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LEARNING OUTCOMES
1. Describe the induction of the neural plate by the notochord and the progressive formation of the neural tube
2. Explain the origin of the neural crest cells, their migration and eventual destinations
3. Show the segmental pattern of nerve development in the spinal cord and the relationship between nerve, and muscle derived from the myotome
4. Outline the segmentation of the brain
5. Describe the congenital malformation of the nervous system, e.g. spinal bifida, cerebellar hypoplasia and hydrocephalus.
6. Outline the development of the nasal passage and the mouth
7. Understand the development of the ear and the eye
NEURULATION AND CRANIO-FACIAL DEVELOPMENT
Ectoderm
Presumptive neural crest
Notochord
The notochord induces the overlying neuroectoderm cell layer to invaginate to form the neural tube
Represents paracrine signals
Neural tube
Neural crest
Neural tube closure begins near the rostral end of the embryo and progresses caudally
Neuro-epithelial layer already showing signs of elongation of cells (notochord not present)
Neural groove forming
Neural tube formed with overlying ectoderm and underlying notochord
DORSAL VIEW
http://www.med.unc.edu/embryo_images/
VENTRAL VIEW
Heart
Foregut
Hindgut
Non-fusedneural folds
Mouse 8 days
Mouse 9 days
Anterior neuropore
Mouse 9 days
caudal neuropore
The neural fold closes from a starting cervical location in both a rostral and caudal direction
Neural crest cells escape the neuroectoderm epithelium and migrate to diverse destinations
Neural crest cells leaving dorsal ectoderm (Gilbert)(Epithelial to mesenchyme transition)
DESTINATIONS OF TRUNK NEURAL CREST CELLS
1. Melanocytes 2. dorsal root ganglion neurones 3. autonomic ganglia neurones 4. Adrenal medulla 5. Submucosal nerve plexus of gut
1 2
3
54
FoxD3, Slug
Wnt6, ectoderm
BMPs
Neural Crest Cell Induction
Ectoderm
Neural crest precursors
Neural tube
Neural crest migration
• Slug activates factors inducing the dissociation of tight junctions
• Migrating cells follow cues from the extracellular matrix
• One set of proteins (fibronectin, laminin) promote migration while ephrin impedes migration (remember lectures on cell adhesion and control of cell division)
The spinal cord develops a segmentation pattern which reflects the pattern of somites.
SEGMENTATION IN THE SPINAL CORD AND PERIPHERAL NERVES
PIG - 38 DAYS
Dorsal root ganglion
Motor efferent
Dorsal horn
Ventral horn
PIG - TERM
Dermis
Muscle
A segmental reflex arc
Sensory neurone
Inter- neurone
1. The notochord produces Sonic hedgehog (Shh) and induces the ventral neural tube to become floor plate and produce Shh 2. The ectodermal cells produce members of the Transforming growth factor (TGF-) family and induce the dorsal neural tube to become roof plate and to start to produce the same proteins3. Two gradients are created of TGF- and Shh 4. Different concentrations of these proteins activate the expression of different sets of genes so that cells differentiate to become inter-neurones and motor neurones
TGF family in ectoderm
TGF family in roof plate
shh in notochord
shh in floor plate
Gradient of TGF family
Gradient of shh
Interneurones
Motor neuronesshh
Dorsal-ventral axis in the Neural Tube
The head also shows a rostral/caudal segmentation pattern but this is less regular and more complex than that of the trunk somites
3 VESICLE STAGE 5 VESICLE STAGE
CEREBRAL HEMISPHERES OLFACTORY LOBES
OPTIC VESICLES, PITUITARY, HYPOTHALAMUS, THALAMUS
FIBRE TRACTS BETWEEN ANTERIOR AND POSTERIOR BRAIN
CEREBELLUM - MUSCULAR COORDINATION PONS - FIBRE TRACTS
MEDULLA OBLONGATA - INVOLUNTARY COORDINATION
SEGMENTATION OF THE HEAD - REGIONS OF THE BRAIN
TEL-
DI-
MES-
MET-
MYEL-
II
VIII
*
FORE
MID
TELENCEPHALON
DIENCEPHALON
MESENCEPHALON
METENCEPHALON
MYELENCEPHALON
II AND VIII ARE CRANIAL NERVES INNERVATING OPTIC VESICLE AND OTIC VESICLES RESPECTIVELY (CIRCLED) * - THE NEUROHYPOPHYSIS WHICH GIVES RISE TO THE NEURAL COMPONENT OF THE PITUITARY
ANTERIOR NEUROPORE
VENTRICLES CONTAIN CEREBROSPINAL FLUID
SEGMENTATION OF THE HEAD REGIONS OF THE BRAIN
