Presentation 24

Neural Development

Lecturer: Dr Lucy Patstonlpatston@unitec.ac.nz

Overview of this PPT and Learning Objectives

Overview• Fetal brain development• Neural tube formation

Learning Objectives• Understand and be able to

reproduce (Ha!) the process of brain development during the three stages of fetal development

• Genetic and environmental influences act on cells throughout development of nervous system• Processes: cell growth, migration, differentiation• Even cell death, axonal retraction help to create the mature brain• Some processes completed in utero, others in first years after birth

(by no means “ready to go” at birth! Gazelle anecdote)

Introduction

• We will only be considering the brain development of babies

• Humans undergo 3 developmental stages:• Pre-embryonic• Embryonic• Fetal (Major brain development occurs very early in this stage)

Developmental Stages in Utero

• Conception to day 14• Fertilization (usually fallopian tube)• Cell begins divisions -> solid sphere

cells• Blastocyst (D) opens into a cavity• Outer layer becomes placenta,

inner cell mass becomes embryo• Implants in uterus (day 7), inner

cell mass forms embryonic disk of ectoderm and endoderm (future brain)!

Pre-embryonic Stage

• Day 15 to end of 8th week• (Organs are formed)• Ectoderm develops into sensory organs, epidermis and nervous

system• Mesoderm develops into dermis, muscles, skeleton, excretory and

circulatory systems• Endoderm develops into gut, liver, pancreas and respiratory system

Embryonic Stage

• Beginning of 9th week to birth• Nervous system develops more and myelination begins

Fetal Stage

• During embryonic stage nervous system tissue coalesces to form a neural tube running down the back of the embryo• When tube closes (right to the ends) brain formation begins• Neural tube formation (Day 18-27)• Brain formation (Day 28 ->)

Formation of Nervous System

• Nervous system begins as longitudinal (head to “tail”) thickening of ectoderm – the neural plate

Neural tube formation (Day 18-27)

• Midline of neural plate moves toward interior, creating the neural groove• Somites begin to form

Neural tube formation

• When folds touch, neural tube is formed• The neural crest separates from the tube and from the remaining

ectoderm

Neural tube formation

The neural crest is a mass of tissue that differentiates into: dorsal root ganglia, spinal nerves, ganglia of cranial nerves, cranial nerves, ganglia of ANS, adrenal medulla and meninges

• Neural tube first closes in cervical region then “zips up” front to back, leaving open ends (neuropores)• (Superior (head) neuropore closes Day 27, inferior (tail) neuropore

Day 30)

Neural tube formation

• Somites spherical clusters of cells adjacent to mesoderm• Anterior part (sclerotome)

becomes vertebrae and skull (e.g., “somite 1” becomes occipital bone; “somite 10” becomes C6)• Posterior part (myotome)

becomes skeletal muscle• Lateral part (dermatome)

becomes dermis

Somites (developing structures)

• By Day 26 the neural tube differentiates into:• Mantle layer: which will become gray matter• Marginal layer: which will become axons of cells in mantle layer and

glial cells• Ependymal layer: which will become the lining of the central canal of

spinal cord and ventricles• (the hole in the tube becomes the ventricles and the central canal)

• When tube and crest have developed both move inside embryo, remaining overlying ectoderm will become skin

Developing Structures

Ependymal layer

• Cells of mantle (middle) layer proliferate inside neural tube and start to separate into dorsal and ventral sections (look familiar?!)• Axons from cells in motor plate grow out of neural tube

and innovate myotome region of a somite

Developing Structures

• Neurons with cell bodies in motor plate become motor neurons (innervate muscles) and interneurons

• Motor plate becomes ventral horn of the mature spinal cord

• Association plate becomes dorsal horn of mature spinal cord

Somite

Adult nervous system

Fetal nervous system

• Neural crest separates into two columns (each side of tube)• Some neural crest cells become peripheral sensory neurons and grow

two “processes”, one connects to spinal cord, one to dermatome of somite

Developing Structures

Developing Structures

As foetus develops the spinal column grows faster than the cord

Cauda equina: The collection of lumbosacral nerve roots that extend inferior to the end of the spinal cord

Filum terminale: A continuation of the dura, pia, and glia connecting the end of the spinal cord with the coccyx