ORAL HISTOLOGY AND EMBYOLOGY lecture - 8

DEVELOPMENT OF THE FACE

OVERVIEW Concept of brachial arch Origin of basic tissues Specific origin of head mesenchymes Development of the face

Craniofacial development is an extraordinarily complex process. It requires the orchestrated integration of multiple specialized tissues, such as The surface ectoderm Neural crest Mesoderm

The co ordination and integration of those tissues will eventually generate: Central and peripheral nervous systems Axial skeleton Dental tissues Musculature Connective tissues of the head and face.

Understanding the development of the structures of the face also requires knowledge of the pharyngeal or branchial arches. These arches form on either side of the foregut and correspond to the primitive branchial arches.

The pharyngeal arch Consists of a core of mesenchyme covered externally by ectoderm and internally by endoderm.

Components of branchial/pharyngeal apparatus: 1)Pharyngeal arches 2)Pharyngeal pouches 3)Pharyngeal clefts/grooves

Pharyngeal Pouches Four well-defined pairs of pharyngeal pouches develop from the lateral walls of the pharynx First pouch (betwen the 1 st and 2 nd arches) ; –external acoustic meatus, tympanic membrane, and eustachian tube Second pouch ; – palatine tonsils Third pouch ; –thyroid and parathyroid glands, Fourth pouch ; –parathryoid gland Fifth pouch ; –becomes incorporated into the fourth

Pharyngeal clefts/grooves (4) 1 st Cleft –external auditory meatus 2 nd -4 th cleft –epicardial ridge and cervical sinus (disappears)

Ectoderm –The ectoderm that contribute to the formation of the face are found around stomodeum around fourth week of embryonic life –Stomodeum is the primitive mouth –The ectoderm around the stomodeum are therefore responsible for the formation of: Face Oral cavity Nasal cavity

Mesoderm –These fills the pharyngeal arches –Derived from three sources 1.Paraxial mesoderm 2.Lateral plate mesorderm 3.Neural crest cells –Paraxial and lateral plate mesoderm → forms the musculature –Neural crest cells → skeletal portion of each arch

There are 5 pharyngeal arches numbered 1, 2, 3, 4 and 6. They developing in “craniocaudal” sequence They start to appears in the following days of embryonic life: the 1st pair on day 22 the 2nd and 3rd pairs on day 24 the 4th and 6th pairs on day 29.

Each pharyngeal arch contains; 1.An arch cartilage 2.An arch artery 3.A paraxial mesoderm component (muscle) 4.An arch-specific cranial nerve.

Each pharyngeal arch, except for the 6th arch, has on its caudal border: »A pharyngeal pouch internally »A pharyngeal cleft externally »A pharyngeal membrane separating the clefts and pouches. The pharyngeal arches and pharyngeal membranes have an outer covering of ectoderm, an inner lining of endoderm and an intermediate core of mesoderm.

The 1st arch cartilages are in 2 swellings: 1.The maxillary swelling Containing the palatopterygoquadrate bar cartilage, giving rise to »greater wing of sphenoid »incus »Maxilla ossify intramembranous 2.The mandibular swelling Containing Meckel’s cartilage, which gives: »malleus »fibrous core of the mandible NB mandible does not ossify from the cartilage.

The 2nd arch cartilage, Reichert’s cartilage –forms the stapes, styloid process, stylohyoid ligament, and parts of the hyoid. –Contain cranial nerve VII –Mesenchyme → form the muscles of facial expression, the middle ear muscles. The 3rd arch –Its cartilage contributes to the hyoid bone. –Tongue –Cranial nerve IX Fourth arch – Tongue –Most of the laryngeal cartilages –Cranial nerve (IX and X nerves) Fifth arch – becomes incorporated into the fourth Sixth arch – Most of the laryngeal cartilages –(IX and X nerves)

Pharyngeal arches

Congenital malformations (Birth defects)

Cleft lip

Bon appetite