Mouse 10 days. http://www.med.unc.edu/embryo_images/
Faint evidence of rhombomere segmentation of met- and myel-encephalon
Di-
Tel-
The forebrain gives rise to the Tel- and Di-encephalon vesicles
Mes-The midbrain gives rise to the Mes-encephalon vesicle
Met-
Myel-
The hindbrain gives rise to the Met- and Myel-encephalon vesicles
CONGENITAL MALFORMATIONS OF BRAIN AND SPINAL CORD
1. Spina bifida Teratogenic factors can block the induction by the underlying notochord of the neural plate. This can lead to failure of closure of the neural tube in the extreme form of Spina bifida The development of the vertebral arches is disrupted and the arches fail to fuse along the dorsal midline giving rise to an open vertebral canal 2. Exencephaly Failure in the closure of the rostral neuropore 3. Cerebellar hypoplasia Viral infection affects cerebellar development 4. Hydrocephalus Poor circulation of cerebrospinal fluids in the aqueducts leads to cranial accumulation and the pressure build-up leads to skull deformation and neuroepithelial atrophy
Normal Spina bifida occulta full Spina bifida
Mouse, 10 days, lateral view
Arch 1: maxillary
Arch 1: mandibular
Arch 2
Human, 30 days, ventro-lateral view
Arch 1: maxillary
Arch 1: mandibular
Arch 2
Arch 3
The branchial arches are bilateral pouches of tissue separated by branchial clefts in the region of the pharynx
http://www.med.unc.edu/embryo_images/
Mouse, 9 days, section, from dorsal view
Mid-brain and vesicle
Oral cavity
Torn edge of oral plate
The branchial arches are separated internally by pharyngeal pouches and externally by branchial clefts
http://www.med.unc.edu/embryo_images/
AORTIC ARCH
MIDLINE
Laryngo-Trachealgroove
Floor of pharynx
1
2
3
Branchial arch
Branchial cleft
Pharyngeal pouch
SEGMENTATION OF THE HEAD THE BRANCHIAL ARCHES AND PHARYNGEAL POUCHES
EACH BRANCHIAL ARCH CONTAINS A CRANIAL NERVE AND AN AORTIC ARCH
THE FOUR PHARYNGEAL POUCHES CORRESPOND TO THE FOUR BRANCHIAL CLEFTS LATERALLY
The branchial arches and clefts and the juxtaposed pharyngeal pouches are a recapitulation of the respiratory anatomy of fish
There are 12 cranial nerves corresponding to the 7 somitomeres and 5 rostral somites of the head region
12 CRANIAL NERVES
GENERAL FEATURES
1. NERVES ORIGINATE IN BRAIN 2. OLFACTORY (I) AND OPTIC (II) ARE BRAIN TRACTS RATHER THAN TRUE NERVES. 3. USUALLY LACK OF UNION OF DORSAL AND VENTRAL ROOTS 4. MAY BE MOTOR OR SENSORY OR MIXED
CERVICAL NERVES
SEGMENTATION OF THE HEAD - THE CRANIAL NERVES
1
2
3
45
67
12
3 4 5
EYE MUSCLES
MUSCLES OF MASTICATION
FACIAL MUSCLES
PHARYNGEAL MUSCLES
CRANIAL NECK MUSCLES
LARYNGEAL MUSCLES
SEGMENTATION OF THE HEAD - CRANIAL NERVES, MOTOR EFFERENTS AND TARGET MUSCLES
TONGUE
7 SOMITOMERES
5 SOMITES
XIIXIIXIIXI
XIX
VIIVI
V
IV
III
III
ROMAN NUMERALS ARE THE CRANIAL MOTOR NERVES ARROWS FROM SOMITOMERES AND SOMITES INDICATE THE MIGRATION OF THE MYOBLASTS OF THE MYOTOME SOME CRANIAL NERVES HAVE SENSORY AFFERENTS I(OLFACTION), II(VISION), V(TOUCH), VII, IX, X (TASTE), VIII (HEARING AND BALANCE)
Motor cranial nerves follow their corresponding myotome to find their adult path
Mouse, 10 days, lateral view
Arch 1: maxillary
Arch 1: mandibular
Arch 2
Surface bulge of sensory ganglion of Cranial nerve V (trigeminal)
Surface bulge of sensory ganglion of Cranial nerve VII (facial)
The bulges of the sensory ganglia of cranial nerves innervating the branchial arches are visible on the surface
http://www.med.unc.edu/embryo_images/
SEGMENTATION IN THE HEAD CRANIAL NEURAL CREST CELLS
DERIVATIVES IN COMMON WITH TRUNK NEURAL CREST UNIQUE DERIVATIVES
1. SENSORY AND AUTONOMIC NERVE GANGLIA 2. SCHWANN CELLS OF PERIPHERAL NERVES 3. MELANOCYTES
1.BONE, DERMIS OF FACE 2. MENINGES OF BRAIN 3. CORNEA OF EYE 4. DENTAL PAPILLAE 5. CONNECTIVE TISSUE COMPONENTS OF BRANCHIAL ARCHES
Cranial neural crest cells give rise to structural components normally associated with the paraxial mesoderm in the trunk
In the facial region, neural crest cells contribute all of the skeletal and connective tissues with the exception of tooth
enamel
Arrows indicate the origin and destinations of neural crest cell populations.
