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Embryonic development Embryonic development of the nematode C. elegans 4 main stages on the growth and development of animals. 1. Gametogenesis • Formation of sperm and egg • Begins with fertilization and ends with birth • Process leading to reproductive maturity (puberty) 2. Embryonic Development Video 3. Maturation 4. Aging --We will concentrate on Embryonic Development-- Stages of Embryonic development 1. 2. 3. 4. 5. 6. 7. Fertilization Cleavage Morulation Blastulation Gastrulation Extraembryonic membrane development Organogenesis Fertilization 1. Acrosomal reaction (recognition) 2. Penetration 3. Cortical reaction 4. Activation of egg Acrosom al Animatio n 1 2 3 4 6 8 10 20 30 40 50 1 Binding of sperm to egg Acrosomal reaction: plasma Membrane depolarization (fast block to polyspermy) Increased intracellular calcium level Cortical reaction begins (slow block to polyspermy) Formation of fertilization envelope complete 2 Increased intracellular pH 3 4 5 Increased protein synthesis 10 20 Fusion of egg and sperm nuclei complete 30 40 Onset of DNA synthesis 60 90 First cell division 1-The acrosomal reaction – – Head of sperm (acrosome) releases hydrolytic enzymes that penetrate the jelly coats of the egg. Sperm releases proteins that bind to receptors on vitelline membrane of the egg. 2- Penetration • 1 This ensure same species fertilization 2 4 3 Acrosomal reaction. Contact. Hydrolytic enzymes released from the acrosome make a hole in the jelly coat. Growing actin filaments protrude from the sperm head and penetrates the jelly coat, Binding to receptors in the egg cell membrane that extend through the vitelline layer. 5 Entry of sperm nucleus. The membrane becomes depolarized, resulting in the fast block to polyspermy. Sperm plasma membrane Sperm nucleus Acrosomal process Basal body (centriole) Fertilization envelope Sperm head Actin Acrosome Jelly coat Sperm-binding receptors Fused plasma Cortical membranes granule Perivitelline Hydrolytic enzymes space Cortical granule membrane Vitelline layer Egg plasma membrane EGG CYTOPLASM Mammals • Follicle cells are release with egg • Sperm must find way through the cells to the “Zone Pellucida” • Sperm contain receptor molecules to move through the Zone Follicle cell Zona pellucida Egg plasma membrane Acrosomal vesicle Sperm basal Sperm Cortical body nucleusgranules EGG CYTOPLASM 3- The cortical reaction – – The fusion sets up a signal transduction pathway that cause large amts of Ca2+ to be release into cytoplasm Ca2+ cause change in cortical granules causing hardening of membrane • 1 This resist the entry of any other sperm 2 4 Entry of sperm nucleus. 3 5 Cortical reaction. cytosol, causing cortical granules Fusion of sperm and egg membranes. Contact Acrosomal reaction. This in leads totoswelling of plasma the the egg fuse with the perivitelline space, hardening membrane and discharge their of the contents. Thisand leadsclipping to swelling vitelline layer, ofof the perivitelline space, hardening of sperm-binding receptors. Thetheresulting vitelline layer, and clipping of fertilization envelope is the slow block sperm-binding receptors. The resulting to polyspermy. fertilization envelope is the slow block to polyspermy. Sperm plasma membrane Sperm nucleus Acrosomal process Basal body (centriole) Fertilization envelope Sperm head Actin Acrosome Jelly coat Sperm-binding receptors Fused plasma Cortical membranes granule Perivitelline Hydrolytic enzymes space Vitelline layer Egg plasma membrane EGG CYTOPLASM Cortical granule membrane Hypothetical pathway for calcium release Fertilization Overview 4. Acrosomal reaction (recognition) Penetration Cortical reaction Activation of egg Polar body discharged – – Sharp rise in Ca2+ causes the egg to begin to develop Humans- completion of Meiosis II. – – Sperm and egg nuclei fuse Substantial increase in the rates of cellular respiration and protein synthesis by the egg cell • Polar body are discharged through membrane Parthenogenesis • Development of an unfertilized egg. • Drone honey bees develop by natural parthenogenesis Haploid males Stages of Embryonic development 1. Fertilization . 