Development 122, 1523-1534 (1996) 1523 Printed in Great Britain © The Company of Biologists Limited 1996 DEV3325 Fates and migratory routes of primitive streak cells in the chick embryo Delphine Psychoyos* and Claudio D. Stern* Department of Genetics and Development, College of Physicians and Surgeons of Columbia University, 701 West 168th Street, New York, NY 10032, USA *Formerly in the Department of Human Anatomy, South Parks Road, Oxford OX1 3QX, UK SUMMARY We have used carbocyanine dyes to fate map the followed the movement of labelled cells during their emi- primitive streak in the early chick embryo, from stages gration from the primitive streak in living embryos, and 3+ (mid-primitive streak) to 9 (8 somites). We show that find that cells destined to different structures follow defined presumptive notochord, foregut and medial somite do not pathways of movement, even if they arise from similar originate solely from Hensen’s node, but also from the positions in the streak. Somite and notochord precursors anterior primitive streak. At early stages (4− and 4), there migrate anteriorly within the streak and pass through is no correlation between specific anteroposterior levels different portions of the node; this provides an explanation of the primitive streak and the final position of their for the segregation of notochord and somite territories in descendants in the notochord. We describe in detail the the node. contribution of specific levels of the primitive streak to the medial and lateral halves of the somites. To understand how the descendants of labelled cells Key words: primitive streak, fate map, notochord, somite, reach their destinations in different tissues, we have gastrulation, cell movements, chick embryo INTRODUCTION are incomplete in that they either concentrate on a single stage or do not cover the entire length of the primitive streak. In higher vertebrate embryos (birds and mammals), the Here we present detailed fate maps of the primitive streak homologue of the amphibian Spemann’s organizer is located of the chick embryo constructed between mid-primitive streak at the tip of the primitive streak, in a structure known as and early somite stages, using carbocyanine dyes. We find that Hensen’s node (Waddington 1932; Hara 1978). This is sub- tissue types normally considered as derivatives of Hensen’s stantiated by comparison of the amphibian dorsal lip of the node, such as notochord and the medial parts of the somites, blastopore with Hensen’s node in terms of the fates of their have some progenitors situated posterior to the node, in the cells, the expression of several genes such as goosecoid, HNF- primitive streak. In addition, we have followed the migration 3β and Sonic hedgehog (see Levin et al., 1995) and their ability of cells labelled in the anterior primitive streak up to the time to induce an ectopic embryonic axis upon transplantation. The that they reach their destination in different tissues. We find finding (e.g. Grabowski 1956) that the embryo can regulate for that cells destined to different structures follow defined extirpation of such an important region is therefore surprising. pathways of movement, which appear to correlate more closely One possible explanation is that regions of the primitive with the tissue to which they will contribute than to their streak just posterior to the node contains some precursors with position in the streak at the time of labelling. the same fates as cells contained in the node. However, although the fate of the primitive streak has been studied exten- sively (Peebles 1898, Gräper 1929; Wetzel 1929, 1936; MATERIALS AND METHODS Kopsch 1934; Pasteels 1937, 1943; Spratt 1942a,b, 1946, 1947,1952, 1955, 1957; Spratt and Codon 1947; Bellairs Embryo techniques and fate mapping experiments 1953a,b; Spratt and Haas 1962a, 1965; Nicolet 1965, 1967, Fertile hens’ eggs (Rhode Island Red × Light Sussex or White 1970, 1971; Rosenquist 1966, 1970a-f, 1971a-d, 1972, 1982, Leghorn) were incubated at 38°C for 12-30 hours to give embryos at + 1983; Orts-Llorca and Collado 1968; Stalsberg and DeHaan stages 3 –9. Embryos were explanted ventral side uppermost in modified New culture (New 1955; Stern and Ireland 1981). For fate 1969; Schoenwolf and Sheard 1990; Schoenwolf et al., 1992; mapping studies, embryos were staged according to Hamburger and García-Martínez and Schoenwolf 1993; García-Martínez et al., Hamilton (1951) and labelled with DiI (Molecular Probes). Methods 1993; Inagaki et al., 1993), these previous studies have all used for this have been described previously (Selleck and Stern 1991; Ruiz different methods to define the site of marking and therefore it i Altaba et al., 1993); briefly, a 0.25% stock of DiI in absolute alcohol is difficult to compare them. In addition, most of these studies was diluted 1:10 in 0.3 M sucrose