One tracked cell (it becomes heart muscle)Oct4-GFP: bright while pluripotentThe landscape: ribs, one per half dayA metaphor, not an anatomy: Waddington's 1957 “epigenetic landscape” built as a sliced terrain board. Every cell carries the same genome; which genes it keeps switched on is the valley it rolls into, and the ridges that rise downhill are why a committed cell rarely climbs back. Distance along the board is time, not space: ruler: 1 tick = 1 day of human development, days 5–28 after fertilisation (the board itself is not to scale). The fork days are textbook human ages: gastrulation sorts cells into ectoderm, mesoderm and endoderm on days 14–16; mesoderm splits toward blood (blood islands, ~day 17) and heart (the heart tube beats from ~day 22); ectoderm toward neurons and skin (neural plate ~day 18); endoderm toward gut lining and liver (liver bud ~day 23). The plates on the front lip, left to right: neuron, skin, blood, heart muscle, gut lining, liver, six of more than 400 human cell types. The green is an Oct4-GFP reporter, a real marker of pluripotency that switches off as cells commit. The tour runs about 170,000× faster than development; twelve cells stand in for thousands, real cells keep dividing on the way, and real valleys are chosen by signals from neighbours, not by chance.
~3.6 × 10¹³成年男性体内细胞数（Hatton 等, 2023）~36 trillion cells in an adult man (Hatton et al., 2023)400+人类细胞类型（同一套基因组）human cell types, one genome~20,000蛋白编码基因，几乎每个细胞都相同protein-coding genes, the same in almost every cellday 14–16原肠胚形成：三个胚层gastrulation: three germ layersday 22心管开始跳动the heart tube starts to beat4 factorsOct4 · Sox2 · Klf4 · c-Myc 四个因子可将细胞重编程回多能状态（2006）Oct4, Sox2, Klf4, c-Myc push a cell back uphill to pluripotency (2006)