0071509674.pdf

integrins. The split occurs through the lamina lucida of the basal lamina,
which contains primarily laminin and its connections with the integrins. At
the light microscopic level, a uniform basement membrane is visible under
epithelia. Ultrastructurally, basement membranes are composed of one or two
electron-lucent areas (laminae rarae) that contain laminin, proteoglycans, and
adhesive proteins. Deep to the lamina rara is the lamina densa with its
electron-dense type IV collagen. The third layer is the reticular layer that is
formed by the underlying connective tissue. This reticular lamina is com-
posed of collagen fibrils formed by the connective tissue below the epithelium
(basement membrane =basal lamina +reticular lamina. The table below sum-
marizes the components of the basal lamina.

Epithelium Answers 165

Type IV collagen 3 α-chains Insoluble structural

support

Laminin 3 chains Bridge between the cells

and type IV collagen

Heparan sulfate Protein core (polypeptide Electrostatic charge

chain) with glycosaminoglycan (anionic sugar side chains

side chains repel one another)

Entactin (nidogen) Single polypeptide chain Bridge between two networks:

laminin and type IV collagen

COMPONENTS OF THE BASEMENT MEMBRANE

Molecular Component

Component Arrangement Function

78.The answer is d.(Junqueira, pp 84–85.)The basolateral membrane is
characterized by the ubiquitous presence of Na+,K+–ATPase, responsible
for generating the Na+, K+gradient of the cell. Na+is pumped out of the
cell, and K+is pumped into the cell by this ATP-dependent pump. Ouabain
is a specific inhibitor of the Na+,K+–ATPase. Radioactive forms of this
inhibitor are used to label Na+,K+–ATPase in the basolateral membrane in
experimental studies. Hormonal receptors (answer a)are found on both
apical and basolateral surfaces. Neurotransmitter receptors are more preva-
lent on the basolateral surfaces. Exocytosis and endocytosis (answers b
and c)may occur across both apical and basolateral membranes, as does
ion transport. The apical surface of cells is covered by a glycocalyx (answer e)
that consists of oligosaccharides linked to glycoproteins and glycolipids