Week 1 Limb Development

Question 1

Which embryonic germ layer gives rise to skeletal muscle?

Answer 1

Mesoderm

Question 2

List the adult derivatives stemming from the subdivisions of a somite

Answer 2

1. Sclerotome: vertebrae
2. Dermomyotome: dermis and muscles of the back, trunk, and limbs
   - dermomyotomes are source of all skeletal muscle precursor cells, myoblasts

Question 3

Describe the embryonic processes by which nerves and muscles appear in limb buds

Answer 3

• limb buds project from body wall, initially palms and soles face each other
• medial: preaxial compartment, lateral: postaxial compartment
• myoblasts from dermomyotomes adjacent to limbs migrate into limb buds and cluster in these compartments
• ventral primary rami at each limb base grow into developing limb bud–>divide into anterior and posterior divisions as they enter limb bud

Question 4

Define the embryonic rationale for the extensor and flexor components of the adult

Answer 4

limb rotation

• upper limbs: rotate laterally (outward), preaxial=anterior/flexors, postaxial=posterior/extensors
• lower limbs: rotate medially. preaxial=posterior/flexors, postaxial=anterior/extensors

Question 5

explain the embryonic rationale for the dermatome patterns seen in the limbs

Answer 5

Upper limbs: C5-T1
-centrally positioned dermatomes drawn out to the distal extremes as limb bud elongates
-C5-shoulder, C6-thumb, C7-index/middle finger, C8-little finger, T1-ulnar side of forearm, and to armpit
Lower limb:
-spiral pattern: L1-S3
Muscles of limbs follow proximal to distal pattern. Proximal muscles are C5/6 (upper limb shoulder), L2-L4 (lower limb thigh), distal muscles hands (C8, T1) and feet (S2, S3)

Question 6

Describe the steps by which a muscle fiber develops from a myoblast

Answer 6

-myoblasts undergo mitosis–>fuse to myotubes, increase in length and girth via formation of myofibrils and fusion of myoblasts–> myofiber
-satellite cells-reserve cells for muscle injury or disease
-

Question 7

Categorize the more common limb malformations

Answer 7

1. Reduction defects:
   - meromelia-part of limb
   - amelia-missing entire limb
2. Duplication defects:
   - polydactyly-extra digits
   - supernumerary-extra limb
3. fusion of digits-syndactyly

Question 8

What are causes of common limb defects?

Answer 8

1. teratogens
2. genetics
3. multifactorial
   - most critical period of limb development is 24-36 days post fertilization