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*
Imperial Cancer Research Fund, London, U.K.; and
Department of Anatomy, Medical School, University of Birmingham, Birmingham, United Kingdom.
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The development of the thymus has been shown to require the expression of a number of genes, including members of the Hox and Pax gene transcription factor families. Mice deficient for Hoxa3 are athymic and have several other pharyngeal organ abnormalities (9, 10). Its expression in both the neural crest mesenchyme and pharyngeal endoderm suggests that Hoxa3 could affect both these cell lineages. Pax1 and Pax9 are expressed in an overlapping pattern in the pharyngeal endodermal cells and continue throughout fetal development. Mice deficient for Pax1 have a hypoplastic thymus and show abnormal thymocyte maturation, whereas Pax9-mutant mice are athymic (11, 12).
Fibroblast growth factors (Fgfs) are a large family of intercellular signaling molecules and mediate their biological responses by binding and activating high affinity cell surface receptors (FgfR) with intrinsic tyrosine kinase activity (reviewed in Refs. 13, 14). Fgfs have been implicated in a number of important cell activities including differentiation, proliferation, and migration (reviewed in Refs. 15, 16). The Fgfs signal through four Fgf receptor genes (FgfR1–FgfR4), but alternative splicing increases FgfR diversity. FgfR2 can generate two isoforms, FgfR2-IIIb and FgfR2-IIIc, with different Fgf-binding specificity and different sites of expression (17). FgfR2-IIIb is located in many types of epithelia and is activated by four known ligands (Fgf1, Fgf3, Fgf7, and Fgf10) that are synthesized predominantly in mesenchyme (17, 18, 19, 20, 21, 22). Gene targeting studies on FgfR2, which encompass both receptor isoforms, have given varying results with peri-implantation lethality at E4.5–E5.5 (23) to early embryonic lethality of embryos showing no limb buds and a defective placenta (24, 25). More recently, FgfR2-IIIb isoform-specific abrogations have shown this isoform is essential for epithelial-mesenchymal interactions during the development of several organs (26, 27, 28, 29, 30).
Mice deficient for FgfR2-IIIb survive in utero to term but cannot survive after birth because they have no lungs. These mice also show a severely hypoplastic thymus of comparable size to the normal E12 thymus. Here, we show that Fgf signaling is essential for thymic epithelial cell proliferation, and although very low numbers of thymocytes are generated, differentiation occurs to the CD4/CD8-double-positive and -single-positive stages.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Gene targeting of isoform IIIb of FgfR2 has been
realized by introducing an internal ribosomal entry site
(IRES)-lacZ gene into exon IIIb as previously
described (30). Briefly, stop codons in different reading
frames were introduced upstream of the IRES. A neomycin-resistant gene,
driven by the HSV-tk promoter and flanked by LoxP sites from plasmid
pL2Neo (a gift of H. Gu, Institute for Genetics, University of Cologne,
Cologne, Germany; Ref. 31), was inserted into the
intron between exons IIIa and IIIb. The resulting construct was
electroporated into 129P2/OlaHsd embryonic stem cells (Fig. 1
B). Neomycin-resistant clones were screened by both
Southern blotting and PCR. Five different lines of mice were used (2G1,
3G9, 2D9, 5A7, and 2G1 Cre). The 2G1 Cre line was obtained by deletion
of the neomycin-resistant gene from the 2G1 line by injecting eggs
derived from a 2G1-heterozygous male crossed with a wild-type Bl6
female with 10 ng/ml of pMC-Cre (kindly provided by H. Gu; Ref.
31). No differences in phenotype between the various lines
were observed. The mice were genotyped as previously described
(30). Fgf10-/- mouse
embryos were kindly provided by D. Ornitz
(Washington University Medical School, St. Louis, MO; Ref.
32).
