Leaky gut syndrome:

Leaky gut syndrome is a term often used in complementary or alternative medicine
circles and by the lay public that is really describing a known entity termed
increased intestinal permeability. Gut permeability is important for health and
prevention of disease. The intestine is lined with a single layer of epithelial
cells, called enterocytes in the small bowel and colonocytes in the large bowel
or colon. These epithelial cells both constitute the intestinal barrier and the
digestive and absorptive lining of the gut.

Intestinal villous and microvilli increases absorptive area:

The small intestine lining has projections called villi that dramatically
increase the surface area of the intestine. Each villous is lined with a single
layer of enterocytes and goblet cells. The enterocytes of the small intestine
have projections on their surfaces known as microvilli where digestive enzymes
reside.

Tight junctions prevent leaky gut:

The cells are joined to each other by tight junctions or zonula occludens that
form a barrier to fluid and proteins except when permitted by signals that open
up the spaces between the cells, the paracellular space. Abnormal tight
junctions result in increased intestinal permeability or what may be called a
leaky gut. This altered intestinal permeability or barrier function is
implicated in several diseases, especially celiac disease and the inflammatory
bowel diseases ulcerative colitis and Crohn