1. Chapter 47: Animal Development
Lauren Bookstaver
Block: F

2. Overview
Development determined by zygote genome & distribution of maternal substance (cytoplasmic determinants) in egg
Location of cells affects early development
Selective gene expression leads to cell differentiation, the specialization of cells in structure & function
Morphogenesis produces body shape

3. Overview (continued): MAJOR CONCEPTS
“After fertilization, embryonic development proceeds through cleavage, gastrulation, & organogenesis”
“Morphogenesis in animals involves specific changes in cell shape, position, & adhesion”
“The development fate of cells depends on their history & on inductive signals”

4. Fertilization
Activates egg & brings together nuclei of sperm & egg, triggering embryonic development
Combines haploid sets of chromosomes
Acrosomal reaction releases hydrolytic enzymes that digest material around egg
Gamete contact/fusion depolarizes egg cell membrane & sets up fast block to polyspermy
Membrane fusion initiates cortical reaction, where rise in calcium ions stimulates cortical granules to release contents outside egg
Forms fertilization envelope that functions as slow block to polyspermy

5. Cleavage
Rapid cell divisions where embryo becomes partitioned into many small cells (blastomeres)
Creates multicellular ball (blastula), which contains fluid-filled cavity (blastocoel)
Meroblastic cleavage- incomplete division of egg in animals w/ yolk-rich eggs (i.e. birds & reptiles)
Produces cap of cells (blastoderm) on top of large, undivided yolk
Holoblastic cleavage- complete division of eggs w/ little amounts of yolk (i.e. mammals, frogs, sea urchins)

6. Gastrulation
Changes in cell motility, shape, & adhesion, resulting in 3-layered embryo (gastrula)
3 embryonic germ layers: outer ectoderm, middle mesoderm, & inner endoderm lining embryonic digestive tract
Has digestive cavity (archenteron)

7. Organogenesis Organs develop from embryonic germ layers
Vertebrates: formation of notochord from condensation of dorsal mesoderm, neural tube from folding of ectodermal neural plate (will develop central nervous system), & coelom from splitting of lateral mesoderm

8. Developmental Adaptations of Amniotes
Amniotes (i.e. reptiles & mammals) create aqueous environment necessary for embryonic development
Develop in fluid-filled sac contained in shell or uterus
Germ layers give rise to 4 extraembryonic membranes: amnion, chorion, yolk sac, & allantois

9. Mammalian Development
Fertilization occurs in oviduct, & embryonic development begins on way to uterus
Eggs small & store few nutrients
Cleavage is holoblastic & show no obvious polarity
Gastrulation & organogenesis similar to birds & reptiles
Blastocyst implants uterus after fertilization & cleavage in oviduct
Trophoblast initiates formation of fetal portion of placenta & embryo develops from single layer of cell (epiblast) in blastocyst
4 extraembryonic membranes homologous to birds & reptiles

10. Cytoskeleton, Cell Motility, & Convergent Extension
Changes in cell shape & position involve reorganization of cytoskeleton
Convergent extension- cells of tissue layer crawl between each other, causing sheet of cells to become narrower & longer

11. Roles of Extracellular Matrix & Cell Adhesion Molecules
Fibers of ECM provide anchorage for crawling cells & tracts that direct movement of migrating cells
Cell adhesion molecules help regulate movement & tissue building by holding cells together
Cadherins are important adhesion molecules

12. Fate Mapping
Developed by Vogt, fate maps of embryos have shown that specific regions of zygote or blastula develop into specific parts of older embryos
“Founder cells” identified that will generate specific tissues
Cell’s developmental potential restricts as development proceeds

13. Establishing Cellular Asymmetries
In nonamniotic vertebrates, major axes established in egg or at fertilization
Unevenly distributed cytoplasmic determinants in egg important in formation of body axes & differences of blastomeres
In amniotes, polarity not established until later
Environment differences play influence initial differences between cells & later body axes
As embryonic development continues, potency of cells becomes limited

14. Cell Fate Determination & pattern Formation by Inductive Signals
Cells influence development of other cells w/ signals that switch on or off sets of genes
Cells & offspring differ based on positional info- molecular cues indicating position
Info in form of signal molecules in “organizer” regions of embryo
(i.e.) Dorsal lip of blastopore in amphibian gastrula (shown above), AER & ZPA of vertebrate limb bud
Influence gene expression in cells that receive them, leading to differentiation