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Radioactive in situ hybridization was performed on dewaxed tissue sections essentially as described previously (33). A mouse FgfR2-IIIc probe (IIIc) was kindly provided by P. Kettunen (University of Bergen, Bergen, Norway) (33). The lacZ probe and the pan-FgfR2 probe from the tyrosine kinase (TK) domain were previously described (30). Fgf10 and Fgf7 probes correspond to full-length cDNA cloned, respectively, in pBluescript KS and pGEM3 (Stratagene, La Jolla, CA, and Promega, Madison, WI, respectively). Plasmids used to generate antisense Pax1 and Pax9 were kindly provided by R. Balling (Max Planck Institute for Immunobiology, Freiburg, Germany) and I. Thesleff (University of Helsinki, Helsinki, Finland), respectively. Plasmids were linearized and antisense single-stranded RNA probes generated with the appropriate RNA polymerase.
Histology
Tissues were fixed overnight in neutral-buffered formalin, dehydrated through ethanol, embedded in paraffin, and 5-µm sections were cut for staining in H&E.
Immunohistochemistry
The 8-µm sections were cut, and when necessary, Ags were unmasked by prior microwaving for 10 min in 0.01 M trisodium citrate (pH 6). For detection of proliferating cells, a standard indirect immunoperoxidase protocol was used using a rabbit polyclonal Ab, NCL-Ki67p (NovoCastra, Newcastle, U.K.), and revealed by a secondary Ab, biotinylated swine anti-rabbit IgG (DAKO, Ely, U.K.) followed by streptavidin-peroxidase (DAKO) and 3,3'-diaminobenzidine (Sigma, St. Louis, MO) visualization. The cells were then counterstained with hematoxylin. Apoptotic cells were detected by the incorporation of TUNEL using the ApopTag Plus in situ apoptosis detection kit (Oncor, Gaithersburg, MD) according to the manufacturer’s recommended protocol.
For cytokeratin 5 detection, thymic lobes were dissected from E17 embryos, embedded in OCT compound (TissueTek; Sakara Finctek Europe BV) and snap-frozen in liquid N2. Next, 5-µm cryosections were cut onto multispot slides and allowed to air-dry for 1 h before being fixed in cold methanol for 30 min. Sections were then triple-labeled for 45 min for Thy1.2-biotin, cytokeratin 5, and pan-cytokeratin, followed by two 5-min washes in PBS. These were then revealed using extravidin-tetramethylrhodamine isothiocyanate, anti-rabbit Ig-7-amino-4-methylcoumarin-3-acetic acid, and anti-mouse Ig-FITC, respectively, for 30 min. After further washes, sections were mounted in SlowFade Light Antifade reagent in glycerol buffer (Molecular Probes, Eugene, OR). These were then photographed using a Zeiss Axioplan epifluorescence microscope (Zeiss, Oberkochen, Germany; x40 objective). The following reagents were used: biotinylated rat anti-Thy1.2 (clone 53-2.1; BD PharMingen, San Diego, CA), rabbit anti-keratin 5 (clone AF138; Covance; Cambridge Bioscience, Cambridge, U.K.), mouse anti-pan cytokeratin (clone C-11; Sigma), extravidin-tetramethylrhodamine isothiocyanate (Sigma), goat anti-rabbit Ig-7-amino-4-methylcoumarin-3-acetic acid (DAKO), and goat anti-mouse Ig-FITC (Caltag, South San Francisco, CA).
Flow cytometric analysis
Thymic lobes were dissected from E17- and E18-heterozygous and
-homozygous mutant embryos and teased using fine cataract knives to
yield a cell suspension. Cells were counted, spun down, and the
supernatant removed before triple labeling for CD4, CD8, and TCR
for 45 min. After washing, cells were fixed in 1% paraformaldehyde.
Samples were subsequently analyzed on a BD LSR flow cytometer (BD
Biosciences, San Jose, CA). Dead cells and debris were excluded by
selective scatter gating. For flow cytometric analysis, the following
Abs were used: PE-conjugated anti-CD4 (clone RM4-5; BD PharMingen),
allophycocyanin-conjugated anti-CD8 (clone 53-6.7;
PharMingen) and FITC-conjugated anti (clone HS7-597; Sigma).
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The generation and characterization of mice deficient for
FgfR2-IIIb has been described previously (30).
In brief, translational termination codons in three reading frames were
placed close to the start of exon IIIb, followed by an
IRES-lacZ gene (Fig. 1
, A and B).
