CHAPTER 10 Cell populations at gastrulation

CONCEPTUS WITH A BILAMINAR EMBRYONIC DISC

At stage 6 the conceptus is composed of the walls of three cavities: the large chorionic cavity is surrounded by a meshwork of trophoblast and developing villi and lined with extraembryonic mesoblast. The chorion, trophoblast and extraembryonic mesoblast enclose the extraembryonic coelom and contain the much smaller amniotic cavity and yolk sac (see Fig. 9.1). These latter cavities abut at the embryonic bilaminar disc where the epithelial epiblast and visceral hypoblast are approximated. A fourth cavity, the allantois, will form as a hypoblastic diverticulum in stage 7. The ‘bilaminar disc’ commonly referred to in embryology texts does not yet possess the definitive layers of embryonic ectoderm and endoderm that will give rise to embryonic structures. Only the epiblast will give rise to the embryo; all other layers produced so far are extraembryonic. The amnion and chorion (and surrounding mesoblast) are part of the extraembryonic somatopleure, whereas the yolk sac, allantois and surrounding extraembryonic mesoblast constitute extraembryonic splanchnopleure. At the junctional zone surrounding the margins of the embryonic area, where the walls of the amnion and yolk sac converge, the somatopleuric and splanchnopleuric layers of extraembryonic mesoblast are continuous.

The terms epiblast and hypoblast are used to make the distinction between the earliest bilaminar disc layers and the later embryonic layers. Epiblast and hypoblast contain mixed populations of cells with little restriction (see p. 193), which establish the placental structures and extraembryonic tissues before the production of embryonic cell lines at gastrulation. The older terminology depicting three germ layers that give rise to the skin, gut lining and intervening tissues is thus incorrect for the bilaminar and trilaminar embryonic disc. The application and the retention of this aged terminology for the early stages of embryology continues to cause confusion and inhibits the development of more pertinent descriptive language to describe these early events.

At early stage 6, the epiblast is producing extraembryonic mesenchyme from its caudal margin. With the appearance of the primitive streak, a process is begun whereby cells of the epiblast either pass deep to the epiblast layer to form the populations of cells within the embryo, or remain on the dorsal aspect of the embryo to become the embryonic ectoderm. Although human embryos do not form a ‘gastrula’ as such, the term gastrulation is used here to denote an early period of development during which significant rearrangements, migrations and folding of the early embryo occur. The primitive streak is the site of organizer cells analogous to those found in embryos which do undergo gastrulation. The appearance of the primitive streak therefore marks the beginning of a period when gross alterations in morphology and complex rearrangements of cell populations occur. During this time, the epiblast will give rise to a complex multilaminar structure with a defined craniocaudal axis. By the end of gastrulation, cell populations from different, often widely separated, regions of the embryonic disc will become spatially related and the embryonic shape will have been produced.

Primitive streak and node

Seen from the dorsal (epiblastic) aspect, at stage 6, the embryonic disc appears elongated. The primitive streak is first seen in the caudal region of the embryonic disc at this stage as a collection of pluripotent cells, orientated along its long axis in the median position, conferring the future craniocaudal axis of the embryo (Figs 10.1 and 10.2). Although the future 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 ‘cranial’ or ‘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, reaches its greatest relative length in stage 7 and its maximum length in stage 8.

Fig. 10.1 Longitudinal section through an early conceptus. Ingression of mesoblast is occurring at the primitive streak and the notochord is ingressing via the primitive (Hensen’s) node.

Fig. 10.2 Transverse section through the embryonic plate at the level of the primitive streak to show the early movement of mesoblast between the epiblast and underlying hypoblast.

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 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.

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 results in complete absence of the notochord and a failure of neurulation.

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 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 to 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. The epiblast cells that do not pass through the streak but remain instead within the epiblast population give rise to the neural and surface ectoderm of the embryo.

Fig. 10.3 The predictive fates of the epiblast cell population at the time the primitive streak is present.

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 which 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 (Müller & O’Rahilly 2003). 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 remaining within the endoderm is not clear.

Notochord

The notochord, also called 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. The cells of the early notochordal process express myogenic markers transitorily 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, as both cells lines ingress at the same time (Figs 10.1, 10.2 and 10.4). In the stage 8 embryo, the ingressing notochordal cells remain in the midline along the cephalocaudal axis. They form a rostral part, which is 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, which is contiguous with the embryonic endoderm and 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. Removal of the notochord results in elimination of the neural floor plate and motor neurones, and expression of sensory cells types.