In all vertebrates, the mouth opening appears rather late in embryonic life after all the primary organ rudiments have already been formed. The mouth opening breaks through where the anterior end of the endodermal part of the alimentary canal touches the ectodermal epidermis beneath the front end of the neural tube. Here the ectodermal epidermis sinks in to form a pocket-like depression, the ectodermal mouth invagination or the stomodeum.
The ectodermal epithelium and the endodermal epithelium fuse in this area and become an oropharyngeal membrane. The membrane becomes very thin and eventually disappears. Forthwith, the boundary between the ectodermal and endodermal epithelia becomes indistinguishable, and it is not an easy matter to determine later which part of the oral cavity is derived from the stomodeum and which from the endodermal gut. There are, however, indications that the stomodeal epithelium extends (in mammals) to about the middle of the tongue ventrally and to the beginning of the pharynx on the dorsal side.
The determination of the stomodeum begins at the end of gastrulation, and the development of the ectodermal mouth invagination is induced by the endoderm when the anterior end of the archenteron comes in contact with the ectoderm of the presumptive mouth region.
No mesoderm ever penetrates here between the ectoderm and endoderm, and the endoderm remains in contact with the ectoderm throughout the subsequent stages until the rupture of the pharyngeal membrane. After the endoderm and ectoderm come in contact in the oral region, the oral ectoderm acquires, to some extent, the ability to differentiate as stomodeum.
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The anterior part of the archenteron may now be removed, and the remaining ectoderm produces a mouth invagination, although there are no endodermal oral and branchial parts behind. Mouth invaginations thus developed are small, and their shape is not normal. If, however, the operation is performed slightly later, in the neurula stage, the stomodeum may develop in an almost completely normal way, with the sole reservation that it does not lead into the endodermal alimentary canal.
As the oral ectoderm acquires the ability for self-differentiation, all the rest of the epidermis loses the competence for development into stomodeum. If the region where the mouth is to develop is covered with a flap of epidermis taken from a different part of the body, the development of the stomodeum is suppressed, in spite of the presence of oral endoderm.
Presumptive epidermis of the stomodeum may develop a mouth invagination in an abnormal position, but it can do so only under certain conditions. In newts, stomodeal epidermis of a neurula, when transplanted alone, will not develop a mouth, but when it is transplanted together with some adjoining endoderm, and if the site of transplantation is not too far from the oral region of the host, an additional stomodeal invagination will be formed by the graft, and this invagination may break through into the endodermal cavity of the host.
In frogs, the determination of the mouth ectoderm proceeds more rapidly than in urodeles, and the capacity for differentiation in the determined direction is higher. Already in the early neurula, the removal of oral endoderm does not interfere with mouth development, and transplants of presumptive mouth ectoderm may develop in a typical way, producing the characteristic horny jaws and horny teeth not only in the vicinity of the normal mouth but also in other locations, such as on the belly.
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For the mouth to be developed in abnormal regions, however, it is necessary to transplant a piece of the transverse neural fold together with the presumptive mouth ectoderm. Only a narrow median piece of the neural fold can support mouth development; lateral neural fold does not have this ability. In addition to the mouth, the adhesive organ also develops.
The transplantation experiments show that the oral endoderm is not the only part which induces the development of the ectodermal mouth invagination. Another factor operating in the immediate environment of the mouth, namely, the transverse neural fold, is also involved.
In this case, there seems to be some analogy to the dual dependence in the development of the lens – the dependence on the head mesoderm as well as the dependence on the eye cup. In the case of mouth differentiation, as with the lens inductor, a certain variation of modes of development in different animals may be attributed to the relative strength of one or the other inducing factor.
Besides giving rise to part of the oral epithelium, the stomodeal invagination furnishes the cells which become the rudiment of the anterior lobe of the hypophysis. The rudiment is formed as a solid bud, or a small pocket (Rathke’s pocket), on the dorsal side of the stomodeal invagination, just in front of the oropharyngeal membrane.
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The rudiment pushes backward through the connective tissue and eventually comes to rest underneath the diencephalon. The original connection with the stomodeum becomes interrupted, while the floor of the diencephalon furnishes the posterior lobe of the hypophysis.
One of the most characteristic differentiations of the oral cavity is the teeth. The rudiments of the teeth consist of an epithelial cap (the enamel organ which secretes the enamel) and a connective tissue papilla which produces the dentine. The enamel organs of the teeth may develop from both the ectodermal and the endodermal epithelia.
