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Flashcards in Development of the limbs Deck (29)
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1
Q

What do somatic mesoderm and somites become?

A

somatic mesoderm –> limb skeleton

somites –> limb musculature

2
Q

When do limb buds appear? Where do they appear?

A

appear at the end of week 4
lower limbs 2 days behind upper limbs
appear on ventro-lateral body walls, extend ventrally (belly)

3
Q

How do limb buds elongate?

A

through proliferation of mesenchyme core (flexibl mesoderm)

4
Q

What is apical ectodermal ridge (AER)? Where is it located and what is its purpose?

A

orchestrates limb development: proximal to distal
keeps mesenchyme immediately underlying it undifferentiated
undifferentiated mesenchyme proliferates –> elongation
proximal mesenchyme begins to differentiate as it is far from AER - can’t receive molecular signal to stay undifferentiated

located at apex of mesenchyme core

5
Q

What does AER induce and what does it mark?

A

induces development of digits in hand/foot plates
AER marks the boundary between dorsal & ventral limb ectoderm - ‘dorsalventro patterning’
ectoderm exerts dorsalising & ventralising morphological changes over the mesenchyme core

6
Q

What are the 3 degrees of symmetry?

A
  1. top & bottom: proximal –> distal
  2. front & back: dorsal –> ventral
  3. side to side: anterior –> posterior
7
Q

What is the apical ectodermal ridge (AER)?

A

thickened ectoderm at the apex of limb bud

8
Q

How does limb bud elongate?

A

through proliferation of mesenchyme core

9
Q

What is the zone of polarising activity (ZPA)?

A
a signalling centre at the posterior base of the limb bud
generates asymmetry in the limbs (anterior - posterior axis determination)
controls patterning (separate dorsal & ventral) & maintains AER
10
Q

Which component controls which axial specification?

A

Anterior - Posterior: ZPA (zone of polarising activity)
Proximal - distal: (AER) apical ectodermal ridge
D-V: ectoderm (dorsal & ventral)

11
Q

Describe the embryo’s axis

A

top-bottom: anterior - posterior
belly-back: ventral - dorsal
away-close: distal - proximal

12
Q

Describe an adult’s axis

A

top-bottom: superior - inferior
front - back: anterior - posterior
close - away: proximal - distal

13
Q

Hand & foot plates: what are digital rays? which process is required?

A
mesenchyme condensation (regress) within plates - cartilaginous models of digital bones formed (digital rays - flaring of limb buds = digits)
apoptosis require of tissue between digits (sculpts interdigital space)
AER only maintained over tips of digits, other break up to prevent proliferation between digits
14
Q

What is the purpose of apoptosis in hand & foot plates?

A

to sculpt interdigital space

15
Q

How are bones formed?

A

signals from AER to remain undifferentiated stop
lateral plate mesoderm condenses & differentiates
cartilage model forms
endochondral ossification (long bones) - osteoblasts - primary ossification centre in diaphysis of long bones

16
Q

Further development of long bones? (after birth)

A

secondary ossification centres appear in the epiphysis

bone growth is maintained by epiphyseal growth plate (cartilage forming regions)

17
Q

Compartmentalisation of muscles & its positional changes during development

A

myogenic precursors from somites migrate into limbs
coalesce into 2 common muscle masses around the newly formed skeletal element (ventral: flexor, dorsal: extensor)
individual muscles split (branch) from common masses

18
Q

Rotation of limbs

A

limbs extend ventrally at first, but as they elongate, they rotate
upper limb: rotates LATERALLY (thumb)
lower limb: rotates MEDIALLY (big toe)

19
Q

Describe the position of the limbs before and after rotation

A

before rotation: upper: thumbs up and elbows out
after rotation: thumbs out and elbows in

before rotation: lower: soles facing in, knees out
after rotation: soles facing down, knees up

20
Q

Limb innervation of upper & lower limb buds

A

upper LB: appears OPPOSITE caudal cercival spinal spinal segments (highest 2)

lower LB: appears OPPOSITE lumbar & sacral spinal segments (lowest 2)

21
Q

When do spinal nerves enter the limb bud? why?

A

enter limb bud early - because without innervation, development stops

22
Q

brachial plexus in relation to the nerves innervating

A

all muscles are compartmentalised, nerves grow into the common muscle masses
all anterior divisions regroup to form medial & lateral chords - innervating flexors (ventral)
all posterior divisions regroup to form posterior chord - innervating extensors (dorsal)

23
Q

what are dermatomes and myotomes?

A

derived from paraxial mesoderm
D: strip of skin supplied by a single spinal nerve
M: muscle / group of muscles supplied by single spinal nerve

24
Q

describe how nerves innervate dermatomes and myotomes

A

Nerve originate from somite segment (spinal root - C5-T1 upper)
as the embryo grows, morphological changes take the spinal nerve with it to innervate a strip of skin / group of muscles
there is no overlap between the segment of skin / muscle innervated by the nerves

25
Q

Limb defects

A
upper limbs (grows ahead 2 days) usually more affected than lower limbs
rare, usually hereditary
can be teratogen - induced malformity of embryo e.g. thalidomide
26
Q

What are some common limb defects?

A

Amelia: complete absence of a limb
Meromelia: partial absence of 1 or more limb structures
Phocomelia: deformity where limbs are attached close to trunk of body - seen when thalidomide given

27
Q

Common digit defects

A

syndactyly: fusion of digits, can be just CT or bones as well (lack of sculpting from apoptosis and AER maintained too long by ZPA)
polydactyly: extra digits, genetic recessive trait

28
Q

Limb development preaxial & postaxial

A

preaxial: thumb (radial) & big toe (tibial) (between C5 & C6)
postaxial: little finger (ulna) & little toe (fibula) (between C8 & T1)

29
Q

How does prenatal limb begin its development?

A

activation of mesenchyme (mesoderm CT) in somatic mesoderm (somatic (top) layer of lateral plate mesoderm)