1. Development
Chapter 47

2. Big Questions:
How does a multicellular organism develop from a zygote?
How is the position of the parts of an organism determined?
How does differentiation of cell type occur in organisms?
How are genetics and development connected? Environment?

3. Developmental Biologists
noted common features of early embryonic stages among animals
demonstrated specific patterns of gene expression that direct cells in a developing embryo to adopt distinctive fates
embryonic development involves common stages that occur in a set order

4. Development is Regulated
The development of an organism is coordinated by sequential changes in gene expression.
Multicellular organisms must regulate:
Cell division
Differentiation
Morphogenesis

5. Fertilization
The formation of a diploid zygote from a haploid egg and sperm
Sperm penetrates
Changes at the egg surface prevent polyspermy.
Ca2 spread across the egg correlates with the appearance of the fertilization envelope

6. Fertilization envelope
Figure 47.4
Experiment
Fertilization envelope
10 sec after fertilization
25 sec
35 sec
1 min
500 μm
Results
Spreading wave of Ca2+
Point of sperm nucleus entry
Figure 47.4 Inquiry: Does the distribution of CA2+ in an egg correlate with formation of the fertilization envelope?
1 sec before fertilization
10 sec after fertilization
20 sec
30 sec
500 μm

7. Video: Calcium Release Following Egg Fertilization

8. Cleavage Rapid division of cells without G1 or G2 stages
So what is physically different about cells?
Q: How do cells know where they are in the embryo, and what they should become?
(a) Fertilized egg
(b) Four-cell stage
Figure 47.6 Cleavage in an echinoderm embryo
(c) Early blastula
(d) Later blastula

9. Cytoplasmic Determinants
Unequal distribution of mRNA and proteins from egg cell
Starts process of cell fate and polarity of organism

10. Cytoplasmic determinants1
Newly fertilized egg 3
Two-cell embryo
Figure Partitioning of P granules during C. elegans development 2
Zygote prior to first division 4
Four-cell embryo
Unequal distribution of proteins determining cell fate
Ex: more proteins in anterior end will signal “head region” to cells

11. Pattern Formation
How do organs develop in their proper positions? How do cells "know" where they are within a developing organism?
Proteins released by cells Morphogens form gradients establishing patterns.

12. Embryonic Induction
Signaling by nearby cells send adjacent cells down a specific developmental path.

13. Presence of the inducer/signaling molecule triggers gene expression (signal transduction) in cell.
From translation of that protein starts differentiation of cell.

14. Morphogenesis “Gastrulation”
3 germ lines established
Differentiation accomplished by the expression of cell type-specific proteins.
“Organogenesis”
Formation of organs

15. Differentiation Control
Genes must be both turned on and turned off at appropriate times and locations.
Regulated by internal and external cues

16. Internal Cues
DNA packing, micro RNA’s and epigenetics can switch genes ON or OFF.
Transcription factors must be present and can be either stimulatory or inhibitory.

17. External Cues (outside the cell)
External cues signal to cells, causing cells to alter their gene expression.

18. Environmental Cues
The presence of particular molecules and conditions in the local environment is required for development to proceed properly.
Ex. The role of temperature and moisture in seed development.
Why do you water a seed after you plant it? Hmm…

19. Hydrangea flowers can change from pink to blue based
Environmental Cues
Changing the environment around the cells can alter gene expression.
Ex: Himalayan rabbits carry the C gene, which is required for the development of pigments in the fur, skin, and eyes, and whose expression is regulated by temperature. Why?
Hydrangea flowers can change from pink to blue based
on soil pH

20. Apoptosis
“Programmed cell death”: Important role in defining borders and openings in the developing organism.

21. Homeobox (HOX) Genes
Genes that serve as master regulators of animal development.
Similar HOX genes found in all animals
Ex: the “head” hox gene regulates head development

22. Hox Gene Ancestry

23. Experimental Evidence
Mutations in similar hox genes caused mutations in same structures.

24. Experimental Evidence
Swapping “leg” hox genes for “antenna” hox gene of a developing fly embryo. Weird!