Head and Neck Mesenchyme for formation of the head region is derived from - PowerPoint Presentation

Slide1

Head and Neck

Mesenchyme for formation of the head region is derived from

1-Paraxial mesoderm

2-Latral plate mesoderm,

3-Neural crest,

4-And thickened regions of ectoderm known as ectodermal

placodes

.

Slide2

Paraxial mesoderm

1-(

S

omites

and

Somitomeres

) forms the

floor

of

the brain case and a small portion of the

occipital

region

.

2-All

voluntary muscles of the

craniofacial

region

3-Dermis

.

4-Connective

tissues in the dorsal region

of the head

.

5- Meninges

caudal to the

prosencephalon

.

Slide3

Lateral plate mesoderm: Forms the laryngeal cartilages (arytenoid and cricoid) and connective tissue in this region.

Neural crest cells

originate in the

1-neuroectoderm of forebrain, midbrain, and hindbrain regions and migrate ventrally into the pharyngeal arches In these locations, they form

midfacial

and pharyngeal arch skeletal structures (

Fig. 16.1

)

Slide4

and all other tissues in these regions, including

2-Cartilage

,

3-Bone

,

4-Dentin,

5-Dendon

,

6-Dermis,

7-

Pia

and arachnoid,

8-Sensory

neurons,

9-And

glandular stroma

..

Slide5

.Cells from ectodermal

placodes

, together with neural crest, form neurons of the fifth, seventh, ninth, and 10th cranial sensory ganglia.

1975

The

most typical feature in development of the head and neck is formed by the

pharyngeal or

branchial

arches.

These arches appear in the fourth and fifth weeks of development and contribute to the characteristic external appearance of the embryo (

Table 16.1

and

Fig. 16.3

).

Slide6

Skeletal structures of the head and face. Mesenchyme for these structures is derived from neural crest (

blue

), lateral plate mesoderm (

yellow

), and paraxial mesoderm (

somites

and

somitomeres

) (

red).

Slide7

Pharygeal arch ,Initially, they consist of bars of mesenchymal tissue separated by deep clefts known as

pharyngeal

clefts

see also.

Simultaneously, with development of the arches and clefts, a number of

outpocketings

, the

pharyngeal pouches

Slide8

PHARYNGEAL ARCHESEach pharyngeal arch consists of a core of mesenchymal tissue covered on the outside by surface ectoderm and on the inside by epithelium of endodermal origin (

Fig. 16.6

). In addition to mesenchyme derived from paraxial and lateral plate mesoderm, the core of each arch receives substantial numbers of neural crest cells, which migrate into the arches to contribute to skeletal components of the face. The original mesoderm of the arches gives rise to the musculature of the face and neck.

Slide9

Thus, each pharyngeal arch is characterized by its own muscular components. The muscular components of each arch have their own cranial nerve, and wherever the muscle cells migrate, they carry their nerve component with them (

Figs. 16.6

and

16.7

). In addition, each arch has its own arterial component

.

(Derivatives of the pharyngeal arches and their nerve supply are summarized in

Table 16.1, p.

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First Pharyngeal ArchThe first pharyngeal arch consists of a

1-

D

orsal

portion,

the maxillary process, which extends forward beneath the region of the eye, and a

2-

V

entral

portion

, the mandibular process, which contains

Meckel's

cartilage (

Figs. 16.5

and

16.8A

). During further development,

Meckel's

cartilage disappears except for two small portions at its dorsal end that persist and form the

incus and malleus

(

Figs. 16.8B

and

16.9

).

Mesenchyme

of the maxillary process gives rise to the premaxilla, maxilla, zygomatic bone, and part of the temporal bone through membranous ossification (

Fig

. 16.8B

).

Slide12

The mandible is also formed by membranous ossification of mesenchymal tissue surrounding Meckel's cartilage. In addition, the first arch contributes to formation of the bones of the middle ear (see

Chapter 18

Slide13

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Musculature of the first pharyngeal arch includes the muscles of

1-Mastication

(temporalis, masseter, and

pterygoids

),

2-Anterior

belly of the digastric,

3-Mylohyoid

,

4-Tensor

tympani, and

5-Tensor

palatini

. The nerve supply to the muscles of the first arch is provided by the mandibular branch of the trigeminal nerve (

Fig. 16.7

). Since mesenchyme from the first arch also contributes to the dermis of the face, sensory supply to the skin of the face is provided by ophthalmic, maxillary, and mandibular branches of the trigeminal nerve.