at 45°C and this injected by air 1524 D. Psychoyos and C. D. Stern pressure through a micropipette made by pulling a 50 µl capillary Examination of embryos, fixation and histology (Sigma) in a vertical electrode puller. The position of the labelling Embryos were explanted into phosphate-buffered saline (PBS; pH site was determined (see below) and the embryos cultured at 38°C in 7.4) in Sylgard (Dow Corning)-coated dishes and fixed in PBS con- a humid chamber for up to 30 hours, when they had reached stages taining 0.25% glutaraldehyde and 4% formaldehyde. They were then 9-13. The fate of the labelled cells was assessed both in whole mounts examined as whole mounts and photographed as double exposures under fluorescence illumination and after photooxidation of the dye using epifluorescence and bright-field optics on Ektachrome or Fuji and histological processing (see below). 1600 ASA film. Slides were then scanned into a Dell Pentium P-90 computer using a Kodak RFS 2035 Professional Plus film scanner. Definition of the injection site The illustrations in this paper were made using Adobe Photoshop Immediately after injection, each embryo was staged and the length (Adobe Systems, Inc) and printed on a Tektronix II SDX dye subli- of the primitive streak measured (Fig. 1). We plotted the position of mation printer. the injection site both in terms of distance (in mm) from the tip of When photographing labelled embryos as whole mounts, some light Hensen’s node and as a percentage of the length of the primitive scattering occurs from cells in other layers of the embryo, particularly streak. When constructing fate maps from these sets of data and at the time of labelling, when the intensity of emission is great. It is comparing the two approaches, we found no significant difference therefore important to note that some of the injections illustrated, par- between the two methods in terms of the positions of the boundaries ticularly for time = 0 hours, appear larger than they are in reality. between different prospective regions. Furthermore, there are large A total of 36 embryos labelled with DiI were processed histologi- variations in the length of the primitive streak for any one stage. For cally to confirm the location of the labelled cells. For this, DiI was these reasons, we chose to use the relative method to present the data. photoconverted by exposure to the excitation wavelength in the presence of 3,3′–diaminobenzidine (DAB; Aldrich) in 0.1 M Tris (pH Observation of migratory pathways 7.4) as described previously (Ruiz i Altaba et al., 1993), the embryos Immediately after labelling (see above), embryos were photographed embedded in Paraplast and sectioned at 10 µm. Sections were pho- (see below). They were then placed in a humid chamber at 38°C. At tographed on Kodak T-MAX 100 film, and the negatives obtained intervals from one to a few hours, embryos were removed from the were scanned and printed as described above. incubator and rapidly staged and photographed. The removal of the embryos from the incubator, the repeated changes in temperature and the exposure to fluorescent light did not appear to affect development RESULTS substantially; the average time required for the addition of new somites was 104.8±17 minutes (data from 27 embryos), which is 689 embryos were labelled, of which 448 were used for similar to that in New (1955) culture (in control embryos, the average analysis. The remaining embryos were not considered because time of somite formation was 94.7±14 minutes; n=13) and in ovo they either had not developed normally or had died before the (Menkes and Sandor 1969; Primmett et al., 1989; Birgbauer et al., desired stage. The results obtained are summarised in Fig. 2 1995). and specific examples shown in Fig. 3. At all stages, the area 0 (posterior end) to 75% of the length of the primitive streak contributes mainly to lateral plate mesoderm and extraembryonic tissues. By contrast, from stages 3+ to 4, the most anterior quarter of the primitive streak contributes largely to axial tissue types, such as notochord, foregut, head mesenchyme and somites. From stage 4+, this region of the streak contributes mainly to somitic mesoderm. The following sections are organized according to the tissue type to which labelled cells contributed. Chordamesoderm, prechordal plate, foregut and head mesenchyme Our results show that cells contributing to notochord are localized in the anterior end of the primitive streak (from 75- 92% of the length of the streak), as well as in Hensen’s node (see Selleck and Stern, 1991). Between stages 4 and 4+, there is a reduction in the number of notochord progenitors in the streak: within the region 75-92%, 18/39 (46%) injections at stage 4 produced labelled cells in the notochord, as opposed to only 3/22 (14%) of injections performed at stage 4+ in this Fig.