IX. A young human embryo (Embryo Dobbin) with head-process and prochordal plate

The enigmatic structure known as the prechordal plate and also the precursors of the notochord were reassessed in 101 human embryos of stages 8–14; 36 were controlled by precise graphic reconstructions. Various measurements were made and the appearance of median structures was tabulated. The prechordal plate, which has been unequivocally found first at stage 7, is usually detectable at stage 8 as a highly developed mesendodermal mass in contact with the floor of the neural groove. At stages 9 and 10 the plate is related to neuromere D1. Cellular migration laterad at stages 9–11 gives rise to the bilateral premandibular condensations, which are lateral to the adenohypophysial primordium, and at stages 13 and 14 these condensations are closely related to the future tentorium cerebelli. The notochordal process is first visible at stage 7, and its dorsal part constitutes the notochordal plate at stage 8. At stages 8–10 the notochordal and prechordal plates appear continuous, but they are distinguishable histologically. The notochordal plate becomes intercalated in the endoderm of the foregut and begins to give rise to the notochord at stages 10 and 11. Bifurcation occurs rostrally at stage 12: the dorsal limb disappears, whereas the ventral limb is the definitive continuation. The topographical relationships of the prechordal and notochordal plates, the notochord, the adenohypophysis, and the oropharyngeal membrane are documented. Definitions and pertinent remarks on terminology are included, comparative data are considered, and the origin and derivatives of the prechordal plate are discussed. In addition to giving rise to external ocular muscles, the possibility of contributions to the heart and the tentorium cerebelli is raised. The importance of the plate in the development of the forebrain, as well as in the production of median anomalies such as holoprosencephaly and cyclopia, is stressed.

Section: The Notochord and Its Precursorsmentioning

confidence: 86%

The Prechordal Plate, the Rostral End of the Notochord and Nearby Median Features in Staged Human Embryos

Müller

OʼRahilly

2003

“…As a consequence, the left-right orientation of the embryo cannot be determined on the basis of the series of histological sections alone (see Results) but had to follow previous reconstructions [Cork and Gasser, 2017c]. The embryo "Dobbin" (Carnegie stage 8a) had been fixed after abortion and estimated to be recovered at 17days post coitum (dpc) [Hill, 1931]; the histological sections are part of the collection of the Naturkundemuseum in Berlin, Germany. Embryo "Da1" (Carnegie stage 9 and estimated to be recovered at 21 dpc) had been found intraoperatively during a tubal sterilization [Ludwig, 1928].…”

Section: Human Embryosmentioning

confidence: 99%

“…Human stage 8b embryo (specimen name "Dobbin"). a Dorsal view of the 3D reconstruction in Hill and Florian [1931] displaying the notochord, primitive streak, cut edge of amnion, and proximal parts of the yolk sac. B, D, F, and H indicate the position of the transverse sections shown in b-h (section numbers indicated at the bottom left).…”

Section: Morphology Of Human Embryosmentioning

confidence: 99%

See 1 more Smart Citation

On the Enigma of the Human Neurenteric Canal

Rulle

Tsikolia

Bakker

et al. 2018

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.

“…In another embryo (Peh.,-Hochstetter) of this stage. Hill and Florian [1931] plotted the rostral limit of the neural plate, based on the transition between high and fiat ectodermal cells of the embryonic disc. Further studies of the histological ap pearances of the neural plate in the human embryo are needed.…”

Section: Neural Plate and Groovementioning

confidence: 99%

The initial development of the human brain

OʼRahilly

Gardner²

1979

An account of the early development of the human brain has been prepared from the data available for the Carnegie Collection, as well as from published information from other sources. Although the site of the neural plate can be discerned at stage 7, the first visible indication of the nervous system is the neural groove in certain embryos of stage 8, in which the embryonic disc measures more than 1 mm and the notochordal process at least 0.3 mm. The progressive fusion of the neural folds during stage 10, and the closure of the rostral and caudal neuropores at stages 11 and 12, respectively, are detailed with further precision than hitherto. It is emphasized that the major subdivisions of the human brain do not begin as vesicles, but as enlargements of the open neural folds at stage 9. The relationships of the neuromeres to the otic region, the somites, and the neural crest are clarified and illustrated. The early appearance of the telencephalon medium (before cerebral vesicles have formed) is stressed, and the terminological implications for the subdivisions of the brain are discussed.