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Post Midterm Anatomy > Limb Development > Flashcards

Limb Development Flashcards

1
Q

Appearance of limb buds

A
  • by 4th week, paired upper/lower limb buds from ventrolateral body wall
  • upper day 26-27, lower 1-2 days later
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2
Q

Limb bud structure

A
  • mesencymal core (from somatic layer of lateral plate mesoderm)
  • covered by ectoderm
  • activation of mesenchymal cells starts limb development
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3
Q

Lateral plate mesoderm

A
  • subdivision of intraembryonic (2º) mesoderm

- forms CT of limbs

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4
Q

Intraembryonic mesoderm

A
  • Lateral plate-CT of limbs
  • Paraxial-somites
  • Intermediate-UG system
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5
Q

Derivatives of Somites

A
  • sclerotome: vertebrae & ribs
  • myotome: muscle
  • Dertatome: Connective tissue (dermis)
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6
Q

Activation of mesenchyme

A
  • Induce formation of Apical ectodermal ridge

- mesenchyme condenses to form skeleton and CT

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7
Q

AER

A
  • apical ectodermal ridge
  • ectodermal thickening at apex of each limb bud
  • inductive effect on mesencyme: proliferation (progress zone); growth and develompemtn (proximodistal axis)
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8
Q

Proximodistal axis

A
  • signaling center: AER
  • signaling molecule: Fibroblast growth factors (FGF) -2, -4, -8
  • maintains progress zone
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9
Q

Anteroposterior axis

A
  • signaling center: Zone of Polarizing Activity (ZPA)(cell cluster at posterior border of limb near body wall)
  • signal: Sonic Hedgehog (Shh, induced by Retinoic acid (vit A) from ZPA)
  • correct order of digets (thumb to little finger, radial to medial)
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10
Q

Dorsoventral Axis

A
  • signaling center: dorsal and ventral ectoderms
  • signal: Wnt-7a (dorsal), En-1 (ventral)
  • En-1 inhibits Wnt-7a, restricting it to dorsal ectoderm maintaining orientation of palmar vs dorsal survaces
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11
Q

What signals induce formation of limb buds of forelimbs?

A

TBX5, FGF-10 in lateral plate mesoderm C5-T1 (around somites 16-20)

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12
Q

What signals induce formation of limb buds of hindlims?

A

TBX4, FGF-10 in lateral plate mesoderm L4-S3 (starting around somites 27 and ending caudal to that)

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13
Q

Hand and foot plates form when?

A

5th week or 32 days

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14
Q

Digital ray formation

A

6th week/42 days

-tissues b/w digital rays undergoes apoptosis to form notches

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15
Q

digits are distinct @ …?

A

8th week/56 days

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16
Q

@ week 7

A
  • upper limbs bent at elbow

- fingers short and webbed

17
Q

Positional changes of developing limbs

A
  • 48 days: limbs extend ventrally; hand and foot plates face each other
  • 51 days: upper limbs bent @ elbows, hands curve over thorax
  • 54 days: soles of feet face medially
  • 56 days: elbows point caudally, knees point crainially (limb rotation)
18
Q

Limb rotation

A

90º
upper limb: laterally (flexors-ant, extensors-post, thumb lateral)
lower limb: medially (flexors-post, extensors-ant, big toe medial)

19
Q

Development of cartilagenous bones

A
  • 5th week: chondrification centers
  • 6th week: entire limb skeleton is cartilaginous
  • 7th weeek: 1º ossification centers formed in long bones (osteogenesis) (includes phalanges, metacarpals, metatarsals)
  • Carpal and Tarsal bones: ossification begins in 1st year after birth
20
Q

Ossification of Appendicular skeleton

A
  • Endocondrial ossification (except clavicle)
  • diaphysis @ week 7-8 (most 1º ossification centers form in appedicular skeleton by week 12)
  • epiphysis during 1st 4 years
  • epiphysseal plate b/w: growth plate, adds length, persists til growth is over (end of puberty)
21
Q

Rickets

A

Vitamin D deficeincy–>impaired Ca2+ absorption–>disturbance in soofication of epiphyseal plates (short and deformed limbs)

22
Q

Achondroplasia

A
  • Autosomal dominant 1:15,000
  • premature fusion of epiphyses
  • short stature
  • bowed limbs
  • short trunk
  • enlarged head
23
Q

Development of Myotomes (limbs)

A

2 divisions:

  • Epaxial (dorsal)–>intrinsic muscles of back, dorsal rami
  • Hypaxial (ventral)–>everything else (including limbs and abdomen), segmentation retained in some regions, ventral rami

-limbs: myogenic precursor cells migrate to limb bud and differentiate into myoblasts–>aggregate as muscle mass in limb bud as long bones develop–>separate into flexor and extensor compartments

24
Q

Development of Dermatomes

A
  • motor axons arise from spinal cord and enter limb bud during 5th week
  • sensory axons follow after (use motor axons for guidance)

-neural crest cells–>precursors of schwann cells–>surround motor and sensor nerve fibers in limbs–>form neurolemma and myelin sheaths

dermatome=area of skin supplied by single spinal nerve and spinal ganglion

spinal nerves in segmental bands and orderly distribution recognized even after growth of limbs

25
Q

Syndactyly

A

Cutaneous: webbing of digits (failure of loose mesenchyme b/w digital rays to undergo apoptosis)
Osseous: fusion of bones (failure of notches to form)

Upper limb–> usually 3rd and 4th digits
Lower limb–> digits 2 and 3

26
Q

Polydactyly

A
  • extra digits (supernumerary)
  • relatively common (1:1000 esp. Amish in USA)
  • Disruption fo anteroposterior pattern
  • severe type often has fusion of bones of digits
27
Q

Congenital club foot

A
  • Tailpes equinovarus
  • Foot turned medially and inverted
  • common (1:1000)
  • usually associated with other defects (VACTERL)
28
Q

Amelia

A
  • Complete absence of limbs
  • suppression of limb bud development in 4th week
  • can be caused by thalidomide (drug used to prevent morning sickness in ’60s until link was made)
29
Q

Meromelia

A
  • Partial absence of limbs (some limbs or parts of limbs)

- disturbane of limb development in 5th week

30
Q

Cleft hand and foot

A
  • absence of 1 or more central digits
  • failure of development of ≥1 digitl ray
  • remaining digits are partially or completely fused
  • rare anomaly: lobster-claw deformity
31
Q

Amniotic Band syndrome

A

-tears in aminion result in amniotic bands
-can encircle limbs and digits–>constriction and amputations
-cause unknown (infection speculated)
~1:1200 births

Post Midterm Anatomy (10 decks)

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