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Second Pharyngeal ArchThe cartilage of the second or hyoid arch (

Reichert's cartilage

) (

Fig. 16.8B

) gives rise to

1-The

stapes,

2-Styloid

process of the temporal bone,

3-Stylohyoid

ligament,

4-And

ventrally, the lesser horn and

5-Upper

part of the body of the hyoid bone (

Fig. 16.9

). Muscles of the hyoid arch are the

1-Stapedius

,

2-Stylohyoid

,

3-Posterior

belly of the digastric,

4-Auricular

, and

5-Muscles

of facial expression.

The

facial nerve, the nerve of the second arch, supplies all of these muscles

Slide16

Third Pharyngeal ArchThe cartilage of the third pharyngeal arch produces the

1-lower

part of the body and

2-greater

horn of the hyoid bone

.

The

musculature is limited to the

stylopharyngeus

muscles

.

These

muscles are innervated by the glossopharyngeal nerve, the nerve of the third arch (

Fig. 16.7

).

Slide17

Fourth and Sixth Pharyngeal ArchesCartilaginous components of the fourth and sixth pharyngeal arches fuse to form the

1-Thyroid

,

2-Cricoid

,

3-Arytenoid,

4-

Corniculate

, and

5-Cuneiform

cartilages of the larynx (

Fig. 16.9

). Muscles of the fourth arch

(

cricothyroid

,

levator

palatini

, and

constrictors of the pharynx)

are innervated by the superior laryngeal branch of the

vagus

, the nerve of the fourth arch

. Intrinsic muscles

of the larynx are supplied by the recurrent laryngeal branch of the

vagus

, the nerve of the sixth arch.

Slide18

PHARYNGEAL POUCHESThe human embryo has four pairs of pharyngeal pouches; the fifth

is

rudimentary .Since

the epithelial endodermal lining of the pouches gives rise to a number of important organs, the fate of each pouch is discussed separately. Derivatives of the pharyngeal pouches are summarized in

Table 16.2, p. 273

.

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First Pharyngeal PouchThe first pharyngeal pouch forms a stalk-like diverticulum, the

tubotympanic

recess, which comes in contact with the epithelial lining of the first pharyngeal cleft, the future external auditory meatus (

Fig. 16.10

). The distal portion of the diverticulum widens into a sac-like structure, the primitive tympanic or middle ear P.272 cavity, and the proximal part remains narrow, forming the auditory (

eustachian

) tube. The lining of the tympanic cavity later aids in formation of the tympanic membrane or eardrum (see

Chapter 17

).

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Second Pharyngeal PouchThe epithelial lining of the second pharyngeal pouch proliferates and forms buds that penetrate into the surrounding mesenchyme. The buds are secondarily invaded by mesodermal tissue, forming the

primordium

of the palatine tonsils

(

Fig. 16.10

). During the third and fifth months, the tonsil is infiltrated by lymphatic tissue. Part of the pouch remains and is found in the adult as the

tonsillar

fossa

.m

Slide22

Third Pharyngeal PouchThe third and fourth pouches are characterized at their distal extremity by a dorsal and a ventral wing (

Fig. 16.10

). In the fifth week, epithelium of the

1-dorsal

region of the third pouch differentiates into the

inferior parathyroid gland

, while the

2-ventral

region forms the

thymus

.

Both

gland

primordia

lose their connection with the pharyngeal wall, and the thymus then migrates in a caudal and a medial direction, pulling the inferior parathyroid with it

Slide23

Growth and development of the thymus continue until puberty. In the young child, the thymus occupies considerable space in the thorax and lies behind the sternum and anterior to the pericardium and great vessels. In older persons, it is difficult to recognize, since it is atrophied and replaced by fatty tissue.

The parathyroid tissue of the third pouch finally comes to rest on the dorsal surface of the thyroid gland and forms the inferior parathyroid

Slide24

Fourth Pharyngeal PouchEpithelium of the

dorsal

region

of the fourth pharyngeal pouch forms the superior parathyroid gland. When the parathyroid gland loses contact with the wall of the pharynx, it attaches itself to the dorsal surface of the caudally migrating thyroid as the superior parathyroid gland (

Fig. 16.11

).

