Molecular Patterning During Development Flashcards
How does the embryo predict a description of its future adult form?
It does not
Rather it contains a generative program for making it
Discuss cell differentiation
• Process by which embryonic cells become
different from one another
• Involves the emergence of cell types such as muscle, nerve, skin and fat cells
• Is the achievement of a stable terminal state (not just transitory differences)
• Is characterized by the profile of proteins in that cell
Discuss the hierarchy of stem cells in regards to their potency
(Totipotent and pluripotent also make copies of themselves)
Totipotent I V Pluripotent I V Multipotent
What is potency?
The entire repertoire of cell types a particular cell can give rise to in all possible environments
Define totipotency
‘toti’ = whole. eg. Cells of the very early mammalian embryo; identical and unrestricted; can give rise to any cell of the body (EMBRYONIC)
Define pluripotent
‘pluri’ = more. eg. Inner cells of the blastocyst; less potent; can give rise to many cell types but not all (EMBRYONIC)
Define multipotent
‘multi’ = many. e.g. Blood stem cells;
they give rise to cells that have a particular function (ADULT)
In each level of decision that restricts cell fate cells become committed. In what two stages dos commitment occur?
First stage: specification (reversible)
Capable of differentiating autonomously if placed in isolation BUT can be respecified if exposed to certain chemicals/ signals.
Second stage: determination (irreversible)
Cell 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 signal – cell autonomous signal tells the cell ‘who is it’
Extrinsic signal -a chemical or molecule in the environment gives the cell spatial information, tells the cell ‘where it is’
What is cell fate?
The fate of a cell describes what it will become in the course of normal development.
When a cell “chooses” a particular fate, it is said to be determined, although it still “looks” just like its undetermined neighbours. Determination implies a stable change - the fate of determined cells does not change.
What causes the change from a naive cell to a specified cell?
Cytoplasmic determinants or induction
Describe the change from specified to determined cell
Loss of competence for alternative fates
Describe the change from determined to differentiated cell
Cell specific gene expression
What is competence in cell development?
Ability of a cell to respond to the chemical stimuli.
A cell can lose competence by changes in surface receptor or intracellular molecules
What is the mechanistic basis of fate decisions?
Bivalent chromatin
How early are cell fate decisions made?
As early as the four-cell stage
How many specific regulatory proteins make the many cell types of the body?
Only a few but with different combinations along generations
What is another term for developmental regulatory genes?
Transcription factors
e.g. HOX, SOX, T-box
Describe the process of therapeutic cloning
Creation of ES cells by somatic cell reprogramming
Cells isolated from patient
Nucleus removed from egg cell
Transfer of nucleus from patients cells to the egg
Egg cell will ‘reprogram’ patients DNA
Cell is stimulated to begin division, it is left to divide until the blastocyst stage
Inner cell mass is isolated from the blastocyst and grown in a dish
Describe somatic cell reprogramming by defined factors (creation of iPS cells)
Skin or fibroblasts isolated from patient and grown in dish
Cells treated with reprogramming factors
Wait a few weeks
Pluripotent stem cells are produced
Culture conditions may now be changed to stimulate cells to differentiate into a variety of cell types
What is a pentadactyl limb?
A limb with 5 digits
What is the radiological relevance endochondrial ossification?
Radiologists can determine the skeletal age of a patient by examining the development of epiphyseal plates
Describe the process of endochondrial ossification
A ‘soup’ of cells known as mesenchyme from a group
Mesenchyme differentiates into cartilage in the shape of the to-be bone
Cartilage is pushed to the longitudinal poles of the developing bone as osteoblasts form a primary ossification site in a core of bone surrounded by proliferating chondrocytes and a vascular supply reaches the bone (WEEK 12)
The bone continues to develop, with defined growth plates and secondary ossification centres in the heads of the bone (AT BIRTH)
Discuss intramembranous ossification
Intramembraneous ossification is the formation of bone in fibrous connective tissue (which is formed from condensed mesenchyme cells)
The process occurs during the formation of flat bones such as the mandible and flat bones of the skull
What is mesoderm broadly?
Mesoderm is one of the early embryonic germ layers ectoderm, mesoderm, endoderm
What is mesenchyme?
Mesenchyme is generalised embryonic connective tissue derived from mesoderm
Discuss HOX genes pattern in the body axis
Hox genes are a related group of genes that are expressed along the long axis of the embryo from head to tail.
They are ancient (highly conserved) genes and are therefore present in many animal groups. They have been extensively studied in Drosophila melanogaster
Discuss Limb Development
During embryonic development HOX genes determine the body axis and the position of the limbs along the body axis - Intrinsic factors The products of HOX genes belong to a class of proteins known as transcription factors, which bind to DNA, and thereby regulate the transcription of other genes (e.g. TBX5, TBX4). Once the cranio-caudal position is set, limb growth is regulated along three axes:
- Proximo-distal axis
- Antero-posterior axis
- Dorso-ventral axis
When and where do upper limb buds appear?
