The Body Plan

Question 1

Rostral-Caudal Axis

Answer 1

Question 2

Organogenesis begins around. . .

Answer 2

week 3

Goes from ~3-8 weeks (to the end of the embryonic period)

Question 3

Phylotypic stage

Answer 3

Stage of embryological development at which all vertebrates look essentially the same - almost indistinguishable.

Question 4

Metameric segments

Answer 4

Repeated developmental modules/areas that segment

Question 5

Selector Genes

Answer 5

ex, Hox genes

Confer distinctive regional properties to metameric segments.

Question 6

Hox gene colinearity

Answer 6

The order of Hox genes along the body axis corresponds to their order in DNA.

Question 7

Rhombomeres

Answer 7

Developmental segments of the vertebrate embryo closest to the midbrain. Represents the developing hindbrain and is the origin of the cranial nerves. Neural crest cells also stem from here before they migrate to associated pharyngeal arches.

Question 8

Somites

Answer 8

Regions of the developing spinal cord in all vertebrate embryos.

Question 9

Lineage Compartmentation

Answer 9

Cells and their progeny do not cross compartment/segment boundaries.

Question 10

Rhombomere Boundary Formation

Answer 10

Eph/ephrine signals are bidirectional! It is receptor-receptor interaction. Results in cell-cell repulsion.

Question 11

Somite boundary formation

Answer 11

Also utilizes Eph/ephrin, but rather than differentiating nodes of neural tissue, neural tissue is guided through the already segmented body tissue.

Question 12

Trunk vs Cranial Neural Crest

Answer 12

Question 13

In general, rhobomeres form by ____, while somites form by ____.

Answer 13

In general, rhobomeres form by dividing up the pre-existing neural tube, while somites form by budding off of previous somites.

Question 14

Regionalization of the Somite

Answer 14

Question 15

Somite regionalization mechanism

Answer 15

Wnt and Shh signaling results in transcriptional activation via master transcription factors (for example, MyoD in the Wnt+Shh myotome tissue)

Question 16

Homeotic mutations

Answer 16

Mutations in genes that result in transformation of one body part into another.

Question 17

Why is a homeotic mutation in humans unlikely to generate a second pair of legs as it may in Drosophila?

Answer 17

Human HOX genes operate by combination rather than simple pattern. Humans have 4 sets while Drosophila has only one set, and even among an individual set humans have more overlap.

Question 18

A Hox gene typically exerts its strongest effects near . . .

Answer 18

its rostral limit of expression

Question 19

Morphogen

Answer 19

Concentration/gradient dependent, two or more thresholds.

Question 20

Mechanisms of “despeckling”

Answer 20

Eph+ cells surrounded by ephrin+ cells will transdifferentiate

Generally speaking, when cells find themselves in the wrong part of the body, they apoptose.

Eph+ cells bordered on one side by an ephrin+ cell may be repulsed.

Question 21

“Posterior prevalence”

Answer 21

The more caudal Hox genes tend to have a stronger effect or override the effect of the rostral genes.

Question 22

Oral isotretinoin (13-cis retinoic acid)

Answer 22

Microtia! Ahhhh!!!!

Regular blood tests before reperscription to make sure patient is not pregnant because malformation risk is SO great. Prescribed for severe acne. Time of greatest risk is week 3-8 (organogenesis phase)