Congress Abstracts - Society for Developmental Biology

differentiation potential acting as a molecular switch between osteoblast and chondrocyte cell fates according the culture or
environmental conditions. This study paves the way for developing specific therapeutic approaches for cartilage and bone repair.
Program/Abstract # 527
Evaluating of hematopoietic and mesenchymal stem cell markers during limb bud development
Camargo Sosa, Karen; Marin Llera, Jessica Cristina; Soldevila Melgarejo, Gloria; Chimal Monroy, Jesús (UNAM-Mexico City,
Mexico)
As it is proposed, “undifferentiated zone” of developing limb is the source of skeletal structures, tendons and dermis. Also, interdigital
mesenchymal cells are competent to cartilage differentiation, proven by exogenous implantation of TGF-β soaked beads. Therefore,
these undifferentiated cells have potential to generate several cell types. Another undifferentiated and multipotent population has been
described in adult bone marrow, these are MSC (Mesenchymal Stem Cells) and HSC (Hematopoietic Stem Cells). How and when do
these cells population appear within limbs during embryo development remain unknown. Mouse HSC has been described as Sca-1 and
c-Kit positives, and considered negatives for lineage specific markers (Lin-). By contrast, mouse MSC have not been that well
characterized, but the most accepted immunophenotype in humans, is CD73 + , CD90 + and CD105 + , as well as Lin - . In this work, we
evaluate if described adult HSC and MSC markers are expressed and can be detected by flow cytometry during early limb bud
development. Results showed MSC markers CD73 and CD105 are present in a low percentage of limb bud cells but not CD90 in the
evaluated stage, it will be interesting to determinate if the same cells are expressing both markers at the time they are negative for
hematopoietic lineage specific markers like CD45 also expressed in a low percentage of limb bud cells. In the evaluated stages,
CD117 was not detected but Sca-1 did. Since both markers expressions are necessary for HSC identification, next, we will be looking
for their expressions at later stages. This study was partially supported by CONACyT grants 53484 and 168642, DGAPA-UNAM
grants IN214511 and IN220808.
Program/Abstract # 528
Defining the origins of the hemogenic endothelium, the source of hematopoietic stem cells
Naiche, L.A. (National Cancer Institute, USA); Klarmann, Kimberly; Keller, Jonathan (SAIC-Frederick National Lab,USA);
Lewandoski, Mark (National Cancer Institute, USA)
Differentiation generally proceeds linearly from less to more differentiated cell fates, and exceptions are usually pathological (i.e.
cancer) or artificial (i.e. iPS cells). However, during normal development, hematopoietic stem cells (HSCs) arise from apparently
differentiated, functional endothelial cells in the endothelial walls of specific embryonic vessels, called the hemogenic endothelium.
An unanswered question is the difference between hemogenic and non-hemogenic endothelium, which both express markers of
differentiated endothelium. The vasculature of the placenta and umbilical cord, which are hemogenic tissues, originates in an
embryonic organ called the allantois. We have observed that the bulk of the allantois is formed from cells that express Tbx4 or their
descendants (the Tbx4 lineage), but the allantois-derived endothelium arises from two different lineages. The endothelium of the
umbilical cord and proximal placenta are derived from a distinct population of cells that are not part of the Tbx4 lineage, while the
endothelium of the distal placenta is derived from the Tbx4 lineage. This spatial distribution correlates with observed hemogenic fate
and non-hemogenic fate, respectively. Despite abundant Tbx4 lineage endothelium in the placenta, the Tbx4 lineage does not populate
circulating blood or HSC niches. These results indicate that endothelium can be sorted into two populations: a hemogenic lineage that
has never expressed Tbx4, and the non-hemogenic Tbx4 lineage. Because the Tbx4 lineage is defined several days prior to
hematopoietic activity, isolation of these lineages allows us to investigate the underlying factors that determine the potential of
endothelium for future stem cell differentiation.
Program/Abstract # 529
SOX2 is Required for Correct Pituitary Morphogenesis
Goldsmith, Sam; Rizzoti, Karine; Lovell-Badge, Robin (MRC NIMR, UK)
Pituitary hormonal deficiencies, or hypopituitarism, can be caused in humans and mice by haploinsufficiency of the HMG box
transcription factor SOX2 (Kelberman et al, 2006). During embryogenesis, Sox2 is expressed ubiquitously in the pituitary anlagen or
Rathke’s Pouch, (RP) and is then down-regulated, as endocrine cells differentiate. In the adult gland, a small population of SOX2 +ve
progenitors remains (Fauquier et al, 2008). We, and others, have shown that SOX2 is required for proper pituitary morphogenesis.
Conditional deletion of Sox2 in RP leads to a hypoplastic pituitary, likely caused by a reduction in progenitor proliferation (Jayakody
et al 2012, S. G. unpublished). Using different Cre strains, we show that Sox2 deletion efficiency correlates with further reduction in
pituitary size. The Sine Oculis homeodomain protein SIX6, required for progenitor proliferation in particular in the eye and the
pituitary (Li et al 2002, Tetreault et al 2009), is downregulated upon Sox2 deletion while expression of LHX3, a LIM homeodomain
transcription factor also required for RP progenitor proliferation is maintained. This suggests that SOX2 may directly up-regulate Six6
expression, as shown in other tissues, promoting RP progenitor proliferation. Later on, as endocrine cells differentiate, lineage tracing
experiments show that the endocrine differentiation potential of Sox2 deleted progenitors is impaired. These data suggest that during
normal development SOX2 drives early progenitor proliferation through the upregulation of Six6. Later on, Sox2 deleted progenitors
are largely unable to give rise to endocrine cells. Ongoing experiments address a direct role of SOX2 for Six6 transcriptional
regulation in RP.
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