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Orofacial Embryology by MikeJenny


									Orofacial Embryology

  Prenatal Development
         Websites to check out
Fertilization & Development in Utero

-fertilization in the upper third of the oviduct/fallopian tube
-fertilization = union of egg and sperm
-after fertilization = zygote
-first period of prenatal development = preimplantation period
        -first week of development

                                              -development of the unattached
                                              -immediately prior to the union of
                                              egg and sperm nuclei – the egg must
                                              complete the final stages of meiosis
                                              (meiosis I is completed as the egg
                                              primordium then stops)
                                              -fertilzation result in the combination
                                              of the haploid egg and sperm =
                                              diploid zygote
                                              -called an embryo once it begins to
                                              divide (within 24 hours)
                                              -division = mitosis
    Embryonic terms to consider
•   induction
     – first process to occur during embryogenesis
     – interaction between developing embryonic cells

•   morphogenesis
     – may also be called morphodifferentiation
     – development of form and specific tissues
     – results from migration of embryonic cells and inductive interactions between
       these cells

•   patterning
     – specification of the embryo through segmentation
•   differentiation
     – process of specialization of embryonic cells
•   proliferation
     – controlled levels of mitosis
•   interstitial growth
     – growth deep within a tissue of organ
     – as opposed to appositional growth – growth at the periphery through the
       addition of additional cell layers
        Prenatal development

• 10 lunar months
• three phases (first two = embryonic stage)
  – first – after fertilization and spans the first 4 weeks
     • largely cellular proliferation and migration
     • some differentiation
  – next 4 weeks of development
     • largely the differentiation of all major internal and external
       organs = morphogenesis
     • very vulnerable stage
  – remaining phase – fetal stage
     • largely a matter of growth and maturation
                                                                -preimplantation period –
                                                                first week
                                                                -embryonic stage -
                                                                week 2 to week 8

-union of sperm and egg nuclei (zygote) -> first mitotic cell division (embryo) -> cell
division continues -> formation of the morula
-morula forms a blastocyst or blastula (assymetrical ball of cells with a cavity) ->
implantation into the endometrium
-formation of the blastula marks the beginning of morphogenesis - shaping of the embryo,
migration of dividing cells to specific locations
-blastula = blastocyst - hollow ball of cells
                    -outer layer = trophoblast - forms extraembryonic tissues (e.g. placenta,
                               yolk sac)
                    -inner mass of cells at one end - totipotent embryonic stem cells
                    or embryoblasts
          Second week of development

•   blastocyst increases in size by proliferation
•   a blastocoel or blastocyst cavity forms between the inner cell mass and the
•   differentiation of the inner cell mass ES cells begins
•   results in the formation of a bilaminar embryonic disc comprised of two cell
     – two layers are called an upper epiblast (ectoderm, mesoderm, endoderm) and a
       lower hypoblast (extraembryonic endoderm)
     – above this disk is an upper amniotic cavity and a lower yolk sac (primitive
       hematopoietic organ for the embryo/fetus)
•   trophoblast also begins to differentiate to form a primitive placenta
•   the embryo connects to the developing placenta through a stalk
Third week of development

-the bilaminar embryonic disk converts into a trilaminar
disk of ectoderm, mesoderm, endoderm (gastrulation)
-formation of the primitive streak within the embryonic
disc critical to this formation

-migration of epiblast cells through the primitive streak
towards the hypoblast – eventually creates three tissue layers called the
germ layers
            1. ectoderm
            2. mesoderm
            3. endoderm

                                                                            humans are
       Fourth week of embryonic development
                                                                        -portions of the mesoderm that
                                                                        do not form the notochord
                                                                        divide into sections called
                                                                        somites -> specific body
                                                                        regions and structures
                                                                        -38 somite pairs
                                                                        -give rise to most of the skeletal
                                                                        structures of the head, neck and

