Animal Development Chapter 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 membrane Step 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 Ca 2+ ions from ER Release of Ca 2+ triggers cortical granules in egg membrane Contents of cortical granules harden egg membrane (preventing entry of another sperm)

3. Activation of Egg Release of Ca 2+ 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 receptor 2.Binding of sperm to receptors begins slow block to polyspermy (no fast block) 3.Entire sperm enters egg (not just nucleus)

Fertilization Timeline Sea UrchinHuman 6 hours for sperm to be “enhanced” by woman to meet egg Slow block to polyspermy Completion of meiosis II hours for first cell division

1. CLEAVAGE 2. GASTRULATION 3.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)

1 st cleavage 1 cell  2 cells of equal size 2 nd cleavage 2 cells of equal size  4 cells of equal size 3 rd 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.syndactyly.