http://www.med.unc.edu/embryo_images/
EYE NASAL PIT MAXILLARY DEVELOPMENT (from arch 1) STOMODEUM (mouth) TONGUE MANDIBULAR ARCH(arch 1) (mastication) HYOID ARCH (II) (facial expression)
FEATURES OF THE FACE AND THEIR ORIGINS - 1
1. Unusually, supporting tissue components of branchial arches and face derive from neural crest 2. Muscle contribution is from somitomeres (for example somitomere 4 gives rise to muscles of mastication) 3. Maxillary arch extends inwards to fuse with its bilateral partner and the nasal structures. It forms the bone of the upper jaw and the tissues of the upper lip 4. Mandibular arches fuse to form lower jaw 5. Failure of fusion of maxillary arches and nasal prominences gives rise to cleft lip and palate
The branchial arches contribute to features of the face with their tissue components deriving from both neural crest and myotome
NASAL PIT
LUNG BUD
MANDIBULAR ARCH TONGUE
NASAL CAVITY SECONDARY PALATE ORAL CAVITY TRACHEA OESOPHAGUS
1. Lateral walls of nasal cavity contain olfactory epithelium 2. the rest of nasal cavity is pseudostratified ciliated epithelium derived from the original ectoderm 3. The oral cavity develops partly from ectoderm and partly from endoderm. The fusion point between the two was the position of the (now degraded) oral plate
FEATURES OF THE FACE AND THEIR ORIGINS - 2
The epithelium of the oral cavity derives from both ectodermal and endodermal sources
1. The otic placode invaginates to form the otic vesicle which will become the inner ear 2. The splanchnopleure of the pharynx forms a diverticulum - the first pharyngeal pouch
THE OTIC SENSORY PLACODES - 1
NEURAL GROOVE
OTIC PLACODE
NOTOCHORD
NEURAL TUBE (HINDBRAIN)
PHARYNX
II
VIII
The otic placode is induced ectoderm which invaginates to become the cavity of the inner ear
Components of the middle and outer ear derive from the first pharyngeal pouch and first branchial cleft
THE OTIC SENSORY PLACODES - 2
GANGLION OF CRANIAL NERVE VIII
OTIC VESICLE / INNER EAR (from otic placode)
BONES OF MIDDLE EAR (from 1st pharyngeal pouch)
EXTERNAL EAR (from 1st branchial cleft)
AUDITORY TUBE (from 1st pharyngeal pouch)
Fish have just the inner ear as an organ of balance. The middle and outer ear evolved to receive and transmit sound waves
(A)
(B)
9 day mouse
10 day mouse
9 day mouse
http://www.med.unc.edu/embryo_images/
The otic placode invaginates to form an otic pit and finally the otic vesicle. Its surface aspect is dorsal to the 2nd branchial cleft
The neural tube in the region of the hindbrain induces formation of the otic placode (A) and then otic vesicle (B), dorsolateral to the pharynx
THE DEVELOPMENT OF THE EYE - 1
ROSTRAL NEUROPORE LENS PLACODE
FORE-BRAIN LENS VESICLE INNER/OUTER LAYERS OF OPTIC CUP (this neuroepithelial layer gives rise to the visual retina)
OPTIC STALK
The lens placode is induced ectoderm under the influence of the neuroepithelium of the optic cup
THE DEVELOPMENT OF THE EYE - 2
PRESUMPTIVE CORNEA IRIS
TEMPORARY FUSION OF EYELIDS
LENS
PIGMENTED RETINAL LAYER
NEURAL RETINAL LAYER
FIBRES OF OPTIC NERVE
DEVELOPING EYELID
The neuroepithelium gives rise to the pigmented and neural retinal layers of the visual retina
Mouse 8.5 days
http://www.med.unc.edu/embryo_images/
Mouse 11 days
Mouse 10 days
Neuroectoderm of optic vesicle inducing surface ectoderm to form lens placode
The invaginating lens placode pinches off to form the lens and invagination of the optic vesicle forms the optic cup connected to the brain via the optic stalk
Carlson BM (2003) Patten's Foundations of Embryology
Noden DM, de Lahunta (1985) A Embryology of domestic animals
McGeady TA, Quinn PJ, Fitzpatrick ES, Ryan MT (2006) Veterinary embryology
University of North Carolina web site: http://www.med.unc.edu/embryo_images/
REFERENCES
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