These changes result in a truncated form of FgfR2-IIIb that lacks part
of the ligand binding domain, the transmembrane element, and the TK
domain, whereas expression of the alternatively spliced isoform,
FgfR2-IIIc, remains intact.
The thymus is one of several organs showing developmental abnormality
in FgfR2-IIIb-/-lacZ mice (Fig. 1, C and F). The dissected thoracic regions of E18.5
fetuses show that homozygous mutant mice have a profoundly smaller
thymus, as well as no significant lung development, although the heart,
which lies immediately posterior to the thymus, appears of normal size
(Fig. 1, D, E, G, and H).
To characterize thymus development in more detail, consecutive sagittal
sections from E12–E17.5 fetuses were stained and examined for
histological differences (Fig. 2). By E12
of gestation, the mouse thymus consists of a region of epithelium
surrounded by a capsule, and the whole is embedded in a thick layer of
mesenchyme. At this stage, there were no discernible histological
differences in the thymus of wild-type and
FgfR2-IIIb-/-lacZ mice (Fig. 2, A and B). However, from E12.5 to E17.5, the
thymus of heterozygous and wild-type mice grew progressively larger
(Fig. 2, C, E, and G), whereas the
thymus of homozygous mutant mice remained the same size (Fig. 2, D, F, and H). These data suggest that
FgfR2-IIIb signaling is essential for normal thymus development, and
the block in thymic development occurs between E12 and E12.5.
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The mutant FgfR2 allele contains an
IRES-lacZ cassette in the disrupted IIIb exon, although
-galactosidase activity is below detectable levels in heterozygous
or homozygous embryos (30). Consequently, expression of
FgfR2-IIIb RNA was assessed by radioactive in situ
hybridization on E13.5 heterozygote and homozygote thymus using a IIIb
isoform-specific probe (lacZ) or a pan-specific
FgfR2 TK probe, which detects both FgfR2-IIIb and
FgfR2-IIIc (Fig. 3, A–D). The lacZ and TK probe staining were very
similar, establishing the presence of FgfR2-IIIb RNA in the
thymic epithelium and suggesting no discernible expression of
FgfR2-IIIc in the surrounding mesenchyme. Although encoding
for a defective receptor, FgfR2-IIIb RNA expression was also
detected in the null mice using both the TK (Fig. 3, E and
F) and lacZ (Fig. 3, G and
H) probes. As a negative control for lacZ
staining, the probe was shown to give no signal on sections from
wild-type embryos (data not shown). The finding that
FgfR2-IIIb is expressed by the thymic epithelial cells is
consistent with other FgfR2 expression studies that show
that this receptor is expressed specifically in several epithelial cell
lineages (18, 19). FgfR2-IIIb was not detected
at E12 (data not shown), but was present by E13.5 in the thymic
epithelial compartment in both normal and mutant mice. Expression of
FgfR2-IIIb continued in the thymic cortical region
throughout fetal life, although the progressive infiltration of
lymphoid and mesenchymal cells at later stages of development makes
assignment of FgfR2-IIIb to specific cell lineages
ambiguous. Moreover, FgfR2-IIIb expression was not detected
in the surrounding mesenchyme or on the infiltrating lymphoid cells in
the mesenchyme. These data suggest a strong spatio-temporal correlation
between the loss of FgfR2-IIIb expression in the epithelium
and the onset of thymic hypoplasia.
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, I–L), suggesting that they may induce thymic epithelial
proliferation. However, mice deficient for Fgf7 are viable
and fertile, and no thymic phenotype has been reported
(34). Meanwhile, very recent data showed that mice
deficient for Fgf10 have a small thymus (35).
We have confirmed this observation with an analysis of similar
Fgf10-deficient fetuses (Fig. 4). Embryos were assessed at E12.5 and
E14.5 by standard histological staining procedures, and a significant
reduction in the size of the thymus was observed (Fig. 4, B
and D). As seen in the
FgfR2-IIIb-/-lacZ mice, the parathyroid
appeared to be unaffected (Fig. 4, C and
D). A comparison between the thymus of stage-matched
Fgf10-/- and
FgfR2-IIIb-/-lacZ mice showed that those
from FgfR2-IIIb-/-lacZ mice were
significantly smaller, suggesting than Fgf7 might be able to provide
some compensatory signaling through FgfR2-IIIb in Fgf10-null
mice. These findings support the idea that Fgf10 is the major ligand
activating FgfR2-IIIb in the thymic epithelium, and this signaling is
necessary for normal thymic development.
