Embryonic Chapter 3

Composition

The oropharynx is formed by the combination of the primitive oral cavity (oral pit) and the upper portion of the foregut, called the pharynx.

4th Week (Formation of the Oral Pit)

As the overlying brain expands during the 4th week, the embryonic disk bends down ventrally. This action pushes the developing heart beneath the brain and forms a pit in the midline between the forebrain and heart, creating an oral pocket known as the oral pit.

5th Week (Rupture of the Oropharyngeal Membrane)

The deepest extent of the oral pit is bounded by the oropharyngeal membrane. In the 5th week, this membrane ruptures, opening the primitive oral cavity directly to the tubular foregut to form the oropharynx.

Heart Position

The heart becomes positioned in the thorax below the mandibular arch and begins beating at the end of the 4th week.

Structure & Alignment

Pharyngeal arches bend around the pharynx as parallel bars of tissue, decreasing in size from anterior to posterior. Each arch contains its own blood vessels, muscles, nerve supply, and cartilaginous skeletal elements.

1st Arch (Mandibular Arch)

Forms the mandible and gives off the maxillary processes laterally to form the cheeks. Gives rise to the muscles of mastication (innervated by CN V - Trigeminal).

2nd Arch (Hyoid Arch)

Forms parts of the hyoid bone, as well as components of the external and middle ear. Gives rise to the muscles of facial expression (innervated by CN VII - Facial).

3rd, 4th, 5th, & 6th Arches

Form paired bars that are divided before reaching the midline by the heart. The 3rd and 4th arch muscles develop into the pharyngeal constrictor muscles of the throat (innervated by CN IX and X).

Overgrowth

By the 5th week, tissues of the 2nd and 5th arches grow over the middle arches and grooves, obscuring them externally.

Development of Pharyngeal Grooves & Pouches

The arches are separated externally by vertical pharyngeal grooves (clefts) and internally within the pharynx by endoderm-lined pharyngeal pouches:

Pharyngeal Grooves (External)

1st Groove:

Deepens to form the external auditory canal leading to the middle ear. The membrane at its depth becomes the tympanic membrane (eardrum).

2nd-5th Grooves

Overgrown externally by adjacent arches by the 5th week.

Pharyngeal Pouches (Internal Adult Derivatives)

1st Pouch

Differentiates into the middle ear cavity and Eustachian tube.

2nd Pouch

Differentiates into the palatine tonsils.

3rd Pouch

Differentiates into the inferior parathyroid glands and the thymus.

4th Pouch

Differentiates into the superior parathyroid glands.

5th Pouch

Differentiates into the ultimobranchial body

What important development occurs at the end of the 4th week?

At the end of the 4th week, the primary landmark development is that the enlarging heart becomes positioned in the thorax below the mandibular arch and begins beating.

During the 4th week overall,

several other critical craniofacial and embryonic developments take place:

Formation of the Oral Pit

As the forebrain expands and neural folds develop, the flat embryonic disk bends ventrally, pushing the developing heart beneath the brain and creating a midline pocket known as the oral pit (primitive oral cavity).

Mandibular Arch & Maxillary Processes

The first pharyngeal arch (mandibular arch) forms beneath the oral pit and gives off the maxillary processes laterally to form the cheeks.

Early Vascular Systems

The first and second aortic arch vessels begin to develop during this week.

Tongue Development

The body (anterior two-thirds) of the tongue begins developing from the first pharyngeal arch.

Mandibular Constriction

A midline constriction is normally observed in the developing mandible.

During which weeks do muscle cells start to mark the site of muscle origin?

Muscle cells first become apparent in the first pharyngeal arch during the 5th week. They then spread within the mandibular arch into each muscle site of origin during the 6th and 7th weeks (5th-7th weeks)

The cartilages of the face and craniofacial region arise early in development as the earliest formed skeletal elements in the craniofacial area. Cartilage serves as the initial skeletal component in the embryo before being gradually replaced by bone.

Midline Cartilages of the Cranial Base

The primary cartilages that underlie and support the developing brain form a single cartilaginous continuum in the midline known as the cranial base. These early cartilages include: Cartilaginous nasal capsule (ethmoid), Sphenoid cartilage, Auditory capsules, and Basioccipital cartilages

Later in development, these continuous cartilages separate into individual bones and undergo endochondral bone formation to transform into bony structures.

Pharyngeal Arch Cartilages

Cartilaginous bars also appear early within each pharyngeal arch to provide a temporary structural framework:

First Arch (Meckel Cartilage)

Acts as the initial framework for the mandible and terminates posteriorly into the malleus and incus.

Second Arch (Reichert / Hyoid Cartilage)

Gives rise to the stapes, styloid process, and the lesser cornu and upper body of the hyoid bone. Third Arch Cartilage Forms the greater cornu and lower body of the hyoid bone.

Fourth to Sixth Arch Cartilages

Form the laryngeal cartilages (such as the thyroid and cricoid cartilages).

Cartilaginous Skeletal Development

Early Framework: Cartilage serves as the initial skeletal component in the embryo, forming around the spinal cord to build the vertebral column and within the appendages, cranium, and face.

The Cranial Base

The earliest formed skeletal elements in the craniofacial area arise as a single cartilaginous continuum in the midline underlying the brain. Composed of the cartilaginous nasal capsule (ethmoid), sphenoid, auditory capsules, and basioccipital cartilages, this continuum forms the cranial base.

Endochondral Bone Formation

As the embryo grows, these midline cartilages separate into individual bones and are transformed into bone through endochondral bone development.

Pharyngeal Arch Cartilages

The initial skeleton of the pharyngeal arches also develops as cartilaginous bars. Posteriorly, these bars terminate in enlarged bilateral structures—the malleus, incus, and stapes—which later transform into bone.

Replacement of Meckel Cartilage

The bony mandible develops laterally to the first arch cartilage (Meckel cartilage) and grows posteriorly to join the bony body of the mandible with the cartilaginous condyle, replacing Meckel cartilage.

Early Mandibular Articulation (Weeks 0-20)

For the first 20 weeks of prenatal life, the primary articulation (joint) of the mandible is the contact point between the malleus and incus.

Functional Shift to the TMJ

After 20 weeks of prenatal development, the temporomandibular joint—formed by the articulation of the mandibular condyle and the temporal fossa—becomes functional.

Post-Developmental Persistence

Once adult bone formation is complete, cartilage remains restricted to specific sites, including joint surfaces like the TMJ condyle, the nasal septum, ears, and the heads of long bones.

Cellular Composition

Each connective tissue suture consists of a central zone of proliferating connective tissue cells, flanked by osteogenic cells along the peripheral bony fronts.

Growth & Inactivity

The relative positions of these articulations in the fetal skull are similar to those in an adult skull. All facial sutures eventually become inactive once facial growth is complete.

The Three Types of Facial Sutures

Simple Suture

An uncomplicated band of tissue between bony fronts.

Serrated Suture

An interdigitating type of suture.

Squamosal Suture

A beveled or overlapping type of junction.

Midline Articulations (Synchondroses)

In contrast to the fibrous sutures of the external face, midline articulations contain an interposing band of cartilage rather than just fibrous connective tissue, forming a synchondrosis:

Growth Mechanism

These grow by forming new cartilage in the center of the articulation while transforming cartilage into bone at its periphery. Examples: The ethmosphenoid and sphenoccipital articulations