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  • Mark Hill's site for this prac
  • The earliest stage we can identify the layers that contribute all the structures right through to the postnatal period
  • The limit of the overall size of the head in utero is limited by what fits through the birth canal
  • In postnatal growth, there is unlimited growth of the nose and the ears
  • Understand the contributions of the pharyngeal arches to the face and the neck.
    • Note also that they contribute endocrine glands of the neck
  • Know what each one of the components of the arch will contribute to the face and neck (see his tables and those in Larsen's) - rote learn this
    • Specific content and contributions: arch, cartilage, blood vessels, ectoderm, membrane, pouch, groove, etc
  • We'll also cover the auditory pathway (hearing) - this is the system we'll clinically see a lot of issues in childhood/development
    • The normal human infant has auditory pathway affected by disease (because of normal development)
  • We won't be covering development of smell, taste or vision
  • Then we'll cover abnormalities
  • Chapters 16&17 in Larsen's
  • Only the content on the actual lab pages/lecture pages is what is examinable

Germ layer contributions, week 3

  • Have established the 3 germ layers (ectoderm, endoderm, mesoderm) by week 3
  • Each arch has contributions to it from all 3 germ cell layers
  • Buccopharyngeal membrane - where enoderm and ectoderm are in close apposition (oral cavity)
  • Cloacal membrane - similar, but at other side of embryo

Week 4

  • At the cranial end, we can see the open neural tube (cranial neuropore)
  • At the caudial end, we can see the open neural tube, the caudal neuropore
  • Can see the heart tube
  • Note the stomedeum
  • Swelling lateral to the neural plate at the cranial end, represents the swelling of the first pharyngeal arch. This will fuse together anteriorly to form a U-shaped structure (first feature we can identify on the surface that contributes head and neck structures)
    • Stage 11 (23-26 days)
  • Over the course of week 4, these pharyngeal arches form in sequence (1, 2, 3, 4, 6 in a rostrocaudal sequence). Each arch gradually gets smaller (the first arch is the largest swelling we see)
  • Stage 12
    • 2nd pharyngeal arch starts to appear
    • Dorsal side of the embryo - we see the otic placode (just behind the 2nd pharyngeal arch)
      • These are surface indentations representing columnar epithelium indenting on a small patch. They begin to invaginate to form a cavity/impression/pit. In a later stage, it will close entirely, be separated from the surface (just like the neural tube was separated from the surface ectoderm).
      • In later embryos in week 4, we can't identify the otic placode at the back because it's lost from the surface (dropped in)
  • Day 23-26 (ventral)
    • Bright field image, see a triangular indentation on the ventral surface - the stomedeum
    • On the floor of the stomedeum (indentation) is the buccopharyngeal membrane (floor)
  • Stage 11 SEM
    • Note the first pharyngeal arch
    • Obvious indentation at the front is the stomedeum
    • Triangular structure on the floor of the stomedeum is the buccopharyngeal membrane
    • Even at this early stage, it is
    • Buccopharyngeal membrane is foregut (stomedeum also contributes to oral cavity)
    • First pharyngeal arch has two obvious prominences
      • Largest is the mandibular prominence (anterior) (of the first arch) - lies beneath the buccopharyngeal membrane
        • Each of the mandibular prominences fuses very early on - very rare to have a clefting of it
      • Smaller lateral swelling is the maxillary prominence (of the first arch) - lies above the opening of the buccopharyngeal membrane
    • More common to have a clefting of the maxillary prominence - forms the palate
    • Otic placode lies behind the 2nd pharyngeal arch
    • Stomedeum is anteriorly, lies between forebrain region and developing heart
  • Later stage
    • Black spot - shows where the otic placode is closing, has almost separated completely from the surface
      • Small opening enlarges into an epithelial fluid-filled sac (otic vesicle or otocyst)
      • For some time the otocyst remains as a ball. Superior part will differentiate to form the cochlear duct. Inferior part will differentiate to form the semicircular canals
      • The part that allows you to hear, won't differentiate for a while (even thoguh you've developed the membranous labyrinth - still need to differentiate the organ of Corti later on)
    • Near the frontonasal prominence is the nasal placode (anterior and medial) - will form the sensory portion of nose
    • Optic placode is more laterally to the nasal placode, will develop the eye
  • By the end of week 4, we see clearly:
    • Mandibular and maxillary portions of first arch
    • No otic placode any more (lost)
    • Pharyngeal arches 2, 3, 4
    • Pharyngeal arch will begin to grow over the top of the inferior smaller arches, covering them entirely. Leaves an anatomical landmark (indentation beneath the 2nd pharyngeal arch, growing over those beneath it) - the cervical sinus
    • Surface of arch is covered with ectoderm
    • Beneath the first pharyngeal arch and second arch is the first pharyngeal groove/cleft (nth groove beneath the nth arch)
    • The endoderm pockets from the inside to form the nth pharyngeal pouch
    • The endoderm and ectoderm together are the nth pharyngeal membrane
  • Late stage 13 embryo (at cusp between week 4 and 5); 30 somite pairs
    • Grey line through head region shows you the level of this slice
    • In the center of the embryo head, we see the neural tube - dark purple is the epithelium of the NT. The space within it will form the ventricular cavity
    • When we cut through the head further forwards, we'll see more of the pharyngeal arches. In this case, we see arch 1 (and CN V associated with it)
    • In the hindbrain we see folds in the neural tube, which segment the nervous system (called rhombomeres - segment the whole nervous system, but can only be seen in the hindbrain). The neural crest migration into other tissue is in rhombomeric divisions
    • Outside the neural tube in the tissue around it is the otocyst - a round vesicle inside the head. It is in close apposition to the neural tube (lots of neural contribution). Their will be cranial ganglion associated with the otocyst (visible in this slide), and there will be a neural ganglion associated with each of the pharyngeal arches.
    • Trigeminal ganglion is the largest of the cranial ganglion
    • First pharyngeal arch and second pharyngeal arch and the groove between them on a further slice down
    • Pink line joining the first and second pharyngeal arches together is the first pharyngeal membrane
      • Everything inside the pharyngeal membrane is endoderm (pouch)
      • Everything outside it is ectoderm (groove)
    • Underneath the notochord is the pharynx. Above it is the neural tube
    • Can see an outpocketing of endoderm from the pharynx out to between the first and second
    • Hypopharyngeal eminence in the midline is associated with where the thyroid gland will migrate down from
    • Identify the arch arteries (one for each arch on each arch)
    • Paired dorsal aortas viewed beneath the fourth pharyngeal arch, and the fourth arch arteries empty into it
  • Start at the heart - then have it coming out of a common outflow tract, up into a large aortic sac, then the pharyngeal arch arteries come out into each side (1st, 2nd, 3rd, 4th), into each arch artery, then runs back into paired dorsal aortas (which fuse to form the descending thoracic aorta), down the back of the embryo's length, and out through the placenta
    • Extensive remodelling of arch arteries - the top ones regress to form less significant vascular contributions, while the bottom ones have major contributions to the CVS

