Dr. Robert t romano, D. C

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Dr. Robert T Romano, D.C.Embryology and Development of the Human Neck
The pharyngeal arches are very important embryological structures.
Many important structures are derived from the pharyngeal arches. The 1st pharyngeal arch is the most cranial, and the sixth is the most caudal. The 5th pharyngeal arch is not present in humans.
The first pharyngeal arch gives rise to the muscles of mastication. Those muscles are the temporalis, masseter, and medial and lateral pterygoids. Other muscles derived from the first arch are the tensor tympani and tensor veli palatini, as well as the mylohyoid and the anterior belly of the digastric. Skeletal structures derived from the first arch are the malleus and incus, both of the ear. Innervation derived from the first arch relies on the Trigeminal nerve (CN V – the maxillary and mandibular branches only). Ligaments resulting from the first arch are the anterior ligament of the malleus and the sphenomandibular ligament.
The second pharyngeal arch gives rise to the muscles of facial expression. Those muscles are the buccinator, auricularis, frontalis, platysma, orbicularis orbis, and oculi. Other muscles derived from the second pharyngeal arch include the stapedius of the ear, the stylohyoid, and the posterior belly of the digastric. Skeletal structures derived from the second arch are the stapes (of the ear), the styloid process, lesser cornu of hyoid, and the upper part of the body of the hyoid. Innervation derived from the second arch relies on the Facial nerve (CN VII). Only one ligament is derived from the second arch: the stylohyoid ligament.
The third pharyngeal arch gives rise to only one muscle, the stylopharyngeus. Skeletal structures derived from the third arch are the greater cornu of the hyoid and he lower part of the body of the hyoid. Third arch innervation relies on the Glossopharyngeal nerve (CN IX).
The fourth and sixth pharyngeal arches give rise to some muscles of the neck, such as the cricothyroid, levator veli palatini, the contrictors of the pharynx, intrinsic muscles of the larynx, and striated muscles of the esophagus. Resulting skeletal structures are the thyroid and cricoid cartilage, and arytenoid cartilage, the corniculate cartilage, and the cuneiform cartilage. Note that only cartilaginous structures are derived from the fourth and sixth arches, no bony structures. No ligaments are derived from the final two arches, either. Innervation relies on the Vagus nerve (CN X), with the superior laryngeal branch (4th arch) and the recurrent laryngeal branch (6th arch) both supplying CNS communication to the resultant structures.
Between the pharyngeal arches lie the pharyngeal pouches.
The pharyngeal pouches also play important roles in the embryology of the neck. These pouches develop in a craniocaudal sequence between the arches, and the pouches are paired. For example, the first pouches are found between the first two arches.
The first pharyngeal pouch gives rise to the tubotympanic recess, which will help give rise to the tympanic membrane, tubutympanic cavity, and mastoid antrum, and the pharyngotympanic tube (the auditory canal).
The second pharyngeal pouch gives rise to the palatine tonsil, and not much else.
The third pharyngeal pouch gives rise to the parathyroid gland and thymus, both of which maintain connections to the pharynx during part of the embryological period. Those connections will be obliterated
The fourth pharyngeal pouch develops into the superior parathyroid gland, ultimopharyngeal body, and the parafollicular cells of the thyroid gland (AKA “C cells”).
The fifth pharyngeal pouch degenerates.
Pharyngeal grooves lie in pairs on each side of the embryological neck region.
There are also pharyngeal grooves present during the fourth and fifth weeks. None of the grooves will give rise to any important adult structures other than the first pair of grooves which will result in the development of the external acoustic meatus. The other grooves lie in the cervical sinus and will be obliterated as the neck develops.

