EMBRYOLOGYBY

Dr. THAAER MOHAMMED DAHER ALSAADSPECIALIST IN GENERAL SURGERY

M.B.Ch.B. (MBBS) F.I.B.M.S. )Ph.D.)SENIOR LECTURER

IMS MSU

The Pharyngeal

Arches

Highlights

• Introduction

• Derivatives of the skeletal elements

• Nerves and muscle of the arches

• Fate of ectodermal cleft

• Fate of endodermal pouches

• Development of the thymus

• Development of the parathyroid gland

• Development of thyroid gland

• Timetable of some mentioned events

Highlights (continue)

• Pharyngeal arches?

• Are rod-like thickening of mesoderm present in the wall of the foregut.

• How many arches are there?

• At first they are six aches. The fifth arch disappears and only five remain.

• Where the aches meet?

• The ventral ends of the arches of the right and left sides meet in the middle line in the floor of the pharynx.

Highlights (continue)

• Endodermal /pharyngeal pouches

• In the interval between any two arches, the endoderm is pushed outwards to form a series of pouches.

• Ectodermal cleft

• The surface ectoderm dips inwards opposite each pouch.

Highlights (continue)

• Each pharyngeal arch contains :1. a skeletal element (cartilage).

2. Striated muscle (supplied by a nerve of the arch and an arterial arch.

• The cartilage of the 1st arch (Meckle’s cartilage) gives rise to the incus and malleus.

• The cartilage of the 2nd arch forms a. The stapes.

b. The styloid process.

c. Part of the hyoid bone.

Highlights (continue)

• The cartilage of the 3rd arch forms the greater part of the hyoid bone.

• The cartilage of 4th and 6th arches give rise to the cartilage of the larynx.

• Nerves of the arches:• 1st arch -----------------------mandibular.• 2nd arch ----------------------facial.• 3rd arch ---------------------- glossopharyngeal.• 4th arch ---------------------- superior laryngeal.• 5th arch ---------------------- recurrent laryngeal.

The muscles are supplied by these nerves are derived from the mesoderm of the arch concerned.

Highlights (continue)

• The external acoustic meatus develops from the 1st ectodermal cleft.

• Tubotympanic recess develops from the 1st and 2nd endodermal pouch.

• The middle ear and the auditory tube develops from the tubotympanic recess.

• The palatine tonsil arises from the 2nd pouch.

• The inferior parathyroid gland and the thymus are derived from the 3rd pouch.

• The superior thyroid gland is derived from the 4th pouch.

• The thyroid gland develops mainly from the thyroglossalduct.• Thyroglossal duct is formed as a median diverticulum arising from the floor of

the pharynx (at the foramen caecum0.

Introduction

• After establishment of the head fold, the foregut is bounded ventrally by pericardium and dorsally by developing brain.

• Cranially, it is separated at first separated from the stomatodaeum by the buccopharyngeal membrane.

• When this membrane breaks down, the foregut opens to the exterior through the stomatodaeum.

• At this stage, the head is represented by the bulging caused by the developing brain, and

• While the pericardium occupying the future thorax,

• The two are separated by the stomatodaeum which is the future mouth.

• The neck is not yet present//////////

• The neck is formed by the elongation of the region between the stomatodaeum and the pericardium

Introduction (continue)

• The elongation is due to the appearance of the pharyngeal/ or branchial arches (the mesodermal thickening).

• At this stage, the wall of the foregut is separate from the surface ectoderm by a layer of mesoderm.

• The mesoderm comes to arrange in the form of six bars.• These bars run dorsoventrally in the side wall of the foregut.• Each of these “bars” grows ventrally in the floor of the developing pharynx and

fuses with the corresponding “bar” of the opposite side to form a PHARYNGEAL or BRANCHIAL ARCH.

• In the interval between any two adjoining arches, the endoderm extends outwards in the form of a pouch (endodermal or pharyngeal pouch) to meet the ectoderm which dips into this interval as an ECTODERMAL CLEFT.

• The 1st arch is also called he mandibular arch;

• The 2dn arch is called,the hyoid arch.