This can be proved by experiments similar to the ones already described, in which either the stomodeum or the endodermal oral epithelium is prevented from developing. The mouth invaginations, developing in the absence of the anterior end of the archenteron and the parts derived from it, often have well-differentiated teeth.
When the endodermal oral cavity develops without a stomodeum being formed, or when the two are not in contact, teeth can be found developing in the endodermal epithelium. In cyprinid fishes, one or two rows of teeth develop on the inner surface of the last branchial arch (the pharyngeal teeth). There can be no doubt that the enamel organs in these teeth are of endodermal origin.
In lower vertebrates, the fishes and amphibians, the connective tissue papillae of the teeth project from the inside into the stratified epithelium lining the mouth, and the enamel organs of the teeth develop from the malpighian layer of this epithelium. In mammals, however, the epithelium sinks down at the edges of the jaws into the connective tissue in the form of ridges.
The rudiments of the individual teeth are formed at the edge of the dental ridges, at the expense of a connective tissue papilla and of the layer of innermost cells of the dental ridge which adjoin the papilla and form the enamel organ. The teeth are thus formed and begin to grow deep in the tissue of the jaw. They erupt to the surface only when they have almost reached their full development.
While the cells of the enamel organ may be ectodermal or endodermal, the connective tissue cells of the dental papillae are derived from the neural crest. The neural crest is also the source of skeletogenous cells for the development of the mandibular arch – the quadrate and Meckel’s cartilage. The development of the mouth as a whole depends therefore on the harmonious cooperation of cells coming from different primary rudiments—the foregut, the epidermis, and the neural crest.
The development of the mandibular arch is, at least in part, directly dependent on the influence exercised by the ectodermal mouth invagination. If the ectodermal mouth invagination fails to be formed, the ventral part of the mandibular arch is not formed.
An additional Meckel’s cartilage can develop in connection with a mouth invagination resulting from the transplantation of the presumptive stomodeal ectoderm. The quadrate, on the other hand, may develop even in the complete absence of any sort of oral cavity; it is thus not dependent on the latter.
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The development of structures surrounding the edges of the mouth contributes very much to the formation of the face in man and of corresponding parts in higher mammals. The structures in question are a number of swellings, consisting of actively growing mesenchyme and covered by ectoderm, which are formed around the stomodeal invagination.
At the dorsal edge of the stomodeum medially, there is a slight swelling, the medial frontal process. Lateral to the frontal process on each side there develops a U-shaped swelling encircling the nasal pit. The free ends of the U are directed downward. The inner branch lies just alongside the frontal process and impinges on the edge of the mouth as the medial nasal process.
The outer branch, lying lateral to the nasal pit, is the lateral nasal process, and it does not quite reach the edge of the mouth. Around the angle of the mouth on each side, another U-shaped swelling develops, with the upper branch ending on the edge of the mouth as the maxillary process. The lower branch extends along the lower edge of the mouth as the mandibular process.
The two medial nasal processes grow downward and toward the midline until they fuse and exclude the frontal process from participating in the formation of the edge of the mouth. The maxillary processes grow forward and eventually fuse with the lateral edges of the medial nasal processes, thus completing the upper edge of the mouth. The opening of the nasal pit remains just above the line of fusion of the maxillary and medial nasal processes.
The upper edge of the maxillary process also fuses with the lateral nasal process. The furrow lying between the maxillary process and the lateral nasal process, leads from the angle of the eye to the nasal pit. The in-folded epidermis lining this furrow gives rise to a ridge of epithelial cells, which later becomes hollowed out and establishes a communication between the space underneath the eyelids and the nasal cavity. This is the nasolacrimal duct.
The lower edge of the mouth acquires its final shape after the median fusion of the two mandibular processes. The nasal, maxillary, and mandibular processes, as previously stated, are essentially proliferating masses of mesenchyme covered externally by ectodermal epithelium.
The mesenchyme later ossifies and gives rise to some of the most important parts of the facial skeleton. The mesenchyme in the lower portions of the medial nasal processes ossifies to form the pair of pre-maxillary bones.
The maxillary bones are produced by the ossification of the mesenchyme of the maxillary processes, and the mesenchyme of the mandibular processes gives rise to the mandibular bones. The upper parts of the medial nasal processes together with the medial frontal process develop into the back of the nose.