Embryology Flashcards
When does the upper limb bud appear on the flank of the embryo?
Day 26 as a lateral outgrowth
What does the upper limb bud consist of?
Core of lateral plate mesoderm growing into the overlying ectoderm
What does the mesoderm in the upper limb bud differentiate into?
Bone, cartilage and tendon
What happens to the ectoderm in the upper limb bud?
Thickens at the tip of the bud in the antero-posterior axis - forms the apical ectodermal ridge
What is the antero-posterios axis in the upper limb bud analogous to?
The radio-ulnar axis
How is the upper limb bud supplied initially?
Through a capillary network
How does the upper limb bud’s blood supply develop?
The capillary network coalesces into a main stem artery that drains into a marginal vein
The artery becomes the subclavian-axillary-brachial axis
The vein becomes the basilic-axillary-subclavian axis
What branches does the brachial artery divide into?
Interosseous and Median arteries
Describe the median artery’s blood supply distribution?
Main blood supply to the hand
Later regresses to provide the blood supply of the median nerve
What is the main blood supply to the hand?
Median artery initially
Around D44 it is replaced by the radial and ulnar arteries
When is a paddle-shaped hand present?
D33
What structure appears where skeletal elements will form?
Pre-chondrogenic condensations of mesenchyme
Def chondrogenic: the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondrocytes and begins secreting the molecules that form the extracellular matrix
When does the nerve trunk enter the arm?
D36
What does somitic mesoderm differentiate into?
Muscles
After invading the limbs somitic mesoderm aggregates into dorsal and ventral masses - these differentiate into myoblasts that later become muscles
When does the hand assume a webbed appearance?
D42 - Digital rays are present
What happens during the 7th week (D42 - D49)?
- Upper limb rotates 90 degrees laterally (palm now anterior)
- Elbow flex
- Ossification and digital separation commences
How does digital separation occur?
Apoptosis between the finger rays
When does the upper extremity resemble a miniature adult upper limb?
By the 8th week (D49 - D56)
Where does our understanding of upper limb development come from?
Silkie Chicken limbs
Transplanting tissue within chick embryos
What are the three axes of upper limb development?
- Proximodistal
- Anteroposterior (Known as AP pathway but is actually Posteroanterior)
- Dorsoventral
What is the embryogenesis timeline of upper limb development?
Overall 4-8 weeks
4-6 weeks
Formation: Multiplication and Migration
6-8 weeks
Differentiation
8 weeks to term
Growth
What is the size of the embryo at the time the upper limb bud develops?
Equivalent to a seed
What is the axial positioning of the forelimb?
Between the 8th and 12th somite
Def of somite: segmental axial structures of vertebrate embryos that give rise to vertebral column, ribs, skeletal muscles, and subcutaneous tissues
Which genes control the axial positioning of the forelimb?
HOX and TBOX genes - this ensures the upper limb comes out exactly where is it meant to come out.
Embryo is segmented in a cranial to caudal fashion by HOX transcription factor which permit different body segment-specific organs to develop.
TBX5 is critical for upper limb development
What is the apical ectodermal ridge and what is it’s function?
Condensation of ectodermal tissue which forms the junction between the dorsal and ventral ectoderm
Function: Signalling source for cellular migration
1. Myoblasts
2. Angioblasts
3. Melanocytes
4. Glial and Schwann cells
5. Macrophages
What is the signalling centre for proximodistal axis development?
Apical Ectodermal Ridge (AER)
In what direction does the proximodistal pathway of upper limb development happen?
Distally
Ensures differentiation of the upper limb into a shoulder proximally and digits distally
What is the main protein responsible for the distalisation of the proximodistal axis? And how did they discover it?
Fibroblast Growth Factor (FGF) 2, 4 and 8
Discovered by Niswander in 1994 replaced the AER with FGF soaked beads - upper limb continued to grow
Later through in situ hybridisation they confirmed high concentrations of FGF at the AER.
What is the signalling centre for the anteroposterior axis upper limb development?
Zone of Polarising Activity - cluster of mesenchymal cells in the posterior limb bud margin
Controls the development of the ulna and little finger on one side but radius and thumb on the other.
In what direction does the posteroanterior pathway of upper limb development happen?
Ulnarly
What is the main protein responsible for the ulnarisation of the posteroanterior axis?
