Mouth Development & Induction

Mammal Mouth Development:

            As an example in the human, development of the face depends on epithelial - mesenchymal interactions. Most of the face is formed from the first 2 of 5 pharyngeal arches (Marino, Thomas PhD). Here we will be focusing on the formation of the human mouth.

            The oral cavity, the hole that connects the foregut to the amniotic fluid, develops from the contact of the embryonic endoderm with the ectoderm. This is one of two places in the human which lacks the mesoderm that usually lies between the two embryonic tissues. This contact induces the ectoderm to invaginate, forming the primitive mouth called the stomodeum (K.Tosney).  The cavity is originally covered by the buccopharyngeal membrane which breaks down to leave the open oral cavity (K.Tosney).

            The upper jaw of the human being develops from 5 primordial buds of tissue; the frontonasal process, the two nasal placodes, and the two maxillary swellings (Odaci, Ersan MD, PhD). The lower jaw develops from the two mandibular swellings (Odaci, Ersan MD, PhD). The facial primordia contribute to other parts of the face as well, such as;

• Frontonasal prominence - Forehead and the dorsum apex of the nose

• Lateral nasal prominences - Sides of the nose

• Medial nasal prominences - Nasal septum

• Maxillary prominences - Upper cheek region and most of the upper lip

• Mandibular prominences - Chin, lower lip, and lower cheek regions

• Mesenchyme in the facial prominences - Fleshy derivatives and various bones (Odaci, Ersan MD, PhD)

Human Embryo: The neural crest cells of the brain migrate to the mesodermal cells which together form the facial primordia.  

                                 Face of Chicken Embryo cut through first pharyngeal arch.

                                                                 Used and modified with permission � K Tosney

The frontonasal prominence and the nasal placodes come together to form the medial and the lateral nasal processes. The medial nasal processes then fuse to form the philtrum of the upper lip. (K.Tosney) 

The palate of the mouth is developed from the medial and lateral palantine processes growing out from the sides of the mouth cavity and fusing together when the meet at the middle. The fusion stops near the dorsal side of the head. The fusion of the palate and other parts of the face rely on apoptosis. (K.Tosney) 

The tongue develops on the floor of the oral cavity. Tongue formation requires assistance from four of the pharyngeal arches. It begins as lingual swellings which fuse together. The fusion of the two lateral lingual swelling creates and indentation called the median sulcus. (Hill, Mark Dr.) This is the visible part of a human tongue.       

            Arch 1 - oral part of tongue (anterior 3/2)

            Arch 2 - initial contribution to surface is lost

            Arch 3 - pharyngeal part of tongue (posterior 1/3)

            Arch 4 - epiglottis and adjacent regions

            (Hill, Mark Dr.)

The teeth of the human mouth form as neural crest molar mesenchyme condenses and induces jaw epithelium to thicken. Both the neural crest molar mesenchyme and jaw epithelium follow a series of reciprocal inductions to form teeth. (K.Tosney) 

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Puppeteer Mouth Formation:

�           The most obvious difference between mammals and ourselves, is that mammals only have one head with one mouth, whereas Puppeteers have two heads with one mouth on each head. This is a result of our diencephalon splitting early in embryonic development. The splitting causes two sets of fully functional pharyngeal arches to form.

�            Oddly, although we are such different organisms the formation of our mouth is fairly similar. The same interaction between endoderm and ectoderm causes the opening to our mouths.

�         Puppeteer jaws form from the same primordial facial buds. However, the maxillary swellings protrude farther forward. In the formation of facial bones of our upper and lower jaws grow farther forward from our eye creating what humans would call a �snout�.

�         Some differences can be found in our lips. They too, are developed from the maxillary and mandibular prominences. However, in our lips apoptosis divides our embryonic solid lip into small digit like sections. With the musculature of our face we are able to use our lips as a human might use a hand.

�         The formation of the puppeteer tongue is completed much like humans. The 4 pharyngeal arches participate and the primordial tongue buds fuse. However fusion is restricted along 1/3 of the anterior of the median sulcus. This is how the fork of our tongue is developed. The muscle in the tongue continues to grow outward stretching the tongue until it is longer and more slender then a human tongue.  

�         Our teeth are also slightly different. We have flat teeth, made for the eating of vegetation only. Humans, being omnivores have sharper teeth in the front and molars similar to ours in the back. The reciprocal inductive reactions that occur between the neural crest molar mesenchyme and the jaw epithelium are the same for our teeth as they are for humans.

� Leviathan Studios

(Click on Image to view Original Source)

Sources:

Hill, Mark Dr.. UNSW Embryology. 2005. <embryology.med.unsw.edu.au/Notes/head6.htm>.

Odaci, Ersan, MD, PhD. Face Embryology. 2005. <www.emedicine.com/ent/topic30.htm>

K.Tosney. Lecture Notes. Epithelial Mesenchymal Interactions. 2005.

Marino, Thomas PhD. Development of the Pharyngeal Arches. 2001. < isc.temple.edu/marino/embryology/parch98/parchdev.htm>