-by the end of the third week: trilaminar embryonic disc has a definite orientation
-during the fourth week - embryo begins to form a tubular structure
-in front of the primitive streak is the primitive node – mesodermal cells that will form the
-also have differentiation of cells from the ectoderm forms the neuroectoderm
            -a neural plate forms
            -this plate thickens with proliferation – invaginates centrally and forms the neural groove
            -this groove deepens and is surrounded by the two neural folds
            -development of the neural crest cells from these neural folds
     Fourth week of embryonic
• the neural folds meet superior to the neural
  groove and forms the neural tube
• neural folds also form the cells of the neural
    – migratory population of cells
    – multipotent
    – gives rise to ectodermal tissues and
      mesenchyme in specific areas of the head/face
• embryo folds along this tube – results from
  extensive proliferation of the ectoderm and the
  differentiation of specific tissues at the
  cephalic end
• the anterior end of the neural tube rapidly
  expands to form the beginnings of the
  forebrain, midbrain and hindbrain
• also folds along the rostrocaudal axis
• continued development of the somites
• development of a head fold
• is critical to the formation of the
  primitive oral cavity
    – folding results in the formation of
      the primitive oral cavity =
    – separated from the developing
      and expanding gut by a
      buccopharyngeal membrane
      (or oropharyngeal
• the mesoderm can be divided
  into three distinct types
   – paraxial mesoderm
   – intermediate mesoderm
   – lateral plate mesoderm
• the lateral folding determines
  the disposition of the germ
   – with lateral folding the
     amniotic cavity encompasses
     the entire embryo
   – the paraxial mesoderm
     remains adjacent to the
     developing neural tissue
   – the lateral plate mesoderm
     drops down and cavitates -
     the resulting cavity forms the
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• rostral or head fold
• anterior portion of the neural tube
  expands as the forebrain, midbrain
  and hindbrain
• the neuroectoderm in this region will
  form the olfactory, orbital and
  otic placodes
• the hindbrain forms 8 bulges =
• the paraxial mesoderm in this region
  also segments into somites
• migration of neural crest cells into
  this region provides the embryonic
  connective tissue (mesenchyme)
  required for development of the
  craniofacial structures
• these neural crest cells arise from the
  midbrain and the first two
  rhombomeres as two streams
    Branchial arches
•   also called pharyngeal arches
•   figure 4-11
•   fourth week: development of a
    frontal prominence forms the
•   below this is the formation of the
    first branchial arch
    (mandibular arch)

•   6 pairs – U shaped
     – core of mesenchymal tissue
       formed from neural crest cells that
       migrate in to form the arches
     – covered externally by ectoderm
       and lined internally by endoderm
     – each has its own developing
       cartilage, nerve, vascular and
       muscular components
•   these arches separate the
    stomatodeum from the developing
                       Branchial arches
• separated laterally by branchial grooves/clefts

• medially they are separated by pharyngeal pouches
• first arch (mandibular arch) – maxillary and mandibular
• second arch (hyoid arch) - hyoid bone, part of the temporal bone
  (VII nerve)
        • cartilage = Reichert’s cartilage
        • the mesoderm of this arch will form the muscles of facial expression, the
          middle ear muscles

• third arch –tongue (IX nerve)
• fourth arch –tongue, most of the laryngeal cartilages (IX and X
• fifth arch – becomes incorporated into the fourth
• sixth arch – most of the laryngeal cartilages (IX and X nerves)
               Pharyngeal Pouches
– four well-defined pairs of pharyngeal pouches develop from
  the lateral walls of the pharynx

– first pouch (betwen the 1st and 2nd arches) - external acoustic
  meatus, tympanic membrane, and eustachian tube
– second pouch – palatine tonsils
– third pouch - thyroid and parathyroid glands,
– fourth pouch – parathryoid gland
– fifth pouch -becomes incorporated into the fourth
     Development of the Face
• forms from the fusion of 5 face primordia
  which develop during week 4 and fuse
  during weeks 5 through 8
  – primordia = ectodermal swellings or
    prominences that are filled with mesodermal
    and neural crest cells
     • frontonasal prominence
     • mandibular prominences (2) – from branchial
       arch #1
     • maxillary prominences (2) – from branchial arch
        Development of
           the Face

