6.5 Embryology of Limb Development Flashcards
When and what is Gastrulation?
- Week 3 of development
- Converts the bilaminar embryo into the trilaminar embryo.
What embryonic tissues give rise to the limbs?
The limbs are initially undifferentiated mesenchyme (mesoderm) with an epithelial (ectoderm) covering
When do the limb buds appear?
- End of week 4
- Forelimbs appear first followed bt hind limbs 1 to 2 days later.
When do the limbs rotate and what direction?
- During the 7th week
- Opposite directions
What is the critical period for the limbs?
-Weeks 4 to 8 (4-5 most critical-major anomalies)
Define Critical Period
During the critical period (most of the first trimester), the developing embryo is most susceptible to environmental insults (alcohol, infection, radiation, nutritional deficiencies)
When does body folding take place?
During the 4th week
What two types of folding take place?
1) lateral folding
2) Caniocaudal folding
Define lateral folding
Mesoderm splits. As lateral aspects of embryo fold ventrally, the space between the layers of mesoderm forms future body cavities.
Define Craniocaudal folding
Head and tail regions bend towards each other and embryo becomes āCā shaped.
What forms the gut tube?
Endoderm
What becomes the Somite formations?
Paraxial mesoderm organized into segments known as somitomeres
What are three types of somites?
1) Sclerotome
2) Dermatome
3) Myotome
What does sclerotome become?
Future vertebral segments
What does the dermatome become?
Dorsal portion of somite that forms dermis of skin, segmentally supplied by a spinal nerve
What does the myotome become?
Mesoderm from somite that will differentiate into myoblasts.
What forms the distal border during limb growth?
The AER- Apical ectodermal ridge
What happens that causes finger and toe formation?
Cell death of AER gives way to formation
When does the limb bud flatten to form hand and footplates?
During the 6th week
What direction does each limb rotate during the 7th week?
Upper limbs rotate laterally 90 degrees
Lower limbs rotate medially 90 degrees
What is Achondroplasia?
One form of congenital dwarfism resulting from improper develop,met of cartilage at the end of long bones
What is Amelia?
Complete absence of one or more limbs
What is Phocomelia?
Upper portion of limb is absent or poorly develop
What is Syndactyly?
two or more fused fingers or toes
What is Macrodactyly?
enlargement of one or more digits
What is Polydactyly?
extra digits
What is Ectrodactyly?
fewer than normal digits
What is Clubfoot?
- common: 1 in 5000 live births
- abnormal position of the foot
- can result from compression of the infant in the uterus
What gives rise to the bones of the limbs?
The mesenchyme condenses and cells become chondrocytes
During week 6 what substance foreshadows the bones of the extremities?
Hyaline cartilage
When does ossification begin in the limbs?
The end of the embryonic period.
- Primary ossification: At birth, the diaphysis of the bone is completely ossified.
- Secondary ossification of the cartilaginous epiphyses begins around the time of birth.
What is the Epiphyseal plate?
- A cartilage plate remains between the diaphyseal and the epiphyseal ossification centers.
- Important for growth of long bones
- Once a child has reached the end stages of puberty, the growth plates are essentially closed
What nerve supplies the extensor muscles?
- Radial nerve (C5,C6, C7, C8, and T1)
- Formed from dorsal segmental branches
What nerve supplies the flexor muscles?
- Ulnar (C8, T1) and median nerves (C6, C7, C8, T1)
- Formed from ventral segmental branches.
What happens to muscle formation in limbs during week 7?
Mesenchyme is penetrated by spinal nerves from the same segment derived from.
What does somites made from?
paraxial mesoderm
