1. Development

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

3. Essential Knowledge: Timing and coordination of specific events are necessary for the normal development of an organism, and these events are regulated by a variety of mechanisms.

4. Gamete formation Fertilization Cleavage Gastrulation Organogenesis Growth
Four processes of development:
Determination sets the fate of the cell.
Differentiation is the process by which different types of cells arise.
Morphogenesis shapes differentiated cells into organs, etc.
Growth is an increase in body size by cell division and cell expansion.

5. A program of differential gene expression leads to the different cell types in a multicellular organism
Embryonic development--fertilized eggs give rise to different cell types
Tissuesorgansorgan systemswhole organism
Gene expression orchestrates development; zygoteadult results from cell division, cell differentiation, morphogenesis and then growth
(a) Fertilized eggs of a frog
(b) Newly hatched tadpole

6. Fertilization Formation of zygote – union of male & female gametes
Purpose? Variation/creates a diploid cell
Triggers development of the egg

7. Cytoplasmic Determinants and Inductive Signals
Unfertilized egg—uneven distribution of RNA, proteins, other substances
Cytoplasmic determinants—molecules in oocyte that influence early development
Zygote dividescells have different determinantsdifferential gene expression
Another source of developmental info--environment (e.g. signals from nearby embryonic cells)
Induction--signal molecules from embryonic cells cause transcriptional changes in nearby target cells
Interactions between cells induce differentiation of specialized cell types
blastomere-cytoplasmic determinants distributed/ create first “totipotent” cells

8. Cleavage Embryo divides into cells called “blastomeres”
Hollow ball forms of somatic cells called a blastula
These cells are also called germ cells (totipotent stem cells!!!)
**NO growth – just cell division

9. Morula….Blastula

10. Gastrulation “conversion of blastula into germ layers”
Invagination – blastula forms
horseshoe shape
-Gastrocoel forms (stomach cavity)
-Blastopore forms (“germ hole”)
Gastrula stage forms 2 germ layers: Endoderm & Ectoderm
(incomplete gut)
“Diploblastic”
Comb Jelly

11. Gastrulation: Formation of complete gut
Cavity extends through animal (endoderm and ectoderm layers join)

12. Gastrulation: Formation of Mesoderm (3rd germal layer)
Usually formed by pieces of endoderm breaking away
Animals are termed “Triploblastic”
Triploblastic animals can be Protostomes (blastopore forms mouth first) or Deuterostomes (blastopore forms the anus 1st and mouth is formed 2nd)

13. forms the digestive system, inner lining of respiratory system
Endoderm – inner layer
forms the digestive system, inner lining of respiratory system
Ectoderm – outer layer forms the skin and nervous system
Mesoderm – middle layer
forms muscle, bone, reproductive system, circulatory system, etc.
Acoelomate-no body cavity
Pseudocoelomate- false body cavity
Coelomate-true body cavity (cavity lined with mesoderm)

14. Organogenesis Cell differentiation Formation of coelom (cavity)
Body organs form

15. Development is Regulated
The development of an organism is coordinated by sequential changes in gene expression.

16. One Cell  Organism

17. Cellular Differentiation
Accomplished by the expression of cell type-specific proteins.

18. Stem Cells

19. Transcriptional Regulations
Determination vs. Differentiation
Determination sets the fate of the cell.
Differentiation is the process by which different types of cells arise.

20. Sequential Regulation of Gene Expression During Cellular Differentiation
Myoblasts—make muscle-specific proteins, form skeletal muscle cells.
MyoD--“master regulatory genes”; makes proteins that commit cell to becoming skeletal muscle (50+)
MyoD—TF--binds to enhancers of target genes

21. Pattern Formation
Cells need to establish their position in the developing organism. This is established through protein gradients.

22. Embryonic Induction
Local signals communicated among populations of cells to control their development

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

24. 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.

25. Experimental Evidence
Mutations in normal developmental pathways lead to malformations in embryonic development

26. Experimental Evidence
Transplantation Experiments: Moving regions of a developing embryo affects normal pattern formation.

27. Essential Knowledge: Interactions between external stimuli and regulated gene expression result in specialization of cells tissues and organs.

28. Control of Differentiation
Differentiation is regulated by internal and external cues.
These cues “switch” specific genes “on” and “off” at specific times.

29. Turning On AND Turning Off
Genes must be both turned on and turned off at appropriate times and locations in the developing organism.
Transcription factors, and micro RNA’s both function in regulating gene expression.

30. Pattern Formation: Setting Up the Body Plan/Axis Establishment
Pattern formation--development of spatial organization of tissues and organs—studied in fruit flies
Animal pattern formation starts with formation of major axes
Positional information--molecular cues that control pattern formation-- gives cell location relative to the body axes and neighboring cells

31. The Life Cycle of Drosophila
Head
Thorax
Abdomen
0.5 mm
Pre-fertilization--cytoplasmic determinants determine axes
After fertilization--embryo develops into segmented larva with 3 larval stages
Edward Lewis, Christiane Nüsslein-Volhard, and Eric Wieschaus--Nobel 1995 Prize--decoding pattern formation in Drosophila
discovered that homeotic genes direct the developmental process; studied segment formation
discovered embryonic lethals with lethal mutations
found 120 genes essential for normal segmentation
Dorsal
Right
BODY
AXES
Anterior
Posterior
Left
Ventral
(a) Adult
Follicle cell 1
Egg cell
developing within
ovarian follicle
Nucleus
Egg
cell
Nurse cell 2
Unfertilized egg
Egg
shell
Depleted
nurse cells
Fertilization
Laying of egg 3
Fertilized egg
Embryonic
development
Antenna 4
Segmented
embryo
0.1 mm
Body
segments
Hatching
Leg 5
Larval stage
Eye
Leg
(b) Development from egg to larva
Wild type
Mutant

32. Hox Genes

33. Hox Genes
A family of related genes that serve as master regulators of animal development in all animals on the planet.

34. Internal Cues
Various transcription factors must be present inside a cell to allow for specific genes to be expressed.
Transcription factors can be either stimulatory or inhibitory.

35. External Cues
External cues signal to cells, causing cells to alter their gene expression.

36. Differentiation leads to Divergence
As cells differentiate, the proteins that they express commit them to particular “fates”. These fates are normally irreversible.

37. Don’t Forget The Environment!
The environment of the cell can also play a major role by contributing to gene expression in mature cells.