Organization and Development of the Limbs

Question 1

What are the structures shared by the forelimb and hindlimb?

Answer 1

girdles - (shoulder/pelvic) attachment to axial skeleton

single proximal element (humerus/femur)

double distal element (radius-ulna; tibula-fibula)

carpals/tarsals

metacarpals/metatarsals

phalanges

Question 2

Describe the compartments of the limbs.

Answer 2

separated by deep fascia

muscles within compartments share:

• embryological origins
• nerve and blood supply
• attachment points
• function

Question 3

What is the innervation and muscles of the posterior compartment of the arm?

Answer 3

radial nerve

shoulder and elbow extensors (triceps brachii)

Question 4

What is the innervation and muscles of the anterior compartment of the arm?

Answer 4

musculocutaneous nerve

shoulder and elbow flexion (biceps brachii, brachialis)

Question 5

What is the innervation and muscles of the posterior compartment of the forearm?

Answer 5

radial nerve

wrist and finger extensors (extensor carpi or extensor digitorum)

supinator

Question 6

What is the innervation and muscles of the anterior compartment of the forearm?

Answer 6

median and ulnar nerves

wrist and digit flexors (flexor carpi, flexor digitorum)

pronators

Question 7

Describe the formation of the limb buds.

Answer 7

outgrowths of the body wall

proliferation of the somatopleuric (lateral plate) mesoderm in response to signals from cervical and lumbar somites results in the outgrowth of upper (26 days) and lower (28 days) limb buds

Question 8

What is the role of the ZPA in development?

Answer 8

determines the preaxial/postaxial polarity of the limb

the primary signaling factor is sonic hedgehog (SHH) induced through retinoic acid gradient

areas closest to the ZPA will develop into postaxial structures

areas farther from the ZPA develop into preaxial structures

Question 9

What signal induces formation of the AER?

Answer 9

FGF signals from the mesoderm

Question 10

What does the AER determine in development?

Answer 10

determines the proximo-distal polarity of the limb

also determines the proximo-distal growth of the limb

cells near the AER become “distalized” and cells further away “proximalized” as a result of morphogen gradients

primary signaling factor is FGF (fibroblastic growth factor)

Question 11

Holt-Oran syndrome

Answer 11

heart and forelimb defects due to mutation in Tbx5 gene, which is expressed int eh AER and determines forelimb formation

Question 12

What are the side effects of thalidomide, and what is the name of the defect that results?

Answer 12

thalidomide - destroys developing blood vessels, distruption of FGF/SHH signaling

the resulting limb defect is called amelia/phocomelia

Question 13

What is the result of duplication of the ZPA?

Answer 13

“mirror hand” defect

two sides of development occurs

Question 14

nail-patella syndrome

Answer 14

disruption of “dorsalization” of the limb due to mutation in LMX1 gene

nails don’t develop correctly

Question 15

palmar nail syndrome

Answer 15

palmar nail, lack of flexion creases, dorsalization of the palmar skin

appearance associated with lack of flexion and tapered distal phalanx

Question 16

amniotic band syndrome

Answer 16

caused by strands of the amniotic sac that separate and entangle digits, limbs, or other parts of the fetus

constriction can cause a variety of problems depending on where strands are located and how tightly they are wrapped

Question 17

talipes equinovarus

Answer 17

clubfoot - condition characterized by a foot fixed in adduction, supination, and varus position

as a result, the foot typically is turned inward, and it has a clublike appearance

one of the most common congenital birth defects and has been diagnosed as early as 13 weeks gestation

1/3 of cases are isoolated

however, many are associated with other abnormalities such as central nervous system defects and chromosome abnormalities

Question 18

development of joints

Answer 18

the initial axial mesenchymal condensation differentiates into bone primordia and an interzone, which differentiates in to the connecting material of joints

future areas of joints do not chondrify or ossify

they remain as dense connective tissue

further differentiation depending on the type of joint

Question 19

Where do muscles develop from?

