Molecular Patterning During Development Flashcards
State the principles governing cell differentiation
Generative program
Regulatory proteins work together to form a ‘committee’ to control the expression of a eukaryotic gene.
Combinations of a few gene regulatory proteins can generate many different cell types during development.
Describe what the embryo contains
The embryo does not contain a description of the adult, rather it contains a generative program for making it.
Cell differentiation
Process by which embryonic cells become different from one another.
What does cell differentiation involve ?
Involves the emergence of cell types such as muscle, nerve, skin and fat cells.
It is the achievement of a stable terminally differentiated state (not just transitory differences).
How are differentiated cells characterised ?
Characterised by the profile of proteins in that cell.
Describe cells just after fertilisation
Totipotent
- can form every cell in the body
- placenta too
Potency
The entire repertoire of cell types a particular cell can give rise to in all possible environments.
Toti
Whole cell
Totipotent
Cells of the very early mammalian embryo
Identical and unrestricted
Can give rise to any cell of the body
EMBRYONIC
Describe cells that go on to form the embryo
Pluripotent
- Can’t form placental tissues
Pluri
More
Pluripotent
Inner cells of blastocyst
Less potent
Can give rise to many cell types but not all
EMBRYONIC
Multi
Many
Multipotent
EXAMPLE:
Blood stem cells
They give rise to cells that have a particular function (e.g. red or white blood cells)
ADULT
Order of stem cells and potency
Totipotent
Pluripotent
Multipotent
Commitment
Each decision that restricts cell fate
- cells are committed
Key feature of commitment
Occurs in 2 stages
1. Specification (reversible)
2. Determination (irreversible)
Describe the 1st stage of commitment
Specification (reversible)
Cells are capable of differentiating autonomously if placed in isolation BUT can be respecified if exposed to certain chemicals/ signals.
Describe the 2nd stage of commitment
Determination (Irreversible)
Cells will differentiate autonomously, even when exposed to other factors or placed in a different part of the embryo.
How does a naive cell become specified ?
Intrinsic and Extrinsic Signals
Intrinsic signals
Cell autonomous signal tells the cell ‘who it is’
Extrinsic signal
A chemical or molecule in the environment gives the cell spatial information, tells the cell ‘where it is’
Describe cell fate
The fate of a cell describes what it will become in the course of normal development.
What does determination imply ?
Implies a stable change - the fate of determined cells does not change.
Competence
Ability of a cell to respond to the chemical stimuli.
How can a cell lose competence ?
A cell can lose competence by changes in surface receptor or intracellular molecules.
Process of a cell during development
Naive
Specified
Determined
Differentiated
What happens between naive and specified cell stage ?
Cytoplasmic determinants or induction
What happens between the specified and determined cell stage ?
Loss of competence for alternative fates
What happens between the determined and differentiated cell stage ?
Cell specific gene expression.
Preimplantation
Poised genes
Neurogenesis
Active genes
Postnatal neurogenesis
Silenced genes
Chromatin
DNA double helix
Wrapped around histone proteins
What is bivalent chromatin ?
Histone features that are part of chromatin.
The mechanistic basis of fate decisions.
ONLY occurs at ‘developmental regulator genes’
Developmental regulator genes
Transcription factors that control thousands of other genes.
Bivalent chromatin
Histone features that are part of chromatin. - found around the regulator genes.
What happens when bivalent chromatin is expressed ?
When they are expressed and make choices, these control big differentiation patterns.
K4
GO
K27
STOP
Describe embryonic stem cells
In embryonic stem cells, which haven’t made any fate choices yet:
You would see both kinds of signal, co-located on these genes.
- Closed pattern
- Open pattern (overlayed at the same time)
Closed conformation tag
Corresponds with the gene not being expressed.
Poised genes
IN bivalent chromatin, the genes are ready to either be expressed or not be expressed.
When do fate decisions occur ?
Cell fate decisions are made as early as the 4-cell stage.
Decide:
- pluripotent stem cells
- extraembryonic cells to form placenta
How are many cell types generated ?
Combinations of a few regulatory proteins.
3 regulatory proteins –> 8 different cell types
Transcription factors
Proteins that bind DNA and regulate thousands of other types of proteins.
e.g. HOX, SOX, T-box families
State some developmental regulatory genes
HOX
SOX
T-box
(all transcription factors)
iPS cells
Induced pluripotent stem cells
Therapeutic cloning
Somatic cell reprogramming
Human limb
Pentadactyl limb
5 digits
Describe the phases of endochondrial ossification
Mesenchyme
Cartilage
Osteoblasts and Proliferating chondrocytes
Secondary ossification centre
How is skeletal age determined ?
