Limb Development Flashcards
What limb buds are seen first, upper limb or lower limb?
upper
Where do limb buds first appear?
as elevations on ventrolateral body wall
Where do upper limb buds develop from?
opposite the caudal cervical somites (C5-T1)
Where do lower limb buds develop?
opposite the lumbar and upper sacral somites (L1-S3)
What do limb buds consist of?
mesenchyme covered by ectoderm
What are Apical Ectodermal Ridges (AER)?
- thick band of ectoderm that induces limb development
- control patterning of limb in proximodistal axis
Secretions produced in what induce the formation of the zone of polarizing activity (ZPA)?
AER
What is the zone of polarizing activity?
- group of mesenchyme cells in posterior margin of limb bud
- control patterning of limb in the ant-post axis
Adjacent to AER what is mesenchyme doing?
rapid proliferating
In week 6 digital rays form in mesenchyme of what?
handplate
What happens to the intervals between digits which are filled with loose mesenchyme in week 7?
apoptosis
What happens in week 8?
- 1* ossification centers in diaphyses
- separate fingers and toes visible
When do most 2* ossification centers appear?
postnatally
When do carpels and tarsals ossify?
1st yr after birth
Where doe synovial joints form?
in interzonal mesenchyme between developing bones
What does peripheral mesenchyme form in synovial joints?
joint capsule, synovial membrane, other ligaments
What do central mesenchyme in synovial joints do?
disappears to become joint cavity
What happens with restricted fetal movement?
abnormal limb development
What are the most critical days for limb development?
day 24-36
What is worse, early exposure or late exposure to teratogens?
early
What is thalidomide?
- sedative and antinauseant given to pregnant women
- assoc. with many severe limb defects
- nowadays treatment for leprosy etc
What are some other causes for limb anomalies?
genetics
environmental factors (drugs)
ischemia
low amniotic fluid (oligohydramnios)
What is amelia?
absence of limbs
What is meromelia?
absence of part of a limb
What is cleft hand or foot?
absence of one or more central digits
- failures of digital rays to form
What is syndactyly?
- cutaneous syndactyly
- webbing of skin between digits due to failure to degeneration of webs
- osseous syndactyly = fusion of phalanges (3-4th digits/2-3 toes)
- notches fail to form between developing digital rays
What is brachydactyly?
shortness of the digits
- due to reduction in length of phalanges - assoc. with short stature
What is polydactyly?
more than the usual # of digits
- extra fingers usually medial or lateral - extra toes usually lateral
What is congenital clubfoot?
- deformity involving the talus
- sole of foot faces medially and foot inverted
- restricted mvmt in utero
What is achondroplasia?
- common cause of dwarfism
- disrupted endochondral ossification of long bones
- limbs short and bowed, forehead bulges
How do appendicular mm form?
myogenic cells migrate from myotomes of somites and differentiate in limbs into myoblasts => single large mm mass in each limb
Muscle mass separates into 2 components, what are they?
upper limb
- dorsal component => extensors and supinator
- ventral component => flexors and pronator
lower limb
- dorsal component => extensors and abductors
- ventral component => flexors and adductors
Where do ligaments and blood vessels in appendicular mms derive from?
limb mesenchyme
How do upper limbs rotate?
laterally on longitudinal axis more than 90 degrees
What are the before and after rotation of upper limb?
Before => After
- Elbows: laterally => dorsally
- Thumbs: anteriorly => lateral
- Extensors: dorsal => lateral and posterior aspect
How do lower limbs rotate?
medially on longitudinal axis less than 90 degrees
What are the before and after of rotation on lower limbs?
Before => After
- Knees: lateral => ventrally
- Big toe: anteriorly => medially
- Extensors: dorsally => anterior aspect
What is the motor innervation of limbs?
- ventral motor axons from spinal cord => dorsal/ventral mm masses
- result in formation of brachial and lumbosacral plexuses
What is the sensory innervation of limbs?
-sensory peripheral processes enter limb after motor axons and follow their paths
What surrounds both motor and sensory processes? what does it form?
neural crest cells - differentiate as Schwann cells
What is a dermatome of a limb?
segmental area of skin supplied by a single spinal n. and its ganglion
- both dorsal and ventral ramus
Where do upper limb dermatomes progress?
- down lateral aspect (C5-7 supply preaxial border)
- back up medial aspect (C8-T2 supply postaxial border)
Where do lower limb dermatome progress?
- down anterior aspect (L2-L4 supply preaxial border)
- back up posterior aspect (L5-S3 supply postaxial border)
Where is there minimal overlap in dermatomes?
ventral axial line
- ant on upper limb and posterior on lower limb
What is the cutaneous n. area of limbs?
- skin supplied by a peripheral n.
- usually innervated by terminal br. of more than one spinal n.
What kind of sensory loss occurs with a dorsal root injury?
parietal sensory loss
Where does the primary axial artery form?
- forming as limb bud grows
- br from intersegmental a.
- initially grows adjacent to condensed mesenchyme forming the limb bones
What does the primary axial artery in upper extremity become?
- brachial a in arm
- radial and ulnar aa.
- common interosseous a in forearm
What does the primary axial a in lower limbs become?
- deep femoral a.
- femoral a.
- ant and post tibial aa.
What does the peripheral venous sinus become?
- cephalic and basilic veins in upper extremity
- greater and lesser saphenous vv. in lower extremity
