Animal DevelopmentChapter 47

WHAT’S NEXT?Once copulation ends…

Fertilization• In mammals, sperm travel from the vagina to the fallopian

tube where fertilization will take place

How do we know what happens?• Much of what we know about fertilization comes from the

study of sea urchins• Very similar to mammals:• Deuterostomes

1. Acrosomal Reaction

Step 1: Sperm makes contact with egg.

Step 2: Acrosome on sperm head releases enzymes, actin filaments form acrosomal process

Step 3: Acrosome process binds to receptors on egg membrane; enzymes there digest egg membraneStep 4: Egg membrane and sperm membrane fuse

Step 5: Egg membrane depolarizes and sperm nucleus enters egg Step 6: Cortical reaction hardens egg membrane, blocking entry of other sperm: fast block to polyspermy

2. Cortical Reaction• Happens due to release of

Ca2+ ions from ER• Release of Ca2+ triggers

cortical granules in egg membrane• Contents of cortical

granules harden egg membrane (preventing entry of another sperm)

3. Activation of Egg• Release of Ca2+ ions from ER for cortical reaction triggers egg to

increase rate of metabolism (cellular respiration), protein synthesis

Mammalian Fertilization• A few differences:1. Sperm must get past follicle cells to get to zona pellucida (egg’s

extracellular matrix), where it will bind to a receptor2. Binding of sperm to receptors begins slow block to polyspermy

(no fast block)3. Entire sperm enters egg (not just nucleus)

Fertilization Timeline

Sea Urchin Human

• 6 hours for sperm to be “enhanced” by woman to meet egg• Slow block to polyspermy• Completion of meiosis II• 12-36 hours for first cell

division

1. CLEAVAGE2. GASTRULATION3.ORGANOGENESIS/DIFFERENTIATION

After fertilization, 3 major steps occur on the pathway to a new organism…

Levels of Organization• Zygote• Fertilized egg

• Morula• Solid ball of cells

• Blastula• Hollow ball of cells

• Blastocoel – cavity within blastula

Cleavage• Series of rapid cell divisions following fertilization• Zygote is partitioned off into separate cells• Determinate (cells’ fate has already been determined) or

indeterminate (all cells can become any part of the organism)

• 1st cleavage• 1 cell 2 cells of equal size

• 2nd cleavage• 2 cells of equal size 4 cells of equal size

• 3rd cleavage• 4 cells of equal size 8 cells of equal size• Equatorial (4 cells on top, four cells on bottom)

Egg Zygote 2 cell stage 4 cell stage 8 cell stage

Egg and Zygote Polarity• Most animals have eggs and

zygotes with polarity: • Uneven distribution of mRNA,

proteins, yolk, etc. in cytoplasm• Animal pole: low concentration

of yolk, polar bodies bud from here, head of embryo forms here

• Vegetal pole: yolk concentrated here, becomes posterior of embryo

• Grey crescent: marks dorsal side of future embryo, first cleavage bisects, creates left and right axes

Gastrulation• Folding of blastula to form gastrula• During gastrulation, some cells move inward• Three tissue layers produced: • Endoderm• Mesoderm• Ectoderm

"It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Lewis Wolpert (1986)

Gastrulation

• Archenteron• Primitive gut

• Blastopore• Open end of archenteron,

will become the anus

Organogenesis•Formation of organ

systems from germ layers• In vertebrates:•Neural tube

becomes spinal cord and brain•Neural crest forms

nerves, teeth, skull

Germ Layer Derivatives

How do embryos “know” how to develop?• Homeobox (Hox) genes• Act as a blueprint for a body plan, direct cell differentiation• All organisms have them

Apoptosis

• Programmed cell death• Important in development• “Carves out” facial features, makes appendages possible

Effect of apoptosis during paw development in the mouse. The embryonic region that develops into hands initially has a solid, plate-like structure. Apoptosis eliminates the cells in the interdigital regions, forming the digits. The cells undergoing apoptosis are stained to appear bright green. Abnormal apoptosis can lead to syndactyly.