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From Cell to Organism Embryogenesis 1 Animal Development is an Amazing Process • It is difficult to imagine that each of us began life as a single cell called a zygote • This single cells forms an intricate body made of billions of cell in a unique organized pattern and shape typical to our species • A human embryo at about 6–8 weeks after conception shows development of distinctive features 2 1 mm 3 Development of Developmental Thinking • Man has always wondered how development happens • Aristo suggested gradual formation • As recently as the 18th century, the prevailing theory was called preformation as exemplified by the homunculus • Modern embryology and Cell biology showed that Aristo was right! 4 Model Organisms • Most of our knowledge is on selected species that are representative of a larger group and easily studied • Embryological studies have focused on the sea urchin, frog, zebrafish, chick, the nematode C. elegans, the fruit-fly Drosophila and the mouse 5 Stages of Embryonic Development • Important events regulating development occur during fertilization and the three stages that build the animal’s body Cleavage: cell division creates a hollow ball of cells called a blastula Gastrulation: cells are rearranged into a threelayered gastrula Organogenesis: the three layers interact and move to give rise to organs 6 Fertilization in the Sea-Urchin Basal body (centriole) Sperm head Acrosome Jelly coat Sperm-binding receptors Vitelline layer Egg plasma membrane 7 Fertilization in the Sea-Urchin Basal body (centriole) Sperm head Acrosome Jelly coat Sperm-binding receptors Hydrolytic enzymes Vitelline layer Egg plasma membrane 8 Fertilization in the Sea-Urchin Sperm nucleus Acrosomal process Basal body (centriole) Sperm head Acrosome Jelly coat Sperm-binding receptors Actin filament Hydrolytic enzymes Vitelline layer Egg plasma membrane 9 Fertilization in the Sea-Urchin Sperm plasma membrane Sperm nucleus Acrosomal process Basal body (centriole) Sperm head Actin filament Fused plasma membranes Acrosome Jelly coat Sperm-binding receptors Hydrolytic enzymes Vitelline layer Egg plasma membrane 10 Fertilization in the Sea-Urchin Sperm plasma membrane Sperm nucleus Fertilization envelope Acrosomal process Basal body (centriole) Sperm head Acrosome Jelly coat Sperm-binding receptors Actin filament Cortical Fused granule plasma membranes Perivitelline Hydrolytic enzymes space Vitelline layer Egg plasma membrane EGG CYTOPLASM 11 How to Prevent Polyspermy? • Fusion of egg and sperm induces a rise in internal Ca2+ • This initiates cortical granules to release their contents outside the egg, a process termed cortical reaction • These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy 12 The Calcium Wave and Cortical Reaction EXPERIMENT 25 sec 10 sec after fertilization 35 sec 1 min 500 µm RESULTS Corticle Reaction 10 sec after fertilization 1 sec before fertilization 20 sec 30 sec 500 µm CONCLUSION Point of sperm nucleus entry Spreading wave of Ca2+ Fertilization envelope Calcium Wave 13 Activation of the Egg • The sharp rise in Ca2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cell • With these rapid changes in metabolism, the egg is said to be activated • The sperm nucleus merges with the egg nucleus and cell division begins 14 Cleavage • Fertilization is followed by cleavage, a period of rapid cell division without growth • Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres • The blastula is a ball of cells with a fluid-filled cavity called a blastocoel Fert. env Zygote Blastocoel 4 cell Early Blastula Later Blastula 15 Cleavage Patterns in Animals • The eggs and zygotes of many animals, except mammals, have a definite polarity • The polarity is defined by distribution of yolk (stored nutrients) • The vegetal pole has more yolk; the animal pole has less yolk Xenopus egg 16 Holoblastic and Meroblastic cleavages • Cell division is slowed by yolk • Holoblastic cleavage, complete division of the egg, occurs in species whose eggs have little or moderate amounts of yolk, such as sea urchins, frogs and mammals • Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as fish, reptiles and birds 17 Formation of the body Axes • The three body axes in amphibians are established by the egg’s polarity and by a cortical rotation following binding of the sperm Dorsal Right Anterior Posterior Left Ventral 18 Cortical rotation leads to determination of the axes and cleavage pattern Animal pole Animal hemisphere Vegetal hemisphere Vegetal pole Point of sperm nucleus entry Gray crescent Pigmented cortex First cleavage Future dorsal side 19 Polarity of the Early Embryo - Experiment EXPERIMENT EXPERIMENT Experimental egg (side view) Control egg (dorsal view) Gray crescent Gray crescent Thread RESULTS RESULTS Normal Belly piece Normal 20 From Zygote to Blastula in the Frog 0.25 mm 0.25 mm Frog Cleavage Animal pole Zygote 2-cell stage forming 4-cell stage forming Blastocoel Vegetal Blastula 8-cell pole (cross stage section) 21 The Mammalian Blastula is Asymmetric 22 Gastrulation – The most important event in Life? • Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut • The three layers produced by gastrulation are