Embryology Flashcards
When do the limbs start to develop?
4th week
When and where does the upper limb bud appear?
Day 24
cervical region
When and where does the lower limb bud appear?
Day 25-26
Lumbar and sacral segments
Which genes regulate the patterning of the development of the limbs?
i.e. determine which skeletal elements are formed and making sure the upper/lower limbs are the same on each side
Homeobox (HOX) genes
Where do upper limb buds develop?
Opposite the caudal cervical segments
Where do lower limb buds develop?
Opposite the lumbar and sacral segments
What forms at the apex of each limb bud?
The Apical Ectodermal Ridge (AER) forms from thickening of the ectoderm
Apical Ectodermal Ridge (AER)
It’s a multi-layered epithelial structure that forms from thickening of the ectoderm at the distal end of each limb bud and acts as a major signaling center to ensure proper development of a limb.
After the limb bud induces AER formation, the AER and limb mesenchyme—including the zone of polarizing activity (ZPA)—continue to communicate with each other to direct further limb development.
The limb mesenchyme adjacent to the AER consits of undifferentiated, rapidly proliferating cells whereas mesenchymal cells proximal to it differentiate into blood vessels and cartilage bone models
What do the distal ends of the limb buds flatten into?
Paddle-like hand plates and flipper like foot plates
When do digital rays form in the hands and feet?
Hands - 6th week
Feet - 7th week
What process is responsible for the tissue breakdown in the inter-digital regions?
Apoptosis mediated by bone morphogenetic proteins
When do cartilaginous precursors/models of the limb bones develop within the mesoderm?
6th week
During weeks 8-12 what forms in bones?
The primary centre of ossification
i.e The site where bone begins to form in the shaft of a long bone or the body of an irregular bone
All limb bones grow by Endochondral ossification except one. What is it?
The clavicle (intra-membranous ossification)
In what direction do upper limbs rotate?
Upper limbs rotate 90 degrees laterally so that extensors are on the lateral and posterior surfaces (and the thumb is lateral)
In what direction do lower limbs rotate?
Lower limbs rotate 90 degrees medially so that the extensors are on the anterior surface (and the big toe is medial)
During which week do peripheral nerves grow from the developing limb plexus into the mesenchyme of the limbs?
During the 5th week
Birth defects of limbs descriptive terms
Amelia - total absence
Meromelia - Partial absence
Phocomelia - some long bone absence
Micromelia - All parts present but small
What is the critical period of limb development?
24-36 days after fertilisation
What is polydactyly?
Presence of more than 5 digits on the hands or feet
Extra digit is incompletely formed and lacks muscular
development
May be inherited or teratogen induced
What is syndactyly?
Common birth defect of the hand or foot
Cutaneous Syndactyly (simple webbing between the digits) is a common limb defect
Osseous Syndactyly (fusion of bones) occurs when notches between the digital rays fail to develop
How did Thalidomide affect limb development when given to pregnant women in the past?
Babies were born with absent or deformed long bones
Acts by disrupting cell adhesion and angiogenesis
Club foot
Relatively common birth defect
Sole of the foot is turned medially and the foot is inverted
Causes:
Multi-factorial (genetic and environmental)
Abnormal positioning or restricted movement of the foetus in the utero
