19.1 Face and upper foregut

• Introduction
  
• Development of the facial region
• Oral cavity
• Tongue
• Development of the teeth
• Development of the salivary glands
• Definitive pharynx: Derivatives of the first and second pharyngeal arches
  • Thyroid
  • Middle ear
    
  • Tonsils
• Derivatives of the third and fourth pharyngeal arches
  • Thymus
  • Parathyroid glands and ultimopharyngeal bodies
    
• Lungs

Oral cavity

Following the tearing of the oropharyngeal membrane in the young embryo, the ectoderm-covered stomodeum, which gets surrounded by the mandibular prominences, goes directly over into the endoderm-covered foregut. It is also called a primary oral cavity because at this time no nasal cavity yet exists. (For the forming of the exterior nose, see facial development).

Only through the formation of the nasal sacs are the primary palates (= floors of the two nasal sacs) formed that divide the primary oral cavity into oral and nasal cavities. The wall between the two nasal sacs is called the nasal septum.

Using the two interactive diagrams the relationships can be studied in detail.

• View from the oral cavity towards its roof   (1.9 Mb)
  
• View from the oral cavity towards its roof shown as if it were transparent
  (1.7 Mb)

The adenohypophysial primordium (AH) appears in stage 10 (28 days) in front of the oropharyngeal membrane as thickening of the ectoderm. It develops in the immediate vicinity of the basal plate of the neural tube (prosencephalon [forebrain]).
At the same time a sacculation arises from this part of the basal plate, the neurohypophysis (NH). This basal plate belongs to the future diencephalon (part of the prosencephalon). The AH primordium forms a pouch (Rathke's pouch) in stage 13 (32 days), which remains in constant contact with the NH of the diencephalon and later surrounds this in the front. (1)

Fig. 8
The adenohypophysis forms from a small pouch coated with ectoderm (Rathke's pouch) that forms in front of the oropharyngeal membrane and with the increasing flexion is displaced into the pharyngeal roof.

Subsequently, the AH loses the contact to the ectoderm and, through further embryonic flexion and the expansion of the prosencephalon (= future di- and telencephalon) gets displaced at the cerebral base together with the NH.
With the facial formation through the spreading maxillar and mandibular arches the AH orifice is displaced into the pharynx roof. The skull base (os sphenoideum) forms between the pharynx roof and the AH in the later embryonic period.

Fig. 9
The orifice of the adenohypophysis gets displaced into the pharynx roof by the growing facial processes. In stage 17 (ca. 41 days) the connection is still open, but is obliterated a short time later.