Development Of The Limbs PDF

S2 L1 DEVELOPMENT OF THE LIMBS Lecturer: Mela Salah Designer: Ali Kareem  PRENATAL LIMB DEVELOPMENT Limb Development Begins With The Somite Activation Of Mesenchyme Within The Somatic Layer Of Lateral Mesoderm. This Somatic Mesoderm...

S2 L1 DEVELOPMENT OF THE LIMBS Lecturer: Mela Salah Designer: Ali Kareem  PRENATAL LIMB DEVELOPMENT Limb Development Begins With The Somite Activation Of Mesenchyme Within The Somatic Layer Of Lateral Mesoderm. This Somatic Mesoderm Forms The Limb Skeleton. Somites Form The Limb Musculature.  Mesenchyme LIMB BUDS Core Limb Buds Appear Towards The End Of 4th Week, With The Lower Limbs’ Development Lagging About 2 Days Behind The Upper Ectoderm Limbs’. Limb Buds Appear On The Ventro-lateral Body Wall, And Extend Ventrally At First. They Consist Of A Mesenchyme (Flexible Mesoderm) Core With A Thickened Ectoderm At The Apex, The Apical Ectodermal Ridge (Aer) The Elongation Of The Limb Bud Is Through Proliferation Of The Mesenchyme Core.  AER – APICAL ECTODERMAL RIDGE Critical for limb bud outgrowth Orchestrates limb development Proximal → Distal AER keeps the mesenchyme that is immediately underlying it undifferentiated. Undifferentiated mesenchyme proliferates → Elongation Proximal mesenchyme begins to differentiate into constituent tissues as it is too far away from the AER to receive the signals to stay undifferentiated. Finally AER induces development of the digits within the hand/foot plates and regresses.  ZPA–ZONE OF POLARISING ACTIVITY The ZPA is a signaling centre located at the posterior base of the limb bud. It is responsible for generation of asymmetry in the limbs. Controls both patterning and maintains the AER.  DORSOVENTRAL PATTERNING The AER marks the boundary between dorsal and ventral limb ectoderm. Ectoderm exerts “Dorsalising and Ventralising” influences over the mesenchyme core.  Axes In Limb Development And Control Of Axial Specification Superior Anterior–Posterior – ZPA *Postnatal Proximal-Distal – AER Posterior Dorsal-Ventral - Ectoderm Anterior Inferior  HAND AND FOOT PLATES Digital Rays Mesenchyme condensations within plates forming cartilaginous models of the digital bones AER breaks up and it maintained only over the tips of the digital rays. Interdigital spaces are progressively shaped by Apoptosis.  FORMATION OF BONES Signals from the AER to remain undifferentiated stop Lateral plate mesoderm condenses and differentiates Cartilage model forms Endochondral ossification Primary and Secondary ossification centres  MUSCULATURE Myogenic precursors migrate into the limbs from the somites and coalesce into two common muscle masses around the newly formed skeletal elements.  Ventral – Flexor  Dorsal – Extensor Individual muscles then split from the common masses.  ROTATION OF THE LIMBS Both upper and lower limbs have extensor and flexor compartments, but they are on the opposite sides. As the limbs extend ventrally, they rotate. Upper limb – Rotates Laterally – Thumb is lateral Lower limb – Rotates Medially – Big toe is medial Before rotation – Thumbs up, elbows out / Soles facing in, knees out After rotation – Thumbs out, elbows down / Soles down, knees up  LIMB INNERVATION Upper limb bud appears opposite the caudal cervical spinal segments. Lower limb bud appears opposite the lumbar and sacral spinal segments. Spinal nerves enter the limb bud early in its development. Without this innervation, development stops.  BRACHIAL PLEXUS Muscles are compartmentalised and Nerves grow into common muscle masses. The anterior divisions for the anterior (ventral) compartment regroup to form medial and lateral cords. Therefore medial and lateral cord branches supply flexors. The posterior divisions for the posterior (dorsal) compartment regroup to form the posterior cord. Therefore the posterior cord branches supply extensors.  NAILS Nails develop from nail fields on the tips of the fingers. Nail fields migrate onto the dorsal surface, bringing their nerve supply with them. Dermatomes and Myotomes Dermatome – Strip of skin supplied by a single spinal nerve Myotome – Muscle/Group of muscles supplied by a single spinal nerve  LIMB DEFECTS Upper limb affected more often than lower limb. The lag period of two days means they have different critical periods. Occurrence = 6/10,000 live births. Rare and usually hereditary but teratogen induced defects have been described.  COMMON LIMB DEFECTS Amelia Meromelia Complete absence Partial absence of one of a limb or more limb structures  COMMON LIMB DEFECTS Cleft hand and foot (lobster claw deformity) Club Foot  DIGIT DEFECTS Syndactyly Polydactyly Lack of apoptosis between Too many digits digits Digits fused S2 L1