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CHAPTER 8 Principles of Development 8-1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Organizing cells during development 8-2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Development Development 8-4 Series of progressive changes in an individual from its beginning to maturity Begins when a fertilized egg divides mitotically Specialization occurs as a hierarchy of developmental “decisions” Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fertilization Fertilization and Activation Contact and Recognition Between Egg and Sperm 8-6 A century of research has been conducted on marine invertebrates Especially sea urchins Marine organisms release enormous numbers of sperm in the ocean to fertilize eggs Many eggs release a chemical molecule Attract sperm of the same species Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fertilization Sea urchin sperm Penetrate a jelly layer surrounding egg Next, contacts the vitelline envelope Egg-recognition proteins bind to species-specific sperm receptors on the vitelline envelope Ensures an egg recognizes only sperm of the same species In the marine environment 8-7 Thin membrane above the egg plasma membrane Many species may be spawning at the same time Similar recognition proteins are found on sperm of vertebrate species Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fertilization Prevention of Polyspermy Fertilization cone forms where the sperm contacts the vitelline membrane Sperm head drawn in and fuses with egg plasma membrane Important changes in the egg surface block entrance to any additional sperm Polyspermy, the entry of more than one sperm In the sea urchin, an electrical potential rapidly spreads across the membrane “fast block” 8-8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fertilization 8-10 The cortical reaction follows Fusion of thousands of enzyme-rich cortical granules with the egg membrane Cortical granules release contents between the membrane and vitelline envelope Creates an osmotic gradient Water rushes into space Elevates the envelope Lifts away all bound sperm except the one sperm that has successfully fused with the egg plasma membrane Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fertilization One cortical granule enzyme Causes the vitelline envelope to harden Now called the fertilization membrane Block to polyspermy is now complete Similar process occurs in mammals 8-12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Binding Sperm to Sea Urchin Egg 8-13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sea Urchin 8-14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fertilization After sperm and egg membranes fuse Sperm loses its flagellum Enlarged sperm nucleus migrates inward to contact the female nucleus Fusion of male and female nuclei forms a diploid zygote nucleus 8-15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fertilization Fertilization Sets in motion important changes in the egg cytoplasm 8-16 Fertilized egg called a zygote Zygote now enters cleavage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cleavage and Early Development Cleavage Embryo divides repeatedly Large cytoplasmic mass converted into small maneuverable cells: blastomeres No cell growth occurs, only subdivision until cells reach regular somatic cell size At the end of cleavage 8-17 Zygote has been divided into many hundreds or thousands of cells Blastula is formed Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Types of Cleavage is Determined by Yolk 8-18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cleavage Types Holoblastic Cleavage extends entire length of egg Egg does not contain a lot of yolk, so cleavage occurs throughout egg Example: mammals, sea stars, worms Meroblastic Cells divide sitting on top of yolk Too much yolk and yolk can’t divide Examples: birds, reptiles, fish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Egg Types and Cleavage Isolecithal Mesolecithal Very little yolk, evenly distibuted Use Holoblastic cleavage- full cleavage Moderate yolk Use Holoblastic - full cleavage Telolecithal Have an abundance of yolk Use Meroblastic cleavage - partial cleavage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Development of Sea Urchin 8-21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. An Overview of Development Following Cleavage Blastulation Cleavage creates a cluster of cells called the blastula Blastula stage typically consists of a few hundred to several thousand cells During blastula stage, first germ layer forms In most animals Cells are arranged around a fluid-filled cavity called the blastocoel (blas-to-seal) 8-22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. An Overview of Development Following Cleavage Gastrulation and Formation of Two Germ Layers Gastrulation Results in the formation of a second germ layer Involves an invagination of one side of blastula Forms a new internal cavity gastrocoel Opening into the cavity: Blastopore Gastrula has an outer layer of ectoderm and an inner layer of endoderm 8-23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Generalized Development showing germ layers Incomplete/ Blind Gut Blastopore (Opening) 8-24 Complete Gut Gastrocoel (Cavity) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. An Overview of Development Following Cleavage The only opening into embryonic gut is the blastopore Blind or incomplete gut Some animals retain the blind gut - the opening does not fully extend to other side (flatworms, sea anemones) Most develop a complete gut - in which the opening extends and produces a second opening, the anus 8-25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Generalized Development showing germ layers Incomplete/ Blind Gut 8-26 Complete Gut Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. An Overview of Development Following Cleavage