STREAMING ORGANISM

The cell kinetic characteristic of all epithelia is the same. All are analogs of the crypt-villus unit of the gastrointestinal mucosa. Each unit is nourished by at least one determined uncommitted stem cell (DS ). When the DS divides, one of its progeny replaces the parent and rem ains a DS, while the other starts streaming outward. When entering a d ifferentiation pathway it is called a committed stem cell (CS). Cells in the unit differentiate while streaming. Initially they continue mul tiplying and are called amplifying progenitors (P-cells), then they lo se the capacity to synthesize DNA and become non-dividing (quiescent) end cells (Q-cells). All cells except the DS are transitional and shor t lived (in the crypt they live several days): only the DS is permanen t. Since epithelial tissues are cell kinetic analogs of the crypt, it is assumed here that their neoplastic progression is analogous with th e adenoma-carcinoma sequence of the crypt. Neoplasia starts when a nor mal cell is transformed into a neoplastic. If a transitional cell is h it by a carcinogen and transformed into a neoplastic, it soon will be washed out from the system. Only a transformed DS can maintain the neo plastic trait since it never leaves the crypt. Neoplasia is thus a pat hology of the DS. There are two differentiation scales in the embryo: global and local. The first starts with the fertilized ovum that divid es into stem cells that become more and more determined. Following gas trulation each determined stem cell generates its local progeny of tra nsitional cells that make the tissue units. Determined stem cells are direct descendants of the fertilized ovum, while their transitional pr ogenitors are direct descendants of determined stem cells.