Development of the Nervous System Lesson 4

Development of the Brain n Adult brain structure product of… l Genetic instructions l Cell-to-cell signals l Interaction b/n child and external world n Early development l Before synaptogenesis l Activity-independent l Critical periods ~

Gastrulation & Neuralation n Gastrulation l Invagination of embryo l single sheet of cells  3 cell layers l Formation of notocord n Neuralation l Induction signals from notocord… l To neuroectoderm l Growth of neural precursor cells~

Neural Induction n Induction signals l From embryonic tissue l Modulate gene expression n Retinoic acid l Steroid hormone n Peptide hormones l Fibroblast growth factor (FGF) l Transforming growth factor (TGF) l Sonic hedge hog (shh) ~

Retinoic Acid – Jekyl & Hyde? n Vitamin A derivative l Too little  abnormal development l Too much  abnormal development n Inductive signal l Critical period l Localization ~

Sonic Hedge Hog (SHH) n Induction of motor neurons l Mutations  diseases n Holoprosencephaly (1/16 k births) l Prevents bilateral development l Division of hemispheres l facial features (cyclopean eye) n Medulloblastoma l Most common childhood tumor n Basal cell carcinomas of skin l 750 k/yr ~

Embryological Origin n Embryo has 3 layers l endoderm internal organs l mesoderm muscles & skeleton l ectoderm nervous system & skin ~

Embryological Origin n At 18 days l Neural plate forms l sheet of cells n At 20 days l Neural groove l folds rostral to caudal will form tube ~

Development of N.S. n At 22 days n Neural tube l CNS develops from walls spina bifida - caudal tube fails to close l as tube develops dorsal portion pinches off & forms...~

Development of N.S. n At 24 days n Neural crest l develops into all neurons of PNS n Mesoderm l somites  vertebrae & muscles ~

Differentiation n Specialization of structures n 3 primary vesicles l rostral end of tube l develops into brain n Prosencephalon  forebrain n Mesencephalon  midbrain n Rhombencephalon  hindbrain ~

Prosencephalon n Secondary vesicles form & separate l optic  retinas retina & optic nerve CNS not PNS l telencephalic  telencephalon l remainder  diencephalon ~

Other Primary Vesicles n Mesencephalic  mesencephalon l dorsal - tectum l ventral - tegmentum l tube - cerebral aqueduct n Rhombencephalic l rostral - metencephalon l caudal - myelencephalon l tube - 4th ventricle ~

Neural Tube Defects: Rostral n Neural tube fails to close n anencephaly l skull & brain partially or totally absent l perinatal mortality n Encephalocele l parts of brain protrude outside skull l mental disability depends on extent ~

Neural Tube Defects: Caudal n Spina Bifida l Cleft spine n Spina bifida occulta l 40% of Americans n Spina Bifida Manifesta l 1 in a 1000 births l 0.1% ~ Occulta

Spina Bifida Manifesta n Meningocele (*4%) l SC/nerves not in sac l less severe, repaired n Myelomeningocele (*96%) l SC/ nerves in protruding l paralysis, incontinence, learning disabilities l hydrocephalus can cause severe brain damage ~

Neural Tube Defects: Prevention n Possible genetic component l reoccurrence in families l ethnic/racial relationship n Folic Acid (Folate) Deficiency l 50-70% of cases preventable l CDC guidelines not planning pregnancy: 400  g/day planned pregnancy: 4000  g/day under direction of health care provider ~