Embryology

1. Embryology Cellular and Molecular Mechanisms Involved in Fertilizationand Development

2. unfertilized sea urchin egg fertilized sea urchin egg Egg plasma membrane Vitelline envelope Sea Urchin Fertilization Contains digestive enzymes Animation 43-01

3. Sea Urchin Fertilization Acrosomal enzymes digest jelly coat Bindin on acrosomal process contacts receptors on vitelline envelope Acrosomal process forms by actin polymerization Egg plasma membrane forms fertilization cone to engulf sperm head

4. Preventing Polyspermy • Polyspermy = fertilization of egg by more than one sperm • Blocked in two ways • Fast block to polyspermy: Change in charge across the egg plasma membrane, mediated by influx of sodium ions • Slow block to polyspermy: cortical granules release enzymes and water-absorbing materials to form fertilization envelope around egg, mediated by release of stored calcium from endoplasmic reticulum

5. Sea Urchin Fertilization Cortical granules fuse with plasma membrane and release • Enzymes to catalyze reactions that • break bonds between vitelline envelope and plasma membrane • remove bindin receptors • harden vitelline envelope fertilization envelope • Substances that absorb water and raise vitelline envelope

6. Which structure of the egg has a receptor for sperm? Which structure is directly responsible for bringing the sperm head into the egg ? Which part of the sperm contains enzymes for digesting through the jelly coat surrounding the egg? Bindin Cortical Granule Acrosome Vitelline envelope Fertilization cone Applying Your Knowledge

7. Development • A series of progressive changes that generates the structures of the organism throughout its life cycle • Includes • growth: change in size due to increase in cell number and/or cell size • differentiation: cellular specialization • morphogenesis: establishing body and organ shapes

8. Determination Precedes Differentiation • Determination: commitment of a cell to a particular fate, occurs by the activation of specific genes • Affected by • Cytoplasmic segregation • Induction • Differentiation isthe result of determination

9. Induction: process in which a substance or tissue influences the fate of a group of adjacent cells Optic vesicle induces lens placode Optic cup induces lens Lens induces cornea Transplant of tissue from dorsal lip of blastopore induces second invagination and formation of secondary embryo.

10. Blastula Morula Embryonic Stages • Cleavage: series of rapid cell divisions following fertilization, leading to • morula: ball of cells • blastula: hollow sphere of cells Blastocoel = fluid-filled space Blastomere = individual cell of blastula

11. Yolk Content Affects Cleavage Patterns Blastodisc= layer of cells on top of yolk

12. Orientation of mitotic spindles in early cleavage is dependent on the organization of cytoplasmic determinants in the egg Cytoplasmic Determinants Influence Cleavage Patterns Mosaic Development Regulative Development

13. Cell Adhesion Molecules Influence Blastula Organization • Cell Adhesion Molecules (CAMs) are trans-membrane cell surface proteins that bind to CAMs on other cells. • CAMs are involved in cell migration and producing stable tissue structure. • Experiments have shown that cadherin (one type of CAM) stabilizes the frog blastula.

14. blood vessels, muscle, bone, liver and heart mesoderm nervous system, skin linings of digestive and respiratory tracts primitive gut Embryonic Stages Invagination Gastrulation: cellular movements forming three tissue layers and distinct body axes Sea Urchin Migration

15. Sea Urchin Gastrulation

16. Cytoskeletal Components Directing Invagination

17. Embryonic Stages Gastrulation in the Frog Involution

18. Embryonic Stages Migration Gastrulation in the Chick

19. Neurulation in the Frog Notochord, derived from mesoderm, induces the overlying ectoderm to form the neural tube

20. Cytoskeletal Components Involved with Neurulation

21. Tissues of the Frog Neurula

22. Neurulation in the Chick Embryo

23. Which one represents a hollow ball of cells? Which tissue induces formation of neural structures? Which one involves invagination or involution of cells? Gastrula Neurula Blastula Ectoderm Mesoderm Applying Your Knowledge

24. Morphogenesis • Pattern Formation: organization of differentiated tissues into specific structures • Body segmentation in fruit flies is controlled by • gradients of materials established in the egg • actions of a series of embryonic genes Animation 19-03

25. Genes Affecting Body Segmentation Products of Maternal Effect Genes establish gradients in the egg Gap genes define broad areas and regulate Pair-rule genes Pair-rule genes refine segment locations and regulate Segment Polarity genes Segment Polarity genes determine segment boundaries and orientations Homeotic genes define the role of each segment

26. Mutation in a Homeotic Gene Causes Legs to Grow Where Antenna Should Be

27. Homology between Genes Controlling Segmentation in Fruit Flies and Mice

28. Morphogenesis May Require Apoptosis Apoptosis = Programmed Cell Death

29. Cellular and Molecular Mechanisms Essential to Development • Specificity of receptors and enzymes • Cytoplasmic organization • Cell movement through the action of cytoskeletal elements and cell adhesion molecules • Orientation of tissue layers leading to Induction • Differential Gene Expression

30. Which process leads to a morula? Which process leads to the formation of somites, the precursors to vertebrae? Which process is directly preceded by determination? Differentiation Growth Morphogenesis Applying Your Knowledge