Formation of Mesoderm Animals with two germ layers Most animals add a 3rd germ layer Diploblastic (Endoderm and Ectoderm) Triploblastic Mesoderm 3rd germ layer Forms between the endoderm and the ectoderm Mesoderm arises from endoderm 8-27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Developmental Characteristics Germ Layer Outcomes: Ectoderm Epithelium and nervous system Endoderm Epithelial lining of the digestive and respiratory tract, liver, pancreas, Mesoderm Muscular system, reproductive system, bone, kidneys, blood Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Germ Layer Outcome in mammals 8-29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. An Overview of Development Following Cleavage Formation of the Coelom (see-lom) Coelom The method by which the coelom forms is an inherited character 8-30 Important in grouping organisms based on developmental characters Upon completion of coelom formation Body cavity surrounded by mesoderm Body has 3 tissue layers and 2 cavities Animals Without a Coelom are called Acoelomates (Ex. flatworms) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Developmental Characteristics 8-32 Two major groups of triploblastic animals (animals with 3 germ layers) Protostomes and deuterostomes The groups are identified by four developmental characters Cleavage Patterns (radial or spiral) Fate of Blastopore (mouth or anus) Coelom Formation (split mesoderm or outpocketing mesoderm) Embryo Type (Regulative or Mosaic) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protostomes and Deuterostomes Blastopore Fate Fate of Blastopore Deuterostome embryos Develop a complete gut Blastopore becomes the anus Second opening becomes the mouth Protosome embryos Blastopore becomes the mouth Anus forms from a second opening 8-34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Coelom Formation - mesoderm movement Enterocoely 8-35 Mesoderm sides push outward and expand into a pouch-like coelomic compartment Pouch-like compartment pinches off and forms a mesoderm bound space surrounding the gut Occurs in Deuterostomes ( Sea stars, fish, frogs, etc.) Schizocoely Coelom forms from Endodermal cells move to blastopore and develop into mesoderm Mesoderm seperates or splits to form cavity (coelom) Occurs in Protostome (Earthworms, snails) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blastula and Gastrula Of Embryos 8-37 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vertebrate Development The Common Vertebrate Heritage 8-38 All vertebrate embryos share chordate hallmarks Dorsal neural tube Notochord Pharyngeal gill pouches with aortic arches Ventral heart Postanal tail Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-39 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vertebrate Development Amniotes and the Amniotic Egg Reptiles, birds, and mammals Embryos develop within the amnion Fluid-filled sac that encloses the embryo Provides an aqueous environment in which the embryo floats Protection from mechanical shock Amniotic egg contains 4 extraembryonic membranes including the amnion 8-40 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vertebrate Development In the shelled amniotic egg: Yolk sac Stores yolk Allantois Storage of metabolic wastes during development Respiratory surface for gas exchange 8-41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vertebrate Development 8-42 Chorion Lies beneath the eggshell Encloses the embryo and other extraembryonic membrane As embryo grows Need for oxygen increases Allantois and chorion fuse to form a respiratory surface, the chorioallantoic membrane Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chick Embryo 8-43 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A. Fish Larvae - 1 day old, has large yolk sac B. 10 day old fish larva, developed mouth, yolk sac smaller 8-44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vertebrate Development The Mammalian Placenta and Early Mammalian Development Most mammalian embryos do not develop within an egg shell Develop within the mother’s body Most retained in the mother’s body Monotremes Primitive mammals that lay eggs Large yolky eggs resembling bird eggs Duck-billed platypus and spiny anteater 8-45 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vertebrate Development Marsupials Embryos born at an early stage of development and Continue development in abdominal pouch of mother Placental Mammals Represent 94% of the class Mammalia Evolution of the placenta Required reconstruction of extraembryonic membranes Modification of oviduct Expanded region formed a uterus 8-46 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Extraembryonic membranes of a mammal 8-47 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vertebrate Development Early Stages of Mammalian Development (Human) Germinal Period (1st two weeks) Blastocyst transported by oviduct to the uterus Propelled by ciliary action Around 6th day Blastocyst = 100 cells Contacts uterus By the twelfth day 8-48 Implantation is complete Embryo surrounded by pool of maternal blood Chorion thickens, sends out tiny fingerlike projections Chorionic villi Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Early Development of the human embryo 8-49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vertebrate Development Amnion Remains unchanged Surrounds embryo Secretes fluid in which embryo floats Yolk sac Contains no yolk Source of stem cells that give rise to blood and lymphoid cells Stem cells migrate to into the developing embryo 8-50 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vertebrate Development Allantois Not needed to store wastes Contributes to the formation of the umbilical cord Chorion 8-51 Forms most of the placenta Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Human embryo showing somites - They will differentiate into skeletal muscle and the axial skeleton 8-52
