Þ dye cells in embryo Þ follow the dye Þ if goes to heart, those cells were fated to goto the heart
Does development of the organ require other tissues?
Induction
: other tissues signal the organ to form
The heart is induced by endoderm and the node
Cultured fragments
Mesoderm
Þ no heart
Mesoderm + endoderm
Þ no heart
Mesoderm + node
Þ no heart
Mesoderm + endoderm + node
Þ ++++heart++++
What are the signaling molecules?
Do not know the signaling molecules yet
What are the receptors for the inducing signal?
Candidates have been identified
Induction of the Neural Plate
Are fates written in stone? No
Ectoderm is induced by the node and the mesoderm
Cultured fragments
Ectoderm
Þ no neural plate
Ectoderm + endoderm
Þ no neural plate
Ectoderm + mesoderm
Þ ++++neural plate++++
Ectoderm + node
Þ ++++neural plate++++
What are the signalling molecules?
Noggin and chordin
Receptors for these molecules could not be identified
BMP-4 made by ectoderm inhibits neural plate
Cultured fragments
intact ectoderm
Þ no neural plate
dissociated/reassociated ectoderm
Þ ++++neural plate++++
dissociated/reassociated + BMP-4
Þ no neural plate
intact ectoderm with inactive BMP-4 receptor
Þ ++++neural plate++++
Chordin and Noggin bind BMP-4 and prevent its action
In regions far from the node where noggin and chordin concentrations are low, BMP-4 can bind to receptor and prevent neural plate growth
Patterning the Neural Plate Along the Anterior-Posterior Axis
Neural plate is induced to form posterior CNS structures by retinoic acid made by the node and posterior mesoderm
Cultured fragments
neural plate
Þ forebrain (default)
neural plate + posterior mesoderm
Þ forebrain, midbrain, hindbrain, SC
neural plate + node
Þ forebrain, midbrain, hindbrain, SC
neural plate + retinoic acid
Þ forebrain, midbrain, hindbrain, SC
Retinoic Acid
lipid-like hormone made by node and primitive streak
crosses cell membrane and binds to retinoic acid receptors which are transcription factors that in turn regulate other transcription factors (Hox, Otx, and Emx genes)
The different Hox genes have different sensitivities to retinoic acid
the sensitivity to retinoic acid matches their expression in the embryo
the different sensitivities of Hox genes to retinoic acid give rise to their "nested expression" in the embryo
The Hox Code
Structures at different levels anterior / posteriorly develop because of the combinations of the Hox genes expressed there
Removal of Hox genes by mutations alters the code, and changes what parts form where (homeotic transformations)
Changing retinoic acid concentrations changes the Hox code, and what parts form where
Patterning the Spinal Cord Along the Dorsal-Ventral Axis
Motor = ventral; Sensory = dorsal
If a 2nd notochord is transplanted to a new location next to spinal chord, motor neurons develop next to grafted notochord
Notochord synthesizes a peptide growth factor called sonic hedgehog which induces motor neuron growth
