1. Embryology
Cellular and Molecular Mechanisms Involved in Fertilization and Development

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

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
Sea Urchin Fertilization

6. Applying Your Knowledge
Bindin
Cortical Granule
Acrosome
Vitelline envelope
Fertilization cone
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?

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 is the 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. 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
Blastula
Morula
Blastomere = individual cell of blastula

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

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

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

15. Sea Urchin Gastrulation

16. Cytoskeletal Components Directing Invagination

17. Embryonic Stages
Gastrulation in the Frog
Involution

18. Embryonic Stages
Gastrulation in the Chick
Migration

19. Neurulation in the Frog
Notochord, derived from mesoderm, induces the overlying ectoderm to form the neural tube
Notocord in Chordates
First tissue to fully differentiate
Forms in mid-dorsal mesoderm (chordamesoderm)
Induces formation of neural tube and cranial space in nearby ectoderm (dorsal side)
Replaced by spinal column (cartilage and bone from mesoderm) in vertebrates

20. Cytoskeletal Components Involved with Neurulation

21. Tissues of the Frog Neurula
Differentiation of Mesoderm
Mid-dorsal – chordamesoderm
Dorsal – somites – segmented parts of body
Intermediate – thin layer, contributes to kidney and/or testicular ducts (labeled “nephrotome” in previous slide)
Lateral Plate – two sheets join at mid-ventral line
Somatic – body side
Splanchnic – surroundings of gut
Coelom – body space between two layers of mesoderm b

22. Neurulation in the Chick Embryo

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

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

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. Applying Your Knowledge
Differentiation
Growth
Morphogenesis
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?