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As an assessment of thymic endoderm differentiation into
the epithelial primordium, we examined the expression of two markers of
thymic epithelium, the transcription factors Pax1 and
Pax9. Moreover, mice deficient for Pax1 also show
a reduced thymic size, similar phenotypically to
FgfR2-IIIb-/-lacZ mice, as well as
quantitative alterations in T cell maturation (11).
Although structurally similar and showing a similar thymus-related
expression pattern to Pax1, mice deficient for
Pax9 are athymic (12). To determine whether
these transcription factor genes have an altered thymic expression
pattern in the FgfR2-IIIb-deficient mouse, they were
analyzed using in situ hybridization at E13.5 both in the heterozygous
and homozygous mutant mice. Pax1 and Pax9 were
both strongly expressed by the thymic epithelial compartment of
heterozygous mice (Fig. 5, A–D). Similarly, in FgfR2-IIIb-deficient mice,
expression of both Pax1 and Pax9 occurs normally
and at about the expected levels, consistent with a role upstream of
FgfR2-IIIb receptor signaling (Fig. 5, E–H).
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To investigate the mechanism behind the reduced size of the
thymus, the levels of apoptosis and cell proliferation were analyzed.
Sections were stained using the TUNEL method for the detection of
apoptosis and for Ki67 Ag to measure proliferating cells. The TUNEL
assay revealed a few apoptotic cells in the thymus of both normal and
homozygous mutant mice. As a positive control, other tissues in the
sections were stained for the expected level of apoptosis (Fig. 6, A and B).
Staining with Ki67 showed that nearly all the stromal cells in the
normal thymus stained positive, indicating a high level of
proliferation, consistent with its increase in size (Fig. 6C). However, a significant decrease in staining of thymic
stromal cells in the homozygous mutant was apparent, indicative of a
proliferation block (Fig. 6D). A similar decrease in cell
proliferation was observed in mice deficient for Fgf10 (Fig. 6F), whereas the heterozygous mice displayed normal
proliferation, indicated by the extensive brown immunoperoxidase
staining (Fig. 6E). Because most of the thymic stroma at
E13.5 is composed of epithelial cells, this suggests that Fgf10 is the
main mesenchymal inducer for the thymic epithelial cell proliferation.
However, it is also likely that the reduced proliferation of epithelial
cells has a secondary effect on the proliferation of the infiltrating
lymphoid cells.
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As a preliminary assessment of thymic development, frozen sections
of the thymus from E17 heterozygous and homozygous mutant mice were
stained with a pan-cytokeratin Ab to identify the epithelium,
cytokeratin 5 as a marker of early epithelial cell differentiation, and
Thy1.2 for thymocytes. Although the thymus fails to grow in
FgfR2-IIIb-/-lacZ mice, many of the
epithelial cells remain positive for cytokeratin 5 (Fig. 7, A and B),
suggesting that they remain in a precursor form and may not therefore
be competent to promote T cell maturation (7).
