Embryo - Limb Development Flashcards
1
Q
What are the Characteristics of the Development of Limbs?
A
Basics:
- Limb buds = paddle projections
- Ventrolateral body wall
-
Upper limb buds first
- 26th -27th day
- Opposite to C5-T1 spinal segments
- contribute to brachial plexus
-
Lower limb buds second
- 2 days later
- Opposite to L4-S3 spinal segments
- contribute to sacral plexus
Limb buds:
- Mesenchymal core
- derived: somatic layer of lateral plate mesoderm
- Covered by ectoderm
Note:
- Activation of mesenchymal cells in the lateral mesoderm starts off the process of limb development
2
Q
What are the 3 types of Mesoderm? What do Somites give rise to?
A
Basics:
-
Lateral plate mesoderm
- subdivision of the intraembryonic (2nd) mesoderm
- forms connective tissue of the limbs
-
Paraxial Mesoderm
- divdes into 42-44 pairs of somites
-
Intermediate mesoderm
- urogenital system
Somites give rise to…
-
Sclerotome
- Vertebrae & ribs
-
Myotome
- Muscle
-
Dermatome
- Connective tissue
- Dermis
- Connective tissue
3
Q
What is the importance of an Apical Ectodermal Ridge (AER)?
A
Basics:
-
Activation of mesenchyme induces formation of apical ectodermal ridge (AER)
- mesenchyme condenses
- gives rise to skeleton & connective tissue
- mesenchyme condenses
- Molecular signaling causes formation of AER
- drugs can affect development of the limbs
-
AER = start of limb formation
- loose mesenchyme –> condensed mesenchyme –> cartilage
AER:
- Ectodermal thickening at the apex of each limb bid
- Inductive influence on limb mesenchyme
- stimulates proliferation of mesenchyme
- progress zone
- growth & development of the limbs
- stimulates proliferation of mesenchyme
- Interaction btw AER & Mesenchyme = essential for limb dev.
4
Q
A
5
Q
How do limbs develop from Week 4 - Week 8?
A
Week 4
- Limb buds appear
Week 5
- Hand & foot plates formed
- Chondrification centers form
Week 6
- Digital rays appear
- Tissue btw digits = undergo apoptosis
- form notches
- entire limb = cartilaginous
Week 7
- Upper limbs bend at elbow
- Fingers = short & webbed
- Osteogenesis of long bones = begins via 1° Ossifcation Centers
- Ossification of the carpal & tarsal bones = after 1st year of birth
Week 8
- Digits are distinct & separate
6
Q
How does Upper and Lower Limb Development Progress from Day 26 - Day 52?
A
7
Q
What are the Positional Changes of Developing Limbs?
A
48 Days
- Limbs extend ventrally
- Hand & Foot plates face each other
51 Days
- Upper limbs bend at elbow
- Hands curved over thorax
54 Days
- Soles of feet face medially
56 Days
- Elbows point = caudally
- Knees point = cranially
8
Q
How does Limb Rotation differ from Upper/Lower Limbs?
A
-
Upper Limb
- Rotates 90° laterally
- Flexors = Anterior
- Extensors = Posterior
-
Lower Limb
- Rotates 90° medially
- Extensors = Anterior
- Flexors = Posterior
9
Q
How does Ossification of Appendicular Skeleton Occur?
A
Basics:
- Bones of the limbs & limb girdles are formed = endochondral ossification (except for the clavicle)
- Mesenchymal cells gives rise to a cartilaginous model –> ossifies
Fetus:
-
Diaphysis
- formed by primary ossification center in long bones
- week 7-8
- Epiphysis/carpus/tarsus = still cartilageninous
After Birth
-
Epiphysis
- formed by secondary ossification centers
- appear during 1st 4 years
Child/Teen/Young Adult
-
Epiphyseal plate
- Growth plate btw diaphysis & epiphysis
- addes length to bone
- persists until done growing
- Growth plate btw diaphysis & epiphysis
10
Q
What causes Rickets?
A
-
Vitamin D deficiency leads to impaired calcium absorption by the intestine
- calcium deficiency causes disturbance in ossification of epiphyseal plates
- short & deformed limbs
- calcium deficiency causes disturbance in ossification of epiphyseal plates
11
Q
What is Achondroplasia?
A
- Premature fusion of epiphyses (growth plate)
- short stature
- bowed limbs
- short trunk
- enlarged head
- Autosomal dominant
- 1: 15,000
12
Q
What is Syndactyly/Polydactyly?
A
Syndactyly
-
Cutaneous syndactyly
- Webbing of the digits
-
Osseous syndactyly
- Fusion of the bones
Polydactyly
- Extra digits
- Can be posterior/anterior
- not in the middle
13
Q
A
14
Q
What is Congenital Club Foot?
A
- Talipes equinovarus
- Foot is turned medially and inverted
- Common anomaly
- Gets fixed on its own as child learns to walk
15
Q
What are the difference between Amelia & Meromelia?
A
Amelia
- Complete absence of limbs
- Suppression of limb bud dev.
- Week 4
Meromelia
- Partial absence of limbs
- Disturbance of limb development
- Week 5
16
Q
How do Cleft Hand & Cleft Foot Develop?
A
-
Absence of one or more central digits
- Failure of development of one or more digital rays
- Remaining digits are partially or completely fused
- Rare anomaly
- Lobster‐claw deformities
17
Q
What is Amniotic Band Syndrome?
A
- Tears in the amnion result in amniotic bands
- May encircle fetal limbs & digits
- Ring constrictions
- Amputations
18
Q
How do Dermatomes arise?
A
Basics:
- Motor axons arise from the spinal cord & enter limb bud
- 5th week
- Sensory axons enter limb buds after the motor axons
- Use them for guidance
Dermatomes:
- Neural crest cells
- precursor of schwann cells
- surround moto/sensory nerve fibers in limbs
- form the neurolemma & myelin sheaths
- Dermatome = area of skin supplied by 1 single spinal nerve & ganglion
Note:
- Spinal nerves = distributed in segmental bands
- Orderly sequence –> recognized even after growth of limbs
20
Q
What are Myotomes of Body Somites?
A
- As long bones form, myoblast aggregate and form a muscle mass in each limb bud
- Muscle mass separates into flexor and extensor compartments
Image:
- Epaxial myotome = intrinscic muscles of the back
- Epaxial division = dorsal
