Transcript EMBRYOLOGY - University of St Mark & St John

EMBRYOLOGY 1
YR1 SLT
EMILY BURTENSHAW
Introduction to embryology: 1
• Basic concepts and principles
• Influences on the growth and
development of the embryo
• Critical period concept
• The first 8 weeks: organogenesis
• Development of the nervous system,
skull, face, palate, pharynx and
tongue
Why study embryology??
Implications for SLT:
• A child can suffer from a disability
caused by a genetic disorder or a
teratogen (e.g. deafness; learning
disability…)
• As a therapist, is it important to know
the aetiology of the disorder? Why?
What could be the consequences?
• How do you think the parents might feel
depending on the aetiology?
Pregnancy: measured in
trimesters
In reality…
• Prenatal development: 3 stages of unequal
length.
• Germinal stage: days 1 to 14 ie from
conception to implantation
• Embryonic stage: begins at implantation
approximately 2 weeks after conception
and continues through weeks 3 to 8 (the
period oforganogenesis )
• The Foetal Stage: from 9th week to
the birth (40 weeks from last period,
or 38 weeks from fertilisation)
• Organs grow and continue
differentiation
• Increase in weight
Challenges in studying
embryology:
• Complexity
– CD rom and websites
• Personal experience
• Attitudes and beliefs
• Terminology:
http://www.wordinfo.info/words/index/info/s
earch_box/index
Carnegie stages
Pre-embryonic period
• The first 14 days
• Cells can still repair themselves
• If damaged
either repairs or dies
(spontaneous miscarriage)
Critical period:
A=embryonic period; B= foetal period
http://www.embryology.ch/anglais/iperiodembry/malfoacqui0
1.html
What can affect prenatal
development?
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Chromosomal and genetic factors
Teratogens: maternal disease, drugs
Mutagens: radiation
Other maternal influences on
development:
• Diet, age, chronic illness,
environmental hazards and maternal
emotions
Chromosomal & Genetic
factors
• 90% of abnormal embryos are
spontaneously aborted
• Only 1% of live newborns have a
genetic abnormality
Chromosomal abnormalities
• 23 pairs of chromosomes
• Over 50 different chromosomal
abnormalities
• Too many chromosomes
• Too few chromosomes
• Vast majority of chromosomal
abnormalities are lethal (spontaneous
abortion)
eg:Down Syndrome
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Three copies of chromosome 21
1 in every 600 birth
Learning disability (IQ averages 50)
May have congenital eye, ear, heart
defect
• Distinctive physical features:
protruding tongue, short limbs,
slightly flattened nose…
Eg Angelman’s syndrome and
Prader-Willi syndrome
• Genetic defect on chromosome 15
(maternal in case of Angelman’s
syndrome, paternal in case of
Prader-Willi syndrome)
eg: Klinefelter’s Syndrome
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Sex chromosomes: XXY (boys)
1/500 births
Poor coordination
May have mild learning difficulties
Turner’s syndrome
Genetic disorders
• Through dominant or recessive genes
• Eg Cystic fibrosis
• Phenylketonuria (PKU)
– Lack of enzyme to digest food containing amino
acid phenylalanine (e.g.milk)
– Phenylpyruvic acid accumulates in the body and
attacks the developing nervous system
– Hyperactivity; learning difficulties
– Test routinely given at birth
– Treatment: diet
Mutagens:
• Physical or chemical agents eg ionising
radiation, certain chemicals
• Act on the DNA in cells, especially
during cell division
• Alters the genes
• Mutation can be passed on, if it
occurs in the gametes
Teratogens
• Any disease, drug or other environmental
agent that can harm the developing embryo
or foetus.
• The effect is worse on a body part when
that structure is forming or growing
rapidly
• Critical period: Period when a body part is
most sensitive to teratogenic agents
Maternal disease
• Rubella
– Most dangerous in first trimester
– Blindness, deafness, cardiac
abnormalities, mental retardation
• Syphilis
– Most harmful in middle or later stages
– Miscarriage, serious eye, ear, bone, brain
damage
• Toxoplasmosis
– ¼ adults have this mild disease – similar to
common cold.
