Development and evolution of the vertebrate primary mouth

Abstract

The vertebrate oral region represents a key interface between outer and inner environments, and its structural and functional design is among the limiting factors for survival of its owners. Both formation of the respective oral opening (primary mouth) and establishment of the food-processing apparatus (secondary mouth) require interplay between several embryonic tissues and complex embryonic rearrangements. Although many aspects of the secondary mouth formation, including development of the jaws, teeth or taste buds, are known in considerable detail, general knowledge about primary mouth formation is regrettably low. In this paper, primary mouth formation is reviewed from a comparative point of view in order to reveal its underestimated morphogenetic diversity among, and also within, particular vertebrate clades. In general, three main developmental modes were identified. The most common is characterized by primary mouth formation via a deeply invaginated ectodermal stomodeum and subsequent rupture of the bilaminar oral membrane. However, in salamander, lungfish and also in some frog species, the mouth develops alternatively via stomodeal collar formation contributed both by the ecto- and endoderm. In ray-finned fishes, on the other hand, the mouth forms via an ectoderm wedge and later horizontal detachment of the initially compressed oral epithelia with probably a mixed germ-layer derivation. A very intriguing situation can be seen in agnathan fishes: whereas lampreys develop their primary mouth in a manner similar to the most common gnathostome pattern, hagfishes seem to undergo a unique oropharyngeal morphogenesis when compared with other vertebrates. In discussing the early formative embryonic correlates of primary mouth formation likely to be responsible for evolutionary-developmental modifications of this area, we stress an essential role of four factors: first, positioning and amount of yolk tissue; closely related to, second, endoderm formation during gastrulation, which initiates the process and constrains possible evolutionary changes within this area; third, incipient structure of the stomodeal primordium at the anterior neural plate border, where the ectoderm component of the prospective primary mouth is formed; and fourth, the prime role of Pitx genes for establishment and later morphogenesis of oral region both in vertebrates and non-vertebrate chordates.

Fig. 1

Developmental positioning of the mouth in chordates. The sagittal plane is shown, anterior to the left. In all chordates and around the neurula stages (left box), the nascent primary mouth region is situated anterodorsally, and comprises foregut endoderm and non-neural ectoderm directly adjacent to neural ectoderm. The primary mouth (*) further undergoes individual lineage-specific relocation due to the growth and differentiation of surrounding tissues. In cephalochordates, the anterior larval region is augmented by the rostral prolongation of the notochord and the mouth is shifted to the left side. In urochordates, the mouth stays at its anterodorsal position and differentiates into the oral siphon primordium. The ectoderm directly anterior to the oral siphon gives rise to prominent attachment organs. In vertebrates, the mouth is shifted ventrally by the massive growth and rostral prolongation of the brain. In agnathans, moreover, the extensively growing upper and lower lips together with velar structures (or nasopharyngeal septum in the case of hagfishes) further modify and elaborate the oral region.

Fig. 2

Main modes of primary mouth formation in jawed vertebrates. The sagittal plane is shown, anterior to the left. The initial stage of primary mouth formation is shared among vertebrates, and involves a direct contact between outer ectoderm and foregut endoderm (left box). Its further morphogenesis in diverse vertebrate groups can generally be schematized to proceed in three main alternative developmental modes. The mouth formation via stages of stomodeal invagination and perforation of the oral membrane is the most common. In salamanders, lungfishes and few frog species, the primary mouth forms via the stomodeal collar and horizontal detachment of oropharyngeal epithelia. In ray-finned fishes, primary mouth formation includes a contact between the stomodeal wedge and the endoderm sheet, and the mouth opens via horizontal detachment of these epithelia.

Fig. 3

Details of the stomodeal collar formation during mouth development in axolotl. The prospective oral ectoderm (green channel) was transplanted at early neurula stages from a GFP-transgenic embryo (Sobkow et al. 2006) and fate-mapped during the course of later embryonic development (see Soukup et al. 2008 for a transplantation assay). The magenta channel in (A) and (B) displays basal laminae (fibronectin). Sagittal sections, anterior to the left, black arrows point to the prospective or formed oral opening. (A) Early formation of the stomodeal collar. The outer ectoderm layer covers the oral region, while the inner layer involutes and becomes the basal layer of the future oral cavity. (B) A stage with a well-formed stomodeal collar. (C) Embryo with an almost opened mouth. Prospective oral cavity forms as a cleft inside the oral endoderm mass (oe in A and B), and separates the upper and lower jaws. (D) Open mouth stage. Note the anterior oral endoderm cells that burst outside the mouth and contribute to the lips and adjacent epidermal covering (white arrowheads). Black arrowheads point to the ectoderm internal limits of the former stomodeal collar, now a part of the basal layer of the oral cavity lining. e, eye; ha, hyoid arch; ma, mandibular arch; Mc, Meckel’s cartilage; n, nasal epithelium; oe, oral endoderm; ph, pharyngeal cavity. Staging after Bordzilovskaya et al. (1989).

