula • • • • Morula = solid ball of cells Liquid fills morula, pushes cells outward Blastula = hollow sphere of cells (single layer) Blastocoel = fluid-filled cavity inside of blastula Blastocyst • Blastocyst = blastula in mammals – Trophoblast = outer ring of cells • Embeds in endometrium (implantation) • Produces HCG (to maintain corpus luteum) • Forms chorion (and eventually placenta) – Embryonic disc = inner mass of cells • Inner cell mass (ICM) flattens into disc • Gastrulation & development forms embryo and membranes (besides chorion) PRIMATIVE STREAK Gastrulation (folding inward) occurs along a line (fold) instead of a circular depression Embryonic disc will become the embryo Gastrulation • Group of cells fold into blastocoel (invagination) to create 2-layered embryo • 3rd layer forms between inner & outer layers • 3 primary germ layers: ectoderm (outer), mesoderm (middle), endoderm (inner) • Archenteron = center cavity formed by gastrulation (surrounded by endoderm, will become digestive tract) • Blastopore = opening into archenteron (becomes either mouth or anus) Extraembryonic Membranes • Form outside of the embryo • Chorion: outer membrane for gas exchange (birds, reptiles) or implantation into endometrium (mammals, later becomes placenta) • Allantois: buds off archenteron, below chorion, storage of waste (birds, reptiles) or transport of waste (mammals, later becomes umbilical cord) • Amnion: forms amniotic cavity, filled with fluid to cushion embryo • Yolk sac: contains yolk (birds, reptiles), empty in placental mammals (nutrition from placenta) Organogenesis • Cell differentiation (become specific tissues & organs) • Notochord – Along dorsal surface of mesoderm – Stiff rod to provide support – Vertebrae form from nearby cells • Neural tube – Ectoderm directly above notochord – Neural plate first, then indents (neural groove) before rolling up into neural tube – Develops into central nervous sys. – Cells roll off top, form neural crest (will form teeth, bones, muscles of skull, pigment cells in skin, etc.) Germ Layer Derivatives ENDODERM Lungs, thyroid, pancreas, stomach, colon, bladder, intestines, liver MESODERM Muscle, kidneys, RBC ECTODERM Epidermis, neurons, melanocytes Factors Influencing Development • Embryonic induction – Influence of cells on neighboring cells – Organizers = cells that secrete chemicals to influence development of neighboring cells • Homeotic genes – “Master genes” that control developmental genes – Turn genes on/off that code for substances that directly affect development (control placement and spatial organization of body parts) – Homeobox (180 nucleotides) sequence in homeotic genes • HIGHLY CONSERVED – shows common ancestry • Found in homeotic genes of fungi, plants, and animals Seed Anatomy • Parts of seed: seed coat, endosperm/cotyledons, embryo • Parts of embryo: – Epicotyl: top part, becomes shoot tip – Plumule: young leaves on epicotyl – Hypocotyl: below epicotyl, becomes shoot – Radicle: below hypocotyl, becomes root – Coleoptile: surrounds/protects embryo Seed Germination 1. Dormancy: until encountering specific environmental cues (water, temperature, light, seed coat damage) 2. Absorption of water: seed swells (cracks coat), activates enzymes (initiates respiration) 3. Growth of radicle into roots anchors seedling 4. Elongation of hypocotyl produces shoot 5. Primary growth at tips of roots & shoots (apical meristems) Gibberellins! Root Zones 1. Root tip (cap): protects apical meristem 2. Zone of cell division (meristematic cells) 3. Zone of elongation: new cells absorb water & lengthen 4. Zone of maturation/differentiat ion: cells mature into xylem, phloem, parenchyma, or epidermal cells Similar zones at shoot tip, without a cap