Ectodermal Derivatives

Overview

4^th week, neuralation converts neural plate Þ neural tube

Neural tube Þ brain, spinal cord and neural retina (CNS)

Neural crest cells detach from neural folds and migrate in the embryo Þ neurons and glia of the PNS and pigment cells in the trunk

Neural crest cells Þ facial bone, connective tissue and adrenal chromaffin cells

Thickenings of the ectoderm (placodes) Þ lens of the eye and portions of the ear

Formation of the Neural Tube and Origin of Neurons with Cell Bodies in the CNS

Notochord induces overlying ectoderm to form neural plate, consisting of neurons and glial cells, on day 18

neural tube forms from this, consisting of:

central canal

ventricular cells (Þ ventricles and CSF)

pial surface (Þ outer covering of brain)

Neural tube closes rostrally on day 24, caudally on day 26

if rostral remains open Þ anencephaly

if caudal remains open Þ spina bifida

Origin of Neurons with Cell Bodies Outside the CNS; The Neural Crest Þ PNS

During formation of neural groove, neuroectoderml cells migrate to lateral side of the neural tube

In the trunk, neural crest cells do not leave until after the neural tube has formed

Neural crest cells migrate throughout embryo and give rise to primary sensory neurons of the dorsal root ganglia and cranial nerve ganglia, autonomic neurons of the sympathetic and parasympathetic ganglia, and Schwann glial cells in peripheral ganglia

Thus, give rise to sensory neurons, autonomic neurons, pigment cells, kidney, chromaffin cells

also gut and parasympathetic neurons, bones in face, skin of face

Migration bounded to the migration of somites

Placodes – thickenings of ectoderm, induced by neural tubes Þ give rise to neural cup Þ lens (lens placode)

The Caudal and Intermediate Neural Tube Develops Into the Spinal Cord

Exterior is marginal zone – consists of axons (white matter)

Interior is mantle – consists of neuron cell bodies (grey matter) – most neurons formed in first 3 months

dorsal – alar plate (sensory)
ventral – basal plate (motor)
ependymal layer – neurons and glia in middle

Placodes – thickening of surface ectoderm induced by underlying neural ectoderm

Neural crest cells migrate out of neural tube – form bones and nerves of face, muscle, dermis, pigment cells

CNS (brain, spinal cord, neural retina) and PNS (homeostasis, sensory information, sensory and autonomic gangia)

Overall Organization of the Brain

Anterior (rostral) end divides into three vesicles (forebrain, midbrain, hindbrain) at about 3 weeks development

By 5 weeks, five vesicles form, which give rise to adult structures:

forebrain becomes telencephalon (cerebral hemispheres) and diencephalon (thalamus and neural retina)
midbrain becomes mesencephalon (midbrain)
hindbrain becomes metencephalon (pons and cerebellum) and myencephalon (medulla oblongata)