• The 3rd , 4th ,and 6th arches do not have special names.

• The 5th arch disappears soon after its formation, so that only five arches remain.

Introduction (continue)

• The following structures are formed in the mesoderm of each arch:

1.Skeletal element.

2.Striated muscle.

3.Arterial arch.

Introduction (continue)

• A skeletal element:• This is cartilaginous to begin with.

• It may remain cartilaginous,• It may develop to bone,

• It may disappear.

• Striated muscle:• Supplied by nerve to the arch.• May , or may not, retain its attachment to the skeletal

elements derived from the arch.• May subdivide to form a number of distinct muscles, which

may migrate away from the pharyngeal region.• When they do so, they carry their nerve with them,• And their embryological origin can determined from their

nerve supply.

Introduction (continue)

• An arterial arch:

• Ventral aorta develops ventral to the foregut.

• Dorsal aorta develops dorsal to the foregut.

• Aortic arches (a series of arches) connect ventral and dorsal aortae.

• One arterial arch lies in each pharyngeal arch.• The arrangement of these arteries will be greatly modified ( will be discussed later).

• Each pharyngeal arch is supplied by a nerve.

• In addition to supplying the skeletal muscle of the arch, it supplies sensory branch to the overlying ectoderm and endoderm.

Introduction(continue)!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! You can skip it!!!

• In the human embryo, a double innervation is seen only in the 1st pharyngeal arch.

• In some lower animals, each arch is supplied by two nerves:

A. Post-trematic nerve.

B. Pre-trematic nerve.

(Trema = trench).

The nerve that runs along the cranial border of the arch is called the post-termatic nerve.

The nerve that runs along the caudal border of the arch is called the pre-termatic nerve.

Derivative of the

Skeletal Elements

1. First Arch Derivatives

1. The cartilage of the 1st arch is called Meckel’s cartilage:the following structures are formed from the dorsal endcartilage of the 1st arch:

a) Incusb) Malleusc) Anterior ligament of the malleusd) Sphenomandibular ligamente) Bone (the maxilla, the mandible, the zygomatic bone, the

palatine bone, and part of the temporal bone).///// The ventral part of the cartilage is absorbed. (it is surrounded by the developing mandible and is absorbed)

2. Second Arch derivatives

the following structures 5S are formed from

the cartilage of the 2nd arch:

A. Stapes

B. Styloid process

C. Stylohyoid ligament (from sheath)

D. Smaller (lesser) cornu of hyoid bone

E. Superior part of body of hyoid bone

3. 3rd Arch derivatives

• the following structures are formed from the cartilage of the 3rd arch:

a) Greater cornu of hyoid bone

b) Lower part of the body oh hyoid bone

4. The cartilages of the larynx

• are derived from the 4th and 6th arches

• with possible contribution from the 5th arch

• but, their exact derivation is controversial.

Nerves and Muscles of the Arches

• All muscles derived from a pharyngeal arch are supplied by the nerve of the arch.

• Muscles can be identified by their nerve supply.

• The nerves supply :

• muscles,

• parts of skin

• and mucous membrane derived from the arch.

• The 1st arch has a double nerve supply; (anterior 2/3rd tongue)

• Mandibular nerve is the post-terematic nerve of the 1st arch.

• Chorda tympani is the pre-trematic nerve of the 1st arch

• Anterior 2/3rd of the tongue is derived from the ventral part of the

1st arch.

Arch Nerve of the Arch Muscles of the Arch

First Mandibular Medial + lateral pterygoid, Masseter,

Temporalis, Mylohyoid, Anterior belly of digastric, Tensor tympani, Tensor palati.

Second Facial Muscles of the face, Occipitofrontalis,Platysma, stylohyoid, Posterior belly of digasrtic, Stapedius, Auricular muscles

Third Glossopharyngeal Stylopharyngeus

FourthSixth

Superior laryngeal

Recurrent laryngeal

Muscles of the larynx and pharynx

Fate of Ectodermal Clefts

• 1st cleft: • The ventral part is obliterated.