Sonic Hedgehog (SHH)
What is the signalling centre for the dorsoventral axis upper limb development?
Cells on the Dorsal Ectoderm (non-AER ectoderm) - controls differentiation into dorsal and plantar surfaces.
Removing and replacing ectoderm from a chick limb bud, having reversed its dorsoventral
polarity, will produce a reversal in the polarity of the underlying mesoderm.
In what direction does the dorsoventral development of the upper limb happen?
Dorsally
Which protein controls the dorsoventral development of the upper limb?
WnT-7a protein by the dorsal limb ectoderm and determine dorsal limb identity - fingernails
Encoded by the WNT7A (Wingless-type mouse mammary tumour virus integration site family member 7a) gene
Activates expression of LIM homeodomain factor Lmx1
Describe the molecular control pathway of the upper extremity development of the proximodistal axis.
Apical Ectodermal Ridge secretes FGF
FGF binds on an FGFR
Activates the transcription factor which regulates protein production
Describe Saunder and Summerbell’s contribution to the understanding of the proximodistal axis development
Saunder 1948 and Summerbell 1976 - Removal of the Apical Ectodermal Ridge led to truncation of the upper limb
How does FGF work in proximodistal axis development? And what is the role of retinoid acid?
In direct opposition to Retinoic Acid
RA prevents early upper limb differentiation when the bud is still short
Later as it grows FGF takes over and allows differentiation and further growth.
Who were responsible for the anterior-posterior axis classical experiments? And how?
Saunders and Gasseling in 1968 - Duplication of ZPA
Grafted ZPA cells to the anterior limb margin
Resulted in the development of a mirror image duplication of the limb (321123)
Sharon Tickle in 1981 confirmed the polarizing nature of this zone - there is a gradient of proteins in the ZPA going from the posterior (little finger) to anterior (thumb) limb bud.
The concentration of proteins and time spent near higher concentrations of the proteins in the ZPA determines the identity of the digits
Little finger
Nearest to ZPA
Higher concentration of proteins
Thumb
Furthest from ZPA
Lowest concentration of proteins
1987 Slack thought the protein responsible was Retinoic Acid
Brockes 1991 Negated Slack’s finding and provided evidence it was Sonic Hedgehog (SHH) Protein
Why is it called the Sonic Hedgehog protein?
The ZPA produces Sonic Hedgehog - Hedgehog proteins family
When this protein was removed from flies they developed spikes hence the name “Hedgehog”
The scientist who came up with the name was watching his son play video games and called it Sonic Hedgehog
Name some malformations occurring as a result of abnormal dorsal-ventral axis development.
Ventral Dimelia - duplicated palms
Ventral tissue (Finger tip) is present on the dorsal surface of the digits
Dorsal Dimelia - circumferential nails
Dorsal tissue (nail) is present on the ventral surface of digits
Which transcription factors are responsible for proximodistal development of the upper limb bud?
HOXA 10, 11 and 13
TBX 4 and 5
Which transcription factors are responsible for anteroposterior development of the upper limb bud?
HOXD 10-13
GLI3
Which transcription factors are responsible for dorso-ventral development of the upper limb bud?
LMX1
EN1
What is whole limb duplication or triplication? How is it thought to occur?
Extremely rare condition
Entire limb duplicated or triplicated or the presence of a very proximal split of the limb
Primary error in induction of the proximal distal axis.
1. Disruption in body segmentation
2. Ectopic expression of upper limb induction molecules (TBX5)
What happens to the upper limb after AER resection?
Truncated limb - earlier resection results in more proximal truncation of the upper limb
What happens to the upper limb after AER grafting?
Limb elongation (in chick embryos)
What happens when there are higher Sonic Hedgehog protein concentrations?
More posterior (ulnar) digits are formed
What is the relationship between cells of the Apical Ectodermal Ridge and Zone of Polarizing Activity?
They are interdependent.
FGF from the AER are required for SHH expression
SHH signal maintains AER integrity
So loss of elements often occurs in both PD and AP axes.
Removing and replacing ectoderm from a chick limb bud, having reversed its dorsoventral
polarity, will produce a reversal in the polarity of the underlying mesoderm.
What is an important gene involved in the dorsoventral development of the upper limb bud?
Engrailed 1
Expressed by the ventral ectoderm
Inhibits WnT7a and restricts it’s action to the dorsal ectoderm
