Pharyngeal ArchesPharyngeal Arches

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• That’s why they called it branchial apparatus

• Because branchial mean gill , which is a Greek word

• The head and the neck region of four week human embryo look like the same region in a fish embryo at the same time ( the 4th week )

خيشوم

Pharyngeal apparatus consists of :

Pharyngeal arch

Pharyngeal pouch

Pharyngeal membran

Pharyngeal groove

• Pharyngeal arches begin to develop early in the fourth week as neural crest cells migrate into the head and neck region

• The first pair of pharyngeal arches ( primordium of jaws) appears as an elevation on the surface , lateral to the developing pharynx

• soon other arches appear on each side of the future head and neck region

• By the end oh the fourth week 4 pairs of pharyngeal arches are visible externally

•The fifth and sixth arches are not visible externally

• The pharyngeal arches are separated from each other by a depression called pharyngeal groove or cleft externally , and pharyngeal pouch internally

• They are numbered in craniocaudal sequence

• The first pharyngeal arch ( mandibular arch ) gives a rise to maxillary and mandibular process

• the first pair of pharyngeal arches plays a major role in facial development

Mandibular processMaxillaryprocess

• The second pharyngeal arch ( hyoid arch ) contributes to the formation of hyoid bone

• Each pharyngeal arch consist of a core of mesenchyme

• That is covered externally by ectoderm and internally by endoderm

mesenchyme

• Mesenchyme is derived from mesoderm in the third week

• during the fourth week most of the mesenchyme is derived from neural crest cells

that migrate into the pharyngeal arches

•The pharyngeal arches contribute exclusively in the formation of the :

• face

• Nasal cavities

• mouth

• larynx

• pharynx

• neck

• During the 5th week , the second pharyngeal arch enlarges and overgrows the 4th

and the 3rd arches

• forming an ectodermal depression called cervical sinus

• by the end of 7th week , the 2nd

to the 4th pharyngeal grooves and the cervical sinus are disappeared , giving the neck a smooth surface

2nd arch

A typical pharyngeal arch contains:

1- An aortic arch , an artery that arises from the truncus arteriosus of the primordial heart

2- A cartilaginous rod that forms the skeleton of the arch

3- A muscular component that differentiates into muscles in the head and neck

4- A nerve that supplies the mucosa and muscles derived from the arch

Derivatives of The First Pharyngeal Arch Cartilage

• The dorsal end of first arch cartilage ( Meckel cartilage ) ossifies to form malleus and incus

• The middle part of cartilage forms anterior ligament of malleus and sphenomandibular ligament

• Ventral part of the first arch cartilages form primordium of the mandible

] The cartilage disappears as mandible develops around it [

Derivatives of The second Pharyngeal Arch Cartilage

• The dorsal end of second arch cartilage (Reichert cartilage) ossifies to form the stapes and styloid process of the temporal bone

• The ventral end of second arch cartilage ossifies to form the lesser cornu and superior part of the body of the hyoid bone

Its perichondrium ( fibrous around the cartilage ) forms the stylohyoid ligament

Derivatives of The Third Pharyngeal Arch Cartilage

• The third arch cartilage ossifies to form the greater cornu and the inferior part of the body of the hyoid bone

• The fourth and sixth arch cartilages fuse to form the laryngeal cartilages except epiglottis which develops from hypopharyngeal eminence

• The fifth pharyngeal arch is rudimentary ( disappear later) and has no derivatives

Derivatives of The Pharyngeal Arch Cartilages

Derivatives of Pharyngeal Arch MusclesDerivatives of Pharyngeal Arch Muscles

• The muscles of the first pharyngeal arch forms the muscles of mastication

Derivatives of Pharyngeal Arch Muscles

• The second pharyngeal arch forms the :

1 - stapedius

2 - stylohyoid

3 - posterior belly of digastric

4 - auricular

5 - muscles of facial expression

Derivatives of Pharyngeal Arch Muscles

• The third arch forms the :

stylopharyngeus

Derivatives of Pharyngeal Arch Muscles

• The fourth arch forms :

1 - cricothyroid muscle

2 - levator veli palatini

3 - constrictors of pharynx

Derivatives of Pharyngeal Arch Muscles

• The sixth pharyngeal arch forms the intrinsic muscles of the larynx

Derivatives of Pharyngeal Arch Nerves

• The trigeminal nerve ( the fifth cranial nerve ) supply derivatives of the first pharyngeal arch by it’s caudal two branches

1 - maxillary branch

2- mandibular branch

Derivatives of Pharyngeal Arch Nerves

• The facial nerv ( VII ( supply the second arch

• The glossopharyngeal nerve (IX) supply the third arch

• The vagus nerve (X) supply the fourth and sixth arches by :

1 - superior laryngeal branch supply the 4th

2 - recurrent laryngeal branch supply the 6th

Pharyngeal Pouches

• The primordial pharynxThe primordial pharynx , derived from the foregut

widens cranially where it joins the primordial mouth or stomodeum

• It narrows caudally where it joins the esophagus

• The pharyngeal pouches are balloonlike diverticula that formed on the endodermal side between the pharyngeal archespharyngeal arches