Surprisingly, flow cytometry using cells teased from E17–E18 lobes of
homozygous mutant mice showed that T cell maturation is able to proceed
to the CD4/CD8-single-positive stage (Fig. 7C). Because the
thymuses from FgfR2-IIIb-/-lacZ mice are
extremely small, cells from E17 and E18 were pooled for the analyses;
in heterozygous mice at E17, there were 
3 x
106 cells/lobe and, at E18, 6 x
106 cells/lobe, but from
FgfR2-IIIb-/-lacZ mice, only 15,500
cells/lobe were obtained. With regard to TCR expression, a small
proportion of TCR-positive cells was found in the thymus of
FgfR2-IIIb-/-lacZ mice. Analysis of
TCR expression showed 4 and 7% positive in the heterozygous
and homozygous mutant mice, respectively. Within the TCR
population from FgfR2-IIIb-/-lacZ mice,
8% were CD4-single-positive, and 24% were CD8-single-positive (data
not shown). Similar distributions were found in thymocytes from the
heterozygous mice. These findings clearly demonstrate that
differentiation to the double-positive stage of thymocyte development
occurs in FgfR2-IIIb-/-lacZ mice. A
similar proportion of single-positive cells was also found in both
heterozygous and homozygous mutant mice. However, the total yield of
thymocytes was considerably reduced from the severely hypoplastic
thymus. Consequently, we cannot be certain that positive selection
occurs normally, although we detect low numbers of single-positive
cells that are TCR positive. Further analysis of the T cell
development, which occurs in neonatal mice, was not possible because
these mice die at birth due to agenesis of the lungs.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In a recent study, E12 thymic lobes with intact mesenchyme were shown
to generate all T cell populations in vitro, but lobes from which
mesenchyme has been removed show poor lymphopoiesis (8).
At E13, mesenchyme was shown to start migrating into the epithelial
primordium where it can continue to interact with epithelial cells as
well as thymocytes (8). Previous studies have reported
expression of FgfR2-IIIb in the late gestation thymus
(18, 19), but here we show expression of the
FgfR2-IIIb receptor on the epithelial primordium of the
E13.5 thymus, a stage when only very few lymphoid or mesenchymal cells
have infiltrated the epithelium (41). Furthermore, the
ligands for FgfR2-IIIb, Fgf7 and Fgf10, are present in the
surrounding mesenchyme, indicative of paracrine growth regulation.
Consistent with this suggestion, FgfR2 expression was not
detected in surrounding thymic mesenchyme, which also contains
infiltrating lymphoid cells, nor were Fgf7 and
Fgf10 detected in the epithelial compartment. Significantly,
the block in thymic growth in FgfR2-IIIb-deficient mice
correlates with the stage of development when receptor expression is
first detected. Mice deficient for Fgf10, the major ligand
for FgfR2-IIIb, also have a similar but less severe hypoplasia (Figs. 4 and 6) (35), providing strong circumstantial evidence that
Fgf10 in the mesenchyme acts as the paracrine inducer of stromal
epithelial cell proliferation.
Formation and migration of the thymic primordium appears to be normal, suggesting that the role of the Hox3 paralogs is unaffected (42). This view is supported by expression in the hypoplastic thymus of the transcription factors Pax1 and Pax9, which play important roles at this early stage of thymic morphogenesis (11, 12). Moreover, as T cell infiltration and differentiation of the thymus occurs in the FgfR2-IIIb-/-lacZ mice, the function of the whn gene responsible for the athymic nude mouse mutation also appears unaffected (43). Therefore, we would conclude that FgfR2-IIIb most likely functions downstream of the transcription factor cascade involved in thymus morphogenesis, which includes Hox3 paralogs, Pax1/Pax9, and whn, although this does not exclude a continued requirement for these transcription factors (reviewed in Ref. 44).
Thymic epithelial cells express distinct cytokeratins that reflect
their proliferative potential and maturational status (7).
Most cells express cytokeratins 8 and 18, but a distinct subset also
express cytokeratin 5. This subset contains precursors that generate
the major population of cytokeratin 5-negative cortical epithelial
cells (7, 45). Our studies show that, in the hypoplastic
thymus of mice deficient for FgfR2-IIIb, many of the epithelial cells
fail to progress from the cytokeratin 5/cytokeratin 18-positive stage
(Fig. 7A and data not shown). A similar block in maturation
of thymic epithelial cells has been reported in mice expressing a
human CD3
transgene (7), but in this case, the effect
has been attributed to a failure to generate thymocytes that are
required for epithelial maturation (46). In our studies we
show that, although the epithelial phenotype is similar to that above,
the thymus of FgfR2-IIIb-/-lacZ mice is
capable of supporting thymocyte development. However, we cannot exclude
that the small number of cytokeratin 5-negative cells are both
necessary and sufficient to facilitate the observed thymocyte
maturation. As well as generating CD4/CD8-double- and -single-positive
cells, there is also a small proportion of TCR cells in the
thymus of mutant mice that includes a small subset of the
single-positive populations. Although thymocytes transit the
double-negative 1-double-negative 4 pathway, we cannot exclude
changes in the CD25:CD44 ratios during the earlier highly proliferative
phase of T cell expansion, because in line with the small size of the
thymus, thymocyte numbers are severely reduced.
