Pathway V (Figure 2) involves passage of substances between epithelial cells rather than through them, and for this reason it is designated the paracellular pathway. During full lactation the passage of even low-molecular-weight substances between alveolar cells is impeded by the gasket-like tight junction structures that join the epithelial cells tightly, one to another. During pregnancy, with mastitis and after involution the tight junctions become leaky and allow components of the interstitial space, such as sodium and potassium, to pass unimpeded into the milk, a fact that is sometimes useful in diagnosing breast-feeding problems.
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For many years, scientists have been playing out the ingredients that make breast milk the perfect food for babies. They've discovered to day over 200 close compounds to fight infection, help the immune system mature, aid in digestion, and support brain growth - nature made properties that science simply cannot copy. The important long term benefits of breast feeding include reduced risk of asthma, allergies, obesity, and some forms of childhood cancer. The more that scientists continue to learn, the better breast milk looks.