Developmental biology glossary (Prof. Dale)

Question 1

What is an axis ?

Answer 1

A line in relation to which the embryo (or organ) displays a morphological difference.
E.g.:
- rostral-caudal (head-tail, anterior posterior)
- dorsal-ventral (back-belly)
- left-right

Question 2

What is meant by cell lineage ?

What is the expression “cell lineage analysis” analogous to ?

Answer 2

• cell lineage = “Family tree” showing ancestry of a cell in the embryo, tissue or organ
• more usually reserved for the experimental situation of following a cell, or group of cells, to find their ultimate fate, either –> by direct observation or by some marking method that allows the progeny of the parent cell to be identified
• the expression cell lineage analysis does not formally differ from fate mapping

Question 3

What is commitment in embryology ?

Why is this word not extremely accurate in the field ?

Answer 3

• commitment and determination used interchangeably =
  irreversible decision to differentiate in a particular direction
• some authors –> use “commitment” to describe the process whereby a cell becomes committed to a particular fate, including reversible decisions (specification + determination)

Question 4

What is cell competence ?

Answer 4

Competence = ability of a cell or tissue to respond to a particular inducing signal
May depend on:
- expression of appropriate receptors for inducing factors
- an intact signal transduction cascade
- the epigenetic state of potential target genes (can they respond or not)

Question 5

What is a cytoplasmic determinant ?

What does it guarantee ?

Answer 5

• cyt determinant = substance (usually a transcription factor) localized to part of an egg or blastomere that is asymmetrically distributed during cell division
• guarantees that the cell that acquires it will acquire a particular state of commitment

Question 6

What is determination ?

Answer 6

• Determination = irreversible commitment to a particular cell fate, made by an undifferentiated cell
• The cells developmental potential no longer differs from its fate

Question 7

What is differentiation ?

How can we recognize differentiated cells ?

Answer 7

• differentiation = process whereby a cell acquires its final functional characteristics (=a manifestation of its cell fate)
• a differentiated cell –> expresses characteristic proteins + has a clearly defined morphology that identify it as a member of a defined histological type

Question 8

Define embryonic induction ?

What are examples of molecules involved ?

Answer 8

• Embryonic induction = signalling interaction between one (inducing) group of cells and a 2nd (responding) group of cells ==> responding cells undergo a change of fate
• Inducing factors ~ secreted signalling molecules e.g. TGFß, FGF, Hedgehog + WNT families

Question 9

What is cell fate ?

Answer 9

• fate = cell type(s) normally formed by an embryonic cell (compare w/ concept of developmental potential)
  Note: cells may not be already committed to their normal fate

Question 10

What is a fate map ?
How is such a map elaborated ?
Is this expression commonly applied to single cells or cell groups ?

Answer 10

• fate map = spatial map of the cell fates of different regions of an embryo at a particular stage of development
• made by labelling cells at one stage of development + identifying their distribution + commitment at a later stage (map drawn on the earlier stage)
• usually applied to groups of cells, w/ the term cell lineage more often applied to the collection of fates generated by single cells

Question 11

What is morphogenesis ?

Answer 11

• Formation of a biological structure by changing the relationships of cells/tissues
• Gastrulation + neurulation = examples of morphogenes

Question 12

What is a morphogen ?
What is required for a molecule to be a morphogen ?
What happens otherwise ?

Answer 12

• morphogen = molecule that forms a concentration gradient across a morphogenetic field, evoking different cell fates at different concentrations
• at least 3 different cell types must be formed in response to the gradient (otherwise the molecule is an inducing factor)

Question 13

What is a morphogenetic field ?

Answer 13

• morphogenetic field = portion of an embryo contained w/in well-defined boundaries, which can develop independently, w/o instructive influences from the rest of the embryo
• morphogenetic fields –> capable of regulation ==> any portion of the field can reconstitute the whole field
• as development proceeds, fields subdivide, becoming smaller and more numerous

Question 14

What is an organizer (region) ?

Answer 14

• organizer = embryonic region emitting instructive signals (~morphogen) responsible for regional specification w/in a morphogenetic field

Question 15

What is pattern formation ?

Answer 15

Pattern formation = synonymous w/ regional specification but also includes cell movements + sorting processes

Question 16

What is polarity ?

Answer 16

Polarity = regional differences in cell commitment along an axis of a morphogenetic field

Question 17

What is positional info ?

Answer 17

Positional info –> provides spatial information across a morphogenetic field to specify different cell fates
A positional signal ~ a morphogen.

Question 18

What is potency ?
Can you cite an example ?
How does development act on potency ?

Answer 18

• potency = developmental potential of a cell / the range of all the possible fates that can be produced by the descendants of this cell
• includes both a cells normal fate + additional fates
  that can be achieved through experimental manipulation
• E.g. a cell in the ventral portion of a newly-formed somite, whose normal fate is to become part of the sclerotome + contribute to the axial skeleton –> if the cell is transplanted to the dorsal part of the somite, it will now contribute to dermis or muscle or both –> this cells fate is sclerotome but its potency is sclerotome, myotome, and dermatome
• development = process that gradually restricts the potency of a cell

Question 19

What is regeneration ?

Answer 19

Regeneration = ability of a morphogenetic field or tissue to replace missing parts

Question 20

What is regional specification ?

Answer 20

Regional specification (=positional info) = mechanism by which different cells in a morphogenetic field acquire different developmental commitments

Question 21

What is regulation ?

Do you know an example in amphibians ?

Answer 21

• regulation = the ability of a portion of a morphogenetic field to reconstitute the entire field.
• at early stages of development (when the entire embryo is a single field) –> regulation defines the ability of parts of some embryos to reconstitute the entire embryo
  E.g. in 2-cell-stage amphibians, separation of the two blastomeres –> can allow each blastomere to produce a whole embryo

Question 22

What is specification ?

Why is this complicated to demonstrate for most species ?

Answer 22

• specification = tendency of a cell/group of cells, when cultured in isolation from the rest of the embryo and under “neutral” conditions, to differentiate in a particular direction
• unfortunately, it is experimentally feasible to determine this only for a few species (e.g. amphibians) where “neutral” culture conditions, in a simple saline solution, are possible

Question 23

What are stem cells ?
Where can they be found ?
What different types are there ?

Answer 23

• stem cells = undifferentiated cells capable of both self-renewal (through mitosis) + differentiation.
• found in embryos and adults they may be pluripotent, multipotent, or tri, bi, or unipotent