• primitive
  stomatodeum forms
  a wide shallow
  depression in the
  face – limited in its
  depth by the
       Development of the Upper Face
•   within the fourth week (weeks 5 –
•   the frontonasal prominence
    develops two sets of placodes
    (nasal and lens)
•   formation is dominated by the
    proliferation and migration of
    ectomesenchyme cells
     – development of medial and lateral
       nasal processes or swellings which
       encircle the nasal pits
     – fusion of the medial nasal
       processes at the midline =
       process or process
•   formation of the upper portion of
    the face is faster than the lower
    portion (finally cease to grow at
                Development of the Upper Face
– rapid proliferation of the underlying
  mesenchyme around the placodes
  results in bulges in the frontal
  eminence and produces a horseshoe-
  shaped ridge in each olfactory placode
    • medial and lateral nasal processes
– in between the medial nasal processes
  is where the nose develops = called
  the frontonasal prominence
– the medial nasal processes of both
  sides + the frontonasal prominence
  give rise to the middle portion of the
  nose, upper lip, anterior portion of
  the maxilla and the primary palate
– the medial nasal processes also fuse
  internally and form the
  premaxillary segment
– the frontonasal prominence will also
  form part of the forehead
          Development of the Upper Face

• day 24: development of the
  frontal prominence (covers the
  rapidly expanding forebrain)
   – beginnings of the mandibular
     and maxillary processes from the
     1st branchial arch
   – well-defined boundaries of the
     stomatodeum results
• day 26: well-formed maxillary
  and mandibular processes
• day 27: appearance of the
  nasal placode and the
  odontogenic epithelium
• day 28: localized thickenings
  develop within the frontal
  prominence = olfactory
                    Upper lip formation
• during the fourth week
• fusion of the maxillary processes with
  each medial nasal process
• this contributes to the lateral sides of
  the upper lip – together with the
  medial nasal processes which
  contribute to the medial aspect of the
  upper lip
• the maxillary processes also fuse with
  the lateral nasal processes – results in
  a nasolacrimal groove which
  extends from the medial corner of the
  eye to the nasal cavity
         Development of the Palate
• involves the formation of a
  primary palate, a secondary palate
  and fusion of their processes
• Primary palate
   – forms from an internal swelling of the
     intermaxillary/premaxillary process
     (fusion of medial nasal processes)
• Secondary palate
   – forms from the two lateral palatine
     shelves or processes
   – develop as internal projections of the
     maxillary prominences
                         Primary palate
• fusion of the
  median nasal
  processes gives rise
  to the median
  palatine process –
  fuses to form the
  primary palate

             Secondary Palate
• the common oronasal cavity is bounded
  anteriorly by the primary palate and
  occupied by the developing tongue
• only after the development of the
  secondary palate can oral and nasal
  cavities by distinguished

• three outgrowth appear in the oral cavity
    – nasal septum:
        • grows downward through the oral cavity
        • it encounters the primary and secondary
    – two palatine shelves
• closure of the secondary palate is likely
  to involve the hardening of the palatine
  shelves – mechanism remains unknown
  + the withdrawl of the tongue
Palatine shelves
Cleft lip and
• complex combination of contributions of the frontal prominence
  (forms the bridge), the merged medial nasal prominences (form the
  median ridge and tip of nose), the lateral nasal prominences (form
  the alae) and the cartilage nasal capsule (forms the septum and the
  nasal conchae)
• the external nasal region develops from the superficial alar field –
  gives rise to the alae
• Nasal pits and cavities
    – separate anteriorly from the stomatodeum by fusion of the medial nasal,
      lateral nasal and maxillary prominences – form the nostrils
    – separate posteriorly from the stomatodeum by the oronasal membrane

    – the developing nasal cavities are separated from the oral cavity by the
      intermaxillary process (forms the floor of the nasal cavity)

• Nasal capsule and nasal septum
    – condensation of the mesenchyme within the frontonasal prominence –
      forms the precartilagenous nasal capsule
    – the capsule develops as two masses around the nasal cavities
    – the median mass becomes the progenitor of the nasal septum
    – the lateral masses will form the conchae and nasal alar cartilages
• nasal cavity
   – early development of the face is
     dominated by the proliferation and
     migration of tissue involved in the
     formation of the primitive nasal cavities
   – about 28 days localized thickenings
     develop within the primitive ectoderm of
     the embryo – olfactory placodes
   – rapid proliferation of the underlying
     mesenchyme around the placodes bulges
     the frontal eminence forward and
     produces the nasal pit
   – the lateral arm of this pit = lateral nasal
   – the media arm = medial nasal process
   – in between these nasal pits is the
     frontonasal process – where the nose
   – the two medial processes + the
     frontonasal give rise to the medial portion
     of the nose and upper lip, the anterior
     portion of the maxilla and palate
     Nasal and Paranasal tissues
•   nasal cavity lined with a respiratory mucosa like the rest of the respiratory
•   pseudostratified columnar epithelium with cilia
•   interspersed are goblet cells which rest on the basement membrane

