They are cylindrical apical surface extensions - Jiwani

They are cylindrical apical surface extensions of the plasma membrane of the mucosa’s epithelial cells, and are supported by microfilaments within those cells. Although their small size makes it difficult to see each microvillus, their combined microscopic appearance suggests a mass of bristles, which is termed the brush border. Fixed to the surface of the microvillus membranes are enzymes that finish digesting carbohydrates and proteins.

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There are an estimated 200 million microvillus per square millimeter of small intestine, greatly expanding the surface area of the plasma membrane and thus greatly enhancing absorption. In addition to the three specialized absorptive features just discussed, the mucosa between the villa is dotted with deep crevices that each lead into a tubular intestinal gland (crypt of Lieberkühn), which is formed by cells that line the crevices (see [link]). These produce intestinal juice, a slightly