– Parasite (cat)
– Powerful teratogen
– Serious eye or brain damage, induce
spontaneous abortion
Drugs
• Thalidomide
– In 60s
– Drug against sickness given to pregnant
women in first trimester
– Violent teratogen
– Badly deformed eyes, ears, nose, even
missing limbs
• Smoking
– Low birth weight (less than 2.5 kg)
– Nicotine constricts blood vessels 
Reduces blood flow to placenta
W Eugene Smith, 1972
• Alcohol:
Foetal alcohol syndrome (FAS):
– Microcephaly
– Malformation of heart, limbs, joints, face
– Smaller
– Lower IQ - More likely to have learning
difficulties (IQ < 85)
– Even low drinking, social drinking
Pregnancy testing
• Human chorionic gonadotrophin
• Secreted by chorion of the placenta
from day 8 post fertilisation
– Detected by test kits 14 days after
conception
Foetal assessment:
• Chorionic villus sampling (weeks 1012)
• Amniocentesis (weeks 14-16 plus 4
weeks for results)
• Ultrasound (16-20 weeks)
Iterative processes in
embryological development:
• Cell division (proliferation)
• Cell adhesion
• Separation of cell sheets to form
cavities
• Cell migration
• Cell differentiation
• Cell induction
Apoptosis:
Birth defects:
• Cell proliferation : embryo/organ system
vulnerable to genetic or environmental factors
• Cell migration: cells move into position; can be
affected by matrix through which they travel
• Cell differentiation: cells assume their ultimate
form or phenotype – less vulnerable to insult
♫♪Let’s start at the very
beginning♪♫
Embryonic development:
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Weeks 1 and 2:
Fertilisation
Implantation
Formation of the placenta and the
early embryo
Fertilisation to implantation:
SYMBRYO video
Day 1
Genes ‘switch on’
Day 2
8 to 16 cell stage (morula)
Day 2
Day3
Development of the blastocyst:
Blastocyst stage:
Three sources of embryonic stem cells:
Day 14:
"It is not birth, marriage, or
death, but gastrulation,
which is truly the most
important time in your life."
Lewis Wolpert (1986)
GASTRULATION: a process that lasts 2 weeks
caudal
DAY 15: 0.2mm
Buccopharyngeal
membrane
LEFT
RIGHT
Primitive node
Primitive streak
rostral
The fate of the 3 germ layers:
• Ectoderm: CNS, PNS, epidermis, hair,
nails, sensory epithelium (nose, ear,
eye)
• Mesoderm: part of skull, muscles,
vertebrae, urogenital system, serous
membranes, body wall, limbs
• Endoderm: gut tube and its
derivatives; glands, lungs, liver, gall
bladder, pancreas
Birth defects originating
during gastrulation:
• situs inversus
• Teratoma (formed from epiblast
cells – contain hair, skin, bone,
liver etc cells)
• Caudal dysgenesis
Weeks 3 to 4
• Formation of the neural and gut tubes
• Embryo transformed from a
trilaminar disc into something more
recognisable!
Neurulation:
• Formation of neural plate
• Elevation and curling of lateral edges
• ‘zippering’ and formation of the
neural tube
Formation of the notochord:
Folding of the embryo:
• Dorsal surface – formation of the
neural tube
– ‘zippering’ effect
– cervical first, then caudally
• Ventral surface – formation of gut
tube and body cavities
• Body stalk – eventual umbilical cord
epic neurulation
neurulation video
Final destination neural crest cells:
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Connective tissue and bones of the face and skull
C cells of thyroid gland
Septum of the heart
Odontoblasts
Dermis in the face and neck
Dorsal root ganglia
Sympathetic chain and pre-aortic ganglia
Parasympathetic ganglia of the GI tract
Adrenal medulla
Schwann cells
Glial cells
Arachnoid and pia mater
melanocytes
Week 3: 0.5 mm
Formation of the umbilical ring:
• Proliferation and differentiation of
mesoderm
• Causes ventral folding along sides of
embryonic axis; amnion surrounds embryo
• Formation of gut tube
• Brain grows
• Head and tail folding
• ‘purse strings’ effect
• http://www.embryo.nl/anim%20craniocaud
ale%20kromming.mv
Ontogeny
replicates
phylogeny
Neural crest cells…
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Leave cranial area BEFORE fold closure
Leave the spinal area AFTER closure
Start as ectoderm
Change to loose connective tissue (called
mesenchyme)
• Migrate to destination
• Differentiate into a wide variety of cell
types
In the head and neck region:
• Form ganglia of the cranial nerves
• Connective tissue and some of the
bones of the skull and face
• Dermis in the face and neck
• Odontoblasts
• Arachnoid and pia mater
• Glial cells
Day 17
Fate of the mesoderm:
• Paraxial mesoderm forms paired
somites from occiput caudally along
the length of the neural tube
• In the head region, somitomeres form
part of skull, muscles, vertebrae, and
dermis of the skin.
