Download neural crests

Pharyngeal arches
The rostral and caudal pool
are observed before gastrulation
animation
Neural crests
formation
animation
In the facial region the neural crest cells form
midfacial and pharyngeal arch skeletal
structures and other tissues in this region such
as cartilage, bone, dentin, tendom, dermis, pia
and arachnoid matter, sensory neurons and
glandular stroma.
The neural crest cells
which originate in
neuroectoderm in the
region of three first
brain vesicles
(forebrain, midbrain
and hindbrain) migrate
ventrally into the
pharyngeal arches and
rostrally around the
forebrain and optic cup
into the facial region.
The mesenchyme of head region
is derived from paraxial and lateral plate
mesoderm, neural crests and ectodermal placodes
(thickened regions of ectoderm)
Somitomers (paraxial mesoderm) form the floor of the
brain case and a small portion of the occipital region as
weel as voluntary muscles of the craniofacial region,
the dermis and connective tissues in the dorsal region
of the head, meninges (caudally to
the prosencephalon)
The lateral plate mesoderm (in the rostral region
of the embryo) forms the arytenoid and cricoid
cartilages of the larynx as well as connective
tissue in this region.
The skull can be devided into:
- the neurocranium (forms a protective case around the brain)
- the viscerocranium (forms the skeleton of the face)
The neurocranium can b devided into two
portions:
- the membranous part - consisting of flat bones
- the chondrocranium – forms bones of the base of
the skull
The membranous portion of the skull is derived from
neural crest cells and paraxial mesoderm
The mesenchyme from these two sources invests the
brain and undergoes membranous ossification – and
form membranous bones that contain needle-like bone
spicules (which progressively radiate from primary ossification
centers toward the periphery).
During fetal and posnatal life, membranous
bones of neurocranium enlarge by apposition
(formation of new layers of bone tissue on the
outer surface) and by simultaneous osteoclastic
resorption from the inside.
The flat bones of
neurocranium continue to
grow after birth mainly
because the brain grows.
Microcephaly is usually an
abnormality in which the
brain fails to grow and the
skull fails to expand.
Many children with
microcephaly are severely
retarded.
At birth the flat bones of the neurocranium are
separated from each other by sutures - a narrow seams
of connective tissue.
At points where more than two bones meet, sutures
are wide and are called frontanelles.
Craniosynostosis – cranial abnormalities caused by
premature closure of one or more sutures.
The shape of the skull depends on which of the sutures
closed prematurely.
Early closure of the sagital suture results in frontal and
occipital expansion – and the skull becomes long and
narrow (scaphocephaly)
Premature closure of the coronal suture results in a
short, high skull – acrocephaly (tower skull)
Plagiocephaly – asymmetric craniosynostosis –
is a result of premature closure of coronal or lambdoid
sutures on one side only.
The chondrocranium of the skull
initially consists of a number of
separate cartilages.
Those cartilages that lie in front of
the rostral limit of the notochord
(which ends at the level of the sella
turcica) originate from neural crest
cells.
Those cartilages of chondrocranium
which lie posterior to this limit arise from paraxial mesoderm and
form the chordal chondrocranium.
The base of the skull is formed when
all cartilages of chondrocranium fuse
and then ossify by endochondral
ossification.
The viscerocranium is
formed mainly from the
first pharyngeal arch
- maxillary process (dorsal
portion of the 1st
pharyngeal arch) – gives
rise to the maxilla, the
zygomatic bone, part of
the temporal bone
- mandibular process
(ventral portion of the 1st
pharyngeal arch) – gives
rise to the mandible
At the begining of development the face is small
in comparison with the neurocranium.
During ontogenesis the jaws grow (mainly
because teeth formation) and paranasal air
sinuses develop – and the face loses its babyish
characteristic.
The pharyngeal arches
The pharyngeal (branchial) arches appear in
fourth and fifth weeks of human development,
consist the lateral wall of the pharyngeal gut
(the most cranial part of the foregut) and
contribute to the characteristic external
appearance of the embryo.
The pharyngeal arches consist of bars of mesenchyme
separated by deep clefts (grooves) from outer side and by
pouches from interior.
The pouches penetrate the surrounding mesenchyme, but
do not establiskh an open communication with the external
clefts.
In addition to mesenchyme derived from the paraxial and lateral
plate mesoderm, the core of each arch receives the neural crest
cells, which migrate into the arches to contribute to skeletal
components of the face.
The mesoderm of the arches gives rise to the
musculature of the face and neck – each arch is
characterised by its own muscular component
and has their own cranial nerve.
Wherever the miocytes migrate, they carry their
nerve component with them.
Each arch has also its own arterial component.
First pharyngeal arch
At the end of fourth week,
th e center of the face is
formed by stomodeum,
surrounded by the first pair
of pharyngeal arches.
When the embryo is 42
days old, the stomodeum is
surrounded by five
mesenchymal prominences:
frontonasal prominence,
two maxullary prominences
and two mandibular
prominences.
