Download Bio 2175 Developmental Biology Lecture 17: Organogenesis The

Bio 2175 Developmental Biology
Lecture 17: Organogenesis
The development of teeth as a model for organogenesis.
1. News
a. Discussion Thursday with written prep due
b. Suggested statistical tests for your zebrafish data
i. Fisher’s exact test (categorical data)
1. R.A. Fisher’s wager with Muriel Bristol-Roach, Ph.D: the lady tasting tea
2. e.g. Is observing 8 normal and 0 cyclopic fish sig. diff. from 4 of each?
3. Good for small n’s (chi-squared test OK, but better for larger datasets)
ii. Student’s t-test (quantitative data)
1. How William Sealy Gosset helped make Guinness so delicious
2. Is one mean significantly different from another?
2. Organogenesis (drawings)
a. Initiation
i. How do cells begin forming an organ in a particular time and place?
ii. The highest level instructions are largely unknown
b. Morphogenesis: shaping the organ
c. Differentiation
i. Manifestation of cell types and secreted extracellular products
3. Mammalian tooth development
a. Location (drawing)
i. Oral cavity (also pharynx in non-tetrapods)
1. Stomodeum (ectoderm) and anterior endoderm
b. Early Stages
i. Thickening (placode), Bud, Cap, (Bell)
c. Tissues
i. Epithelium
1. Ameloblasts make enamel
a. Enamel is hypermineralized ECM (hydroxyapatite)
ii. Neural crest mesenchyme
1. Migrates from midbrain
2. Odontoblasts make dentin
a. Dentin is very similar to bone
d. “Classic” molecular model of tooth initiation
i. Fgf8 from epithelium signals to mesenchyme
ii. Pax9 (transcription factor) turns on in mesenchyme
iii. BMP signaling limits where teeth can form
iv. Hox-like code for tooth identity? (probably not)
1. e.g. Molar vs. incisor identity
e. Enamel knot
i. Signaling center (organizer?)
ii. Secretes Shh and Fgf ligands, does not express FgfRs
iii. Cell division (BrdU marker)
Bio 2175 Developmental Biology
Lecture 17: Organogenesis
4. Hen's teeth
a. Can structures reappear in evolution after they are lost? (“Dollo’s Law” says no)
b. Famous paper: mouse mesenchyme + chick epithelium in chick
i. Authors concluded chick epithelium could still make enamel (N = 4/150)
c. Later work
i. Mutation allows partial tooth development
ii. Birds lack genes necessary to make enamel (e.g. Enam)
iii. Perhaps mesenchyme can ingress into the epithelium? (Wang et al., 2011)
5. Fgfs in zebrafish teeth
a. Zebrafish have pharyngeal teeth
b. Gene expression is largely similar between fish and mammals
c. If you block Fgf signaling, teeth don’t form
i. Chemical SU5402 inhibits FgfRs (receptors)
ii. Tooth “marker” dlx2b is not expressed
d. GoF analysis
i. Heatshock construct Hsp70:fgf8:GFP
1. Works (turns on target pea3) but no teeth
ii. Hsp70:fgf10:GFP
1. More dlx2b, more teeth
iii. Fgf8 vs Fgf10 mutation story (D113V)
1. Aspartic acid (polar) to valine (hydrophobic)
6. Reaction-diffusion patterning
a. Alan Turing
i. 20th century mathematician
ii. Helped design Colossus, the first programmable digital computer
iii. Interesting/tragic personal life recent movie
b. A reaction diffusion mechanism
i. Stochastic beginning—cell “randomly” starts to express activator
ii. Slowly diffusing activator
1. Activates expression of itself and inhibitor
iii. Rapidly diffusing inhibitor
1. Only inhibits activator
iv. Different cell fate depending on act vs. inh concentration
v. Complex, two dimensional patterns arise depending on parameters like speed of
diffusion, strength of inhibition, etc.
c. Tooth hypothesis
i. Fgfs as activators
ii. Bmps as inhibitors?
iii. Evolution and Development 2013 paper evidence