Transcript EMBRYOLOGY

EMBRYOLOGY
YR1 SLT
EMILY BURTENSHAW
Introduction to embryology:
• 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
• 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
Critical period:
What can affect prenatal
development?
•
•
•
•
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 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)
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
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
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
Carnegie stages
Three sources of embryonic stem cells:
Day 14:
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:
•
•
•
•
•
•
•
•
•
•
•
•
•
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
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…
•
•
•
•
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
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
viscerocranium
Development of the brain vesicles:
•
•
•
•
•
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
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
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:
•
•
•
•
Micrognathia
Cleft palate (indirectly)
Conductive hearing loss
External ear malformations
formation of the face
formation of the palate
detailed formation of the face