Lecture 2 Flashcards
Limb bud
- Appear in the 4th week as small elevations on the ventrolateral body wall
Limb bud are derived from
- Somatic body wall (lateral plate mesoderm)
Ectoderm covers
- Mesenchyme (embryonic tissue)
Upper limb buds
- Visible in the cervical region ~ 24
Lower limb buds
- Visible in the lumbar region ~ day 28
Limb axis formation
- Directional axes of limb
- Proximal/Distal
- Dorsal/Ventral
- Cranial/Caudal
Proximal/Distal limb axis formation applies to
- Thigh
- Leg
- Foot
Dorsal/Ventral limb axis formation applies to
- Anterior vs. posterior thigh/leg
- Dorsum vs. plantar foot
Cranial/Caudal limb axis formation applies to
- Big toe (cranial, preaxial) to little toe (caudal, postaxial)
Limb bud contains mesenchyme that will form
- Cartilage/bone
- Dermis of the lower extremity
Muscles are derived from
- Somites
- Myoblast migrate into developing limb
Motor neurons are derived from
- Spinal cord
Structures derived from neural crest cells
- Schwann cells
- Melanocytes
- Sensory neurons
- Sympathetics
Limb elongation
- Progresses from proximal to distal
- Thigh (stylopod), leg (zeugopod), foot (autopod)
Early stage of limb elongation
- Limbs appear as flippers on the ventrolateral wall
Ectoderm over distal tip of limb
- Condenses, forming apical ectodermal ridge (AER)
- Represents the dorsal/ventral boundary
Underlying mesenchyme
- Induces the formation of the AER
Apical ectodermal ridge (AER) induces
P-roliferation of underlying mesenchyme
- Elongation of the limb (progress zone)
Mesenchyme obtains
- Positional information about future proximal/distal location
Studies that removed the AER showed
- Formation of a truncated limb
Mesenchyme will differentiate into
- Cartilage (cartilage model precursor to bone)
Pre-axial and post-axial borders
- Location where flexor and extensor compartments of the limb meet in fetal development
Pre-axial (cranial) border
- Medial side (tibial side) of the limb
- Demarcated by the great saphenous vein
Post-axial (caudal) border
- Lateral side (fibular side) of the limb
- Demarcated by the small saphenous vein
Bone formation
- Mesenchyme –> cartilage
- Ossification from cartilage template (most bones) or connective tissue
- Once ossifying, visible on radiograph
Ossification from cartilage template/connective tissue
- Tips of distal phalanges (ungal tuberosity, tufts)
- Develops from intramembranous ossification
Joint formation
- Forms from mesenchyme between cartilage templates of future bone
Mesenchyme between cartilage templates of future bone can differentiate into
- Collagen (fibrous)
- Hyaline cartilage (synchondrosis)
- Joint cavity (synovial)
Synovial joint
- Development of an interzone between cartilage templates (trilaminar)
- Cavitation develops within interzone
Limb muscles
- Derived from hypaxial myogenic precursors
- Migrate ventrally along dorsolateral body wall
- Migration starts week 4/5
- Muscle mass increases by mitosis until mid-fetal period
2 condensations formed by myoblast
- Dorsal mass (extensors)
- Ventral mass (flexors)
Dorsal mass (extensors) initially located
- Posteriorly (dorsally) before limb roation
After limb rotation, dorsal mass (extensors) located
- Posterior in upper limb
- Anterior in lower limb
- Not lower limb girdle (pelvic girdle)
Ventral mass (flexors) initially located
- Anteriorly (ventrally) before limb rotation
After limb rotation, ventral mass (flexors) located
- Anterior in upper limb
- Posterior in lower limb
- Not lower limb girdle
Innervation of developing limb
- Ventral rami from spinal cord segments migrate into developing limb
- Axons innervate muscle masses before they split into individual muscle
Dorsal branches (divisions) of ventral rami innervate
- Dorsal muscle mass
Ventral branches (divisions) ventral rami innervate
- Ventral muscle mass
Subcostal nerve
- Ventral rami T12
- Cutaneous supply to superior anterolateral thigh
Lumbar plexus branches
- Iliohypogastric (ventral division of L1)
- Ilioinguinal (ventral division of L1)
- Genitofemoral (ventral divisions of L1,2)
- Lateral femoral cutaneous (dorsal divisions of L2,3)
- Nerves to psoas major muscle (dorsal divisions of L2-4)
- Femoral (dorsal divisions of L2-4)
- Nerve to iliacus muscle
Obturator (ventral divisions of L2-4) - Accessory obturator (ventral divisions of L3,4)
Part of L4 joins with L5 ventral rami to form
- Lumbosacral trunk
Sacral plexus branches
- Superior gluteal (dorsal divisions of L4-S1)
