1. 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

2. 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

4. 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)

7. 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

9. 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

14. 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

15. 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

17. 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