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

Abstract

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.

Segmentation in staged human embryos: the occipitocervical region revisited

“…1995). Important differences occur, for example, with regard to: (1) the prechordal plate and the notochordal process (Müller & O’Rahilly, 2003); (2) the absence of neural crest in the human forebrain, although present in the rat (Bartelmez, 1960); and (3) the different pattern of closure of the neural folds (O’Rahilly & Müller, 2002). It is also important to keep in mind that until now ‘molecular genetics has not solved the problem of head segmentation.…”

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“…Paralleling the process of neural tube closure is early head patterning and midline specification, which begins around E7.5 (73) in mice or at stage 7-10 (wk 3-4) of human development (70,74). Failure of this process leads to HPE and associated midline facial defects, such as cyclopia or proboscis.…”

Molecular effects of lithium exposure during mouse and chick gastrulation and subsequent valve dysmorphogenesis

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