Answer 19

muscles cells are somitic - 42-44 pairs of somites form from the paraxial mesoderm lying adjacent ot the neural tube

each myotome portion of the somite divides into a small dorsal epaxial and a larger ventral hypaxial division

Question 20

Where do bones, ligaments, and connective tissues of muscles develop from?

Answer 20

intrinsic to the lumb bud (somatic)

the arise form the lateral plate mesoderm

Question 21

Where do all limb muscles develop from? What is the innervation?

Answer 21

hypomere

innervated by ventral rami

*the primitive embryonic limb has two main muscular compartments - dorsal and ventral

Question 22

What nerves supple the limb muscles?

Answer 22

ventral rami of the spinal nerves

limb muscles are hypomere in origin, so the mesenchyme of hte limb bud is invaded by the ventral rami of the adjacent spinal nerves C4-T1 (brachial plexus) for the upper limb bud and L1 to S3 (lumbosacral plexus) for the lower limb bud

Question 23

What are the general features of muscles of the embryological dorsal compartment?

Answer 23

extensor muscle

posterior division of ventral ramus spinal nerve

Question 24

What are the general features of muscles from the embrological ventral compartment?

Answer 24

flexor muscle

anterior division of ventral ramus spinal nerve

Question 25

Describe the structure of the brachial plexus.

Answer 25

C5,6,7,8 and T1 all come in to create the plexus

Question 26

What are the changes in position of the limbs before birth?

Answer 26

lims first grow outwards at a point, will bend in and down leg rotates ouward and feet come in together ventral of surface of the leg end up in the back - posterior in adults hamstrings, calf muscles were embryologically ventral, now anatomical position and posterior quadriceps on the front, embryologically dorsal, now ventral

Question 27

flexion

Answer 27

reduce the angle between (or approximate) two embryonically ventral surfaces

Question 28

extension

Answer 28

reduce the angle between (or approximate) two embryonically dorsal surfaces

Question 29

limb rotation stage 1

Answer 29

the limbs begin as projections from the body in a **coronal orientation** (knees and elbows facing dorsally each limb bud thus has **two surfaces** - ventral and dorsal separated by two borders (cranial and caudal) the cranial border is called the **preaxial border** and the caudal border is called the **postaxial border** the preaxial digits, the thumb (pollex) and the big toe (hallux) face cranially

Question 30

limb rotation stage 2

Answer 30

towrads the end of the sixth week, the upper and lower limb buds bend inward at the future elbos and knees they project at right angles to the trunk with the elbows and knees pointing laterally and the palms and soles facing the trunk

Question 31

limb rotation stage 3

Answer 31

during the seventh and eight weeks, before any joints are fully developed, the limbs rotate about their long axes and become adducted to the trunk the upper limb rotates laterally 90 degrees so that the elbows now face caudally, while the lower limb rotates medially 90 degrees so the knees face cranially

Question 32

limb rotation stage 4

Answer 32

unfolding the limbs results in the adult anatomical position where the preaxial border of the upper limb (thumb) is on the lateral side and the embryonic ventral surfcae is still ventral (or anterior) in the lower limb, however, the preaxial border (big toe) is medial and the embryonic ventral surface is now dorsal (or posterior), and the embryonic dorsal surface (black) is now anterior

Question 33

anterior division of nerves innervating the limb muscles

Answer 33

supply those regionsof the limb that represent the embryonic ventral (flexor) surface

Question 34

posterior division of nerves innervating limb muscles

Answer 34

supply those regions of the limb that represent the embryonic dorsal (extensor) surface

Question 35

What is the orientation and innervation of the gluteals?

Answer 35

dorsal ompartment in embryo, still posterior in adult posterior division nerves (superior and inferior gluteal nerves)

Question 36

What is the orientation and innervation of the hip adductors?

Answer 36

ventral compartment in embryo, medial compartment in adult, anterior division nerve (obturator)

Question 37

What is the orientation and innervation of the hip flexors?

Answer 37

anterior thigh compartment in adult, but supplied by posterior division nerve (femoral)

Question 38

What is the orientation and innervation of the hip extensors?

Answer 38

posterior thigh compartment but are supplied by an anterior division nerve (tibial nerve)