Stomodeum

Abstract: The restricted expression of the Ptx1 (Pitx1) gene in the posterior half of the lateral plate mesoderm has suggested that it may play a role in specification of posterior structures, in particular, specification of hindlimb identity. Ptx1 is also expressed in the most anterior ectoderm, the stomodeum, and in the first branchial arch. Ptx1 expression overlaps with that of Ptx2 in stomodeum and in posterior left lateral plate mesoderm. We now show that targeted inactivation of the mouse Ptx1 gene severely impairs hindlimb development: the ilium and knee cartilage are absent and the long bones are underdeveloped. Greater reduction of the right femur size in Ptx1 null mice suggests partial compensation by Ptx2 on the left side. The similarly sized tibia and fibula of mutant hindlimbs may be taken to resemble forelimb bones: however, the mutant limb buds appear to have retained their molecular identity as assessed by forelimb expression of Tbx5 and by hindlimb expression of Tbx4, even though Tbx4 expression is decreased in Ptx1 null mice. The hindlimb defects appear to be, at least partly, due to abnormal chondrogenesis. Since the most affected structures derive from the dorsal side of hindlimb buds, the data suggest that Ptx1 is responsible for patterning of these dorsal structures and that as such it may control development of hindlimb-specific features. Ptx1 inactivation also leads to loss of bones derived from the proximal part of the mandibular mesenchyme. The dual role of Ptx1 revealed by the gene knockout may reflect features of the mammalian jaw and hindlimbs that were acquired at a similar time during tetrapod evolution.

Abstract: Ptx1 is a member of the small bicoid family of homeobox-containing genes; it was isolated as a tissue-restricted transcription factor of the pro-opiomelanocortin gene. Its expression during mouse and chick embryogenesis was determined by in situ hybridization in order to delineate its putative role in development. In the head, Ptx1 expression is first detected in the ectoderm-derived stomodeal epithelium at E8.0. Initially, expression is only present in the stomodeum and in a few cells of the rostroventral foregut endoderm. A day later, Ptx1 mRNA is detected in the epithelium and in a streak of mesenchyme of the first branchial arch, but not in other arches. Ptx1 expression is maintained in all derivatives of these structures, including the epithelia of the tongue, palate, teeth and olfactory system, and in Rathke's pouch. Expression of Ptx1 in craniofacial structures is strikingly complementary to the pattern of goosecoid expression. In addition, Ptx1 is expressed early (E6.8) in posterior and extraembryonic mesoderm, and in structures that derive from these. The restriction of expression to the posterior lateral plate is later evidenced by exclusive labelling of the hindlimb but not forelimb mesenchyme. In the anterior domain of expression, the stomodeum was shown by fate mapping to derive from the anterior neural ridge (ANR) which represents the most anterior domain of the embryo. The concordance between these fate maps and the stomodeal pattern of Ptx1 expression supports the hypothesis that Ptx1 defines a stomodeal ectomere, which lies anteriorly to the neuromeres that have been suggested to constitute units of a segmented plan directing head formation.

Abstract: Few treatments are known that perturb the dorsoventral axis of the sea urchin embryo. We report here that the dorsoventral polarity of the sea urchin embryo can be disrupted by treatment of embryos with NiCl2. Lytechinus variegatus embryos treated with 0.5 mM NiCl2 from fertilization until the early gastrula stage appear morphologically normal until the midgastrula stage, when they fail to acquire the overt dorsoventral polarity characteristic of untreated siblings. The ectoderm of normal embryos possesses two ventrolateral thickenings just above the vegetal plate region. In nickel-treated embryos, these become expanded as a circumferential belt around the vegetal plate. The ectoderm just ventral to the animal pole normally invaginates to form a stomodeum, which then fuses with the tip of the archenteron to produce the mouth. In nickel-treated embryos, the stomodeal invagination is expanded to become a circumferential constriction, and it eventually pinches off as the tip of the archenteron fuses with it to produce a mouth. Primary mesenchyme cells form a ring in the lateral ectoderm, but as many as a dozen spicule rudiments can form in a radial pattern. Dorsoventral differentiation of ectodermal tissues is profoundly perturbed: nickel-treated embryos underexpress transcripts of the dorsal (aboral) gene LvS1, they overexpress the ventral (oral) ectodermal gene product, EctoV, and the ciliated band is shifted to the vegetal margin of the embryo. Although some dorsoventral abnormalities are observed, animal-vegetal differentiation of the archenteron and associated structures seems largely normal, based on the localization of region-specific gene products. Gross differentiation of primary mesenchyme cells seems unaffected, since nickel-treated embryos possess the normal number of these cells. Furthermore, when all primary mesenchyme cells are removed from nickel-treated embryos, some secondary mesenchyme cells undergo the process of “conversion” (Ettensohn, C. A. and McClay, D. R. (1988) Dev. Biol. 125, 396–409), migrating to sites where the larval skeleton would ordinarily form and subsequently producing spicule rudiments. However, the skeletal pattern formed by the converted cells is completely radialized. Our data suggest that a major effect of NiCl2 is to alter commitment of ectodermal cells along the dorsoventral axis. Among the consequences appears to be a disruption of pattern formation by mesenchyme cells.