called embryonic germ layers The ectoderm forms the outer layer The endoderm lines the digestive tract The mesoderm partly fills the space between the endoderm and ectoderm 23 Gastrulation in the Sea-Urchin: Stage 1 – Mesenchyme Formation Future ectoderm Future mesoderm Future endoderm Animal pole Blastocoel Mesenchyme cells Vegetal plate Vegetal pole 24 Gastrulation in the Sea-Urchin Stage 2 – Archenteron Initiation Future ectoderm Future mesoderm Future endoderm Blastocoel Archenteron Blastopore 25 Invagination is a Result of Apical Constriction 26 Archenteron Elongation is Due to Cell Movement 27 Gastrulation in the Sea-Urchin: Stage 3 – Archenteron Completion Future ectoderm Future mesoderm Future endoderm Ectoderm Mouth Mesenchyme (mesoderm forms future skeleton) Digestive tube (endoderm) Anus (from blastopore) 28 Gastrulation in the Sea-Urchin: From Blastula to Larva Key Future ectoderm Future mesoderm Sea-star Gasrulation Future endoderm Archenteron Animal pole Blastocoel Blastocoel Filopodia pulling archenteron tip Blastocoel Archenteron Blastopore Mesenchyme cells Ectoderm Vegetal plate Vegetal pole Mouth Blastopore 50 µm Mesenchyme cells Mesenchyme (mesoderm forms future skeleton) Digestive tube (endoderm) Anus (from blastopore) 29 Fate Map of a Frog Blastula 30 Frog Gastrulation: Stage 1- Invagination via Dorsal lip SURFACE VIEW CROSS SECTION Animal pole Blastocoel Dorsal lip of blastopore Key Dorsal lip of blastopore Blastopore Future ectoderm Future mesoderm Early gastrula Vegetal pole Future endoderm 31 Frog Gastrulation: Stage 2 - Archenteron and A-P Axis Formation CROSS SECTION SURFACE VIEW Blastocoel shrinking Archenteron Key Future ectoderm Future mesoderm Future endoderm 32 Frog Gastrulation: Stage 3 - Complete Entry of Endoderm and Mesoderm together with Ectoderm Epiboly CROSS SECTION SURFACE VIEW Ectoderm Blastocoel remnant Mesoderm Endoderm Archenteron Key Blastopore Future ectoderm Future mesoderm Future endoderm Late gastrula Blastopore Yolk plug 33 Hans Spemann and the Organizer EXPERIMENT RESULTS Dorsal lip of blastopore Pigmented gastrula (donor embryo) Nonpigmented gastrula (recipient embryo) Primary embryo Secondary (induced) embryo Primary structures: Neural tube Notochord Secondary structures: Notochord (pigmented cells) Neural tube (mostly nonpigmented cells) This is one of the most famous experiments in biology! 34 Chicken Gastrulation Dorsal Fertilized egg Primitive streak Anterior Left Embryo Right Yolk Posterior Ventral Primitive streak Epiblast Future ectoderm Blastocoel Endoderm Migrating Hypoblast cells (mesoderm) YOLK 35 Neural folds Neural fold Neural plate Neurulation: Neural Tube Closure 1 mm Neural Neural fold plate Neural crest cells Notochord Ectoderm Outer layer of ectoderm Mesoderm Endoderm Neural crest cells Archenteron (a) Neural plate formation Neural tube (b) Neural tube formation 36 Cell Shape Change Drive Neural Tube Formation Actin filaments 37 The Tailbud Embryo: Organogenesis Eye Somites Tail bud Neural tube Notochord Coelom SEM Neural crest cells Somite Archenteron (digestive cavity) 1 mm (c) Somites 38 Chick Organogenesis Eye Neural tube Notochord Forebrain Somite Heart Coelom Archenteron Endoderm Lateral fold Mesoderm Blood vessels Ectoderm Somites Yolk stalk These layers form extraembryonic membranes (a) Early organogenesis Yolk sac Neural tube YOLK (b) Late organogenesis 39 So What did we get from the Germ Layers? ECTODERM Epidermis of skin and its derivatives (including sweat glands, hair follicles) Epithelial lining of mouth and anus Cornea and lens of eye Nervous system Sensory receptors in epidermis Adrenal medulla Tooth enamel Epithelium of pineal and pituitary glands MESODERM ENDODERM Notochord Skeletal system Muscular system Muscular layer of stomach and intestine Excretory system Circulatory and lymphatic systems Reproductive system (except germ cells) Dermis of skin Lining of body cavity Adrenal cortex Epithelial lining of digestive tract Epithelial lining of respiratory system Lining of urethra, urinary bladder, and reproductive system Liver Pancreas Thymus Thyroid and parathyroid glands 40 Get Enveloped – Life on Land • During amniote development, four extraembryonic membranes form around the embryo: The chorion functions in gas exchange The amnion encloses the amniotic fluid The yolk sac encloses the yolk The allantois disposes of waste products and contributes to gas exchange 41 In the Shell Amnion Allantois Embryo Albumen Amniotic cavity with amniotic fluid Shell Yolk (nutrients) Chorion Yolk sac 42 Mammalian Development: Implantation of the Blastocyst Endometrial epithelium (uterine lining) Uterus Inner cell mass Trophoblast Blastocoel 43 Mammalian Development: Invasion of the Blastocyst Expanding region of trophoblast Maternal blood vessel Epiblast Hypoblast Trophoblast 44 Mammalian Development: Formation of the bicistronic cavities Expanding region of trophoblast Amniotic cavity Epiblast Hypoblast Yolk sac (from hypoblast) Extraembryonic mesoderm cells (from epiblast) Chorion (from trophoblast) 45 Mammalian Development: Gastrulation and Envelopment Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm Atlantois 46 The Mammalian Placenta 47 The Wonderful Result The Human Embryo 48
