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BIOL 4260 Human Evolu3onary Anatomy Lecture 12: Limb Development Lecture 2: Fossil Record
Outline
•  Limb Evolution
•  Limb Development
•  Limb Function
A Few Definitions
•  Appendicular skeleton – girdles & limbs
•  Pectoral girdle – bony articulation of the upper limb with
the axial skeleton
•  Pelvic girdle - bony articulation of the lower limb
•  Arm - upper aspect of upper limb (versus forearm)
•  Leg - lower portion of the lower limb (versus thigh)
Pectoral Girdle
Pelvic Girdle
Forelimb & Hindlimb
... first there were fins..
First jawed and finned fishes > 400 mya
Homologous upper limb structures derived from tetrapod
Die Bauplan
HOX
genes
regulate
shape and
type of
bones
Limbs Are Serial Homologues
Proximal
Distal
Limb Girdles
Development in
tetrapods allowed
head mobility,
independent
movement of limbs
Much larger in
terrestrial animals bear weight and
provide attachment
sites for muscles
Limb
Development
•  Limb buds
•  Patterning of limb
elements
•  Patterning of limb
musculature
Limb Buds
@ 4 weeks, C4 - T1/2
@ 5 weeks, L1/2 - S3
Embryological Origins
•  Outpocket of ectoderm
and mesoderm
•  Dermomyotomes: skin
and hypaxial muscles
•  Lateral plate mesoderm:
bones
•  Similar “recipe”
for
upper and lower limbs
Signaling molecules and gene expression
1. Upper limb outgrowth initiated
by TBX5 & FGF10
2. Bone morphogenic proteins
induce apical ectodermal ridge
(AER), axes established
3. Fibroblast growth factors
induce distal growth
4. More proximal areas slow
division rates and differentiate
Pinky vs.Thumb?
ZPA - gene sonic hedgehog - regulates
position of thumb- if misexpressed - creates
mirror image
Limb
Development
•  Limb buds
•  Patterning of limb
elements
•  Patterning of limb
musculature
Budding Muscles
Note:
Black dorsal
Grey ventral
Elevators
Depressors
Transformation into
tetrapod forelimb
Basic
organization
remains
Some dorsal muscles
migrate onto back
Some ventral muscles
migrate onto chest
Transformation into
mammalian forelimb
Bring forelimb under
body: 90 degree lateral
rotation
Dorsal muscles become
caudal (posterior)
Ventral muscles become
cranial (anterior)
Limb moves under
the body to improve
locomotion
Quadrupeds
pronate forearm to
locomote
Anatomical
Position
Forelimb:
•  Pollex is lateral
•  Developmentally
ventral muscles typically anterior
•  Developmentally
dorsal muscles typically posterior
Innervation follows...
Developmentally Ventral
Shoulder & arm:
Pectoralis major
Pectoralis minor
Coracobrachialis
Biceps brachii
Brachialis
Superficial forearm:
Pronator teres
Flexor carpi radialis
Palmaris longus
Flexor digitorum superficialis
Flexor carpi ulnaris
Deep forearm:
Flexor digitorum profundus
Flexor pollicis longus
Pronator quadratus
Developmentally Dorsal
Shoulder & arm:
Supraspinatus
Extensor carpi radialis longus
Infraspinatus
Extensor carpi radialis brevis
Subscapularis
Extensor digitorum
Latissimus dorsi
Extensor digiti minimi
Teres major
Extensor carpi ulnaris
Teres minor
Brachioradialis
Deltoid
Triceps brachii
Anconeus
Superficial forearm:
Deep forearm:
Abductor pollicis longus
Extensor pollicis longus
Extensor pollicis brevis
Extensor indicis
Supinator
Lower Limb
Transformation into
mammalian hindlimb
Bring hindlimb under
body: 90 degree medial
rotation - opposite
direction of forelimb
Most dorsal muscles
become cranial
(anterior)
Most ventral muscles
become caudal
(posterior)
Limb moves under
the body to improve
locomotion
Quadruped foot is in
correct position to
locomote
Anatomical
Position
Hindlimb:
•  Hallux is medial
•  Developmentally
ventral muscles posterior and medial
•  Developmentally
dorsal muscles anterior and lateral
Developmentally Ventral
Pelvis and thigh:
Gracilis pectineus
Adductors (longus, brevis,
magnus)
Obturator externus
Obturator internus
Gemellii ms.
Quadratus femoris
Biceps femoris (Long)
Semitendinosus
Semimembranosus
Posterior leg:
Gastrocnemius
Soleus
Plantaris
Popliteus
Flexor digitorum longus
Tibialis posterior
Flexor hallucis longus
Developmentally Dorsal
Pelvis & thigh:
Psoas major
Iliacus
Tensor fasciae latae
Gluteus minimus
Gluteus medius
Piriformis
Gluteus maximus
Sartorius
Quadriceps femoris
Biceps femoris (short)
Anterior leg:
Tibialis anterior
Extensor hallucis longus
Extensor digitorum longus
Peroneus tertius
Lateral leg:
Peroneus longus
Peroneus brevis
Limb Functions
Mammalian Limbs
Limbs shifted from lateral position to below trunk reduced pectoral girdle helps suspend thorax
Bipedalism
Reinforcement of
lower limb stability,
strength
Less demand on
upper limb, reduction
in weight, retained
mobility
Limb Functions
•  Mammals highly efficient - parasagittal movements
•  Locomotion:
Quadrupeds: walking, running, climbing, jumping,
leaping, crawling, swimming, flying, gliding
Bipeds: obligate walking and running
•  Tool use: possible motivating force for human
bipedalism - freeing of hands