Principles of Determination Continued

Question 1

what is lateral inhibition? give an example

Answer 1

lateral inhibition allows a celll to gain an advantage in determination.

example: determination of neuroblasts (will become neurons) in ventral neurogenic region of Drosophila early gastrula

Question 2

lateral inhibition of drosophila - how are cells determined to become neuroblasts?

Answer 2

cells w/ potential will synthesize Achaete and scute proteins - basic helix-loop-helix proteins (transcription factors) - proneural genes.

will also synthesize receptor protein Notch and ligand Delta - these are NOT soluble, but located on surface (this is juxtacrine signaling)

Question 3

why will cells that produce more Delta than neighbors gain a determinant edge?

Answer 3

Delta activates a Notch-mediated signaling cascade, results in inhibition of neuroblast genes which are required for neuroblast differentiation

Question 4

what else compounds lateral inhibition with Delta production?

Answer 4

expression of neuroblast genes is also required for production of Delta - neighbors of “overproducer” are at even bigger disadvantange.

since no Delta production, they become epidermal cells by default

overproducer becomes a neuroblast (neuroblast genes remain active), it leaves epithelium + moves to basal side of future epidermis

Question 5

is this signaling mechanism fundamental?

Answer 5

yes - used in other tussues + many other species as spacing mechanism

ex: bristle spacing in Drosophila, hair cell spacing in inner ear, patterning of feathers on chicks

Question 6

what is embryonic induction, what is it generally a result of?

Answer 6

coordination of the construction of a tissue/structure accomplished by one group of cells communicating an organizing change in an adjacent group of cells

generally due to local interctions between non-eq cells (i.e., paracrine signaling)

Question 7

what is an example of embryonic induction? - give a brief overview of process. what gene is involved in this process?

Answer 7

lens formation - in many species (including amphibians, mammals), optic vesicle (extension of dev. brain) induces overlying ectoderm to form eye lens. thus, the optic vesicle is the inducer

ectoderm must be competent to respond to inductive signals from optic vesicle

Question 8

described experiments that determined where Pax6 is needed (and make a statement about it)

Answer 8

lens formation was observed in four different types of mice - wild type mice (no mutations), Pax6-mutant ectoderm + WT optic layer, WT ectoderm, Pax6-mutant optic layer, and double Pax6-mutants.

it was shown that when ectoderm had a mutation in Pax6, lens formation would never occur. thus, Pax6 is needed in the ectoderm

Pax6 is needed in overlying surface ectoderm (responder) for it to be competent to respond to optic vesicle (inducer) to form lens tissue.

Question 9

describe further induction of the optic vesicle to the ectodermal layer

Answer 9

OV secretes BMP4, which is perceived by receptors on the membranes of ectodermal cells. perception leads to transcription of Sox2 (+ other) transcription factors, allows for expression of lens-specific genes.

one of Pax6’s actions may be to upregulate synhtesis of BMP4 receptors.

Question 10

is the inductive relationship of the OV and ectodermal cells reciprocal? if so, describe.

what else must a Pax6 mutant be missing?

Answer 10

yes - a previous interaction between neural plate + ectoderm was required in order for latter to start expressing Pax6.

eventually, lens will begin to serve as inducer for OV - stimulates OV to become optic cups, walls of which will differentiate into retinal tissue.

Pax6 mutants will not only lack lenses, but also retinas - lenses will not exist to serve as inducers!

Question 11

what are imaginal discs? where are they formed?

Answer 11

determined tissue - packets of epithelial tissue fated to become legs, wings, atennae, genitalia, etc.

formed in pockets of larval epithelium

Question 12

how do these packets begin and form?

Answer 12

begin as 20-50 cells, divide by mitosis during larval period until they consist of tens of thousands of cells by the end of the larval period

during metamorphosis, are hormonally stimulated to undergo eversion, unfold + form a cuticle

Question 13

how has determination of these discs been proven? what does transplantation allow for?

Answer 13

determintion during larval development proven via transplantation into body cavity of a host larva, where they will respond to homrones of host. results in formation of extra adult structure in body cavity of host

transplantation into body cavity of adult allows for proliferation of disc by mitosis (but not eversion - proper homones not present)

Question 14

what is the phenomenon of transdetermination?

Answer 14

determination of a disc is stable after many transfers, but over time, fidelity will decrease and the wrong structure may form from disc.

Question 15

how does transdetermination relate to homeotic mutations?

Answer 15

diff than homeotic mutations - these ar emutations in TF genes that result in formation of wrong appendages in wrong place.

transdetermination does not involve mutations but rather cells accumulate Wingless signaling protein ( = Wnt), which is upstream of B-catenin (therefore leads to transcriptional activation).

Question 16

creation of chimeras in more detail - describe cell predicted fate of the fused embryos

Answer 16

cells on outside (from trophoblast) of one 16-celled embryo can be combined w/ cells from inside of another (ICM), will usually end up carrying out predicted fates.

can occasionally contribute to “other” portion.

Question 17

what do chimera experiments highlight?

Answer 17

highlights how regulative/flexible mammalian development is and that mammalian cells undergo a gradual and prolonged process of determination

Question 18

what is “fated” to become neural plate and at what stage?

Answer 18

at blastula + early gastrula stage, neural plate rudiment shown by lineage analysis to be fated to become neural plate - will form neural tube which ultimately becomes brain + spinal cord.

Question 19

describe the very general process of gastrulation

Answer 19

early stage - begins at depression known as blastopore (“below”, i.e. toward vegetal pole, equator where grey crescent is). vegetal cells buckle inward

mid stage - animal and equatorial regions expand toward blastopore rim, equatorial cells migrate inside

late stage - eventually, three embryonic layers have formed.

Question 20

what forms the epidermis and neural plate?

Answer 20

ectoderm forms epidermis + neural plate. neural plate forms from dorsal ectoderm, found between blastopore and animal pole.

Question 21

describe the first two steps of neurulation

Answer 21

formation of an elevated area on dorsal side of embryo = neural plate (occurs after gastrulation)

neural plate closes into tube (neural tube), which will be internalized + give rise to brain and spinal cord.

Question 22

Spemann’s transplantation experiments - when did he find the prospective neural plate tissue was determined?

Answer 22

early gastrula stage - transplanted prospective neural plate from donor into prospective epidermis region of early gastrula host - became epidermis

late gastrula - formed brain and spinal cord no matter where it was placed into a host late gastrula

Question 23

how were Spemann’s findings confirmed? what does this mean?

Answer 23

isolation experiments confirmed findings - cells typical of brain are formed from pros. neural plate tissue from late gastrulas in saline

pros. epidermis behaves similarly, producing epidermal structures like gland cells
pros. neural plate taken before gastrulation is complete will form epidermal structures in saline

thus, neural cell determination occurs somewhere between early gastrulation and late gastrulation - more meticulous transplantations would be required to determine when exactly during gastrulation determination occurs.