2. 3. 4. 5. 6. 7. Cleavage Morulation Blastulation Gastrulation Extraembryonic membrane development Organogenesis Cleavage • Cleavage partitions the cytoplasm of one large cell – Into many smaller cells called blastomeres Fertilized egg. Shown here is the zygote shortly (a) before the first cleavage division, surrounded by the fertilization envelope. The nucleus is visible in the center. Four-cell stage. (b)Remnants of the mitotic spindle can be seen between the two cells that have just completed the second cleavage division. (c) Morula. After Blastula. A single layer of cells surrounds a large further cleavage (d)blastocoel cavity. divisions, the embryo is a Although not visible multicellular ball that is here, the fertilization still surrounded by the envelope is still fertilization envelope. present; the embryo The blastocoel cavity has will soon hatch from begun to form. (12 cells) it and begin swimming. Blastulation •Cleavage results in the formation of a multicellular stage called a blastula. •The blastula of many animals is a hollow ball of cells Blastocoel Cleavage Cleavage Eight-cell stage Zygote Blastocoel Endoderm Ectoderm Gastrula Blastopore Gastrulation Blastula Cross section of blastula Blastulation & Gastrulation Blastocoel Eight-cell stage Zygote -cavity of the blastula Cleavage Cleavage Blastocoel Blastula Cross section of blastula Endoderm Ectoderm Gastrula Blastopore Gastrulation Gastrulation Animation •A rearrangement of the embryo in which one end of the embryo folds inward, expands, and eventually fills the blastocoel •The three layers produced by gastrulation Frog Blastula Gastrulation • The three layers produced by gastrulation • Archentron • Blastopore – Mouth or anus Archenteron (primitive gut) Gastrulation in a sea urchin Key Future ectoderm Future mesoderm Future endoderm Mesenchyme cells Vegetal plate Animal pole Blastocoel Vegetal pole Blastocoel Filopodia pulling archenteron tip Archenteron Blastopore Mesenchyme cells Blastocoel 50 µm Archenteron Ectoderm Mesenchyme: (mesoderm forms future skeleton) Mouth Blastopore Digestive tube (endoderm) Anus (from blastopore) Extraembryonic membrane development • Birds, reptiles, & mammals. – Develop within a fluid-filled sac that is contained within a shell or the uterus – Called amniotes • Extraembryonic membranes develop outside the embryo proper: – – – – Chorion Allantois Amnion Yolk sac –Amnion –Chorion Allantois Yolk sac Amnion. The amnion protects the embryo in a fluid-filled cavity that prevents dehydration & cushions mechanical shock. Allantois. The allantois functions as a disposal sac for certain metabolic wastes produced by the embryo. The membrane of the allantois also functions with the chorion as a respiratory organ. (placenta in mammals) Embryo Amniotic cavity with amniotic fluid Shell Chorion. The chorion and the membrane of the allantois exchange gases between the embryo and the surrounding air. Oxygen and carbon dioxide diffuse freely across the egg’s shell. Albumen Yolk sac. The yolk sac expands over the yolk, a stockpile of nutrients stored in the egg. Blood vessels in the yolk sac membrane transport Yolk nutrients from the yolk (nutrients) into the embryo. Other nutrients are stored in the albumen (the “egg white”). Amniotic Egg A critical evolutionary development for terrestrial animals is the reptilian amniotic egg, now also characteristic of birds and some mammals. The developing embryo, protected from drying out, can survive outside of water and in a variety of habitats. The yolk provides it with food, and the albumin supplies water and nutrients. Wastes are released to the allantois, an extension of the embryonic gut. Oxygen diffuses easily through the thin outer shell of the egg; its passage to the embryo is regulated by the chorion. Human 1. Decidua capsularis 2. Uterine wall 3. Uterine cavity 4. Placenta 5. Decidua parietalis 6. Decidua basalis 7. Chorion leave 8. Embryo 9. Connecting stalk 10. Yolk sac 11. Chorion 12. Amnion 13. Chorionic cavity 14. Amniotic cavity Organogenesis • Process by which cells continue to differentiate, producing organs from the three embryonic germ layers. • Three kinds of