Additional information:

As the branchial apparatus develops, the first and second pharyngeal arches grow in a cranial to caudal fashion, creating the epipericardial ridge. The epipericardial ridge contains the mesodermal rudiments of the sternocleidomastoid, trapezius, and the infrahyoid and lingual musculature. The nerves of the epipericardial ridge are the hypoglossal and spinal accessory. The proliferation of mesoderm in this area eventually causes overgrowth and narrowing of the third, fourth, and sixth arches into an ectodermal pit, known as the cervical sinus of His. This sinus eventually becomes obliterated. If it fails to be obliterated or is partially obliterated, a branchial sinus, cleft, or cyst of types II, III, or IV may develop.

Specific Developments:
Tongue development

  • tongue=group of muscles covered by mucosa

  • formed by endoderm and mesoderm of 1st – 4th arch

  • mesoderm from occipital somites (NOT somitomeres); precursor muscle cells migrate to region of tongue; innervated by GSE component of XII cranial nerve

  • mucosa: anterior endoderm lining the first four pharyngeal arches; innervation depends on arch derivation

    • anterior and posterior separated by a terminal sulcus

      • midpoint of sulcus: foramen cecum position of thyroid outgrowth

    • mucosa of anterior 2/3 of tongue comes from first archCN V

    • mucosa of posterior 1/3 of tongue comes from third and 4th archCN IX, X

    • special taste of anterior 2/3CN VII

    • Special taste of posterior 1/3CN X

  • Tongue freed from floor of mouth by extensive degeneration of underlying tissue

    • Midline frenulum anchors tongue to floor of mouth

Thyroid Gland

  • Arises from foramen cecum

  • Descends along front of pharyngeal gut

  • Remains connected to tongue by narrow canal: thyroglossal duct (obliterated later)

  • Descends just caudal to laryngeal cartilages

  • Functions during early fetal period

Facial development

  • Nose

    • At time of anterior neural tube closure, mesenchyme around forebrain, frontonasal prominence (FNP), has smooth rounded extended contour

    • Nasal placodes (thickening of surface ectoderm to become peripheral neural tissue) develop on frontolateral aspects of FNP

    • Mesenchyme swells around nasal placodemedial and lateral nasal prominence (nasomedial and nasolateral processes)

    • Nasal prominencesnose

  • Mouth

    • Stomadeum (primitive oral cavity) forms between frontonasal prominence and first pharyngeal arch

    • First pharyngeal archdorsal maxillary prominence and ventral mandibular prominence

    • Maxillary prominence will merge with medial nasal prominences, pushing them closer to cause fusion

      • Fused medial nasal prominences will form midline of nose and midline of upper lip (philtrum) and primary palate (carries first four teeth)

  • Nasolacrimal structures

    • Maxillary and lateral nasal prominences separated by deep furrow: nasolacrimal groove

    • Ectoderm in floor of groove forms epithelial cord, detaches from overlying ectoderm

      • Epithelial cord canalizesnasolacrimal duct

      • Upper end widenslacrimal sac

    • After detachment of cord, maxillary and lateral nasal prominences merge with each othernasolacrimal duct runs from medial corner of eye to inferior meatus of nasal cavity

    • Maxillary prominences enlargecheeks and maxillae

    • Lateral nasal prominencesalae of nose

Secondary Palate development

  • Main part of definitive palate formed by two shelf-like outgrowths from maxillary prominences

  • Outgrowths=palatine shelves

    • Appear during 6th week

    • Directly obliquely downward on each side of tongue—moves down when mandible gets bigger

    • 7th week: ascend to attain horizontal position , fuse  secondary palate

    • anteriorly: shelves fuse with triangular primary palate (front 4 teeth)

    • incisive foramen: midline landmark b/w primary and secondary palate

    • at time palatine shelves fuse, nasal septum (outgrowth of median tissue of frontonasal prominence) grows down and joins cephalic aspect of newly formed palate