Diagram of pharyngeal arches and membranes

  • Each of the arches is filled with mesenchyme (embryonic CT)
  • Mesoderm unsegmented in the head region. The additional component is neural crest
    • Neural crest cells migrated from the dorsal side (hindbrain) into the pharyngeal arch in a rhombomeric fashion
      • Can't easily identify, not expected to
    • The neural crest here is called ectomesenchyme (ectodermal in origin, forms neural tissue, but also muscular and bony connective tissues that are normally formed by mesoderm in other parts of the body)
  • Each arch has:
    • Artery
    • Cranial nerve
    • Large bar of cartilage (U-shaped structure)

Arch cartilages

  • First arch cartilage (Meckel's cartilage)
    • The posterior parts form the malleus and incus - initially formed as a single bony structure, and later in development a joint forms between these two bones
    • The rest of the cartilage around the mandibular area acts as a temporary template for the jaw - but it DOESN'T OSSIFY, it is just present to maintain the shape of the mandible. The mandible will form by intramembranous ossification beside it. The cartilage itself regresses and is lost.
  • Second arch cartilage (Reichert's cartilage)
    • The ends of it also extend dorsally and form the stapes (the third ossicle of the middle ear) -- the three middle ear ossicles have different embryonic origins
    • The remainder of Reichert's cartilage also degenerates, it also contributes the superior parts of the hyoid
  • Third arch cartilage contributes the inferior parts of the hyoid
  • Fourth arch cartilage forms the cartilage rings of the neck

Structures derived from arches

Arch

Nerve

Muscles

Skeletal

Artery

1
(maxillary/mandibular)
trigeminal (V)

mastication
(temporalis, masseter, medial pterygoid, lateral pterygoid)

Meckel's cartilage
mandible modelmalleus, incus

maxillary(terminal branches)

2
(hyoid)
facial (VII)

facial expression
(buccinator, platysma, stapedius, stylohyoid, digastric posterior belly)

Reichert's cartilage
stapes, styloid process, lesser cornu of hyoid, upper part of body of hyoid bone

stapedial (embryonic)
corticotympanic (adult)

3
glossopharyngeal (IX)

stylopharyngeus

greater cornu of hyoid, lower part of body of hyoid bone

common carotid, internal carotid (root)

