limb development Flashcards
evolution of tetrapod limb?
fin fold/wolffian evolved into pectoral fin then chiridium - struts
how does the limb bud form
precursors of skeletal elements and tendon sheaths, muscle and vessels migrate, limb and girdle bone.
how does the limb bud elongate
proximo-distal differentiated is mediated by the apical ectodermal ridge as limb lengthens the AER induces the fate of the underlying tissue
what is the limb composed of
a stylopod, zengopod and autopod
what happens with progressively earlier ablasion of AER
causes loss of more proximal elements
late ablasion?
no autopod
early ablasion
no zengopod or autopod
how is the craniocaudal orientation generated
by zone of polarising activity lying caudally. grafting a ZPA on the cranial edge of the limb bud results in a mirror image duplication
what is polydactyly in cats caused by
ectopic medial expression of ZPA related genes
how is limb patterning achieved
apoptic digit separation is programmed cell death, number of digits is dependent on species
what is the condition where digits do not separate
syndactyly
what is the name of the mechanism of bone formation in most of the trunk and limbs
bone endochondral ossification
what are the 3 main stages of bone endochondral ossification
mesenchymal (fetal connective tissue) condensation > cartilage model > ordered mutlicentre process of ossification
outline the whole process of bone endochondral ossification 7
1 blasternal condensation 2 chondrification 3 hypertrophy 4 early mineralisation 5 vascular invasion 6 further lengthening of diaphysis 7 secondary centres in epiphyses
blasternal condensation
large tightly packed mesenchyme cells
chondrification
cells differentiate into chondrocytes - THE CARTILAGE MODEL
hypertrophy
chrondrocytes at centre will stop dividing and swell = interstitial growth. connective tissue around the model is compressed into perichondrium the inner surface of which is chondrogenic = appositional growth
early mineralisation
central chondroblasts start to mineralise the matrix. perichondral cells become osteoblasts > cortical bone collar > compact bone
vascular invasion
through cortical bone accompanied by osteoblasts. these lay down primary spongiosa which eventually becomes cancellous bone
further lengthening of diaphysis
chrondrocyte proliferation and hypertrophy, marrow cavity enlarges
secondary centres in epiphyses
same process: chondrocyte hypertrophy vascular invasion > ossification
what is the difference of the bone endochondral ossification in epiphyses
no marrow cavity in epiphyses. diaphysis thickens due to traction on periosteum. leaves thin discs of cartilage between epiphysis and diaphysis
what are growth plates
cause most prenatal and post natal growth and are found at each end of most long bones and vertebral centra (only in accessory carpal and tibular tarsal)
when do growth plates close
after maturity - bridged by bone, and are perforated by vessels leaving a radiological mark/scar
which species have 2 epiphysis in the embryo digit
MCIII, MTIII, P1 and P2 all have two
epiphyses in the embryo. Each only has one by birth – at the ‘fetlock end’.
These close between 6-18 months
Unclear if P3 ever has true epiphyses or growth plates (does in humans).
what are the functional types of growth plates
in compression e.g. radius, tension e.g. ulnar, ‘flake epiphysis’ late forming and variable e.g. iliac crest. evolutionary relics?
what are evolutionary relic growth plates
not true growth plates, but fusions between bones e.g. scapular
why is the scapular different?
distal parts are limb bone like but the blade is dermomyotome
what is the structure of growth plates
- reserve cartilage - hyaline 2 proliferation - columns form 3 maturation - stop dividing and swell 4 hypertrophy and calcification 5 cartilage degeneration with osteoblast invasion 6 osteogenic zone - mineralisation on the cartilage specules the metaphysis.
SHAFT LENGTHENS @ DIAPHYSIS
where do joints form
between pre existing gaps between blastemal condensations
outline joint formation
1 a mesenchyme joint plate infills the gap 2 joint plate cavitates and differentiates into capsule and synovial membrane 3 synovial cells are worn off the articular surfaces by fetal movements in utero
where are muscles formed
limb muscle precursors invade into the limb bud from the myotomes at epimere and hypomere
what is angular limb deformity
in fast growing animals and bones such as ulnar and tibia. growth is from the conical distal growth plates leading to a deformed limb of overgrowth and a slight bend in the bone
how are bones repaired after a fracture
have to let the bone remodel and regrow. controlled by cytokines and hormones