•   very vascular lamina propria – warms the air
•   roof of the nasal cavity is a specialized area that contains the olfactory
•   on the medial wall are the three nasal conchae

•   paranasal sinuses
     –   frontal, sphenoid, maxillary and ethmoid sinuses
     –   provide mucus for the nasal cavity
     –   respiratory mucosa of ciliated pseudostratified columnar epithelium
     –   but is thinner than the nasal mucosa – also has fewer goblet cells
     –   no erectile tissue
Development of Sinuses and Nasal
• paranasal sinuses
  – some develop during late fetal life
       • frontal and sphenoid not present at birth
       • at 2 years the two most anterior ethmoid sinuses grow into
         the frontal bone – visible on X-rays by age 7
       • two most posterior ethmoid sinuses grow into the sphenoid
       • sinuses are important in the size and shape of the face
         during infancy and the resonance of the voice
  –   the rest develop after birth
  –   form as outgrowths of the wall of the nasal cavity
  –   become air-filled extensions in the adjacent bones
  –   the original openings of these outgrowths persist as
      the orifices of the adult sinuses
                  Maxilla formation
•   centers of ossification develop in the mesenchyme of the maxillary
    processes of the first branchial arch

•   spreads posteriorly below the orbit towards the developing zygoma and
    anteriorly toward the future incisor region and superiorly to form the
    frontal process

•   ossification also spreads into the palatine process to form the hard palate

•   at the union between the palatal process and the main body of the
    developing maxilla is the medial alveolar plate – together with the
    lateral plates – development of the maxillary teeth

•   a zygomatic or malar cartilage appears in the developing zygomatic
    processes and contributes to the development of the maxilla
Development of the Lower Face
• within the fourth week
• two bulges form inferior to the
                            Mandible formation
•   the cartilage of the first branchial arch
    associated with the formation of the
    mandible = Meckel’s cartilage

•   6 weeks: Meckel’s cartilage forms a rod
    surrounded by a fibrocellular capsule

•   the two cartilages do not meet at the midline
    but are separated by a thin line of cartilage =
•   on the lateral aspect of this symphysis – a
    condensation of mesenchyme forms

•   at 7 weeks intramembranous ossification
    begins in this mesenchyme and spreads
    anteriorly and posteriorly to form the bone
    of the mandible
•   the bone spreads anteriorly to the midline of
    the developing lower jaw – the bones do not
    fuse at the midline – mandibular symphysis
    forms (from meckel’s cartilage)
     –   which fuses shortly after birth
•   the ramus develops from rapid ossification
    posteriorly into the mesenchyme of the first
               Mandible formation
-Meckel’s cartilage does NOT contribute directly to the ossification of
   the mandible
   -posterior extremity – malleolus of the inner ear
   -portion persists as the sphenomandibular ligament
   -significant portion is resorbed entirely
   -most anterior portion near the midline may contribute to the jaw
   through endochondral ossification
-growth of the mandible until birth is influences by the appearance of
   three secondary (growth) cartilages
   1. condylar – 12th week, developing ramus by endochondral
   ossification, a thick layer persists at birth at the condylar head
   (mechanism for post-natal growth of the ramus = endochondral)
   2. coronoid – 4 months, disappears before birth
   3. symphyseal – appears in the connective tissue at the ends of the
   Meckel’s cartilage, gone after 1 year after birth
          Development of the Tongue
•   begins to develop about 4 weeks
•   localized proliferation of the
    mesenchyme results in formation of
    several swellings in the floor of the oral
•   the oral part (anterior two-thirds)
    develops from the fusion of two distal
    tongue buds or lateral lingual
    swellings and a median tongue
    bud (tuberculum impar)

•   the pharyngeal part or root of the
    tongue (posterior one-third) develops
    from the copula and the                      B.As #1,2 and 3   hypobranchial arch
    hypobranchial eminence (forms                                  overgrows the 2nd arch
    from the 2nd, 3rd and 4th branchial

•   these parts fuse (adult = terminal
•   muscles of the tongue arise from
    occipital somites which migrate into
    the tongue area


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