4 and 6
superior laryngeal and recurrent laryngeal branch of vagus (X)

intrinsic muscles of larynx, pharynx; levator palati

thyroid, cricoid, arytenoid, corniculate and cuneform cartilages

4 - aortic arch, right subclavian
6 - ductus arteriosus, pulmonary (roots)

Structures derived from Pouches

Pouch

Overall Structure

Specific Structures

1

tubotympanic recess

tympanic membrane, tympanic cavity, mastoid antrum, auditory tube

2

intratonsillar cleft

crypts of palatine tonsil, lymphatic nodules of palatine tonsil

3

inferior parathyroid gland, thymus gland

4

superior parathyroid gland, ultimobranchial body

5

becomes part of 4th pouch

Later embryo

  • Ventral view of the embryo stages 16, 17, 18 (week 6-7)
    • Mandibular component of the first arch (lower jaw; fused in midline); maxillary component of the first arch appears as a little tag on the lateral surface
    • Above this is the frontonasal prominence (overlying the neural tube). It has within it the nasal region (bilaterally), and can see the nasal epithelium sinking in (nasal placode)
      • Nasal region has a medial and lateral portion
      • The region between the nasal regions bilaterally will contribute the philtrum
  • 5 components of the face:
  1. Lt and Rt maxillary portion of PA 1
  2. Lt and Rt mandibular portion of PA 1
  3. Philtrum
  • Failure of the maxillary regions to fuse with the philtrum (from FNP), there may be bilateral or unilateral clefting (either of the lip or the palate)
  • Primary palate - the anterior region of the palate inside the oral cavity
  • Later on, the maxillary segments give off a flat sheet to extend in and fuse together

Week 8

  • At the end of embryonic development, on either side of the tongue, we see palatal shelves. Each maxillary element contributes a shelf, which will grow towards the midline, and before the foetal period, they lie beside/underneath the tongue. First event in their development is elevation (extend above the tongue), then grow toward the midline, then they fuse in the foetal period (we see fusion of the palate inside the oral cavity).
  • On the dorsal side of the oral cavity, we see the auditory/eustachian tube forming on either side (between the middle ear and oral cavity), which will eventually form a space around the ossicles (in the late embryo, the middle ear ossicles are still embedded in bone and can't move -- later the space grows around them)
  • Can note also the stapedius muscle
  • The features buried deep in the temporal bone (chondrocranium) represent the result of the otocyst - we see a cut through the cochlear duct. We see also the ganglia associated with it (acoustic ganglia), and the portion of the ganglia associated with balance.
    • We see also the internal auditory meatus (where the ganglia sit)
  • Laterally and posteriorly, we see the utricle and saccule. Note that all these spaces are still connected together - this is just a cross section
  • Outside the utricle and saccule, we see cuts through the semicircular canals.
  • On the surface/side of the head, we see the cartilage of the auricle and the external auditory meatus
  • Boxes indicate other images that are blown up elsewhere
  • Can see the superior, middle and inferior conchae of the nose, and the optic nerves extending from the diencephalon

Serial views of the external ear

  • The next series of images are external views of the lateral side of the head viewed from the left. In the fifth week, our first pharyngeal arch and 2nd have been numbered. As we follow this on, we see the maxillary portion, mandibular and the 2nd arch. This represents centre around which our auricle will form
    • The EAM is derived from that first groove/cleft, and at the floor of this groove is the first pharyngeal membrane which will form the tympanic membrane of the ear.
  • In stage 16 we see a series of small bumps
  • In stage 17 (week 6 of development), we can easily see these 6 bumps (auricular hillocks - 3 on the first arch and 3 on the second). After the 6th week, these bumps fuse on the surface, making a ring-like structure centred around the EAM. We still don't see a clear auricle, but an elevation around the EAM (the floor of which is the tympanic membrane).
  • Note the relative position of the EAM in relation to the lower jaw.
  • In week 8 of development, we can clearly see a structure that looks like the external ear (wrapped around the EAM, but still down at the level of the lower jaw).
  • In stage 23 we see something that looks like a normal ear
  • At the end of the embryonic period we still have an ear at the level of the jaw. There is a foetal change in position (to the level of the eye)
  • Each of the auricular hillocks corresponds to a different anatomical region of the auricle. It is a complex structure, susceptible to developmental abnormality (tags on the ears are checked for)

