Embryo Limb Development Flashcards
describe limb organization:
-skeletal elements
-skeletal muscles
fibrous connective tissue
neurvasculature
ski
skeletal elements: centrally positioned bones with a specific pattern in each limb segment
skeletal muscles: surround and attach to the skeletal elements. typically grouped into flexor and extensor compartment
fibrous connective tissue: subdivides limb into compartments
neurovasculature structures: specific pattern of nerves, arteries, and veins found within each limb and limb compartment
skin: covers the surface of each limb
how are limbs developed? what layer of gastrulation?
what week?
On the sides of the embryo, limb fields of SOMATIC INTERMEDIATE MESODERM secrete FGF-8 —this induces FGF-10 in the LATERAL mesoderm
FGF-10 induces the expression of FGF8 in the ECTODERM
so in 4th week the limb fields give rise to limbs
what are limb buds and when are they visible?
-what do they consist of?
visible at the end of the 4th week. small outpecketings from the VENTROLATERAL wall.
-consist of mesenchymal core (from the lateral plate mesoderm) covered by a layer of surface ectoderm
somatic mesoderm vs somitic mesoderm?
somatic mesoderm gives rise to the connective tissue and cartilage of bone
somitic mesoderm from somite hypaxial myotome gives rise to the muscle of the limb
what is the AER
apical ectodermal ridge : thickening of the surface ectoderm at the apex of the limb bud
-it causes the adjacent mesenchyme to remain as a population of undifferentiated, rapidly proliferating cells
limb elongation occurs where?
-as the limb grows, cells further from the AER will differentiate into cartilage and bone.
occurs in proxmial-distal manner due to epithelial mesenchymal interactions between the AER and the adjacent mesenchyme
when does tissue formation and limb organization begin?
week 5
tissues form and myoblasts (somite derived) will migrate into the limb to form muslces and neurons will develop
what factors does the AER secrete to promote limb elongation?
what are proximal distal patterning signaling molecules?
AER secretes
FGF 2, 4, 8 to promote limb elongation
proximal-distal signaling molecules are Wnt and BMP (remember they were in neurulation for ventralizing factors released from the lateral plate)
and HOx containing genes
limb development occurs simultaneously in what 3 lines / axes:
anterior-posterior
ventral-dorsal
proximal- distal
describe limb development around the proximal-distal axis
AER and adjacent somatic mesoderm are responsible
what happens if you remove or duplicate AER
removal causes limb truncation
duplication causes segment duplication
what dictates whether a segment is lower or upper. how do we know this?
Progress zone mesenchyme (rapidly undifferentiated proliferating cells near the AER)
experiment: subsitutzed PZM of leg for wing and got a leg so PZM dictates whether a limb segment is upper or lower
what dictates development in the anterior-posterior axis
-digits develop in this axis
preaxial and post axial borders from along this axis.
-at the posterior side of the limb you find MESODERM with zone of polarizing activing or ZPA.
ZPA produces SHH with a retinoid quality which directs organization of limb along the anterior-posterior axis
-SHH induces asymmetric expression of Hox genes which mediate the pattern of digit formation
think ZPA–SHH—HOX
what causes polydactyly
- an additional ZPA like signal can set up a secondary focus of Hox gene expression.
- secondary site of Sonic hedgehog expression in anterior limb bud mesoderm si though to cause polydactyly
this is in anterior-posterior limb development
describe dorsal-ventral axis development
this is for skeletal muscles, neuromuscular and limb compartmentalization
-mediated by asymmetric expression of genes in the dorsal and ventral ectoderm.
skeletal muscle precursors are derived from SOMATIC MESODERM and give rise to myoblasts which are later dorsal and ventral pre muscle masses
