Embryology of head and neck



3.6: Embryology of head and neck


B.M. Ashwini, Padma Challa Ramprakash, A. Amritha



Overview


The embryology of the human body is very vast including several intricate details and multiple sequential steps. Its thorough knowledge helps us to understand the pathophysiology and radiological imaging appearance of several variants and anomalies. So in this chapter, we will briefly go through the embryology of head and neck and its radiological importance.


Branchial apparatus embryology


This is discussed under Paediatric Neck Chapter 3.7.


Embryology of ear (please also refer to temporal bone chapter for further reading): Internal ear


First indication – at 22 days – thickening of surface ectoderm on each side of rhombencephalon – forms otic placodes (Fig. 3.6.1).


Image
Fig. 3.6.1 Schematic diagram representing the development of the otic placode.

Otic placodes invaginates – otic or auditory vesicles or otocysts formed. (Fig. 3.6.2A and B). The otocysts further give rise to the ventral and dorsal components.


Image
Fig. 3.6.2 Schematic diagram representing the development of the ear. (A) and (B) Invagination of the otic placode to form otic vesicles or otocysts. (C) A small group of cells split away from the statoacoustic ganglion which supply sensory organ of corti, saccule, utricle and semi circular canals.

During formation of otic vesicle, a small group of cells split away and form the statoacoustic ganglion – which supply sensory cells of organ of corti, saccule, utricle and semicircular canals (Fig. 3.6.2C).


Derivatives of ventral and dorsal components together constitute the membranous labyrinth. (Flowchart 3.6.1) shows the division of otocysts and their respective derivatives.


Image
Flowchart 3.6.1 Flow chart representing the division of otocysts into the ventral and dorsal components and their derivatives.

Saccule, cochlea and organ of corti


Sixth week – saccule forms a tubular outpouching at its lower pole called cochlear duct – which in turn penetrates the surrounding mesenchyme in a spiral fashion and towards the end of eighth week – it completes 21⁄2 turns.


Ductus reuniens – remains as a connection between saccule and cochlear duct.


At seventh week – cells of cochlear duct differentiate into the spiral organ of corti – which serves to convert sound impulses to electric impulses.


The mesenchyme surrounding the cochlear duct – differentiates into cartilage – which around 10th week – forms two perilymphatic spaces namely (Fig. 3.6.3A and B),




  • • Scala vestibuli
  • • Scala tympani

Image
Fig. 3.6.3 (A), (B) and (C) Schematic diagram representing the development of the saccule, cochlea and organ of corti.

Cochlear duct is separated from scala vestibuli by vestibular membrane (Fig. 3.6.3B and C).


Cochlear duct is separated from scala tympani by basilar membrane (Fig. 3.6.3A–C).


Lateral wall of the cochlear duct is attached to the surrounding cartilage by a spiral ligament and its median wall is connected to and supported by modiolus (future axis of bony cochlea).


Epithelial cells of cochlear duct form two ridges namely the inner and outer ridge.




  • • Inner ridge – forms spiral limbus
  • • Outer ridge – forms one row of inner and three to four rows of outer hair cells (sensory cells), which in turn are covered by tectorial membrane. These sensory cells and the tectorial membrane together form the organ of corti.

Utricle and semicircular canals


During the sixth week the flattened outpouchings of the utricular part of the otic vesicle form the semicircular canals.


One end of each canal dilates to form the crus ampullare; the other end, which does not dilate, forms the crus nonampullare.


Cells in ampulla – form crista ampullaris, a crest – which contain sensory cells responsible for impulses generated by equilibrium. Similar sensory areas, called maculae acusticae, develop in walls of utricle and saccule. Impulses generated by change in position are then sent to the vestibular fibres of cranial nerve VIII.


Middle ear


Tympanic cavity and auditory tube

The distal part of the endoderm of first pharyngeal pouch expands in a lateral direction – forms a tubotympanic recess – which in turn widens – forms tympanic cavity.


Proximal part of endoderm of the first pharyngeal pouch – narrows – forms auditory or Eustachian tube.


Ossicles

Ossicles remain embedded in the mesenchyme till the eighth month.


When the ossicles are free of surrounding mesenchyme – endodermal epithelium connects them in a mesentery-like fashion to the wall of the cavity.


Tensor tympani, which is the muscle of malleus – is innervated by mandibular division of trigeminal nerve.