• The dorsal part:

1. Epithelium lining of the external auditory meatus.

2. Pinna

• 2nd arch,

• Grows much faster than the succeeding arch and comes overhand them.

• The space between the overhanging 2nd arch and the 3rd, 4th, and 6th arches is called cervical sinus.

• Cervical sinus;• The lower overhanging border of the 2ns arch fuses with

tissues caudal to the arches.

• The side of the neck becomes smooth.

• The cavity of cervical sinus become normally obliterated.

• Part of it may persist and give rise to branchial cyst.

• The cyst may open onto the surface ---- branchial fistula.

• Rarely it may open into the lumen of the pharynx in the region of the tonsil.

Fate of Ectodermal Clefts

Fate of Endodermal Pouches

• 1st pouch;• The dorsal part is obliterated by formation of the tongue.

• The ventral part receives contribution from the 2nd pouch,

• These two form a diverticulum, called;

• Tubotympanic recess. (proximal and distal part).

• Proximal part --- auditory tube (pharyngotympanic).

• Distal part ----- middle ear cavity (including the tympanic antrum).

The derivatives of the branchial pouches. Note that the inferior parathyroid migrates downwards from the 3rd pouch whereas the superior parathyroid (4th pouch) remains stationary.

2nd pouch• The epithelium of the ventral part contributes to the

formation of the tonsil.

• The dorsal part --- tubotympanic recess.3rd pouch gives rise to the inferior parathyroid glands.

4th puocho Gives origin to the superior parathyroid glands.o May contribute to the thyroid gland.

Fate of Endodermal Pouches (continue)

• 5th = ultimobranchial pouch

• Seen for a brief period during development.

• In some species it gives rise to the ultimobranchial body.

• Its fate is controversial in human.

• Generally it is believed to be incorporated into the 4th pouch.

• 4th + 5th pouches form the

• Caudal pharyngeal complex. This gives rise to the:

1. Superior parathyroid glands.

2. Parafollicular cells of the thyroid gland.

Fate of Endodermal Pouches (continue)

Development of the Thymus• Arise from the epithelium of the 3rd pharyngeal pouch.• Relatively large at birth.• Continues to increase in weight till puberty.• Then it gradually undergoes atrophy.• Early in development, the 3rd pouch is cut off, both from the pharyngeal wall

and from the surface ectoderm.• After separating from the inferior parathyroid rudiment, each thymic rudiment

has;• Thinner cranial part and broader caudal part.• Thinner portion forms the cervical part of the thymus;• Boarder parts of both sides enter the thorax and become united to each other

by connective tissue.• Thymic endoderm is invaded by vascular mesoderm.• Vascular mesoderm contains numerous lymphoblasts.• Mesenchymal invading breaks up the thymic tissue into isolated masses, and

gives the thymus its lobulated appearance.• Accessory thymic tissue may develop from fragmentation oh the cervical part.• Accessory thymic tissue, present in relation to the SPG, and is believed to rise

from the 4th pouch.

Development of Parathyroid Glands

• Inferior parathyroid glands; parathyroid III

• Develop from endoderm of the 3rd pharyngeal pouch.

• Superior parathyroid glands; parathyroid IV.

• Develop from endoderm of the 4th pharyngeal pouch.

• Superior parathyroid glands are constant in position.

• Inferior parathyroid glands are carried caudally due to its closely related to thymus.

• As thymus descends caudally parathyroidIII becomes caudal to parathyroid IV.

The normal sites of the parathyroid glands (posterior aspect).

• Thyroid gland develops mainly from the thyroglossal duct.

• Parafollicular cells are derived from the caudal pharyngeal complex (from 4th and 5th pharyngeal pouch).

• After formation of the pharyngeal arches, the medial ends of the two mandibular arches are separated by

• Tuberculum impar =

A midline structure in the floor of the pharynx.

• Immediately the tuberculum impar the epithelium of the floor of the pharynx shows a thickening.

• This thickening is soon depressed to form a diverticulum called the thyroglossal duct.

