Formation of the body axes in the early mammalian embryo

Mechanisms of axis formation and mesoderm induction during the gastrulation period of embryonic development are at the centre of this project. Using the rabbit as the main model organism and its anterior marginal crescent (AMC) in the pre-primtive-streak embryonic disc as a landmark (Fig. A, Viebahn et al. 1995), we intend (1) to describe functional relationships between transcription factors (such as brachyury, goosecoid, ANF) and growth factors (BMPs, nodal, activin, etc.) during mesoderm formation (Fig. B), (2) to establish a concise fate map in the late pre-streak stages of the mammalian embryonic disc (cf. Viebahn et al. 2002), and (3) define signals responsible for a head organizer property resident in the AMC (cf. Knoetgen et al. (1999). The results are compared to those of other vertebrate classes (birds, amphibia, fish) to reveal possible evolutionary relationships which might give clues as to other mechanisms acting, for example, as safeguards in mammalian development. Interdisciplinary relationships exist with the pathology of malignant tumours (epithelio-mesenchymal transformation) and with malformations of the body axes (Siamese twins, left-right asymmetries). This project is funded by the DFG priority programme 1027 "Evolution of developmental processes".

	A Morphology of a stage 2 rabbit embryo after osmium fixation and resin embedding. AMC = anterior marginal crescent (Viebahn et al. 1995), PGE = posterior gastrula extension (Viebahn et al. 2002).
	B Expression domain (blue) of the transcription factor brachyury in the PGE area of a stage 2 rabbit embryo.

References:
Viebahn, C., B. Mayer and M. Hrabé de Angelis (1995) Signs of the principal body axes prior to primitive streak formation in the rabbit embryo. Anat Embryol 192: 159-169
Knoetgen, H., C. Viebahn and M. Kessel (1999) Head induction in the chick by primitive endoderm of mammalian, but not avian origin. Development 126: 815-825
Viebahn, C. (1999) The anterior margin of the mammalian gastrula: Comparative and phylogenetic aspects of its role in axis formation and head induction. Curr Top Dev Biol 46: 63-103
Viebahn, C., C. Stortz, S. M. Mitchell and M. Blum (2002) Low proliferative and high migratory activity in the area of Brachyury expressing mesoderm progenitor cells in the gastrulating rabbit embryo. Development 129: 2355-2365

Differentiation of germ cells (germ line)

Differentiation of germ cells during embryonic development is under investigation using the mitochondrium-associated epitope PG2 which is defined by a monoclonal antibody (Viebahn et al. 1998). This epitope is specifically expressed in germ cells of the rabbit from their earliest appearance as primordial germ cells (PGC) during gastrulation. Initially, the epitope is expressed - similar to the transcription factor Oct4 - in all cells of the embryo (Fig.) and has thus, together with its exclusive expression in primordial and adult germ cells, the characteristics of a germ line marker. This project aims at (1) cloning the PG2 epitope, (2) determining time point and mode of the establishment of the germ cell lineage in mammalian embryos other than the mouse, and (3) at isolating permanent embryonic germ cell lines for targeted mutagenesis in the rabbit. This research is funded by the DFG Priority Program 1109 'Embryonic and Somatic Stem Cells - Regenerative Systems for Cell and Tissue Repair' and by the DAAD to support a cooperation with INRA, France.

Immunofluroescence staining (red) of the mitochondrial antigen PG2 outlining individual mitochondria (arrowheads) in 4 blastomeres of a 8-cell rabbit embryo. White arrows mark DAPI-stained DNA in nuclei of blastomere (top) and polar body (bottom).

References:
Viebahn, C., A. Miething and H. Wartenberg (1998) Primordial germ cells of the rabbit are specifically recognized by a monoclonal antibody labelling the perimitochondrial cytoplasm. Histochem Cell Biol 109: 49-58.
Schäfer-Haas, A. and C. Viebahn (2000) The germ cell epitope PG-2 is expressed in primordial germ cells and in hypoblast cells of the gastrulating rabbit embryo. Anat Embryol 202: 13-23
Ricken, A. M. and C. Viebahn (2002) Stage-Specific Expression of the Mitochondrial Germ Cell Epitope PG2 During Postnatal Differentiation of Rabbit Germ Cells. Biol Reprod 67: 196-203.

The embryological basis of biomedical ethics

In an attempt to provide a legal basis for assisted reproduction techniques in humans, the German law for the protection of human embryos (Embryonenschutzgesetz) defined in 1991 the unification of the pronuclei (syngamy) as the time point from which human embryonic life is to be protected from experimental or other research. However, depending on the scientific progress or even religious or political ideas, other time points of development may be declared to be criticial with regard to the protection of developing human life as well: (1) penetration of the oocyte by the spermium; (2) first transcription from the newly assembled embryonic nuclear DNA at the 2- to 4-cell stage; (3) start (on day 6 of development) or completion (day 14) of implantation as the beginning of intrauterine development typical for mammals; (4) the appearance of the primitive streak (on day 14) establishing the basis for the development of separate individuals or (5) formation of the neural tube (neurulation, week 4) as the hallmark of the high cognitive development of the human race. Our department aims at monitoring the coverage of biological and ethical questions relating to early human development and to assisted reproductive techniques in the media and tries to support the discussion with material that explains the biomedical facts.

Fate map (Anlageplan) of early human development up to organogenesis stages (cf. Viebahn, 2002).

Viebahn, C. (2002) Eine Skizze der embryonalen Frühentwicklung des Menschen.
In: Damschen G, Schönecker D (Hrsg.) Der moralische Status menschlicher Embryonen, de Gruyter, Berlin, p269-277.
Dermietzel, R., C. Viebahn (2003) Kapitel "Allgemeine Entwicklungslehre". In: Benninghoff-Drenckhahn, Anatomie, 16. Auflage, Urban & Fischer, München