Mechanisms Of Cell Fate Specification

Question 1

How are different tissues formed? - Aristotle

Answer 1

Cracked open chicken eggs each day during their three week incubation period and provided two possible explanations:

Preformation and epigenesis

Question 2

Preformation (Aristotle)

Answer 2

Organs and tissues are preformed and correctly positioned in the fertilised egg. They simply enlarge during embryonic development

Question 3

Epigenetics (Aristotle)

Answer 3

Organs and tissues are added progressively during development, with complexity increasing over time (he preferred this explanation)

Question 4

How are different tissues formed (William Harvey - physician to king James I and Charles I 1578-1657)

Answer 4

Examined pregnant deer on hunting trips and also studied in detail chick and insect embryos

He published “Exertationes de generatione animalium” is 1651 in which he:

• observed that embryos form following combination of female eggs and male semen (and not menstrual blood and semen and Aristotle believed)
• observed that embryos increase in complexity over time, strongly supporting Aristotle theory of epigenesis

Question 5

How are different tissues formed?- Antonie van Leeuwenhoek (1632-1723)

Answer 5

Despite Harvey’s evidence, 17th and 18th century
scientists preferred preformation. van Leeuwenhoek, who discovered spermatozoa in 1677:

• believed he could see ‘nerves, arteries and veins’
within them demonstrating that animals are
preformed in sperm.

Question 6

How are different tissues formed?- Marcello Maplighi (1628-1694)

Answer 6

Marcello Maplighi was the first person to observe
chick development under the microscope (1672):

• he made beautiful drawings of chick development (look at slides)

• he ignored the evidence of his own work and
  concluded that chicks were preformed in the
  fertilised egg!

Question 8

How are different tissues formed? George von Dassow

Answer 8

In the 19th century the debate swung decisively in favour of epigenesis:
• More powerful microscopes and the development of dyes with which to stain embryos allowed embryologists to study the early stages of development in more detail.

• All living things are composed of cells and new
cells arise only by division of old cells

• Eggs and sperm are single cells - cannot contain
preformed organs and tissues composed of many
cells

Question 9

Developmental commitment

Answer 9

A progressive restriction of developmental potential

Look at slides for diagram

          (Pluripotent)        (Oligopotent) Toti    —————-> multi —————-> unipotent  All cell                    Many cell                    One cell Types                       Types                           Type

Question 10

Developmental commitment; stages of commitment

Answer 10

Specification, determination, differentiation

Question 11

Specification

Answer 11

commitment to a particular fate that can be changed if cells are moved to a new environment

Question 12

Determination

Answer 12

commitment to a particular fate that cannot be changed

Question 13

Differentiation

Answer 13

cells acquire their functional characteristics

Question 14

Mechanisms of commitment

Answer 14

• localised determinants
• embryonic inductions
• morphogens

Question 15

Mechanisms of commitment: localised determinants (August Weismann 1834-1914 German evolutionary biologist)

Answer 15

Weismann proposed the germ plasm theory which postulated that :

• Inheritance only takes place by means of the
germ cells which contain germ plasm

• Germ plasm contains germ plasm ‘determinants’

• Determinants specify the fates of somatic cells

• Genetic information cannot pass from soma to
germ plasm and on to the next generation - the
Weismann barrier

Question 16

Mechanisms of commitment: localised determinants (Edward Conklin 1863-1952- American

Answer 16

Conklin was studying ascidian development

Conklin found that eggs of certain ascidian species contained a crescent of yellow cytoplasm that was subsequently found in larval muscle cells:
Styela partita

He suggested that the yellow cytoplasm contained a muscle determinant

Question 18

Mechanisms of commitment: localised determinants (hiroki Nishida and Kaichiro Sawada Nature 2001)

Answer 18

C. elegans has an invariant lineage

C. elegans displays asymmetric segregation

Asymmetric segregation depends on PAR proteins

PAR proteins are conserved regulators of asymmetric divisions and polarity

Question 19

Mechanisms of commitment: embryonic inductions (Hans Driesch 1867-1941- German biologist)

Answer 19

Hans Driesch was studying sea urchin embryos

He repeated Roux’s experiments by dissociation

• This was the first demonstration of regulative
development. It means that sea urchin eggs
cannot be a mosaic of localised determinants

Regulation suggests that cells communicate:
• Each cell of the 4-cell sea urchin (and frog and
mammalian) embryo has greater potential than
they normally exhibit. They can form more tissues when isolated from their neighbours than when left in contact with them.
• This suggests cells communicate with each other to restrict their potency - cell signalling!

• The process whereby a cells fate is changed by
signals from an adjacent group of cells is known
as embryonic induction.
• It requires two types of cell, a signalling cell and a responding cell.
• The ability of a cell to respond to a signal is
known as competence.

Question 20

Mechanisms of commitment: embryonic inductions (Pieter Nieuwkoop (1917-1996)
Dutch developmental biologist)

Answer 20

Pieter Nieuwkoop was interested in mesoderm
specification and showed in amphibian embryos that it was induced:

• Isolated animal and vegetal fragments
differentiate into epidermis and endoderm
respectively
• When combined, mesoderm is induced by the
vegetal fragment

Question 21

Mechanisms of commitment: embryonic inductions

Answer 21

A Xenopus animal cap assay was developed to identify mesoderm inducing factors (MIF):
• The cap elongates when mesodermal tissue is formed as the induced notochord undergoes convergent-extension

Question 22

Mechanisms of commitment: embryonic inductions - (MIFs)

Answer 22

MIFs include members of the Transforming Growth Factor ß family and members of the Fibroblast Growth Factor family:

• Activin
• Bone Morphogenetic Protein 2
• Bone Morphogenetic Protein 4
• Growth and Differentiation Factor 3
• Nodal
• Vg1

• FGF1
• FGF2
• FGF4
• FGF8

Question 23

Mechanisms of commitment: embryonic inductions

Answer 23

C. elegans also has inductive interactions

Question 24

Mechanisms of commitment: morphogen gradients

Answer 24

The neural tube has a distinctive dorsal-ventral pattern of neural subtypes that can be revealed by the distribution of distinct transcription factors:

• The ventral midline lies just above the notochord and contains a specialised
structure called the floor plate.Above this, in a distinctive order, are a number of
different ventral interneurons

• The notochord induces ectopic floor plate and notochord when grafted to the side of a neural tube

Question 25

Shh

Answer 25

Shh is a morphogen: • It induces multiple cell fates in a concentration dependent manner

Question 26

Summary

Answer 26

Different organs and tissues are not preformed in the egg, they are acquired gradually with complexity increasing with time (epigenesis). Cell fates are determined by localised determinants, inducing factors, or morphogens. Localised determinants are usually transcription factors that are localised to specific parts of the egg cytoplasm. They specify the fate of cells that inherit them. In ascidians, muscle is specified by Macho-1, encoded by an mRNA that is localised to the yellow cytoplasm. Embryonic induction is mediated by extracellular signals, usually a secreted protein. In amphibians the mesoderm is induced by Nodal protein secreted by the vegetal hemisphere. Cell fate in the ventral half of the neural tube is specified by a morphogen from the adjacent notochord. The morphogen is the secreted protein Sonic Hedgehog.