Foetal growth

  • We still need to grow substantially the head
  • There is significant growth in head circumference (50mm at early foetal, to 350mm circumference at birth)
    • Can measure on ultrasound to measure normal growth of the head (check for foetal growth restriction etc)
    • Relatively linear growth in the second and third trimesters in terms of circumference
  • Skull will ossify, but there will be broad, unfused sutures at the fontanelles, to allow the baby to squeeze through the birth canal (can therefore be born with cone heads, which reshape later on)
  • Two forms of ossification: Red=bone, blue=cartilage in a lateral view of the head. We see the nose/nasal septum, the eye, ossification around the eyeball, the ossifying mandible (via intramembranous ossification, not Meckel's cartilage!!), can see maxillary region
  • Intramembranous ossification of each of the bony plates of the neurocranium/cranial vault spreads out from a single point in the plate, out to the sutures where it will fuse postnatally with adjacent plates
    • Cranial vault = intramembranous
  • Lying across the base of the skull, there is a blue region representing the base of the skull (chondrocranium, which forms by endochondral ossification)
    • Platform on which the brain sits = endochondral
  • Mandible ossifies directly from the CT in the jaw (intramembranous)
  • In brown/grey face photo
    • Can see nasal septum, pituitary gland and the lamina terminalis before it (where the anterior neuropore closed), optic stalk
    • Can see brainstem and cerebellum
    • Ossification fo the vertebral column (axis/atlas)
    • Ossification of the bony palate (maxilla/mandible)
    • Soft palate at the back
    • Cartilages of the neck

Foetal stage of palate formation

  • Secondary stage of palate formation
  • Week 9 - palatal shelves form in midline
  • Clefting of the lip is either unilateral or bilateral, but clefting of the palate is in the midline (shelves fail to fuse)
  • If the primary palate doesn't fuse correctly, then the secondary palate won't be able to fuse in the anterior region (nothing to fuse into on the anterior surface)
  • Can have cleft lip, ?cleft palate, or cleft lip and cleft palate together
  • Shelves elevate above the tongue, then extend to midline, then fuse in the midline, not only with each other, but also with the nasal septum in the middle (T-shaped fusion in the midline)
    • This is the secondary palate fusion event

Hearing

  • A lot of the development of the auditory pathway and structures of hearing occur in the foetal period
  • Need to form a cavity around the auditory ossicles
  • The EAM during early development goes through a stage where the epithelium proliferates and blocks the tube (meatal plug)
  • Through foetal development, we see the inner ear structures developing - form all the components of hearing (organ of Corti) -- all we had was a cavity before this
  • Ossification of the surrounding cartilage, and establishment of the central neural pathway, up to the auditory cortex (one of the last things to develop)
    • Brainstem pathway develops relatively early in the neonate, but late in the foetal stages
  • Even by week 26 we can see the auditory pathway in the brainstem is intact, but the auditory cortex isn't mature (no way to interpret the sound)
  • Late in the foetal period (week 28) - we measure a brainstem response in this pathway, with different latency to in the adult (moves slower) -- later in development, the foetus is able to sense vibration (startle response). It's not really hearing -- it's bony conduction, not air conduction (the middle ear is still filled with fluid so the ossicles can't move).

Postnatal development

  • CT scan of the skull of a newborn
    • Large anterior fontanelle
    • Smaller posterior fontanelle
    • All fuse at different times, anterior is last to fuse
    • Sutures close up over time, and in old age you can't notice them
  • There is a great diagram showing both the adult structures and the embryonic contributions to these structures beneath it

Eustachian tube

  • Adult ear - 45 degree slope, longer, muscles (tensor paliti and levator palati) open it to equalise pressure (it's not always open)
  • Infant ear - 10 degree slope (skull has to grow and will tilt the auditory tube; infective agents from oral/nasal cavity can be easily transmitted to middle ear; doesn't drain as easily), narrower
    • Infection in oral cavity can cause adenoids etc to close in on the eustachian tube, preventing drainage
    • Only have tensor paliti
    • Easily closed by inflammation of the wall of the eustachian tube as well
    • Hearing loss (delaying neural development/learning), damage to ossicles

Therefore hearing testing is important in newborns (as of 2002 it's brought in).

Newborn hear testing

  • Computer calculates an ABR scan measuring whether or not the brainstem pathways for hearing are intact and working

Abnormalities

  • Treacher Collins, Pierre Robins
  • Classes of facial abnormalities: Sinuses, cysts, fisula, vestiges
  • Ventral view of an infant in utero - can see primary clefting of upper lip
  • The issue with clefting isn't that it's irreparable (it is), but it causes problems in feeding (making a liquid seal, and fluid going down the windpipe)
    • Before repairing, wait for the face to grow first, so that it doesn't distort development of the face
  • Both cleft lip and cleft palate are in the top 10 congenital abnormalities each year
  • FASD: neurological effects (big problem). Clinical features: flat face, broad nose, epicanthal folds, narrow upper lip, classic railroad track appearance of external ear, abnormal curvature of the little finger, ears positioned too high (idea: premature maturation of the cartilage of the face)
    • Can have FASD without the facial features