ABSTRACT
The adenohypophysis is derived from a structure called the Rathke's pouch, which is an invagination of the hypophyseal placode. Hedgehog (Hh) and retinoic acid (RA) signals as well as several transcription factors have been suggested to play a role in the development of the adenohypophysis. We have therefore examined the expression pattern of Sonic hedgehog (Shh), the hedgehog receptor Patched2 (Ptc2), the retinoic acid producing enzyme Retinaldehyde dehydrogenase3 (Raldh3) and four transcription factors, Pitx2, Isl1, Lim3 and Pax6 in chick embryos from head fold stage to embryonic day (E) 4.5. We show that already at the head fold stage, Ptc2 is expressed in prospective hypophyseal placodal cells and that Shh is expressed in the underlying mesoderm. Moreover, Shh continues to be expressed in tissues surrounding the prospective adenohypophysis, and Ptc2 is expressed in prospective hypophyseal cells. Lim3 and Pax6 are expressed from stage 10 in the prospective hypophyseal placode, whereas Pitx2 starts to be expressed before stage 10. Pitx2 is together with Pax6 expressed in the entire domain of the Rathke's pouch. Raldh3 is detected at the 20 somite stage and is together with Lim3 expressed in the anterior part of the Rathke's pouch. Isl1 is expressed in the most posterior part of the hypophyseal ectoderm in a complementary pattern to Raldh3 and Lim3.
Subject(s)
Chick Embryo , Hedgehog Proteins/metabolism , Pituitary Gland, Anterior/embryology , Pituitary Gland, Anterior/metabolism , Transcription Factors/metabolism , Aldehyde Oxidoreductases/metabolism , Animals , Eye Proteins/metabolism , Gene Expression , Homeodomain Proteins/metabolism , Mesoderm/metabolism , PAX6 Transcription Factor , Paired Box Transcription Factors/metabolism , Patched Receptors , Receptors, Cell Surface/metabolism , Repressor Proteins/metabolismABSTRACT
Spatial gradients of extracellular signals are implicated in the patterning of many different tissues. Much less is known, however, about how differences in time of exposure of progenitor cells to patterning signals can influence different cell fates. Bone morphogenetic protein (BMP) signals are known to pattern embryonic ectoderm. The olfactory and lens placodes are ectodermal structures of the vertebrate head. By using an explant assay of placodal cell differentiation, we now provide evidence that BMP signals are required and sufficient to induce olfactory and lens placodal cells from progenitor cells located at the anterior neural plate border. We also provide evidence that time of exposure of these progenitor cells to BMP signals plays a key role in the differential specification of olfactory and lens placodal cells.
Subject(s)
Bone Morphogenetic Proteins/metabolism , Lens, Crystalline/embryology , Olfactory Pathways/embryology , Signal Transduction/physiology , Animals , Cell Differentiation/physiology , Chick Embryo , Ectoderm/cytology , Ectoderm/metabolism , Fibroblast Growth Factors/metabolism , Lens, Crystalline/cytology , Lens, Crystalline/metabolism , Neurons/cytology , Neurons/metabolism , Olfactory Pathways/cytology , Olfactory Pathways/metabolism , Stem Cells/cytology , Stem Cells/metabolismABSTRACT
The organisation of the telencephalon into its major structures depends on its early regionalisation along the dorsoventral axis. Previous studies have provided evidence that sonic hedgehog (SHH) is required for the generation of telencephalic cells of ventral character, and that sequential WNT and fibroblast growth factor (FGF) signalling specifies cells of dorsal telencephalic character. However, the signalling mechanisms that specify telencephalic cells of an intermediate character remain to be defined. We provide evidence here that retinoic acid has a crucial role in specifying telencephalic progenitor cells of intermediate character.
Subject(s)
Telencephalon/embryology , Tretinoin/metabolism , Animals , Cell Division , Chick Embryo , Cloning, Molecular , DNA, Complementary/metabolism , Fibroblast Growth Factors/metabolism , Homeodomain Proteins/genetics , Homeodomain Proteins/metabolism , Immunohistochemistry , In Situ Hybridization , Mice , Models, Biological , Organ Culture Techniques , Protein Structure, Tertiary , Signal Transduction , Telencephalon/metabolism , Time Factors , Transcription Factors , Transcriptional ActivationABSTRACT
Dorsoventral patterning of the telencephalon is established early in forebrain development and underlies many of the regional subdivisions that are critical to the later organization of neural circuits in the cerebral cortex and basal ganglia. Sonic hedgehog (Shh) is involved in the generation of the ventral-most telencephalic cells, but the identity of the extrinsic signal(s) that induce dorsal character in telencephalic cells is not known. Here we show in chick embryos that sequential Wnt and fibroblast growth factor (FGF) signaling specifies cells of dorsal telencephalic character.
