Rishi Desai, MD, MPH

Development of the limbs

Chapter 29 Embryology: Body System Structures  DEVELOPMENT OF THE LIMBS osms.it/limb-development Limb buds: overview ▪ End of week 4: limb buds visible on ventrolateral body wall ▪ Limb bud structure ▫ Mesenchymal core: forms connective tissue, bones ▫ Myotomes: form muscles ▫ Ectoderm cover: forms epidermis of skin Connective tissue & bones ▪ Development determined by series of interactions between ectoderm, mesenchyme ▪ Ectoderm at the limb apex proliferates, forms apical ectodermal ridge (AER) → induces adjacent mesenchyme to remain undifferentiated, rapidly proliferating cells ▫ AKA undifferentiated zone ▪ Ectoderm further inﬂuences mesenchyme ▪ Mesenchyme differentiates into cartilage, muscle; forms three components proximodistally ▫ Stylopod: becomes humerus/femur ▫ Zeugopod: becomes radius/ulna, tibia/ ﬁbula ▫ Autopod: becomes carpals, metacarpals, metatarsals ▪ Week 6: limb bud apexes ﬂatten, become hand, foot plates ▪ Fingers, toes formed via localized apoptosis induced by AER ▫ Separates hand, foot plates into ﬁve parts ▪ Mesenchyme underneath differentiates into chondrocytes ▫ Chondrocytes form primary hyaline cartilage models of future bones ▪ As chondrogenesis stops, joint formation induced ▫ Condensed mesenchyme differentiates into dense ﬁbrous tissue (forms articular cartilage, synovial membrane, menisci, ligaments of joint)  Limb muscle development ▪ Derived from dorsolateral cells of somites ▫ AKA myotomes ▫ Myotomes from C4-T2 migrate to upper limb ▫ Myotomes from L2-S2 migrate to lower limb ▪ During migration to limbs, myotomes form two compartments ▫ Anterior condensation → ﬂexor, pronator muscles of upper limb; ﬂexor, adductor muscles of lower limb ▫ Posterior condensation → extensor, supinator muscles of upper limb; extensor, abductor muscles of lower limb ▫ Ventral primary branches of spinal nerves, mesenchyme divide; form dorsal, ventral branches to these compartments ▫ Week 7: limbs rotate ▪ Upper limb rotates 90° laterally ▫ Lower limb rotates 90° medially  Figure 29.15 Limb muscle development: myotomes migrate to limbs, forming anterior and posterior condensations that are innervated by ventral and dorsal branches of the spinal nerve's primary ventral branches.  OSMOSIS.ORG 227   

Human development days 1-4

Human development days 4-7

Human development week 2

Human development week 3

Weeks 1-4

Development of the digestive system and body cavities

Development of the fetal membranes

Development of the placenta

Development of the umbilical cord

Development of twins

Hedgehog signaling pathway

Early structures

Ectoderm

Endoderm

Mesoderm

Germ layers

Early development

Development of the cardiovascular system

Fetal circulation

Cardiovascular system

Development of the ear

Development of the eye

Development of the face and palate

Pharyngeal arches, pouches, and clefts

Eyes, ears, nose, and throat

Development of the gastrointestinal system

Development of the teeth

Development of the tongue

Gastrointestinal system

Development of the integumentary system

Integumentary system

Development of the axial skeleton

Development of the muscular system

Musculoskeletal system

Development of the nervous system

Nervous system

Development of the renal system

Renal system

Development of the reproductive system

Reproductive system

Development of the respiratory system

Respiratory system

Organ system development

Development of the limbs

Videos

Notes