Development of the Skull:
• Neurocranium (protective covering of the
brain) derived from paraxial mesoderm
– Membranous portion (flat bones)
– Cartilagenous portion = Chondrocranium (base
of the skull)
• Viscerocranium (skeleton of the face – the
‘middle third’ - & including the mandible)
derived entirely from neural crest cells
Week 4: 4-5mm
viscerocranium
Development of the brain vesicles:
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Cranial end of neural tube expands
Neural tube closure complete in week 4
Brain vesicles form the future brain
Anterior vesicle: prosencephalon
This subdivides: telencephalon (future
cerebral hemispheres) and diencephalon
(optic and thalamic tissues and other
structures)
Primary Brain Vesicles
(rhombencephalon)
Further development of the
NS
• The nervous system continues to
develop and changes occur up until
the early 20’s
• The main changes include:
• Myelination
• Formation of synapses
• Synaptic pruning
• Apoptosis
Synaptic pruning:
• Role of microglia is to…..?
• They also pluck off or ‘prune’ some of
the synapses between neurons
• In conjunction with apoptosis of
neurons, ensures that only the mostused ie strongest connections, remain
• Keeps the brain operating efficiently
Synaptic pruning occurs:
• Prenatally
• In childhood
• At puberty
• “use it or lose it”
• Learning causes synaptic connections
to increase in strength
Glial cells in the CNS
Manatomy.com
Implications for SLT:
• Aberrant synaptic pruning may be at
the root of MND, MS
• AD – by the time it is identified,
people have lost over HALF their
synapses
• Children with ASD have increased
cerebral volume, ? ? have not
undergone the same extent of
synaptic pruning?
Myelination:
• Starts late in embryonic development
and continues into adolescence/early
adulthood
• Forebrain the last part to complete
myelination
• Q what is the function of the myelin
sheath?
THE SPINAL CORD
• Extends from the foramen magnum to the
level of the second lumbar vertebra.
• Shorter than the vertebral column because it
does not grow as rapidly during embryonic
development.
• Because the spinal cord is shorter, spinal nerves
do not always exit the vertebral column at the
same level as their origin in the spinal cord.
By the end of the 4th week:
• Neural folds have closed
• Head region distinguished by
presence of 3 brain vesicles
• Lens and otic placodes for eye and
ear development;
• Primitive oral cavity (stomatodeum)
• 3 pairs of pharyngeal arches
Week 4: 4-5mm
Formation of the pharyngeal arches:
• Neural crest material grows from the
rhombencephalic region
(rhombomeres)
• Migrates and forms 6 paired bands
• These form the pharyngeal arches
• Each is accompanied by its own
artery, nerve and cartilage
First arch deformities:
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Micrognathia
Cleft palate (indirectly)
Conductive hearing loss
External ear malformations
Abnormal neural crest migration/differentiation:
Treacher-Collins
Robin sequence
Goldenhar syndrome
• First AND second pharyngeal arches
malformed
• Many other syndromes
• http://www.headlines.org.uk/
FORMATION OF THE FACE:
formation of the face
FORMATION OF THE PALATE:
formation of the palate
detailed formation of the face and palate
6 weeks
8 weeks
8 weeks
10 weeks
Week 6: 8mm
Your task:
• To understand how the pharyngeal
arches develop and contribute to the
structures of the face and palate
• When you study the head and neck in
the following weeks, to relate the
fully developed structures (surface
anatomy) to their development in
utero