In summary, signaling through the FgfR2-IIIb receptor on thymic epithelial cells is crucial for thymic development. Mesenchyme, which initially surrounds the epithelium, produces Fgf7 and Fgf10 capable of activating this receptor, although the genetic evidence suggests the latter is the major ligand. While in the limb, FgfR2-IIIb signaling appears to be crucial for cell survival (30), and in the thymus it is required for proliferation of the thymic epithelium after E12.5. The lack of epithelial cell proliferation and the associated retention of primarily cytokeratin 5/cytokeratin 18-positive cells does not block the maturation of thymocytes to the CD4/CD8-double-positive stage, although the yield of cells is severely reduced. It is more difficult to be sure that normal positive selection occurs because of the gestational age and low cell numbers. It seems likely that the effect on thymocyte yield is secondary to the failure of epithelial proliferation, but we cannot exclude the possibility that there are direct effects on thymocytes themselves. These findings suggest that different paracrine signals may be required for growth and differentiation of the thymic epithelium. As mesenchyme also infiltrates the thymic stroma, these cells might have additional direct effects on thymocyte development, because they express an extracellular matrix able to activate thymocyte integrins (8). In this context, it will be interesting to investigate whether Fgfs can wholly substitute for the effect of removal of mesenchyme on lymphopoiesis in cultures of E12 thymus.
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Acknowledgments |
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Footnotes |
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2 Address correspondence and reprint requests to Dr. Clive Dickson, Imperial Cancer Research Fund, 44 Lincoln’s Inn Fields, London WC2A 3PX, U.K. E-mail address: dickson{at}icrf.icnet.uk
3 Abbreviations used in this paper: E, embryonic day; Fgf, fibroblast growth factor; IRES, internal ribosomal entry site; TK, tyrosine kinase.
Received for publication April 30, 2001. Accepted for publication June 18, 2001.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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 D. Zhang, J. Kosman, N. Carmean, R. Grady, and J. A. Bassuk FGF-10 and its receptor exhibit bidirectional paracrine targeting to urothelial and smooth muscle cells in the lower urinary tract Am J Physiol Renal Physiol, August 1, 2006; 291(2): F481 - F494. [Abstract] [Full Text] [PDF]
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J. J. Munoz, D. Alfaro, J. Garcia-Ceca, L. M. Alonso-C, E. Jimenez, and A. Zapata Thymic Alterations in EphA4-Deficient Mice J. Immunol., July 15, 2006; 177(2): 804 - 813. [Abstract] [Full Text] [PDF]
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 A. Marguerie, F. Bajolle, S. Zaffran, N. A. Brown, C. Dickson, M. E. Buckingham, and R. G. Kelly Congenital heart defects in Fgfr2-IIIb and Fgf10 mutant mice Cardiovasc Res, July 1, 2006; 71(1): 50 - 60. [Abstract] [Full Text] [PDF]
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G. O. Gillard and A. G. Farr Features of Medullary Thymic Epithelium Implicate Postnatal Development in Maintaining Epithelial Heterogeneity and Tissue-Restricted Antigen Expression J. Immunol., May 15, 2006; 176(10): 5815 - 5824. [Abstract] [Full Text] [PDF]
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O. Alpdogan, V. M. Hubbard, O. M. Smith, N. Patel, S. Lu, G. L. Goldberg, D. H. Gray, J. Feinman, A. A. Kochman, J. M. Eng, et al. Keratinocyte growth factor (KGF) is required for postnatal thymic regeneration Blood, March 15, 2006; 107(6): 2453 - 2460. [Abstract] [Full Text] [PDF]
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M. Mohtashami and J. C. Zuniga-Pflucker Cutting Edge: Three-Dimensional Architecture of the Thymus Is Required to Maintain Delta-Like Expression Necessary for Inducing T Cell Development J. Immunol., January 15, 2006; 176(2): 730 - 734. [Abstract] [Full Text] [PDF]
