Fig. 2. Zebrafish, chick and mouse notochords express caveolin. (A-E) Ultrastructural analysis of caveolae in the notochord during embryonic development. (A) 30-hour zebrafish embryo showing the notochord at low magnification. Arrowheads indicate the sheath surrounding the notochord cells. The notochord cells contain large vacuoles (v) surrounded by a thin layer of cytoplasm. The three boxed areas show the cell-cell contact regions within the notochord where the highest density of caveolae are found. (B) Peripheral area of the notochord of a 48-hour embryo showing several cell-cell contact regions densely covered in caveolae. Four different cells are indicated (1-4). Caveolae in cell 2 are indicated by dots; red dots indicate caveolae on the plasma membrane oriented towards the centre of the notochord (in contact with cell 3), yellow dots indicate caveolae on the cell surface in contact with cell 4. Note the difference in density on the two surfaces of the cell with more caveolae oriented towards the centre of the notochord away from the sheath (s). (C,E) Central area of 48-hour embryos showing high density of caveolae close to longitudinally running filaments (small arrowheads in C). Three different cells are indicated in C (1-3). Note the putative desmosomes (large arrowheads). (D) 24-hour embryos show a lower density of caveolae. (F,G) Paraffin-embedded sections from zebrafish (72 hours, F; inset shows different focal plane), chicken (Stage 24, G), and mouse (E10.5, H) labelled for caveolin; sc, spinal cord. Caveolin expression is high in the notochord of all three species. Bars, 10 µm (A); 1 µm (B); 500 nm (D,E); 50 µm (F-H).