Development of Thyroid Gland

• The site of the diverticulum is now seen as a depression called the foramen caecum.

• The diverticulum grows down in the midline into the neck.

• Its tip bifurcates.

• Proliferation of the cells of this bifid end gives rise to the

• Two lobes of the thyroid gland.

• The developing thyroid gland comes into intimate contact with

The caudal pharyngeal complex and fuses with it.

• Cells arising from this complex are believed to give origin to the parafollicular cells of the thyroid.

Development of Thyroid Gland (continue)

Anomalies of the Thyroid gland

1. Anomalies of the shape.

2. Anomalies of the position.

3. Ectopic thyroid tissue.

4. Remnants of the thyroglossal duct.

Anomalies of the shape

1. Isthmus may be absent

2. One of the lobe may be very small, or absent

3. Pyramidal lobe:a) Regarded as a normal structure.

b) May arise from the isthmus, or from one of the lobes.

c) May have no connection with rest of the thyroid.

d) May be divided into two or more parts.

e) Its extent vary from a short stump to a process reaching the hyoid bone.

Anomalies of the position

1. Lingual thyroid.

2. Intra-lingual thyroid.

3. Suprahyoid thyroid.

4. Infrahyoid thyroid.

5. Intrathoracic thyroid.

when thyroid tissue is present in the anomalous position, an additional thyroid may or may not be present at the normal site.

Normal and abnormal sites of the parathyroid glands (lateral view).

Ectopic thyroid tissue

• Small masses of thyroid tissue may be present at abnormal sites.

• Thyroid tissue may be observed in the:

• Larynx, trachea.

• Esophagus.

• Pons.

• Pleura.

• Pericardium.

• Ovaries.

Lateral aberrant thyroids =

Masses of ectopic thyroid tissue have been described in relation to the deep cervical lymph node.

Remnants of the thyroglossal duct

• These remnant may persist and lead to the formation of the following:

1. Thyroglossal cyst, that may be anywhere along the course of the duct.

2. Thyroglossal fistula, internal papilla (opening) at foramen caecum.

3. Carcinoma of the thyroglossal cyst.

Timetable of Some Events Mentioned in this Chapter

Age Developmental events

4th week (22nd day) Appearance of the 1st and 2nd arches

5th week (29th day) Four arches are seenThyroid, parathyroid and thymus are start forming

7th week Thyroid gland reaches its definitive position

pharyngeal arch derivatives

• The mesodermal core of each pharyngeal arch differentiates into three main types of tissue, and each arch becomes associated with a particular cranial nerve.

• The tissues are:• Skeletal tissue

(eg: cartilage, bone, ligaments).

• Muscle tissue (striated musculature, but not all under voluntary control).

• Arterial arch (which may or may not be converted into a definitive major artery).

pharyngeal pouches

• on the inside of the embryonic pharyngeal region are endodermally-lined grooves between adjacent pharyngeal arches.

• They contribute to the development of a surprising diversity of structures, ranging from the middle ear cavity to endocrine glands and components of the lymphatic system.

• The last two pouches are difficult to distinguish clearly, and are often considered as a single unit.

pharyngeal clefts

• are ectodermally-lined grooves on the outside of the embryonic pharynx.

• Only the first cleft is important - it develops into the external auditory meatus of the ear and provides the outer epithelium of the tympanic membrane.

• (It used to be believed that the remaining clefts were ‘submerged' beneath overgrowing folds and then disappeared, but more recent work shows that the clefts simply ‘fill out' by proliferation of the underlying mesoderm).!!!!!!!

Pouch Derivatives• 1st middle ear cavity, endodermal aspect of tympanic membrane,

pharyngotympanic tube.

• 2nd palatine tonsil.

• 3rd inferior parathyroid gland, thymus.

• 4th & 5th superior parathyroid gland, parafollicular cells of thyroid gland.

• Cleft Derivatives• 1st external auditory meatus, ectodermal aspect of tympanic

membrane.

• 2nd – 4 cervical sinus.

MalleusIncus

Stapes

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