Upper Limb buds appear on approximately day 24 between somites C5-T1
When and where do lower limb buds appear?
Lower limb buds appear on approximately day 28 between somites L1-S2
When are all major components of the limbs present?
By week 8 all major components of the limbs are present
Medial rotation of the LL is complete
Discuss rotation of the limbs
• Development of fore and hind limbs is similar
• In week 7 the forelimbs rotate 90° laterally and the hind limbs
rotate 90 ° medially.
• Results in the flexor compartments being anterior in the upper limb
and posterior in the lower limb
• The sole of the foot is equivalent to the palm of the hand
• Big toe (hallux) is equivalent to the thumb (pollux)
What is proximo-distal development controlled by?
Apical Ectodermal Ridge AER
What do limb buds consist of?
- Core of mesenchyme derived from parietal layer of lateral plate mesoderm
- Ectoderm which forms the outer covering of the limb (epidermis)
- Ectoderm is thickened at the ‘apex’ of the developing limb to form the Apical Ectodermal Ridge (AER)
What does AER control?
Proximo-distal development
Discuss the Apical Ectodermal Ridge (AER)
- The AER is a key structure in limb development
- It induces the underlying tissue to remain as a population of undifferentiated, rapidly proliferating cells- known as the PROGRESS ZONE
- As cells move further away from the AER they will begin to differentiate into cartilage and muscle
- This differentiation results in proximo-distal development
Discuss AER Proximo-distal development
1 HOX-8 controls the position of the limb
on the long axis of the body
2 Initiation of outgrowth of the fore limb is
controlled by the TBX5 gene and FGF-10
3 AER secretes FGF4 and FGF8 to maintain the progress zone and the further development of the proximo- distal axis
4 As growth progresses, mesenchymal cells are left behind the advancing ridge (and its influence) and so they begin to differentiate
What controls the development of the anterograde-posterior axis (Cranio-caudal axis)?
Zone of Polarizing Activity (ZPA)
Discuss the Zone of Polarizing Activity (ZPA)
The antero-posterior axis iiis regulated by the Zone of Polarizing Activity (ZPA)
Cluster of cells near the posterior border of the limb form the ZPA which regulates the AP axis
It ensures that the thumb grows on the cranial (anterior) side of the limb bud
ZPA expresses the protein sonic hedgehog (SHH). ZPA moves distally with the AER
Expts. in chicks show that adding a ZPA to the limb bud results in mirror image duplication of digits
Discuss the development of the doors-ventral axis
BMPs in the ventral ectoderm induce EN1
EN1 represses WNT7 restricting its expression to the dorsal limb ectoderm
WNT7 induces LMX1 which then specifies the cells to be dorsal
What determines the shape of bones?
Hox Genes
Discuss Hox Genes determining the shape of bones
Variations in the combinations of HOX genes ensure that the:
• Upper and lower limbs are different TBX5 (upper limb), TBX4 (lower limb)
• Patterns for the proximal (arm) middle (radius and ulna) and distal (hand) are
defined
Expression of the HOX genes is dependent on SHH, FGFs and WNT7a
What is the importance of apoptosis in limb development?
To ‘cut away’ unneeded areas such as the webs between fingers and toes
Discuss incidence of limb defects
• Congenital heart defects are the most common non- chromosomal malformation (6.5 /1000)
• Limb malformations occur in approx. 3.8/1000 live births
• UL abnormalities more common that LL
• Limb defects are often associated with other abnormalities
affecting CVS, GU system and craniofacial structures
• Amelia complete absence of the limbs
• Meromelia partial absence of the limbs
• Phocomelia absence of long bones
• Micromelia segments are abnormally short
• Cause may be hereditary but also by environmental (teratogens)
• Affects the progress zone with failure of cell division (weeks 4&5)
What is Amelia?
Complete absence of the limbs
What is Meromelia?
Partial absence of the limbs
What is Phocomelia?
Absence of long bones
What is Micromelia?
Limb segments are abnormally short
Discuss the thalidomide tragedy
Between 1957 and 1962 in the UK, Canada, Germany, Japan thalidomide was prescribed as a sleeping pill. An increase in the incidence of limb abnormalities was reported …..
12,000 babies survived, with phocomelia (flipper-like arms or legs) associated also with intestinal atresia and cardiac abnormalities
Phocomelia or amelia
What is Holt Oram Syndrome?
Holt Oram Syndrome-TBX5 mutations lead to defects in limb development
Upper limb deformities, heart defects
What is Brachydactyly?
Short digits
What is Syndactyly?
Fused digits - failure of apoptosis (1 in 2,000 births)
What is Polydactyly?
Extra digits
What is Cleft Foot?
‘lobster claw’ deformity. - Cleft down centre
