On the Enigma of the Human Neurenteric Canal.

Existence and biomedical relevance of the neurenteric canal, a transient midline structure during early neurulation in the human embryo, have been controversially discussed for more than a century by embryologists and clinicians alike. In this study, the authors address the long-standing enigma by high-resolution histology and three-dimensional reconstruction using new and historic histological sections of 5 human 17- to 21-day-old embryos and of 2 marmoset monkey embryos of the species Callithrix jacchus at corresponding stages. The neurenteric canal presents itself as the classical vertical connection between the amniotic cavity and the yolk sac cavity and is lined (a) craniolaterally by a horseshoe-shaped "hinge of involuting notochordal cells" within Hensen's node and (b) caudally by the receding primitive streak epiblast dorsally and by notochordal plate epithelium ventrally, the latter of which covered the (longitudinal) notochordal canal on its ventral side at the preceding stage. Furthermore, asymmetric parachordal nodal expression in Callithrix and morphological asymmetries within the nodes of the other specimens suggest an early non-cilium-dependent left-right symmetry breaking mode previously postulated for other mammals. We conclude that structure and position of the mammalian neurenteric canal support the notion of its homology with the reptilian blastopore as a whole and with a dorsal segment of the blastopore in amphibia. These new features of the neurenteric canal may further clarify the aetiology of foetal malformations such as junctional neurulation defects, neuroendodermal cysts, and the split notochord syndrome.

Paraxial Nodal Expression Reveals a Novel Conserved Structure of the Left-Right Organizer in Four Mammalian Species.

Nodal activity in the left lateral plate mesoderm is a conserved sign of irreversible left-right asymmetry at early somite stages of the vertebrate embryo. An earlier, paraxial nodal domain accompanies the emergence and initial extension of the notochord and is either left-sided, as in the chick and pig, or symmetrical, as in the mouse and rabbit; intriguingly, this interspecific dichotomy is mirrored by divergent morphological features of the posterior notochord (also known as the left-right organizer), which is ventrally exposed to the yolk sac cavity and carries motile cilia in the latter 2 species only. By introducing the cattle embryo as a new model organism for early left-right patterning, we present data to establish 2 groups of mammals characterized by both the morphology of the left-right organizer and the dynamics of paraxial nodal expression: presence and absence of a ventrally open surface of the early (plate-like) posterior notochord correlates with a symmetrical (in mice and rabbits) versus an asymmetrical (in pigs and cattle) paraxial nodal expression domain next to the notochordal plate. High-resolution histological analysis reveals that the latter domain defines in all 4 mammals a novel 'parachordal' axial mesoderm compartment, the topography of which changes according to the specific regression of the similarly novel subchordal mesoderm during the initial phases of notochord development. In conclusion, the mammalian axial mesoderm compartment (1) shares critical conserved features despite the marked differences in early notochord morphology and early left-right patterning and (2) provides a dynamic topographical framework for nodal activity as part of the mammalian left-right organizer.

Expression of Nodal, Cripto, SMAD3, phosphorylated SMAD3, and SMAD4 in the proliferative endometrium of women with endometriosis.

BACKGROUND: Nodal is a growth factor of the transforming growth factor ß superfamily that is expressed in high turnover tissues, such as the human endometrium, and in several malignancies. The effects of Nodal are modulated by the coreceptor Cripto and mediated by SMAD proteins. This study evaluated the gene and protein expression of Nodal, Cripto, total and phosphorylated (p) SMAD3, and SMAD4 in the proliferative endometrium of women with and without endometriosis. METHOD: Total RNA was isolated and complementary DNA synthesized from eutopic endometrium of women with (n = 15) and without (n = 12) endometriosis, followed by quantitative real-time polymerase chain reaction (PCR) to evaluate the gene expression of Nodal, Cripto, SMAD3, and SMAD4. Western blot was used to evaluate the protein levels of Nodal and Cripto, and immunohistochemistry was performed to localize SMAD3, pSMAD3, and SMAD4. RESULTS: Although Nodal expression was unchanged in women with endometriosis, real-time PCR indicated lower gene expression of Cripto (fold change 0.27, P < .05) in the endometriosis group. This difference, however, was not maintained at protein expression level as assessed by Western blot. The immunostaining of total SMAD3 was reduced in the endometriosis group (P < .01), but the localization of pSMAD3 and the nuclear staining of SMAD4 were unchanged. CONCLUSION: These findings suggest that the Nodal signaling pathway has subtle changes in the endometrium of women with endometriosis, but this imbalance may not cause functional damage as it seems not to affect the nuclear expression of SMAD4.

The POU domain transcription factor POU3F1 is an important intrinsic regulator of GDNF-induced survival and self-renewal of mouse spermatogonial stem cells.

Continual spermatogenesis relies on a pool of spermatogonial stem cells (SSCs) that possess the capacity for self-renewal and differentiation. Maintenance of this pool depends on survival of SSCs throughout the lifetime of a male. Response to extrinsic stimulation from glial cell line-derived neurotrophic factor (GDNF), mediated by the PIK3/AKT signaling cascade, is a key pathway of SSC survival. In this study, we found that expression of the POU domain transcription factor POU3F1 in cultured SSCs is up-regulated via this mechanism. Reduction of Pou3f1 gene expression by short interfering RNA (siRNA) treatment induced apoptosis in cultured germ cell populations, and transplantation analyses revealed impaired SSC maintenance in vitro. POU3F1 expression was localized to spermatogonia in cross-sections of prepubertal and adult testes, implying a similar role in vivo. Through comparative analyses, we found that expression of POU5F1, another POU transcription factor implicated as essential for SSC self-renewal, is not regulated by GDNF in cultured SSCs. Transplantation analyses following siRNA treatment showed that POU5F1 expression is not essential for SSC maintenance in vitro. Additionally, expression of NODAL, a putative autocrine regulator of POU5F1 expression in mouse germ cells, could not be detected in SSCs isolated from testes or cultured SSCs. Collectively, these results indicate that POU3F1, but not POU5F1, is an intrinsic regulator of GDNF-induced survival and self-renewal of mouse SSCs.