The maxillary process is a dorsal portion of the 1st
pharyngeal arch - extends beneath the region of the
eye and gives rise to the premaxilla, maxilla, zygomatic
bone and a part of temporal bone (though
membranous ossification)
The mandibular process is a ventral portion of the 1st
pharyngeal arch – contains Meckel’s cartilage –
mesenchyme surrounding this cartilage form the
mandible by membranous ossification.
Meckel’s cartilage finally disappears except for two
small portions – at dorsal end – which form the incus
and the maleus.
Muscular component of the 1st pharyngeal arch
forms the muscles of mastication, anterior belly
of digastric muscle, mylohyoid, tensor tympani
and tensor palatini – innervated by mandibular
branch of trigeminal nerve.
Dermis of the face is supply by ophtalmic,
maxillary and mandibular branches of the
trigeminal nerve.
Second pharyngeal arch
The Reichert’s cartilage - cartilage of 2nd
pharyngeal arch (hyoid arche) gives rise to the
stapes, styloid process of the temporal bone,
stylohyoid ligament, lesser horn and upper part
of the body of the hyoid bone.
Muscles of the 2nd pharyngeal arch (stapedius,
stylohyoid, posterior belly of digastric, auricular,
and muscles of facial expression) are supplied by
facial nerve
Third pharyngeal arch
The cartilage of the 3rd pharyngeal arch gives
rise to the lower part of the body and greater
horn of the hyoid bone.
From muscular component of this arch
originates stylopharyngeus muscles – innervated
by glossopharyngeal nerve.
Fourth and sixth pharyngeal arches
The cartilaginous components of those arches fuse and
form cartilages of the larynx ( thyroid, cricoid,
arytenoid, corniculate and cuneiform).
Muscles of fouth arch (cricothyroid, elevator palatini,
constrictor of the pharynx) are innervated by superior
laryngeal branch of the vagus.
Intrinsic muscles of the larynx are innervated by the
recurrent laryngeal branch of the vagus (the nerve of
sixth arch)
Pharyngeal clefts
Only first cleft contributes to the definitive
structureof the embryo – penetrates the
underlying mesenhyme and and gives rise to the
external auditory meatus.
The epithelium lining the bottom of the meatus
particitates in formation of the eardrum.
The cervical sinus – ectodermal epithelium of
2nd, 3rd and fourth clefts form common cavity
that lose the contact with the outside and finally
disappears.
Lateral cervical cyst
Treacher Collins syndrome (TCS) or
mandibulofacial dysostosis
found in about 1 in 50,000 births
a rare autosomal dominant congenital disorder characterized
by craniofacial deformities, such as absent cheekbones.
downward-slanting eyes
micrognathia (a small lower jaw)
conductive hearing loss
underdeveloped zygoma
drooping part of the lateral lower eyelids
malformed or absent ears
Mutations in the TCOF1, POLR1C, or POLR1D
genes can cause Treacher Collins syndrome
TCOF1 codes for treacle protein
Haploinsufficiency of the treacle protein leads to
a depletion of the neural crest cell precursor,
which leads to a reduced number of crest
cells migrating to the first and second
pharyngeal arches.
These crest cells play an important role in the
development of the craniofacial appearance
DiGeorge syndrome (DGS)
a syndrome caused by the deletion of a small
piece of chromosome 22
Cardiac abnormality (especially tetralogy of Fallot)
Abnormal facies
Thymic aplasia
Cleft palate
Hypocalcemia/Hypoparathyroidism
Pierre Robin syndrome (PRS)
is a congenital condition of facial
abnormalities in humans.
The 3 main features are
- cleft palate
- micrognathia (a small jaw)
- glossoptosis (airway obstruction
caused by backwards
displacement
of the tongue base)
genetic anomalies at chromosomes 2, 11, or 17
Pharyngeal pouches
First pharyngeal pouch
forms a stalklike diverticulum – tubotympanic
recess – which comes in contact with the
epithelium lining the first pharygeal cleft (the
future external auditory meatus).
Finally 1st pharyngeal puch forms the middle
ear cavity, auditory (eustachian) tube and
partially eardrum.
Second pharyngeal pouch
forms the palatine tonsil and tonsilar fossa
Third pharyngeal pouch
branches into dorsal and ventral wings
The dorsal wing of 3rd pharyngeal pouch
differentates into the inferior parathyroid
gland, the ventral wing of this pouch forms
the thymus.
Both primodia lose their connection with
pharyngeal wall and start to migrate in a
caudal and medial direction.
Fourth pharyngeal pouch
Epithelium of the dorsal wing of the fourth pharyngeal
pouch forms the superior parathyroid gland, epithelium
of ventral wing (which originates from fifth pharyngeal
pouch) of this pouch gives rise into ultimobranchial
body, which incorporates into
thyroid gland and differentiate
into C cells producing calcitonin.
The tongue
Thyroid formation
animation
Lingual thyroid