Week 13, Monday

All of this will be on the final, but not on our test on Thursday

Limb Development ppt

Week1:

• Fertilization of the ovum by a sperm

• Cleavage

• Fermoation of the blastocyst

• Implantation Of y gore into the wall fo the uterus

• Differentiation of trophoblast cells

Week 2:

• Completing of implantation

• Rapid proliferation and differentiation fo the trophoblast

• Formation fo the primary umbilical vesicle, amniotic cavity and bilaminar embryonic disc

Week 3:

• primitive streak formation

• Proliferation and migration of epiblast cells into the space between the epiblast and hypoblast (the Trilaminar disc)

  • These migrating cells form the mesodermal

  • The non-migrating epiblast forms the ectoderm

• Hypoblast cells form the endoderm

Week 4:

• folding in the median and horizontal planes converts the flat trilaminar embryonic disc into a C-shaped, cylindrical embryo

Limb Bud Development:

• Begins at the end of the 4th week of gestation

  • Appear as outpockets of the venrolateral body wall

  • Forelimb (upper li,b) buds appear first, and the Hindlimb (lower limb) buds appear 1-2 days later

• The limb b us initially consist of mesenchymal core derived from parietal (somatic) layer of lateral plate mesodermal, which will form the bones and CT of the limb

• This core is covered by a layer of ectoderm

• The bud continues to grow through the proliferation of cells, and the ectoderm at the distal border thickens to form the apical ectodermal ridge (AER)

  • AER forms a progressive zone that influences the limb development to proceed Proximodistally

  • Directs the overall shape of the future limb

Hand- and Footplates:

• by the 6th week, the terminal portion of the bud becomes flattened to form the hand- and footplates

Digit formation:

• Mesenchymal cells begin to condense to form cartilaginous digital rays

• The digits form when apoptosis in the AER separates its ridge into parts

• Additional cells death occurs between the rays to form 5 digits

Limb Rotation:

• the limbs rotate in opposite directions during the 7th week

  • Upper limb

    • Rotates 90* laterally

    • Extensor muscles lie on the posterior side, thumb positioned laterally

  • Lower Limb:

    • Rotates 90* medially

    • Extensor muscles lie on the anterior side, great toe positioned medially

Cartilage formation:

• once the shape of the hands and feeet are established, the Mesenchymal condenses and the cell differentiate into chondrocytes

• The first hyaline cartilage models form the 6th week

Ossification:

• the bones of the extremities undergo endochonral ossification beginning at the end of the embryonic period

• The primary centers of ossification are present in all long bones of the limbs by the 12th week

Muscle development:

• lateral plate mesoderm: gives rise to CT, bone and some blood vessels

• Myotonic cells (myoblasts) in the myotome part of the somite migrate to form the hypomeres (hypoxia mm.) and epimeres (epaxial mm)

  • Myoblast migration:

    • Migrate from the myotome part of the somite into the limb

• Limb muscle:

  • After migrating, myogenic tisue forms te ventral (flexor) and dorsal (extensor) muscle masses

Somite levels associated with developing libs:

• the li,b muscles are derived from the myotome part of the following somites

  • Upper li,b

    • C4-T2

  • Lower limb

    • L1-S2

Innervation:

• motor neuron atonal sprouts form ventral horn cells migrate from eh Sural tub,a dn follow the migrating mass of muscle cells

• Motor neurons enter the limb buds during the 5th week, and are quickly followed by sensory neurons

• Neural crest cells (precursors of Schwann cells) then migrate and surround the motor and sensory fibers of the limbs

Dermatomes:

• Dermatome patterns result form rotation and differential growth of the limbs

• Adjacent Dermatomes overlap one another by approximately 50%, therefore, damage to a single nerve may only involve a small part of the dermatome

• Actual distributions will vary from patient to patient

• The distribution of peripheral cutaneous nerves carry multiple spinal levels and will therefore supply many dermatomes

Limb Malformations:

• Occur 6 in 10,000 live births

• Most are caused by genetic defects

• ~3:1 upper limb v lower limb

  • Meromelia

    • Partial absence of a limb

  • Amelia

    • Complete absence of a limb

  • Micromelia

    • All segments are present, but are shorter than normal

• Syndactyly (webbing)

  • Dysmorphology results from AER dysfunction and failure in programmed cellular death

• Polydactyly

  • Dysmporphologies results from AER dysfunction

  • More than 4 AER zones

Genetic Control of Limb Development:

• the position of limb development is controlled by HOX genes

• Retinol acid also plays a roll in lib development by acting as an intracellular signaling molecule