Embryology Flashcards
When do the limb buds appear?
What do they arise from?
Upper limb buds - Day 24
Lower limb buds - Day 25/26
Develop due to activation of mesenchymal cells
What genes regulate the development of the skeletal element of limbs?
The homeobox genes
HOX genes regulate the patterning of growth - shown in the diagram
Describe how the limb buds elongate
What forms at the apex of each bud?
Elongate due to the proliferation of Mesenchyme
At the apex of each bud - the ectoderm thickens to form an A__pical Ectodermal Ridge (AER__)
Where do the upper and lower limb buds form?
Upper limb buds develop opposite the caudal cervical segments
Lower limb buds form opposite the lumber and sacral segments
This diagram shows the limb buds developing
Describe what is happening
Darker, centre is mesenchyme:
At the distal end (which is adjacent to AER) there are rapidly proliferating, undifferentiated mesenchymal cells
Proximally, the mesenchymal cells differentiate into blood vessels and cartilage bone models
What type of structure is the AER?
Multilayered epithelial structure
How do the hands and feet develop from the limb buds?
Firstly - the distal ends of the limb buds flatten into paddle-like hand plates and flipper-like foot plates
Secondly - by the end of 6th week, mesenchymal tissue in hand plates has condensed to form digital rays
During the 7th week, digital rays are formed in the foot plates
Describe how your fingers and your tootsie toes are formed xx
- Hand paddles
- Digital rays
- Notches between digital rays
- Webbed fingers/toes
- Seperate fingers/toes
When does development of the fingers and toes happen?
What cellular process is responsible for the tissue breakdown of the interdigital regions as the fingers/toes form?
What mediates this?
4th to 8th week
Apoptosis
mediated by bone morphogenetic proteins
Formation of bones is one of the final stages of limb development
Briefly describe how bone formation works (and when)
Cartilaginous precursors of the limb bones develop by chondrification within the mesoderm in the 6th week
Primary centres of ossification appear in weeks 8 to 12 in response to growth factors
What type of bone formation takes place when limb bones form?
Endochondrial ossification for all limb bones except the clavicle (membranous)
How do the developing limbs rotate?
Upper limbs rotate 90o laterally - so that the extensors (triceps etc) are on the lateral & posterior side
Lower limbs rotate 90o medially so that the extensors (Quads etc) are on the anterior surface
Describe how the nervous system aspect of the limbs develop
How do the dermatomes do
During the 5th week, peripheral nerves grow from the developing limb plexus (brachial and lumbosacral) into the mesenchyme of the limbs
The dermatomes on the limb bud (on diagram) are kinda taken entirely with the limb as it grows
Define what the following conditions are
a) amelia
b) meromelia
c) phocomelia
d) micromelia
a) amelia - total absence of limbs
b) meromelia - partial absence
c) phocomelia - absence of some long bones
d) micromelia - all parts present but small
What is polydactyly?
Is it inherited or teratogen induced?
Presence of more than 5 digits on the hands or feet
Extra digit is incompletely formed and lacks muscular development
Can be either teratogen or inherited
Define what the following are…
- Cutaneous syndactyly*
- Osseus syndactyly*
Cutaneous syndactyly (webbed feet or fingers) is common (esp in feet) - where there is simple webbing between the digits
Osseus syndactyly (fusion of bones) is rarer and occurs when notches between digital rays fail to develop
