1. Chapter 00 Animal Development Biology 102 Tri-County Technical College Pendleton, SC

2. Functions of Fertilization Forms a diploid (2N) zygote [defined as single cell resulting from union of sperm and egg or + and – cells] from haploid (N) sets of chromosomes from two individuals Triggers onset of embryonic development Typically occurs in oviduct of female

3. Acrosomal Reaction Is discharge of hydrolytic enzymes from vesicle in acrosome of sperm cell Enzymes enable acrosomal process to elongate and penetrate jelly coat of egg Protein coated tip (bindin) attaches to specific receptors on vitelline layer of egg (just external to plasma membrane) This provides species specificity for fertilization

4. Acrosomal Reaction Visual

5. Cortical Reaction Fusion of egg and sperm membranes stimulates series of changes in egg’s cortex known as cortical reaction Fusion stimulates release of Ca 2+ from egg cell’s endoplasmic reticulum Signaling pathway also leads to production of IP 3 (inositol triphosphate) and DAG (diacylglycerol) which opens ligand-gated calcium channels on ER

6. Cortical Reaction, cont. High Ca 2+ [ ] results in change in egg’s cortical granules (special vesicles) Increase in Ca 2+ causes cortical granules to fuse with plasma membrane and release contents into perivitelline space outside plasma membrane Contents of cortical granules include enzymes that separate vitelline layer from plasma membrane

7. Cortical Reaction III Osmosis causes perivitelline space to swell Swelling elevates vitelline layer and other granule enzymes harden it to form a fertilization membrane This slow block to polyspermy consists of fertilization membrane and other changes to egg’s surface

8. Cortical Reaction Visual

9. Preventing Polyspermy Acrosomal reaction causes sperm and egg’s plasma membrane to fuse which allows sperm to enter the egg **Na + flows into egg cell and changes its membrane potential This depolarization of egg’s PM prevents other sperm cells from uniting with egg This fast block to polyspermy operates until slow block provided by cortical reaction can function

10. Changes in Activated Egg Sharp rise in cytoplasmic Ca 2+ [ ] incites metabolic changes that activates egg Cellular respiration and protein synthesis rates > Cytoplasmic pH changes from slightly acidic to slightly alkaline due to H + extrusion Sperm nucleus within egg swells and merges with egg nucleus to form zygote (actual fertilization) DNA replication begins and 1 st division occurs in about 90 minutes

11. Gastrulation Involves extensive rearrangement of cells which transforms blastula (hollow ball of cells) into three-layered embryo called the gastrula Set of common cellular changes involved with all animals Changes in cell motility, shape, and cellular adhesion to other cells and to molecules of extracellular matrix

12. Gastrulation, cont. Some cells at or near surface move to more interior location Three layers produced by gastrulation are embryonic tissues called embryonic germ layers These three cell layers (primary germ layers) will eventually develop into all parts of the adult animal

13. Gastrulation III Ectoderm (outermost) develops into nervous system and outer layer of skin in adult animals Endoderm (lines archenteron) develops into lining of digestive tract and associated organs (liver, pancreas) Mesoderm (middle) develops into kidneys, heart, muscles, inner layer of skin, and most other organs

14. Germ Layers Visual

15. Germ Layer Fate

16. Amphibian Embryonic Development Neural tube and notochord are first organs to develop in frogs/other chordates Dorsal mesoderm above archenteron condenses to from notochord Ectoderm above rudimentary notochord thickens to form neural plate that sinks below embryo’s surface Neural plate rolls into neural tube which will become brain and spinal cord

17. AED, cont. Notochord elongates and stretches embryo lengthwise and functions as core around which mesoderm cells that form vertebrae gather Organogenesis continues as ectoderm gives rise to epidermis, epidermal glands, inner ear, and eye lens Mesoderm gives rise to notochord, coelom lining, muscles, skeleton, gonads, kidneys, and most of circulation system

18. AED III Endoderm forms digestive tract lining, liver, pancreas, and lungs Neural crest forms from ectodermal cells which develop along border where neural tube breaks off from ectoderm These cells migrate and form pigment cells in skin, some bones and muscles of skull, teeth, adrenal medulla, and parts of peripheral nervous system End result is aquatic, herbivorous tadpole

19. AED Visual

20. AED Visual II

21. Extraembryonic Membranes Chorion forms from trophoblast and surrounds embryo and all extraembryonic membranes Amnion forms as dome above epiblast and encloses embryo in fluid-filled cavity Yolk-sac encloses fluid-filled cavity but no yolk

22. Membranes, cont. Yolk sac membrane is early site of blood cell formation Allantois develops from outpocketing of rudimentary gut and is incorporated into umbilical cord where it forms blood vessels that transport oxygen and nutrients from placenta to embryo and waste products from embryo to placenta

23. Organogenesis Various regions of 3 germ layers develop into rudiments of organs during organogenesis Three kinds of morphogenetic changes occur: FOLDS, SPLITS, AND DENSE CLUTTERING (condensation) are first evidence of organ building

24. F, S, and C Visual

25. Cell Location and Orientation Morphogeneis is major aspect of development Only in animals does it involve movement of cells Movement of parts of cell can bring about changes in cell shape and enable cell to migrate in an embryo Changes in both cell shape and position involved in cleavage, gastrulation, and organogenesis

26. Location and Orientation, cont. Bilaterally symmetrical animals have anterior- posterior axis, dorsal-ventral axis, and left and right sides Establishing basic body plan is first step in morphogenesis Prerequisite for development of tissues/organs In mammals, polarity does NOT seem to be determined until after cleavage In most other species, it is established earlier

27. L and O III Inductive signals play major role in pattern formation (development of animal’s spatial organization) Arrangement of organs/tissues in their characteristic places in 3-dimensional space Molecular cues controlling pattern formation (positional information) tell a cell where it is with respect to animal’s body axes and help determine how cell and its descendents will respond to future molecular signals ****Works so very well most of the time