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The extreme anterior of the deuterostome embryo is unusual in that ectoderm and endoderm are directly juxtaposed, without intervening mesoderm. In all vertebrates, this region gives rise to the anterior pituitary, the primary mouth and, in most frogs, to the mucus-secreting cement gland. Using the frog Xenopus laevis as a paradigm, we suggest that, initially, the extreme anterior forms a homogenous domain characterized by expression of pitx genes. Subsequently, this domain becomes subdivided to form these three different structures under the influence of different inductive signals from surrounding tissues.
Fig. 1.
Schematic representation showing the relationship of the extreme anterior derivatives, cement gland, primary mouth and anterior pituitary during Xenopus laevis development. A common, undetermined anterior region is shaded purple with stripes, the endoderm is yellow, the cement gland is dark blue, the presumptive primary mouth is turquoise and anterior pituitary anlagen is pink. (A) Sagittal section and (A′) frontal view of a late gastrula stageembryo. A hypothesized common extreme anterior domain contains the future cement gland, primary mouth and anterior pituitary. The same region expresses pitx1. The endoderm is directly juxtaposed in this extreme anterior region. (B) Sagittal section and (B′) frontal view of an early tailbud stage. The cement gland has begun to differentiate, but the anterior pituitary and primary mouth appear as a continuous domain (purple striped) called the adenohypophyseal-stomodealanlagen. (C) Sagittal section and (C′) frontal view of an early tadpole stage. The future anterior pituitary and primary mouth are morphologically distinct. The presumptive PMo ectoderm begins to invaginate and the anterior pituitary anlage is displaced posteriorly. (D) Sagittal section and (D′) frontal view of a mid-tadpole stage. The primary mouth region is thinner, and after the invaginated stomodeum is formed a burst of cell death takes place. The future anterior pituitary is displaced further posteriorly. (E) Sagittal section and (E′) frontal view of a mid- to late tadpole stage. The primary mouth is formed after perforation of the region, opening the foregut cavity to the external environment. The anterior pituitary has lost its connection with the oral roof and has made a new contact with the region of the ventraldiencephalon fated to become the posterior pituitary. At this point many of the endocrine hormones and peptides are being produced. Abbreviations: ANR; anterior neural ridge, PMo; primary mouth, hpf; hours post fertilization, approximate timing at 22 °C.
Fig. 2.
Schematic representation demonstrating the inductive signals that influence the development of the presumptive primary mouth. Signals from three different regions in the early tailbud embryo (NF st. 24; 26 hpf) are necessary for PMo formation. Signals from: (1) the dorsal region, possibly a neural origin (purple arrow), (2) the posterior region, primarily consisting in endoderm (yellow arrow), and (3) lateral regions, mesoderm and/or neural crest from the first branchial arch (red arrow).
