Cell populations at gastrulation

Primitive streak and node

Seen from the dorsal (epiblastic) aspect at stage 6, the embryonic disc appears elongated. At this stage the primitive streak is first seen in the caudal region of the embryonic disc as a collection of pluripotent cells, orientated along its long axis in the median position and conferring the future craniocaudal axis of the embryo ( Figs 10.1 – 10.2 ). Although the future rostral (cranial) and caudal regions of the embryo are well within the boundaries of the embryonic disc, it has become the practice to term the region of the disc closest to the streak ‘caudal’, and the region of the disc furthest from the streak ‘rostral’. With the development of the streak, the terms medial and lateral can be used. The relative dimensions of the primitive streak and the fates of the cells that pass through it change with the developmental stage. Thus, the streak extends half way along the disc in the stage 6 embryo, reaching its greatest relative length in stage 7 and its maximum length in stage 8.

Formation of the primitive streak is induced by the underlying visceral hypoblast, which remains beneath the streak even at later stages. The primitive streak may be considered to be generally homologous with the blastopore of lower vertebrates (e.g. amphibia), with the nodal region corresponding to the dorsal lip. Experiments clearly show the lip of the blastopore to be a dynamic wave front on which cells are carried into the interior to form the roof of the archenteron, a situation analogous to ingression through the node of the prechordal plate and endoderm. The primitive streak similarly may be considered analogous to the coapted, or fused, lateral lips of the blastopore, and the cloacal membrane and its immediate environs are considered analogous to the ventral lip of the blastopore.

At the primitive streak, epiblast cells undergo a period of intense proliferation, the rate of division being much faster than that of blastomeres during cleavage. Streak formation is associated with the local production of several cell layers, extensive disruption of the epiblast basal lamina: increase in adhesive plaques and gap junctions; synthesis of vimentin; and loss of cytokeratins by the emerging cells.

As the epiblast cells proliferate, two ridges are formed on each side of the primitive streak, which appears to sink between them. The lower midline portion of the streak is termed the primitive groove. The process by which cells become part of the streak and then migrate away from it beneath the epiblast is termed ingression and is an example of epithelial to mesenchymal transformation.

The primitive node, or Hensen’s node, is the most rostral region of the primitive streak. It appears as a curved ridge of cells similar in shape to the top of an old-fashioned keyhole. Cells ingressing from the ridge pass into the primitive pit (the most rostral part of the primitive groove), and then migrate rostrally beneath the epiblast. The primitive node has been recorded in all stage 7 human embryos; it produces axial cell populations, the prechordal plate, notochord, embryonic endoderm and the medial halves of the somites. Experimental removal of the node in avian embryos results in complete absence of the notochord and a failure of neurulation.

Studies on the dynamics of ingression at the primitive streak have shown movement of the underlying extracellular tissues as well as of the overlying cells. Re-examination of the processes of cell movement relative to the surrounding extracellular molecules in embryos has found that vertical and convergent extension motion patterns, previously thought to be limited to the epiblast, also occur in sub-epiblastic extracellular matrix fibronectin fibrils ( , ).

Position and time of ingression through the primitive streak

Studies of cell fate have shown that epiblast cells that will pass through the streak are randomly located within the epiblast layer before their ingression, and that epiblast fate is determined at or before the time of ingression through the streak, indicating that passage through the primitive streak is the most important factor for future differentiation.

The position and time of ingression through either streak or node directly affect the developmental fate of cells. Passage through the streak is specified according to time and position, e.g. via the node, or rostral, middle or caudal regions of the streak. Cells that ingress through the primitive node give rise to the axial cell lines, the prechordal mesenchyme and notochord, and the endoderm and the medial halves of the somites. The rostral portion of the primitive streak produces cells for the lateral halves of the somites, whereas the middle streak produces the lateral plate mesoblast. The adjacent caudal portion of the streak gives rise to the primordial germ cells (which can be distinguished histologically and histochemically) and the most caudal portion of the streak contributes cells to the extraembryonic mesoblast until the somites are visible. A composite of the information on the position of ingression through the streak and node is shown in Fig. 10.3 . Epiblast cells that do not pass through the streak but remain within the epiblast population give rise to the neural and surface ectoderm of the embryo.

Prechordal plate

The earliest cells migrating through the primitive node and streak give rise to both the embryonic endoderm and the notochord. The prechordal plate is first seen at stage 7. It has been defined as a localized thickening of the endoderm rostral to the notochordal process, although it is seen as a highly developed mesenchymal mass in contact with the floor of the neural groove, rostral to the notochordal process, rather than as an epithelial layer. The prechordal plate is a temporary collection of cells that underlies the neural plate during stage 9. It is composed of cells that are similar to, or larger and more spherical than, the ingressing endodermal cells ( ). In stage 8 embryos, the prechordal plate is up to eight cells deep and extends along the long axis of the embryonic disc. By stages 9 and 10 the cells at the lateral edge of the plate have begun to migrate laterally as free mesenchymal cells and the plate reduces in height to two cells deep. At stage 11 the migrating prechordal mesenchyme forms bilateral premandibular mesenchymal condensations and is no longer a median structure. The extent of prechordal cells that remain within the endoderm has yet to be established.

Notochord

The notochord (chordamesoderm, the head process or chorda) arises from epiblast cells of the medial part of the primitive node. It passes through several stages during development, i.e. notochordal process and notochordal plate, before the definitive notochord is formed ( ). The cells of the early notochordal process express myogenic markers transiently as they migrate beneath the epiblast, but later they become epithelial, forming junctions and a basal lamina. The notochordal cells are intimately mixed with endodermal cells, because both cell lines ingress synchronously (see Figs 10.1 – 10.2 ; Fig. 10.4 ). During stage 8 the ingressing notochordal cells remain in the midline along the cephalocaudal axis. They form a rostral part composed of a cell mass continuous with the prechordal mesenchyme; a mid-portion in which cells are arranged in a tube with a central notochordal canal; and a caudal epithelial layer of cells, the notochordal plate, contiguous with the embryonic endoderm, which forms a roof to the secondary yolk sac. There is a transitory opening between the primitive node (and amniotic cavity) and the secondary yolk sac called the neurenteric canal (so named because its upper opening is in the future caudal floor of the neural groove, and its lower opening is into the archenteron, which is the primitive gut). It may still be found at stage 9 and the site of the neurenteric canal can be recognized in stage 10 embryos. The ingression of notochordal cells at the primitive node is matched by specification of the overlying neural ectodermal cells, and the notochordal plate is thus matched in length by the future neural floor plate. Both the notochord and the region of the floor plate of the neural tube may arise from a common progenitor cell. The early notochord is important for the maintenance and subsequent development of the neural floor plate and the induction of motor neurones. Experimental removal of the notochord results in elimination of the neural floor plate and motor neurones and expression of sensory neurones.

Caudal eminence

From stages 9–10 the region between the neurenteric canal and the cloacal membrane (see below), including the primitive streak, is termed the caudal eminence. It consists of the caudal region of the trunk, composed of mesenchyme derived from the primitive streak and epiblast, and covered with surface ectoderm. Whereas ingression of cells through the primitive streak gives rise to the prechordal and notochordal plates, and cells rostral to the neurenteric canal (see above), the cells of the caudal eminence arise from local division of a mesenchymal population positioned caudal to the neurenteric canal. The caudal portions of the notochord, that form later in development when secondary neurulation processes begin, arise from these cell populations, sometimes termed the caudoneural hinge or junction. This tissue is thicker and more advanced in differentiation than the tissues derived from the early primitive streak.