Fig. 4

Opening of the primary mouth in axolotl and perforation of the oral membrane. The oral ectoderm is in the green channel (as in Fig. 3), sagittal sections, anterior to the left. (A, B) The newly identified oral membrane consists of the outer ectoderm layer and the anterior cells of the oral endoderm lining. It is situated superficially at the anterior-most end of the oral cavity as an epithelial connection between the upper lips and the region external to the lower lips. (C) The oral membrane is perforated at stage 43 and its remnants can be observed only temporarily joining the surrounding epithelia. (D) Laterally, the remnants of the oral membrane indicate its former position at the upper, and in front of, lower lips. (E) Histological section of the axolotl with an already ruptured oral membrane but with a not yet fully separated endoderm epithelia inside the mouth, which form epithelial bridges. Arrowheads point to the remnants of the former oral membrane and asterisks mark epithelial bridges. b, brain; ha, hyoid arch; ll, lower lip; Mc, Meckel’s cartilage; ul, upper lip.

Fig. 5

The main mode of primary mouth formation in gnathostomes (shark) compared with mouth formation in lampreys and hagfishes. The sagittal plane is shown, anterior to the left. The shark illustrates mouth formation via stomodeal invagination and perforation of the oral membrane. In lampreys, similar morphogenesis occurs including deeply invaginated stomodeum, but the forming oral membrane is made complex by the velum, which represents a part of the secondary mouth. This holds true for hagfishes as well. Here, however, the primary mouth formation arguably appears entirely in the endoderm domain, with ectoderm reaching this area rather late via the subcephalic cleft. The forming oral membrane later perforates to open the separate oropharyngeal and nasopharyngeal cavities. ll, lower lip; npc, nasopharyngeal cavity; nps, nasopharyngeal septum; opc; oropharyngeal cavity; ul, upper lip; v, velum.

Fig. 6

Phylogenetic distribution of the primary mouth formation characteristics. Phylogenetic relationships after Near (2009), where the ‘craniate hypothesis’ is preferred with hagfishes as a sister group to lampreys + gnathostomes. See the text for coding of the characters. Opening of the primary mouth via rupture of the double-layered oral membrane (character 1c) is an ancient plesiomorphic character of craniates. Stomodeal invagination (character 2b) is probably apomorphic for lampreys and gnathostomes, while it was further modified once into the stomodeal wedge (in the ray-finned fish lineage and most notably in teleosts, character 3c) and separately several times into the stomodeal collar (in lungfishes, salamanders and some frogs, character 3b).

Fig. 7

Relationship between embryonic development, amount of yolk and primary mouth formation. Position of the primary mouth is marked by an asterisk. The amniote, shark and hagfish embryos develop separately from the yolk-ball (dark yellow), and their oral region is situated in a distinct head process far from the yolk. In caecilians and lampreys, the yolk is internalized during gastrulation and all endoderm cells contain yolk platelets (dark yellow/white hatching). As in the previous case, however, the primary mouth is situated in a distinct head process. In frogs, salamanders and lungfishes, the yolk is internalized, but the head develops adjacent to the trunk region without a distinct head process. All the above-mentioned lineages, except for salamanders and lungfishes, develop their primary mouth via stages of stomodeal invagination and rupture of the oral membrane (Fig. 2). In salamanders, lungfishes and some frogs, however, the primary mouth forms via stages of stomodeal collar development and later horizontal detachment of the oropharyngeal epithelia. Ray-finned fish embryos develop their oral region in close proximity to the yolk, and no head process is apparent at early stages. Their primary mouth develops via stages of stomodeal wedge formation and horizontal detachment of the oropharyngeal epithelia due to the extremely high amount of yolk and general compression of the oropharyngeal region.

Fig. 8

Induction and molecular specification of the prospective primary mouth region in vertebrates. (A) Anterolateral view of the early amphibian neurula cut through the median plane with the demarcation of endoderm (yellow), mesoderm (red) and ectoderm (blue) regions. The left and right parts show relation of the different ectoderm regions (see the colour code) to the external morphology. The activity of the major signalling pathways is depicted in various regions. The prospective oral ectoderm (part of the pan-placodal region) is specified by a simultaneous upregulation of Fgf signalling from the neural plate and downregulation of Bmp and Wnt signalling by the inhibitors secreted from the cephalic mesoderm. (B) Anterior view at the amphibian neurula (same colour coding as in A). The subregions of the pan-placodal region are not strictly determined yet with occurrence of overlapping areas of several fates. The diagram at the right shows the dorsoventral extent of expression of several transcription factors around the median plane. cg, cement gland; nc, neural crest region; olf, olfactory placode; ppr, pan-placodal region; st-ad, stomodeo-adenohypophyseal placode. Expression data in (B) redrawn from Schlosser (2005, .