Chapter 47: Animal Development Theories of embryonic development Preformation Belief that egg or sperm contain a miniature embryo/adult form Epigenesis Embryo develops gradually from an egg Developmental plan influenced by Zygote genome Maternal mRNA Cytoplasmic determinants Cell signaling influences gene expression

Fertilization Acrosomal Reaction Cortical Reaction Activates the egg brings nuclei of egg & sperm together begins embryo metabolic reactions Acrosomal Reaction Acrosome at sperm tip releases hydrolytic enzymes which digest egg jelly coat/vitelline membrane Ion channels open in egg membraneNa+ flows in depolarizing membrane prevents other sperm from fusing with egg (fast block polyspermy) Cortical Reaction Fusion of egg & sperm stimulates a series of changes in egg cortex Formation of fertilization envelope to function as a slow-block to polyspermy

Fertilization in mammals Activation of the egg Sharp rise in Ca2+ in egg causes: Increased cell respiration & protein synthesis Sperm & egg nuclei to fuse DNA replication & first cell division 90 minutes in sea urchins & frogs 12-36 hours in mammals Can be stimulated to occur without sperm Fertilization in mammals Fertilization is internal (fallopian tubes) Secretions of female tract enhance sperm motility Sperm migrates into zona pellucida (3D matrix of egg) Sperm binds to & depolarizes egg to prevent polyspermy Diploid nuclei form after 1st cell division (12-36 hours after sperm bind)

Partitions the zygote into smaller cells Cleavage Partitions the zygote into smaller cells Rapid succession of mitotic cell divisions without G1 & G2 First 5-7 divisions form cluster of cells= morula Fluid cavity blastocoel forms in morula Hollow ball of cells with fluid within= blastula Polarity & planes Vegetal pole high yolk concentration forms at posterior end of embryo Animal pole low concentration of yolk forms the anterior end of embryo Gray crescent near equator Types of cleavage Meroblastic- incomplete division; found in yolk-rich eggs Holoblastic- complete division; found in eggs with little or moderate amounts of yolk

Rearranges the cells in the blastula Gastrulation Rearranges the cells in the blastula forms a 3 layered embryo with a primitive gut (archenteron) Invagination distributes the cells into layers open end of archenteron= blastopore (future anus) Germ layers Ectoderm outermost layer forms nervous system, skin, hair, nails, eye lens Endoderm inner layer gives rise to digestive lining, pancreas, liver, thyroid, lungs, bladder Mesoderm middle layer gives rise to muscles, skeleton, gonads, excretory & circulatory systems 3 layer embryo= gastrula Organogenesis Formation of organs from germ layers Localized morphological changes in tissue & cell shape Neural tube & notochord first to appear & elongate in chordate embryo Blocks of somites from which vertebrae & muscles will form appear along the notochord

Embryos develop in a fluid-filled sac within a shell or uterus Amniotes Embryos develop in a fluid-filled sac within a shell or uterus Formation of extraembryonic membranes (life support system for embryo); occurs simultaneously with gastrulation Amnion fluid protecting the embryo from temperature & movement shocks Chorion gas exchange Allantois gas exchange & metabolic waste storage incorporated into umbilical cord Yolk sac blood vessels carry food to the embryo

Mammalian development Avian development Yolk contains food supply Blastodisc= embryo forming portion of egg on upper surface Primitive streak- longitudinally thickens along blastodisc Extraembryonic membranes Mammalian development Fertilization & cleavage occurs in oviducts Blastocyst reaches uterus & implants at day 7 Extra-embryonic membranes Organogenesis begins with formation of neural tube, notochord, & somites

Morphogenesis Involves specific changes in cell shape, position, & adhesion; reorganization of the cytoskeleton Morphogenetic movements partially guided by extra-cellular matrix Cell Adhesion Molecules (CAM) contribute to selective association of cells with one another Cytoplasmic determinants help establish body axes & differences among cells of the early embryo Inductive signals drive differentiation & pattern formation in vertebrates The “organizer” influences & induces the development of other cells Inducers turn some genes on & other off