3397.6.1 Transforming Growth Factor-β
The first mesoderm-inducing factor, called Vegetalizing Factor, was purified from
9–13 day old chicken embryos (Tiedemann and Tiedemann 1959 ). This 30 kDa
protein was purified based on its ability to induce muscle, pronephros, and noto-
chord when applied as an insoluble pellet to salamander gastrulae (Born et al. 1972 ).
Interestingly, the type of mesoderm induced depended upon the concentration or
duration of exposure to Vegetalizing Factor (Asashima and Grunz 1983 ; Grunz
1983 ). Treatment of Xenopus animal caps with low doses or short exposures of
Vegetalizing Factor induce ventral mesodermal cell types, like blood or heart, and
higher doses or longer exposures induced dorsal mesodermal tissues, like noto-
chord. The highest doses or the longest exposures induce ectoderm to form endo-
derm. A similar activity was secreted into the media by XTC cells, a line of
transformed fibroblasts from tadpoles, and by the mouse leukemia cell line, WEHI-3
(Smith 1987 ; Godsave et al. 1988 ).
7.6.1.1 Activin
The first indication of the molecular identities of these mesoderm-inducing factors
(MIF) came from the observation that TGF-β2 could mimic their activities in a
variety of assays, including mesoderm induction from Xenopus animal caps (Rosa
et al. 1988 ; Smith et al. 1988 ). Biochemical purification and peptide sequencing of
the Vegetalizing Factor, XTC-MIF, and the WEHI-3 MIF revealed that Activin-A
was responsible for all three activities (Albano et al. 1990 ; Huylebroeck et al. 1990 ;
Smith et al. 1990 ; Tiedemann et al. 1992 ). Analysis of the expression patterns of
activin genes in Xenopus showed that transcripts encoding Activin-B, but not those
for Activin-A, were expressed at the correct time and place to induce mesoderm
(Thomsen et al. 1990 ; Dohrmann et al. 1993 ). Since Activin-B had the same ability
as Activin-A to induce mesoderm from animal cap ectoderm, Activin-B became the
first candidate molecule for the endogenous mesoderm-inducing factor (Thomsen
et al. 1990 ). Furthermore, Activin induces mesoderm in a dosage dependent man-
ner, as predicted by the transplant experiments (Green and Smith 1990 ; Green et al.
1992 ). Activin is also expressed in fish oocytes, and can induce the mesoderm when
overexpressed in zebrafish and medaka embryos (Schulte-Merker et al. 1992 ; Ge
et al. 1993 ; Wittbrodt and Rosa 1994 ). Activin is expressed in the chicken hypoblast
and in the primitive streak, when mesoderm is being specified (Mitrani et al. 1990b;
Stern et al. 1995 ). Exposure to Activin can induce an entire body axis, including a
notochord and somites, to form in a cultured chick epiblast (Albano et al. 1993 ).
Activin can also induce an ectopic primitive streak or when locally misexpressed in
a whole embryo (Ziv et al. 1992 ; Cooke et al. 1994 ). This indicates that the Activin
pathway has a highly conserved function in vertebrates and can induce mesoderm
when activated at the proper place and time of development. In mice, Activin is
expressed in the oocytes and early blastocyst, but not in the embryo during
7 Establishment of the Vertebrate Germ Layers