L62-07 Early Development SLIDES - Lecture 62 Animal...

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Unformatted text preview: Lecture 62 Animal Development The study of development... Follows events from a single cell (zygote) to an adult (billions of cells) Focus is on questions about: 1. the morphology at different stages: (how do organs, limbs etc. arise) 2. the mechanisms of differentiation 3. the role of the genome in development Sequence of Events Fertilization Cleavage Gametes to zygote blastomeres, to blastula & blastocoel Gastrulation gastrula, with blastopore, establish germ layers, & archenteron neurulation, organs and organ systems formation Organogenesis Major Trends 1) Increase in cell number 2) Increase in complexity and specialization. 3) How is it controlled? Information is carried in DNA, but how is its expression regulated? Sea urchin genome confirms kinship to humans and other vertebrates Purple sea urchin Strongylocentrotus purpuratus Genome of 814 megabases 23,500 genes Complex immune system 11,500 genes turned on during first 2 days of development An estimated 95% of 283 transcription factors were functioning before larval formation Source: Science 10 November 2006 Vol. 314 48 hours embryo Sea urchin genome confirms kinship to humans and other vertebrates More than 900 genes designed to sense light and odors - similar to vertebrates More complex innate immune defense than vertebrate's Has adaptive immune defense genes but not producing antibodies Pluteus larva Source: Science 10 November 2006 Vol. 314 Light sensing proteins, yet not sensing light Events of Fertilization in Sea Urchin Sperm contacts egg Acrosome membrane breaks Enzymes released to digest jelly layer Polymerization of actin filaments forms acrosomal process Acrosomal process binds to vitelline envelope Outpocketing of plasma membrane forms fertilization cone Fusion of sperm & egg plasma membrane Na+ channel in egg cell membrane opens; influx of Na+ into cell (depolarization, the fast block to polyspermy) Events of Fertilization (cont'd) Fusion of sperm & egg plasma membrane Release of Ca++ from ER Exocytosis of cortical vesicles Fusion of sperm and egg nuclei (Karyogamy, t=20 min) Enzymes released into perivitelline space break bonding between plasma membrane and vitelline envelope Vitelline and plasma membranes separate Formation of fertilization membrane Release of sperm binding receptors Slow block to polyspermy t=~30 sec Entry of sperm nucleus into egg First mitotic division of zygote Further development Fig. 47.3 A) Sperm makes contact with protective jelly coat. B) Acrosomal membrane breaks down. Digests jelly coat. C) Actin filaments polymerize, forming acrosomal process that extends to vitelline envelope. D) Bindin receptors attach to vitelline envelope. E) Forms fertilization cone that allows sperm entry. Calcium wave Calcium signaling pathway Fig. 11.12 Calcium signaling pathway Fig. 11.12 Calcium signaling pathway Fig. 11.12 Fertilization in mammals - Sperm mature in the female reproductive tract - Sperm then penetrate the follicle cells and the zona pellucida - Zona pellucida is hydrolyzed by acrosomal enzymes - Human sperm will even fertilize a hamster egg lacking zona pellucida! - Recognition between sperm and egg surface proteins is not species specific Cleavage in Sea Urchin up to Blastula Cleavage in sea urchin: A) Before first cleavage B) First cleavage (2 cells) C) Third cleavage (8 cells - radial) D) 16 cells (cleavages unequal) E) 32-cell (morula) F) Hatched blastula Lytechinus variegatus Chick Blastulation Dense yolk causes incomplete cleavage in chick Frog Development: Cleavage cuts the embryo into smaller cells - The first three divisions cut at right angles to each other in eggs with little yolk & the divisions are approximately equal - Asymmetric after the 2nd division in those with more yolk - Continued divisions produce the morula (mulberry) - More divisions produce a blastula, with blastocoel (cavity) inside morula Cross section of a blastula in frog Cleavage in a frog embryo Frog Development Animal hemisphere (more pigment, less yolk) Vegetal hemisphere (less pigment, more yolk) Sperm penetration determines axis Cleavage & orientation Gray crescent: created by rotation of outer (cortical) cytoplasm relative to inner cytoplasm Xenopus laevis Early Cleavage in Urchin and Frog Cleavages: cell division but no growth; increase in surface area to volume; 1st & 2nd: vertical; 3rd: horizontal (cells unequal) In Chick: In Fruit Fly: Importance of Yolk Complete cleavage in sea urchin: yolk is evenly distributed. Frog: yolk is concentrated at the vegetal pole. Incomplete cleavage in bird: disk forms on top of yolk. Superficial cleavage in Drosophila: massive yolk. Nuclei divide but there is no initial cytokinesis; migrate to embryo borders to create layer of cells. Urchin Gastrulation Transforms blastula: Involves large scale cell movement Establishes three germ layers Creates cavity that becomes primitive gut Sea urchin gastrulation: NOTE! 1. At vegetal pole, cells detach and move inward, the vegetal plate invaginates to form the blastopore (future anus) 2. Archenteron (primitive gut) is hollow, and will form the mouth at the other end 3. Mesenchyme cells drag the archenteron across blastocoel Cell movement by filopodia Cell extension: filopodia (small processes with contractile filaments). Summary of sea urchin early development - FLASH video Urchin Development to Pluteus Larva Chick gastrulation to early organogenesis Fig. 47.13 Fig. 47.15 Frog Gastrulation Frog Organogenesis Note Orientations Frog neural tube formation Late gastrulation and early neurulation in frog Types of Cell Movements in Gastrulation Morphogenesis: development of form, results from the movements of cells 1. Invagination 2. Evagination microfilament/microtubule complexes that produce cellular movement Inward Cell Movements Invagination Evidence for constricted cells in the vegetal plate Sea urchin blastula, later with primary mesenchyme cells undergoing invagination at the vegetal pole. dense actin-myosin filaments near kink in vegetal plate Ingression Cell adhesion: attachments by desmosomes attachments through surface recognition molecules Ingression: Groups of cells may suddenly lose adhesion, and slide as group (primary mesenchyme cells) Formation of neural tube Microfilaments Single-cell shape changes underlie many aspects of morphogenesis In the neural plate: microtubules extend and microfilaments contract to fan the cells and create the tube Microtubules Fig. 47.19 Three Primary Germ Layers Ectoderm: epidermis, lining of mouth and rectum, hair, sweat glands lens, brain, nervous system, inner ear, tooth enamel Mesoderm: muscle, notochord, skeleton, cartilage, gonads, outer layer of internal organs,dermis heart, blood vessels, blood, kidney Endoderm: Inner lining of gut, respiratory tract, and reproductive tracts, liver, pancreas, thyroid, urinary bladder. Fig. 47.16 ...
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