The

ventral region

of the fourth pouch gives rise to the

ultimobranchial

body, which is later incorporated into the thyroid gland. Cells of the

ultimobranchial

body give rise to the

parafollicular

, or C, cells of the thyroid gland. These cells secrete calcitonin, a hormone involved in regulation of the calcium level in the blood

Slide25

PHARYNGEAL CLEFTSThe 5-week embryo is characterized by the presence of four pharyngeal clefts (

Fig. 16.6

), of which only one contributes to the definitive structure of the embryo. The dorsal part of the first cleft penetrates the underlying mesenchyme and gives rise to the

external auditory meatus

(

Figs. 16.10

and

16.11

). The epithelial lining at the bottom of the meatus participates in formation of the eardrum (see

Chapter 18

).

Slide26

Active proliferation of mesenchymal tissue in the second arch causes it to overlap the third and fourth arches. Finally, it merges with the

epicardial

ridge in the lower part of the neck (

Fig. 16.10

), and the second, third, and fourth clefts lose contact with the outside (

Fig. 16.10B

). The clefts form a cavity lined with ectodermal epithelium, the

cervical sinus

, but with further development, this sinus disappears.

Slide27

TONGUEThe tongue appears in embryos of approximately 4 weeks in the form of two lateral lingual swellings and one medial swelling, the

tuberculum

impar

These three swellings originate from the

first pharyngeal arch

. A second median swelling, the copula, or

hypobranchial

eminence, is formed by mesoderm of the second, third,

and

ventral

part

of

the fourth arch. Finally,

a third median swelling

, formed by the posterior part of the fourth arch, marks development of the epiglottis.

Slide28

As the lateral lingual swellings increase in size, they overgrow the tuberculum

impar

and merge, forming the anterior two-thirds, or body, of the

tongue m

.

Since the mucosa covering the body of the tongue originates from the first pharyngeal arch, sensory innervation to this area is by the mandibular branch of the

trigeminal nerve

. The body of the tongue is separated from the posterior third by a V-shaped groove, the terminal sulcus (

Fig. 16.17B

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Slide30

The posterior part, or root, of the tongue originates from the second, third, and part of the fourth pharyngeal arch. The fact that sensory innervation to this part of the tongue is supplied by the

glossopharyngeal

nerve indicates that tissue of the third arch overgrows that of the second.

The epiglottis and the extreme posterior part of the tongue are innervated by the

superior

laryngeal nerve

,

reflecting their development from the fourth arch. Some of the tongue muscles probably differentiate in situ, but most are derived from myoblasts originating in occipital

somites

. Thus, tongue musculature is innervated by the

hypoglossal nerve.

Slide31

The general sensory innervation of the tongue is easy to understand. The body is supplied by the trigeminal nerve

, the nerve of the first arch; that of the root is supplied by the glossopharyngeal and

vagus

nerves, the nerves of the third and fourth arches, respectively. Special sensory innervation (taste) to the anterior two thirds of the tongue is provided by the

chorda tympani branch of the facial nerve,

while the posterior third is supplied by the glossopharyngeal nerve

Slide32

THYROID GLANDThe thyroid gland appears as an epithelial proliferation in the floor of the pharynx between the

tuberculum

impar

and the copula at a point later indicated by the foramen cecum (

Figs. 16.17

and

16.18A

). Subsequently, the thyroid descends in front of the pharyngeal gut as a

bilobed

diverticulum (

Fig. 16.18

). During this migration, the thyroid remains connected to the tongue by a narrow canal, the

thyroglossal

duct. This duct later disappears

Slide33

With further development, the thyroid gland descends in front of the hyoid bone and the laryngeal cartilages. It reaches its final position in front of the trachea in the seventh week (

Fig. 16.18B

). By then, it has acquired a small median isthmus and two lateral lobes. The thyroid begins to function at approximately the

end of the third month,

at which time the first follicles containing colloid become visible. Follicular cells produce the colloid that serves as a source of

thyroxine

and

triiodothyronine

.

Parafollicular

, or C, cells derived from the

ultimobranchial

body (

Fig. 16.10

) serve as a source of calcitonin.

P.279

Slide34