morphogenetic changes Neural Neural plate fold First to form-the neural tube – Folds – Splits – Dense clustering (condensation) Neural crest Neural crest Neural tube Figure 47.14b Outer layer of ectoderm (b)Formation of the neural tube. Infolding and pinching off of the neural plate generates the neural tube. Note the neural crest cells, which will migrate and give rise to Numerous structures. • Early in vertebrate organogenesis – The notochord forms from mesoderm and the neural plate forms from ectoderm Figure 47.14a Neural folds LM 1 mm Neural fold Neural plate Notochord Ectoderm Mesoderm Endoderm Archenteron (a) Neural plate formation. By the time shown here, the notochord has developed from dorsal mesoderm, and the dorsal ectoderm has thickened, forming the neural plate, in response to signals from the notochord. The neural folds are the two ridges that form the lateral edges of the neural plate. These are visible in the light micrograph of a whole embryo. Chick Organogenesis Factors that Influence Embryonic Development 1. Cytoplasmic Determinants (who’s gonna be what) differently differentiated cell lineages Uneven cytoplasmic distribution Substances that are inherited during cleavage set “stage” for development proteins as well as mRNAs found in the cytoplasm Location during cleavage can also play a role 2. Embryonic Induction (cell signaling) Ability of one group of embryonic cells to influence the development of another group of embryonic cells 3. Homeotic, Homoeobox, or Hox Genes MASTER genes that control the expression of genes responsible for specific anatomical structures. Fish, Frogs, Birds and Humans Classic studies using frogs -Gave indications that the lineage of cells making up the three germ layers created by gastrulation is traceable to cells in the Epidermis blastula Central nervous system Epidermis Notochord Mesoderm Endoderm Blastula (a) Neural tube stage (transverse section) Fate map of a frog embryo. The fates of groups of cells in a frog blastula (left) were determined in part by marking different regions of the blastula surface with nontoxic dyes of various colors. The embryos were sectioned at later stages of development, such as the neural tube stage shown on the right, and the locations of the dyed cells determined. Later studies -Marked individual blastomeres during cleavage and then followed it through development Cell lineage analysis in a tunicate. In lineage analysis, an individual (b) cell is injected with a dye during cleavage, as indicated in the drawings of 64-cell embryos of a tunicate, an invertebrate chordate. The dark regions in the light micrographs of larvae correspond to the cells that developed from the two different blastomeres indicated in the drawings. Subcortical Rotation and the "Gray Crescent" -Formation of the grey crescent of the amphibian egg. -The dense, yolky vegetal deep cytoplasm rotates with respect to the overlying cortex. Gray crescent - Region of intermediate pigmentation in the marginal zone of the amphibian egg caused by a shift in the pigmented egg cortex toward the site of sperm entry; marks the future site of the dorsal lip of the blastopore. Website •Development of embryo occurs in a flat disc (blastodisc) that sits on top of the yolk. •A primitive streak forms instead of a gray crescent. •Cells migrate over the primatitive strek and flow inward to form the archentron. •Extraembryonic membranes form. A human embryo at approximately 6–8 weeks after conception 1 mm • Early embryonic development in a human – Endometrium (uterine lining) Inner cell mass Trophoblast Blastocoel Proceeds through four stages Expanding region of trophoblast Maternal blood vessel Epiblast Hypoblast Trophoblast 1 Blastocyst reaches uterus. Expanding region of trophoblast Amniotic cavity Amnion Epiblast 2 Blastocyst implants. Extraembryonic 3 membranes start to form and gastrulation begins. Hypoblast Chorion (from trophoblast) Extraembryonic mesoderm cells (from epiblast) Allantois Yolk sac (from hypoblast) Amnion Chorion Ectoderm Mesoderm Endoderm Gastrulation has produced a three4 layered embryo with four extraembryonic membranes. Yolk sac Extraembryonic mesoderm 4 weeks 6 weeks