Presentation 24 Neural Development Lecturer: Dr Lucy Patston lpatston@unitec.ac.nz

Overview of this PPT and Learning Objectives Fetal brain development Neural tube formation Understand and be able to reproduce (Ha!) the process of brain development during the three stages of fetal development

Introduction 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)

Developmental Stages in Utero 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)

Pre-embryonic Stage 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)!

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

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

Formation of Nervous System 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 ->)

Neural tube formation (Day 18-27) Nervous system begins as longitudinal (head to “tail”) thickening of ectoderm – the neural plate

Neural tube formation 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 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 formation 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)

Somites (developing structures) 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

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 Ependymal layer

Developing Structures 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 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

Fetal nervous system Somite Adult nervous system

Developing Structures 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 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