8.1 Organ Systems and Integration

An emergent property arises from interactions among components. Cardiac muscle cells can contract individually, but a chamber that contracts in a coordinated sequence can create pressure and pump blood. Epithelial cells can each transport ions, but an aligned sheet with apical and basal surfaces can move solutes directionally across an organ. Emergence does not imply that lower levels are irrelevant; it means that component identity alone is insufficient without organization and interaction.

Organization is hierarchical. Similar specialized cells and their extracellular material form tissues; multiple tissue types arranged for a common purpose form an organ; and cooperating organs form an organ system. The stomach, for example, contains epithelial, connective, muscle and nervous tissues. Its digestive function depends on secretion, mechanical mixing, blood supply and neural or hormonal regulation acting together.

A stem cell can self-renew and can generate differentiated descendants. Totipotent cells can produce every embryonic and extra-embryonic cell type needed for a complete organism; the zygote and its earliest descendants have this capacity. Pluripotent cells can form cell types from all three embryonic germ layers but cannot independently form all supporting extra-embryonic tissues. Multipotent adult stem cells generate a narrower family of cells associated with a tissue.

Adult stem cells remain in specialized microenvironments called niches. Hematopoietic stem cells in bone marrow replenish red blood cells, platelets and immune-cell lineages. Stem cells near hair follicles contribute to recurring hair growth and skin repair. Signals from surrounding cells and extracellular material balance dormancy, self-renewal and differentiation. Removing a cell from its niche can therefore change its behavior.

Stem-cell therapy must solve more than the problem of making the desired cell. Cells must survive, integrate into the correct tissue, respond appropriately and avoid immune rejection or uncontrolled division. Cord-blood stem cells can rebuild blood-forming tissue after destructive treatment, but a limited cell number and tissue matching constrain use. Embryonic sources raise ethical questions because obtaining the cells may destroy an embryo; induced pluripotent cells reduce some ethical and matching problems but retain technical risks.