EMB ch4 Head & Neck Development

What structure forms when the oropharyngeal membrane disintegrates during the fourth week?

The stomodeum, or primitive mouth

What are the five facial processes that develop around the stomodeum?

One frontonasal process, two maxillary processes, and two mandibular processes

Which facial processes are the first to fuse and form the mandibular arch?

The paired mandibular processes

What structure does the mandibular arch develop into?

The lower dental arch, or mandible

What surface landmark on the adult mandible marks the site of fusion of the mandibular processes?

The mandibular symphysis

What cartilage forms within each side of the mandibular arch during its growth?

Meckel cartilage

By what process does the bony mandible form in close association with Meckel cartilage?

Intramembranous ossification

What is the mandibular arch also known as, in terms of pharyngeal/branchial arch numbering?

The first branchial or pharyngeal arch

Which cranial nerve innervates the muscles derived from the mandibular arch?

The trigeminal nerve, cranial nerve V

Name the four muscles of mastication that arise from the mesoderm of the mandibular arch.

Masseter, temporalis, and the medial and lateral pterygoids

From what does the maxillary process arise on each side of the stomodeum?

Increased growth of the mandibular arch tissue

What thickened patches of ectoderm form on the frontonasal process and give rise to the nose?

Nasal placodes

What two processes form around each nasal placode/pit?

The medial nasal process and the lateral nasal process

Which processes fuse with each other and with the maxillary process to form the upper lip and philtrum?

The two medial nasal processes fuse with each other and each fuses with the maxillary process on its side

What facial groove between the nose and upper lip results from medial nasal process fusion?

The philtrum

What clinical condition results from failure of fusion between the maxillary process and the medial nasal process?

Cleft lip

What is the difference between a unilateral and bilateral cleft lip?

Unilateral cleft lip occurs on one side of the midline; bilateral cleft lip occurs on both sides

What tissue factor can prevent mesenchyme from growing together across two fusing facial processes?

Trapped ectodermal tissue that fails to disappear within the fusing processes

What are the three components of the branchial (pharyngeal) apparatus?

Branchial arches, branchial grooves, and pharyngeal pouches

How many branchial arches are there, and which one is often poorly defined or absent?

Six arches total; the fifth pouch/structure is rudimentary or absent

What is the difference between branchial grooves and pharyngeal pouches?

Grooves are external indentations between arches; pouches are internal, endoderm-lined evaginations between arches

How many pairs of pharyngeal pouches develop, and which pair is absent or rudimentary?

Four pairs develop; the fifth pharyngeal pouches are absent or rudimentary

What structure does the first pharyngeal pouch give rise to?

The auditory (pharyngotympanic) tube

What structure does the second pharyngeal pouch give rise to?

The palatine tonsils

What structures do the third and fourth pharyngeal pouches give rise to?

The parathyroid glands and the thymus gland

What is notable about the embryonic origin of the thymus gland?

It arises mainly from pharyngeal pouch endoderm but also has a contribution from ectoderm

What happens to the thymus gland's relative size during puberty?

It starts to diminish in relative size

What clinical condition results from persistence of second branchial groove tissue that should have disappeared?

A branchial cleft cyst

Where in the neck do branchial cleft cysts classically present?

Along the anterior border of the sternocleidomastoid muscle, often near the angle of the mandible

What are the typical clinical features of a branchial cleft cyst?

A slowly enlarging, painless swelling in the lateral neck

What cell type migrates into the facial and pharyngeal region and forms the core mesenchyme of the facial processes and arches?

Neural crest cells (NCCs)

What is the key difference between facial process fusion and the type of fusion seen in a furrow between two adjacent swellings on the same surface?

Facial process fusion joins two separate structures from two different surfaces, while furrow fusion involves tissue on the same surface flattening and disappearing rather than joining separate structures