Limb embryology week 2

Question 1

What are the 3 classifications of common congenital limb abnormalities? What are the 2 most common congenital abnormalities of limb development?

Answer 1

1. reduction deficits: failure of part of or the entire limb to develop

duplication deficits: extra limb elements are present

dysplasia: malformations of parts of the limb
2. syndactyly and polydactyly are the most common congenital abnormalities of limb development
polydactyly: extra digits. individual has more than 5 fingers or toes
syndactyly: 2 or more digits are fused togheter. webbing typically involves only a skin connection. much less commoon for there to be connections btwn adjacent bones. most common webbing is btwn second and third toe

Question 2

What embryonic structures/cells do bone, muscle, and connective tissue originate from?

Answer 2

primitive streak mesoderm including somites, lateral plate mesoderm, and diffuse mesenchyme produced during gastrulation

Question 3

What cell layer are somites associated with? What do somites differentiate into (just list)?

Answer 3

1. paraxial mesoderm
2. sclerotome, dermamyotome (dermatome, myotome)

Question 4

What cell layer do sclerotome cells come from? When do they differentiate to form their structures and what structures do they form?

Answer 4

1. somites of paraxial mesoderm
2. are located on deep aspect of somites. differentiate at 3 weeks of gestation. dissociate and migrate around the nueral tube and notocord to differentiate into chondroblasts, cartilginous precursors of vertebrae

Question 5

What cell layer do dermamyotome cells come from? What structures do they form?

Answer 5

1. differentiated somites of paraxial mesoderm
2. dermamyotome separates into a muscle forming myotome and a dermatome that becomes the connective tissue dermis of the skin

see slide 4 of notes

Question 6

What does the neural tube form in adults? The notocord?

Answer 6

neural tube: spinal cord

notocord: nucleus pulposus of vertebrae. gel like substance composed of water and loose collagen network that resists compression

Question 7

What role does the notocord play in the embryo? During the formation of what cell layer is the notocord formed?

Answer 7

1. inductive organ formation
2. mesoderm

Question 8

True or false: dermatomes and mytomes are associated with a segmented pair of spinal nerves.

Answer 8

True.

Question 9

Limb buds develop as paddle liek extensions of the ventrolateral body during week 4 of development. What are limb buds composed of? What portion of the limb bud iniates growth and development of the limbs through inductive influence? What compartments are limb buds divided into as it relates to flexors and extensors and where they are located in the body and what is the consequence?

Answer 9

1. mesenchyme capped by ectoderm
2. apical ectodermal ridge (AER). inductive influence on mesenchyme
3. limb buds are divided into a ventral flexor compartment and a dorsal extensor compartment. reason why upper and lower extremites flex in opposite directions

Question 10

True or false: How muscles are organized relates to how they are innervated.

Answer 10

True. In the embryo, formation of muscle and neuralation are tied processes

Question 11

Limb bone development begins with condensations of mesenchyme from the ventrolateral body wall. Cells from these condensation diffenitate into _____ that convert mesenchymal primordia into cartilaginous precursors of bone. The cartilage is eventually replaced by bone through ____ _____.

Answer 11

1. chondroblasts
2. endochondral ossification

Question 12

Muscle cells of the limbs are derived from _____ cells and the connective tissue elements of the muscle (tendons) are derived from local _____ cells.

Answer 12

1. myotome
2. mesenchymal

Question 13

True or false: Like the somites from which they are derived, myotomes have a distribtution in the embryo and each segment is innervated by a spinal nerve.

Answer 13

True.

Question 14

Adjacent mytomes fuse to form individual skeletal muscles. What is the consequence of this as it pertains to innervation (including plexus formation).

Answer 14

Result is most muscles are innervated by more than one spinal segment. This occurs by innervation from multiple spinal segments (back, abdominal muscles) or the joining of multiple spinal segments into single nerves in the brachial and lumbosacral plexuses for innervatoin of limbs musculature.

Question 15

In the embryo, the upper limb rotates 90 degrees laterally so that the ventral, flexor compartment faces (anteriorly/posteriorly). The lower limb rotates 90 degrees medially so that the ventral, flexor compartment faces (anteriorly/posteriorly) and the extensors are in the front.

Answer 15

1. anteriorly
2. posteriorly

see figure

Question 16

Ventral rami of the brachial plexus is separated into anterior and posterior divisions. This results in the separation of myotomes in limb buds into ____ and ____ compartments.

Answer 16

flexor and extensor

Question 17

Nerve branches of anterior divisions of the brachial plexus tend to go to (flexor/extensor) compartment muscles and branches of posterior division tend to go to (flexor/extensor) compartment muscles. The same pattern of innervation is true for the lower limbs and the lumbosacral plexus. However, one must remember that flexor musculature is on the (anterior/posterior) aspect of the lower extremity and extensor musculature in on the (anterior/posterior) aspect of the lower extremity

Answer 17

1. flexor
2. extensor
3. in lower ext: flexors are posterior
4. in lower ext: extensors are anterior

Question 18

True or false: Overlapping of dermatomes occurs as fibers of adjacent nerves are distriubted to skin.

Answer 18

True. For instance, the top half of the L2 dermatome also gets innervation from the L1 and the botom half of the L2 dermatome also gets innervation from L3.

Question 19

Question 20

Spinal nerves divide into ___ and ___ rami.

Answer 20

dorsal (posterior) and ventral (anterior)

Question 21

What to dorsal and ventral rami innervate and how are their innervations patterned (as it relates to dermatomes)? What is special about the innervation of ventral rami?

Answer 21

dorsal rami from various spinal levels separte from each other and innervate synovial joints of the vertebral column, muscles of the deep back (intrinsic muscles) and the skin overlying them in a segmental manner.

ventral rami innervate the rest of the body (excluding the head which is for cranial nerves). ventral rami distributed to the trunk innervate skin and muscles in a segmental pattern. However, for innervation of the limbs, the majority of ventral
rami merge with ventral rami of adjacent segments to form a plexus (brachial / lumbosacral). Here their fibers (axons) intermingle and form multi-segmental peripheral nerves (e.g. the median nerve contains axons from ventral rami of
C5, C6, C7,C8, and T1 and the femoral nerve contains axons from ventral rami of L2, L3, and L4).

Question 22

Although spinal nerves lose their identity in the plexus, fibers arising from a particular spinal cord segment are ultimately distributed to a single dermatome, even though these fibers are conveyed to the skin in a peripheral nerve.
It is thus important to distinguish between the dermatomal pattern of cutaneous innervation and the cutaneous innervation pattern of peripheral nerves. Why?

Answer 22

If a patient’s sensory loss corresponds to a dermatomal region rather than the area innervated by a peripheral nerve, the location of the lesion is likely more central
(i.e. close to the spinal cord) rather than somewhere along the course of a peripheral nerve.