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C. C. Bleul and T. Boehm BMP Signaling Is Required for Normal Thymus Development J. Immunol., October 15, 2005; 175(8): 5213 - 5221. [Abstract] [Full Text] [PDF]
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 H. Xu, F. Cerrato, and A. Baldini Timed mutation and cell-fate mapping reveal reiterated roles of Tbx1 during embryogenesis, and a crucial function during segmentation of the pharyngeal system via regulation of endoderm expansion Development, October 1, 2005; 132(19): 4387 - 4395. [Abstract] [Full Text] [PDF]
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 M. De Felice and R. Di Lauro Thyroid Development and Its Disorders: Genetics and Molecular Mechanisms Endocr. Rev., October 1, 2004; 25(5): 722 - 746. [Abstract] [Full Text] [PDF]
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P. T. Tsai, R. A. Lee, and H. Wu BMP4 acts upstream of FGF in modulating thymic stroma and regulating thymopoiesis Blood, December 1, 2003; 102(12): 3947 - 3953. [Abstract] [Full Text] [PDF]
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R. M. Ned, W. Swat, and N. C. Andrews Transferrin receptor 1 is differentially required in lymphocyte development Blood, November 15, 2003; 102(10): 3711 - 3718. [Abstract] [Full Text] [PDF]
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 W. E. Jenkinson, E. J. Jenkinson, and G. Anderson Differential Requirement for Mesenchyme in the Proliferation and Maturation of Thymic Epithelial Progenitors J. Exp. Med., July 21, 2003; 198(2): 325 - 332. [Abstract] [Full Text] [PDF]
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 N. Dainiak, J. K. Waselenko, J. O. Armitage, T. J. MacVittie, and A. M. Farese The Hematologist and Radiation Casualties Hematology, January 1, 2003; 2003(1): 473 - 496. [Abstract] [Full Text] [PDF]
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M. Erickson, S. Morkowski, S. Lehar, G. Gillard, C. Beers, J. Dooley, J. S. Rubin, A. Rudensky, and A. G. Farr Regulation of thymic epithelium by keratinocyte growth factor Blood, October 16, 2002; 100(9): 3269 - 3278. [Abstract] [Full Text] [PDF]
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D. B. Klug, C. Carter, I. B. Gimenez-Conti, and E. R. Richie Cutting Edge: Thymocyte-Independent and Thymocyte-Dependent Phases of Epithelial Patterning in the Fetal Thymus J. Immunol., September 15, 2002; 169(6): 2842 - 2845. [Abstract] [Full Text] [PDF]
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S. Rossi, B. R. Blazar, C. L. Farrell, D. M. Danilenko, D. L. Lacey, K. I. Weinberg, W. Krenger, and G. A. Hollander Keratinocyte growth factor preserves normal thymopoiesis and thymic microenvironment during experimental graft-versus-host disease Blood, June 28, 2002; 100(2): 682 - 691. [Abstract] [Full Text] [PDF]
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D. Min, P. A. Taylor, A. Panoskaltsis-Mortari, B. Chung, D. M. Danilenko, C. Farrell, D. L. Lacey, B. R. Blazar, and K. I. Weinberg Protection from thymic epithelial cell injury by keratinocyte growth factor: a new approach to improve thymic and peripheral T-cell reconstitution after bone marrow transplantation Blood, May 29, 2002; 99(12): 4592 - 4600. [Abstract] [Full Text] [PDF]
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D. U. Frank, L. K. Fotheringham, J. A. Brewer, L. J. Muglia, M. Tristani-Firouzi, M. R. Capecchi, and A. M. Moon An Fgf8 mouse mutant phenocopies human 22q11 deletion syndrome Development, January 10, 2002; 129(19): 4591 - 4603. [Abstract] [Full Text] [PDF]
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F. Vitelli, I. Taddei, M. Morishima, E. N. Meyers, E. A. Lindsay, and A. Baldini A genetic link between Tbx1 and fibroblast growth factor signaling Development, January 10, 2002; 129(19): 4605 - 4611. [Abstract] [Full Text] [PDF]
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