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Fig. 1. Summary tailbud fate maps of the marginal zone and
pertinent Nieuwkoop/Faber (N/F) stages for Xenopus laevis. (A) The
summary fate map for stage 6-30 of Dale and Slack (1987a,b) taken
from Smith(1989). (B) The composite fate map of the marginal zone.
In the ventral marginal zone, a small amount of somitic mesoderm is
present, while lateral plate mesoderm, which includes blood,
predominates. This composite map, shown in textbooks, review
articles and seminars is an amalgamation of many fate maps
superimposed onto the boundary lines of the original fate map (stage
10-22) compiled by Keller (1976). See text for further description.
(C) Keller’s revised map (1991). The ventral marginal zone includes
significant prospective somitic tissue (shown in red on the ventral
side), while leading edge mesoderm (orange), which includes lateral
plate, intermediate and head mesoderms, resides in the vegetal
portion of the marginal zone. (D) N/F stage 30 tailbud. (E) N/F stage
6 (32-cell stage), with the blastomere nomenclature of Nakamura and
Kishiyama (1971) indicated. (F) N/F stage 41 tadpole. A, anterior;
ap, animal pole; vp, vegetal pole; D, dorsal; LEM, leading edge
mesoderm; LP,= lateral plate; N, notochord; P, posterior; S, somites;
V, ventral.
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Fig. 2. Tailbud and tadpole stage, whole-mount embryos, labeled
with RDA at stage 6, photographed under combined epifluorescence
and epi-illumination to reveal labeled and unlabeled tissue. Dorsal is
up and anterior to the left in all images. Faint red is the embryo;
bright red is RDA label. (A,B) B4- and C4-labeled, stage 28-30
embryos. No label is present in the head. Trunk somites (s), lateral
plate and the tailbud anlagen are heavily labeled. The region
including the anterior VBI (abi) is unlabeled. (C) B4-labeled, stage
41 tadpole. Trunk/tail somites, tail epidermis and the proctodeum are
heavily labeled. The anterior VBI (abi) is unlabeled. (D) C4-labeled,
stage 41 embryo. Trunk/tail somites and epidermis are labeled. The
region including the posterior VBI (pbi) and posterior lateral plate is
labeled. Labeled blood cells are fixed in the heart (asterisk) of the
C4-injected embryo.
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Fig. 3. Primitive blood in stage 39 tadpoles. Dorsal is up and anterior to the left in
all images. (A) GFP-expressing blood cells in the tail of a C1-injected embryo.
Three fluorescent images captured with a SIT camera, superimposed on a phase
image of the embryo. The first fluorescent exposure was used for background
subtraction, the second pseudo-colored red and the third pseudo-colored green. The
image shows individual, labeled cells circulating in a tail vein. Arrows of different
shapes indicate individual cells at the two time points. One labeled blood cell
(asterisk) moves out of the field of view between the two time points. (B) C1-
injected embryo. Notochord, somites, head mesoderm and the anterior, ventral
region (arrow) including the region of the anterior VBI (vbi) are labeled. (C) Tail of
C1-injected embryo. GFP-expressing, circulating blood in the caudal artery and
vein. (D) B1-injected embryo. Notochord, brain, floorplate and somites are
labeled. The VBI region (arrow) and leading edge mesodermal derivatives are
unlabeled. (E) Tail of B1-injected embryo. Exposure time is the same as in C. No
labeled blood circulates in the vasculature. The notochord and floorplate are labeled.
(F) A4-injected embryo. Progeny include tail somites and ventral epidermis. (G) A4-
derived, labeled blood circulates in the contralateral gill arches of the embryo shown
in F. Combined epifluorescence and epi-illumination is used in images in B, D and F.
The images in C, E and G are epifluorescence only.
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Fig. 4. Circulating, fluorescent cells are erythrocytes.
(A) A control embryo, stage 40, stained for hemoglobin
with benzidine. Vessels such as the vitelline veins
(arrow) are heavily labeled. (B) Circulating cells
collected by tail bleed from a C1-injected embryo. (C) A
subset of collected cells express GFP. All GFP-positive
cells stain with benzidine (D), indicating they contain
hemoglobin.
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Fig. 5. In situ hybridization for aT3 globin at stage 40. (A) Control
embryo. Cells within the vasculature and the posterior VBI express
globin. The anterior VBI has already disintegrated. (B) Li+-
generated, DAI 9-10 embryo expresses globin. (C) D2O-generated,
DAI 10 embryo, with its radial heart (ht), expresses globin.
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Fig. 6. (A). Provisional Xenopus laevis tadpole fate map for
mesoderm, projected onto stage 6. In the absence of a complete fate
map for stage 41, our results for somites and primitive blood are
combined with data for other mesodermal derivatives (e.g. notochord,
head and heart mesoderms) from Moody’s stage 6-34 fate map
(1987b). Somites (S, red) map to the animal region of all sectors of
the marginal zone (blastomeres C1-C4, B1-B4 and A1-A4). Codistribution
of somitic mesoderm (red) with notochordal mesoderm
(N, reddish brown – blastomeres C1, C2, B1, B2, B3, A1, A2 and
A3) is indicated by the mixed coloring of the notochordal territory.
Blood arises from leading edge mesoderm (orange), situated in the vegetal region of the marginal zone, with a variable contribution from
blastomeres A4 and A3, indicated by orange asterisks. Anterior leading edge mesoderm, including head and heart mesoderms (yellow asterisks)
and the anterior portion of the ventral blood islands (abi), arises from C1, C2, D1 and D2 blastomeres. Posterior leading edge mesoderm,
including the posterior region of the ventral blood islands (pbi) and probably the pronephros, arises from blastomeres C3, C4, D3 and D4. As
there is no clear evidence at present that leading edge mesoderm cells are specified as particular cell types (e.g., blood) while residing in the
vegetal region of the marginal zone of pre-gastrula embryos, the abbreviations for tissues shown within the LEM territory are meant to indicate
general, not exact, distributions of the prospective tissues. For example, the precise relationship between the prospective head and heart
mesoderms is unknown. They lie near the upper blastopore lip, but whether both mesoderms reach the median, or whether head mesoderm lies
at the median while heart mesoderm resides more laterally, is not known. (B) Morphogenesis of the leading edge mesoderm (after Keller,
1991), including the prospective blood islands. Only the marginal zone is shown, from stage 10 to ca. stage 12.5. The animal pole (AP) is held
in a fixed position. Dorsal mesoderm is red and leading edge mesoderm (comprised of head, intermediate and lateral plate mesoderms) is
orange. Migration of the leading edge mesoderm and involution of the dorsal mesoderm begin at the upper lip around stage 10, and spreads to
the lower lip by stage 10.5. Asterisks mark the future position of the ventral blood islands at the ventral midline, where the vanguard of the
migrating leading edge coalesces into the blood islands. See text for fuller explanation. abi, anterior ventral blood islands; AP, animal pole; UL,
upper lip; LL, lower lip; Hd, prospective head mesoderm; Ht, prospective heart mesoderm; pbi, posterior ventral blood islands; Pn, prospective
pronephros.
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