Characterisation of the endoderm populations seen in EBs and EPLEBs by comparison with the endoderm populations of the 6.5 and 7.5 d.p.c. embryo.

<p><b>A.</b> (<b>i</b>) A 1 μm transverse section across the distal tip of a 6.5 d.p.c mouse embryo showing the distinctive morphology of the visceral endoderm (ve) surrounding the inner pluripotent cell core (ep). Size bar represents 40 μm. (<b>ii</b>) 1 μm transverse section across the extraembryonic region of a 7.5 d.p.c embryo showing the visceral endoderm (ve) surrounding the extraembryonic mesoderm (eem) and extraembryonic ectoderm (eee). Parietal endoderm (pa) is indicated by an arrow. (<b>iii</b>) Transmission electron micrographs (TEM) of visceral endoderm from the extraembryonic region of a 7.5 d.p.c. embryo, showing the typical cuboidal cell morphology with large apical vacuoles and dense microvilli on the apical surface, which can be seen at the top of the figure. Size bar represents 10 μm. <b>B.</b> (<b>i</b>) Longitudinal section of a 7.5 d.p.c. late-streak stage embryo. Size bar represents 200 μm, posterior to the right, parietal endoderm (pa) indicated by an arrow. (<b>ii</b>) Detail of (i), showing the trilaminar structure of the egg cylinder, with an outer layer of definitive endoderm (de), middle layer of mesoderm (m) and inner layer of ectoderm (ec) Size bar represents 40 μm. (<b>iii, iv</b>) TEM of definitive endoderm, showing an outer, squamous, cell layer of endoderm with a sparse decoration of microvilli on the apical surface. Inset shows the surface of the cells at a higher magnification. Size bars represent 10 μm (inset 2 μm). (<b>v, vi</b>) TEM of parietal endoderm, showing a dispersed, squamous, cell population in close contact with Reichart's membrane (rm), indicated by an arrow. The surface of the parietal endoderm is devoid of microvilli. Inset shows the surface of the cells at a higher magnification. Size bars represent 10 μm (inset 2 μm). <b>C.</b> (<b>i, ii</b>) TEM of the surface populations of cells observed on a day 5 EB. Cells appear reminiscent of the visceral (<b>i</b>) and parietal (<b>ii</b>) endoderm populations of the embryo. Size bars represent 10 μm. <b>D</b>. (<b>i</b>) Toluidine blue-stained 1 μm section of an EPLEB at day 5 of differentiation. Size bar represents 200 μm. (<b>ii</b>) Detail of (<b>i</b>) showing the squamous outer cell layer. (<b>iii, iv</b>) TEM of the outer layer of cells of EPLEB on day 5 of differentiation, showing an outer, squamous, cell layer with sparse microvilli on the apical surface, reminiscent of the morphology of the embryonic definitive endoderm. Size bars represent 10 μm (<b>iii</b>) and 5 μm (<b>iv</b>).