Unit 2

• Zygote, single cell fertilized egg, undergoes cleavage, which is rapid cell division to generate a blastula
• Blastula is a hollow ball containing thousands of cells that undergoes gastrulation, which is where the cell invaginate
• Gastrula contains 3 tissue source layers

  • Endoderm
  • Mesoderm
  • Ectoderm
• Endoderm: epithelial lining of digestive tract and lungs, endocrine glands, liver
• • Mesoderm: muscle, bones, reproductive system
• • Ectoderm: nervous system, skin
• • Neural plate (strip of ectoderm), precursor of PNS and CNS
• • Neurulation: Neural plate folds and fuses into a tube which sits below (dorsal)  the rest of the ectoderm, which will become skin
• • Neural Crest cells, differentiate from ectoderm and migrate outward which will become all neurons will cell bodies in the PNS
  • E.g. Schwann cells, dorsal root ganglion cells
• • Somites, mesodermal cell precursors to muscle and cartilage
• • Prosencephalon
  • Gives rise to the telencephalon and the diencephalon
• Mesencephalon is midbrain
• Rhombencephalon is hindbrain
• Tectum is roof and tegmentum is floor
• Induction is when cells can direct the development of neighboring cells
• • DLB induces the differentiation of ectoderm into neural plate
• • When ectoderm cells are separated, they become neural
• • Addition of BMP triggers the differentiation of ectodermal tissue to epidermal cells
• BMP signaling represses neural fate and leads to epidermal fate
• • Inhibition of BMP inhibit the differentiation of ectodermal tissue cells to neural cells
• • BMP inhibitors in the dorsal ectoderm de-repress neural fate
• The concentration of BMP has to be high enough to stop the inhibit the neural
• BMP, chordin/noggin are morphogens
• Morphogens are

  • Signaling molecules that spread from a localized source and form a concentration gradient that influences the fate of cells in developing tissue, often by influencing the activity of transcription factors, which alter the repertoire of genes that are expressed in a cell
• Chordin & Noggin are BMP Inhibitors
• Morphogens pattern developing tissue, signaling molecules with non-uniform distributions influence cell fate
• BMP signaling can activate transcription factors, which can change cell’s activities, shape and etc.
• Sonic hedgehog is a morphogen
• Shh is secreted by the notochord and floor plate, producing a concentration gradient
• Conc. gradient of a single molecule Shh can regulate the patterned differentiation of many cell types
• • High concentration of shh = more motor neurons
• • Low conc. Of shh = more interneurons
• • If shh were dorsal, then the dorsal would be efferent and ventral would be afferent
• • At each concentration of Shh, expression of different transcription is either repressed or induced, contributing to cellular differentiation along D-V axis
• • FGF8 (morphogen in telencephalon) is a growth factor protein that is secreted rostrally, which establishes a concentration gradient
• FGF8 reacts with Pax6 and Emx2
• • Pax6, Emx2 are transcription factors and the GF8 gradient results in opposing expression pattern
• Pax6, Emx2 contribute to overall anterior-posterior patterning of the cortex
• • Pax6 -> motor and somatosensory
• • Emx2 -> auditory and visual