Stapedius muscle, which is the muscle of stapes – is innervated by facial nerve (nerve to the second pharyngeal arch).


During late fetal life – tympanic cavity expands dorsally by the vacuolization of surrounding tissues – forms tympanic antrum.


After birth – epithelium of tympanic cavity – invades the bone of the developing mastoid process – forming epithelium-lined air sacs – pneumatization. (Flowchart 3.6.2) representing the origins of various parts of the middle ear.


Image
Flowchart 3.6.2 Flowchart representing the origins of various parts of the middle ear.



RADIOLOGICAL VIGNETTE



Most important is to assess size and shape of middle ear cavity.


If the middle ear cavity is air-containing, assessment of its size and shape are easy to make. But in congenital anomalies, middle ear contains undifferentiated embryological mesenchyme (thick glue-like substance), which cannot be radiologically differentiated from soft tissue, retained mucus or serous otitis media.


External ear


It is derived from six auricular hillocks – three on each pharyngeal arch 1 and 2. Originally formed in the neck, it later migrates in cranial direction during development of mandible. (Table 3.6.1) shows the derivatives of the auricular hillocks.




  • Hillocks 1, 2, 3 – Originate from first pharyngeal arch.
  • Hillocks 4, 5, 6 – Originate from second pharyngeal arch.


TABLE 3.6.1


Derivatives of the Six Auricular Hillocks
























Hillock Auricle Component
1 Tragus
2 Helix
3 Cymba concha
4 Concha
5 Antihelix
6 Antitragus

External auditory meatus:

It develops as invagination of first pharyngeal arch tissue. (Flowchart 3.6.3) represents the development of the external auditory meatus. (Table 3.6.2) depicts a tabular column of the various degrees and components of the external auditory canal deformity.


Image
Flowchart 3.6.3 Flowchart representing the development of the external auditory meatus.


TABLE 3.6.2


Table Describing the Various Degrees of the External Auditory Canal Deformity


























Slight (small EAC) Moderate (absent EAC) Severe (absent EAC)
Hypoplastic tympanic membrane Small middle ear cavity Hypoplastic middle ear cavity
Hypoplastic tympanic bone Atretic plate Severe ossicular deformity
Small middle ear cavity Deformed malleus and incus Poor mastoid aeration
Varying ossicular deformity Well-aerated mastoid
Well-aerated mastoid



RADIOLOGICAL IMPORTANCE



Estimate thickness of atretic plate from coronal section imaging.


Determine position of abnormal auditory meatus in relation to surrounding structures such as the middle ear and the TMJ.


It is important from surgical perspective – if pinna is low lying in relation to middle ear cavity – approach for surgery may change.


Thyroid gland





  • • First endocrine gland to develop at around 4 weeks.
  • • Thyroid begins as an endodermal proliferation at the floor of the primordial pharynx which is later indicated by the foramen caecum.
  • • Descends anterior to the pharyngeal gut as thyroglossal duct (i.e. future pharynx) → inferior end of thyroglossal duct (TGD) forms a bilobed diverticulum which leads to the formation of thyroid lobes.
  • • The course of the TGD begins at the foramen caecum (which is located at the junction of anterior two-thirds and posterior one-third of the tongue); then descends in the midline of the neck, first anteriorly to and then inferiorly and posterior to the primordial hyoid bone -> anterior to the thyrohyoid membrane, thyroid cartilage and trachea.
  • • The gland remains connected to the tongue during this migration by the thyroglossal duct.
  • • It reaches to its adult position which is in front of the trachea in the seventh week.
  • • The gland gives rise to the follicular cells of the thyroid tissue that produce thyroid hormones.
  • • The ventral recess of the fourth pharyngeal pouch gives rise to the ultimobranchial body.
  • • The ultimobranchial body gives rise to the parafollicular or C cells, which produce calcitonin.

Radiological importance





  • • Thyroglossal duct cyst – occurs due to failure of obliteration of a part of the thyroglossal duct. It is seen as a midline swelling which moves with deglutition.
  • • Ectopic thyroid – can be found anywhere along the thyroglossal duct; most common site: at the level of hyoid, followed by lingual/sublingual thyroid. Hemiagenesis- a single lobe of thyroid may fail to develop.

Salivary glands




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Mar 25, 2024 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on Embryology of head and neck

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