Radiologists can determine skeletal age of a patient by examining the development of epiphyseal plates
Primary ossification centre
Week 12
Secondary ossification centre
At birth
What is intra-membranous ossification ?
The formation of bone in fibrous connective tissue (when formed from condensed mesenchyme cells)
When does intra-membranous ossification occur ?
This process occurs during the formation of flat bones, such as the mandible and flat bones of the skull.
What is mesoderm ?
One of the early embryonic germ layers.
What is mesenchyme ?
Mesenchyme generalised connective tissue derived from mesoderm.
HOX genes
HOX genes are a related group of genes that are expressed along the long axis of the embryo from head to tail.
Where have HOX genes been studied ?
Drosophila melanogaster
Function of HOX genes
During embryonic development, HOX genes determine the body axis and the position of the limbs along the body axis - Intrinsic factors.
Product of HOX genes
Belongs to a class of proteins known as transcription factors that binds to DNA and thereby, regulate the transcription of other genes.
State the 3 axis of limb growth
- Proximo-distal axis
- Antero-posterior axis
- Dorso-ventral axis
Describe upper limb development
Upper limb buds appear on approximately day 24, between somites C5-T1
Describe lower limb development
Lower limbs appear on approximately day 28 between somites L1-S2.
When are major components of limbs present ?
BY week 8
Also Medial rotation of the LL is complete
Describe rotation of the forelimbs and hind limbs
In week 7:
- Forelimbs rotate 90degrees LATERALLY
- Hind limbs rotate 90degrees MEDIALLY
Descriptive terms used in embryology
Proximo - Distal
Anterior - Posterior
Dorso - ventral
What does rotation of the forelimbs and hind limbs result in ?
Results on the flexor compartments being anterior in the upper limb and posterior in the lower limb.
What is proximo-distal development controlled by ?
Apical Ectodermal Ridge (AER)
What is AER ?
Ectoderm is thickened at the ‘apex’ of the developing limb, to form the Apical Ectodermal Ridge.
What does the limb bud consist of ?
Core of mesenchyme - derived from parietal layer of LPM
Ectoderm - forms epidermis
Function of AER
Controls proximo-distal development
Progress zone
AER induces the underlying tissue to remain as a population of undifferentiated, rapidly proliferating cells.
What happens as cells move further away from AER ?
They will begin to differentiate into cartilage and muscle.
This differentiation results in proximo-distal development.
Function of HOX-8
Controls the position of the limb on the long axis of the body.
Function of TBX5 and FGF-10
Initiation of outgrowth of the fore limb is controlled by these genes.
What does AER secrete ?
AER secretes FGF4 and FGF8 to maintain the progress zone and the further development of the proximo-distal axis.
What happens as growth progresses ?
Mesenchymal cells are left behind the advancing ridge, and so they begin to differentiate.
Function of ZPA
Controls the antero-posterior axis (from thumb to little finger)
ZPA
Zone of polarising activity
What is the ZPA ?
Cluster of cells near the posterior border of the limb.
ZPA regulates the AP axis.
What does ZPA express ?
The protein sonic hedgehog.
ZPA moves distally with the AER.
What genes are involved in the dorso-ventral axis ?
BMPs - bone morphogenic proteins
Where are BMPs found ?
BMPs in the ventral ectoderm, induce EN1.
Function of EN1
EN1 represses WNT7 restricting its expression to the dorsal limb ectoderm.
Function of WNT7
WNT7 induces LMX1 which then specifies the cells to be dorsal.
Key function of HOX genes
HOX genes determine the shape of bones
What do variations in the combinations of HOX genes ensure ?
Upper and Lower limbs are different:
TBX5 - upper limb
TBX4 - lower limb
Patterns for the proximal, middle and distal hand are defined.
What is the expression of HOX genes dependent on ?
SHH
FGFs
WNT7a
What are limb defects often associated with ?
Other abnormalities affecting:
- CVS
- GU system
- Craniofacial structures
Syndactyly
Failure of programmed cell death of some skin tissue between the digits.
Amelia
Complete absence of limbs
Meromelia
Partial absence of the limbs
Phocomelia
Absence of long bones
Micromelia
Segments are abnormally short
Causes of limb defects
Causes may be:
- Hereditary
- Environmental (teratogens)
Result of limb defects
Affects the progress zone with failure of cell division (weeks 4 and 5)
Causes of thalidomide
Prescribed as a sleeping pill
Results of thalidomide
Increased incidence of limb abnormalities
Also associated with intestinal atresia and cardiac abnormalities.
Phocomelia or Amelia
Holt Oram Syndrome
TBX5 mutations lead to defects in limb development
Upper limb deformities –> heart defects
Brachydactyly
short digits
Syndactyly
Fused digits
Failure of apoptosis
Polydactylyl
Extra digits