Ear development

  • three distinct parts in embryo

    • external ear: sound-collecting organ

    • middle ear: sound conductor from external to internal ear

    • internal ear sound waves nerve impulses; registers changes in equilibrium

  • external ear: 3 parts

    • EAM: first pharyngeal cleft

    • Eardrum or tympanic membrane

      • 3 layers of cells

        • ectodermal epithelial lining of bottom of EAM

        • endodermal epithelial lining of tympanic cavity

        • intermediate layer of connective tissue

      • major part attached to handle of malleus

    • Auricle/external ear

      • Develops from 6 mesenchymal proliferations at dorsal ends of 1st and 2nd pharyngeal arches

        • 1st arch=top of ear

        • 2nd arch=bottom of ear

      • Swellings=auricular hillocks

  • Middle ear:

    • Tympanic cavity and auditory tube forms from 1st pharyngeal pouch

    • Ossicles: from cartilages of 1st and second arches

      • 1st: malleus and incus

        • tensor tympani (muscle of malleus)—CN V

      • 2nd arch: stapes

        • stapedius muscle—CN VII

      • embedded in mesenchyme for most of gestation

      • 8th month: mesenchyme degenerates; endodermal epithelial lining of tympanic cavity envelops them and connects them to wall of cavity in mesentery-like fashion

  • Inner Ear

    • 4th week: thickening of surface ectoderm on both sides of hindbrain

    • thickenings=otic placodes

      • invaginate to form otic vesicles (otocysts)

      • divide into ventral component and dorsal component

        • ventral componentsaccule and cochlear duct (organ associated with hearing)

        • dorsal componentutricle/semicircular canals (organ associated with equilibrium) and endolymphatic duct

  • cochlear duct

    • 6th week: initiates as outgrowth of saccule

    • penetrate surrounding mesenchyme in spiral fashion (2.5 turns)

    • surrounding mesenchymecartilage

      • undergoes vacuolization

    • scala vestibule and scala tympani form, surround cochlear duct

      • filled with perilymph to receive mechanical vibrations of ossicles

      • mechanical stimuli activates sensory cells in cochlear duct

      • impulsesauditory fibers of CN VIII

  • semicircular canals

    • utricle: three flattened outpocketings, which lose central core

    • three semicircular canals formed

      • slightly different angles to each other

      • sensory cells arise in ampulla at one end of each canal, in utricle and saccule

      • change in position of body generates impulse in cell

      • impulses carried to vestibular fibers of CN VIII

Development of Eye

  • outgrowth of brain

  • 4th week: two depressions evident on each of forebrain hemispheres

  • as anterior neural folds, optic pits elongateoptic vesicles

    • connected to forebrain by optic stalks

  • surface ectoderm inducedlens placode, invaginates with optic cup

  • invagination of optic vesiclesbilayered optic cup: neural and pigmented layer of retina

  • optic stalk is deficient ventrally to contain choroids fissure to allow blood vessels into eye (hyaloid artery)

    • artery feeds growing lens

    • will degenerate; distal part breaks downno feeding of lens in adult

  • 7th week: chorid fissure closes and walls fuse as retinal nerves become bigger

    • optic stalk and contentsoptic nerve

  • 3 layers of eye

    • sclera/cornea

      • sclera: continuous with dura mater around optic nerve

      • cornea develops in anterior 1/5 of eye after separation of lens vesicle from surface ectoderm

        • extension of sclera

    • iridopupillary membrane (anterior) /choroid (posterior)

      • membrane separates anterior chamber from posterior chamber

      • choroid: vasculature

    • iris/retina

      • iris: outgrowth of distal edge of retina

        • retina is folded over

        • pigmented layeriris

      • retina

        • direct derivation of optic cup

        • 2 layers

          • neural layer

            • 3 layers

            • thick, stratified

          • pigmented layer

            • cuboidal

          • intraretinal space communicates with ventricles

  • mesenchyme around eyetwo tiny sets of muscles

    • ciliary muscles: control size of lens via zonular fibers

    • papillary muscles: control amount of light entering pupil

    • innervated by CN III

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