- Inferior gluteal (dorsal divisions of L5-S2)
- Nerve to piriformis (dorsal divisions of S1,2)
- Perforating cutaneous nerve (dorsal divisions of S2,3)
- Sciatic (dorsal divisions of L4-S2 and ventral divisions of L4-S3)
- Posterior cutaneous nerve of thigh (dorsal divisions of S1,2 and ventral divisions of S2,3)
- Nerve to quadratus femoris (ventral divisions of L4-S1)
- Nerve to obturator internus (ventral divisions of L5-S2)
- Pudendal (ventral divisions of S2-4)
Digit formation
- Zone of polarizing activity (ZPA)
- Signaling center that forms in the dorsal mesenchyme just below AER
- Controls pattern formation on an anterior/posterior axis
- Hallux vs. little toe
Axis artery (axial)
- Primary vessel that supplies the developing limb
Axis artery arises from
- Dorsal root of umbilical artery
Axis artery pathway
- Passes along posterior aspect of developing limb to plantar foot
External iliac forms from
- Dorsal root of umbilical artery
- Femoral artery then forms from external iliac artery
- Travels to posterior thigh and communicates with axial artery
Once femoral artery forms and communicated with axial artery
- Axis artery proximal to this disappears
- Inferior gluteal and ischiadic artery persist
Axial artery eventually becomes
- Major supplier to lower extremity through its communication with popliteal artery
Dorsal root of umbilical artery becomes
- Common iliac
Remnants of axis artery
- Inferior gluteal
- Popliteal
- Ischiadic artery (sciatic artery)
- Fibular
5th week
- Hand and foot plates develop (hands develop earlier)
- Flat and paddle shaped
6th week
- Joints become more observable
- Digital rays of hand plate develop
- Limbs move to a more ventral position
7th week
- Digital rays of the foot develop
- Limb rotation
Limb rotation
- Knee/elbows initially face laterally
- Upper/lower limbs rotate in opposite directions
8th week
- Fetal position is attained by the end of the week
- Mesenchyme between digits degenerates (apoptosis)
Upper limb rotation (7th week)
- Rotate laterally/externally to face caudally
Lower limb rotation (7th week)
- Rotate medially/internally so the knees face cephalically
Tarsal coalition
- Complete or partial union between 2 or more tarsal bones
- Developmental fusion that results from incomplete or faulty mesenchymal segmentation
Tarsal coalition development
- Genetically programed to develop
- Calcaneonavicular and talocalcaneal are most common
Tarsal coalition classification
- Classified by the type of tissue that bridges the bones
- Fibrous (syndesmosis)
- Cartilaginous (synchondrosis)
- Bony (synostosis)
Syndactyly
- Congenital malformation caused by the the failure of differentiation of digits
- Mesenchyme doesn’t separate
Syndactyly effects
- Fusion of toes (or fingers)
- May be single or multiple
- Can affect skin and soft tissue, or soft tissue and bone
Cutaneous syndactyly
- Failure of mesenchyme degeneration between digits
- Causes webbing between digits
- Can be complete or partial
More of a functional problem in the hand
Osseous syndactyly
- Failure of mesenchyme to segment in the foot
Amelia
- Absence of an entire limb (failure of formation)
- Can be 1 or multiple
Amelia is caused by
- Interruption of the limb formation process (prevented or interrupted early on)
Tetra-amelia
- Absence of all limbs
Meromelia
- Partial absence of limb
- Can occur at any level of the limb
Phocomelia (“seal limb”)
- Type of meromelia where rudimentary hands or feet are attached to trunk
The various types of missing limb or limb malformations can result from
- Vascular interruption
- Genetic
- Teratogens
Teratogens
- Agents that can cause malformation of embryo
- Thalidomide, chemicals, radiation
Congenital absence of bone
- Fibula (fibular hemimelia)
- Can be partial or complete
Cleft foot/hand (ectrodactylyl
- “Lobster Claw Foot (hand)”
- Rare anomaly
- Central conical defect from periphery to tarsals
- Presentation varies
- Affects central rays
- Surgically addressed early on to prevent pathological adaptation
Polydactyly (“many digits”)
- Formation of extra digits
- Can be due to genetic defect or part of a syndrome
- Different appearances
Polydactyly in lower limb
- Pre-axial
- Post-axial
- Centra
Pre-axial polydactyly
- Extra digit towards the hallux
Post-axial polydactyly
- Extra digit towards little toe
- More common
- Surgically corrected
Central polydactyly
- Extra digit involves digits 2, 3, or 4
Macrodactyly (“large digits”)
- Enlargment of digits
- Overgrowth of bone/tissue