• The pairs of pouches develop in a craniocaudal sequence between the arches

Pharyngeal Pouches

• The first pair of pouches lies between the first and second pharyngeal arches

• There are four well defined pairs of pharyngeal pouches

• The fifth pair is absent or rudimentary

Pharyngeal Pouches

• The connection between the endoderm of the pharyngeal pouchespharyngeal pouches and the ectoderm of the pharyngeal pharyngeal groovesgrooves form a double layered membrane called the pharyngeal membrane

that separate the pharyngeal pouches from the pharyngeal grooves

Derivatives of First Pharyngeal Pouch

• The first pharyngeal pouch expands into an elongate tubotympanic recess

Derivatives of First Pharyngeal Pouch

The expanded distal part of this recess contacts the first pharyngeal groove first pharyngeal groove , where it contributes to the formation of the tympanic membrane (eardrum)

The cavity of the tubotympanic recess gives rise to the tympanic cavity and mastoid antrum

Derivatives of Second Pharyngeal Pouch

•The second pharyngeal pouch is largely obliterated

( disappear ) as the palatine tonsils develop

• Part of the cavity of this pouch remains as the tonsillar sinus or fossa

Derivatives of Second Pharyngeal Pouch

The The endodermendoderm of the pouch of the pouch proliferates and grows into the proliferates and grows into the underlying mesenchymeunderlying mesenchyme

the central parts of these buds the central parts of these buds form crypts form crypts

Derivatives of Second Pharyngeal Pouch

• The endoderm of the pouch forms the surface epithelium and the lining of the tonsillar crypts

• At about 20 weeks the mesenchyme around the crypts differentiates into lymphoid tissue

• These tissues soon organize into the lymphatic nodules of the palatine tonsil

Derivatives of Third Pharyngeal Pouch

• The third pharyngeal pouch expands and develops a :

1- solid dorsal bulbar part

2- hollow elongate ventral part

• Its connection with the pharynx is reduced to a narrow duct that soon degenerates

• By the sixth week the epithelium of :

1- each dorsal bulbar part begins to differentiate into inferior parathyroid gland

2 - each elongate ventral parts begins to differentiate into primordia of thymus

• These bilateral primordia of thymus come together in the median plane to form thymus

• It descends into the superior mediastenum

• The bilobed form of thymus remains throughout life

Separately encapsulated and each lobe has its own blood supply, lymphatic drainage and nerve supply

Derivatives of Third Pharyngeal Pouch

• The primordia of thymus and inferior parathyroid glands lose their connections with the pharynx and migrate into the neck

• Later the inferior parathyroid glands separate from the thymus and lie on the dorsal surface of the thyroid gland

superior

inferior

Derivatives of Fourth Pharyngeal PouchDerivatives of Fourth Pharyngeal Pouch

• The fourth pharyngeal pouch also expands into

1- dorsal bulbar part

2- elongate ventral parts

• Its connection with the pharynx is reduced to a narrow duct that soon degenerates

• By the sixth week, each dorsal part develops into a superior parathyroid gland

It lies on the dorsal surface of the thyroid gland

Derivatives of Fourth Pharyngeal PouchDerivatives of Fourth Pharyngeal Pouch

•The parathyroid glands derived from the third pouches descend with the thymus

and are carried to a more inferior position than the parathyroid derived from the fourth pouches

This explains why the parathyroid glands derived from the third pair of pouches are located inferior to those from the fourth pouches

The Fifth Pharyngeal PouchThe Fifth Pharyngeal Pouch

• When this develops , this rudimentary pouch becomes part of the fourth pharyngeal pouch and helps to form the ultimopharyngeal body

Histogenesis of Parathyroid Gland

•The epithelium of the ]bulbar dorsal parts[ of the third and fourth pouches proliferates during the fifth week

• Forming small nodules on the dorsal aspect of each pouch

• soon the Vascular Vascular mesenchymemesenchyme grows into (enter ) these nodules, forming capillary network

• The chief or principal cells differentiate during the embryonic period and regulate fetal calcium fetal calcium metabolismmetabolism

•The oxiphil cells differentiate 5 to 7 years after birth

Histogenesis of Parathyroid Gland

• The elongated ventral part of each fourth pouch develops into ultimopharyngeal body

• Its cells disseminate ( spread ) within the thyroid gland , giving rise to parafollicular cells

•They are also called “C” cells because they produce calcitonin that regulate normal calcium level in body fluids

• “C” cells differentiate from neural crest cells

Pharyngeal grooves

• During the fourth and fifth weeks, head and neck region of the human embryo exhibit four pharyngeal grooves or clefts on each side

• These grooves separate the pharyngeal arches externally

• Only first pair persists as the external acoustic meatus ( external auditory canal )

• The other grooves normally obliterated ( disappear ) with the cervical sinus as the neck develops

Pharyngeal Membranes

• Pharyngeal membranes appear in the floor of the pharyngeal grooves

• These membranes formed where the epithelia of the grooves and pouches approach each other

• The endoderm of the pouches and ectoderm of the grooves are soon separated by mesenchyme

• Only first pharyngeal membrane becomes the tympanic membrane, others obliterate ( disappear )

pure s

pirit

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