structures of varying density and diameter, although their diameter is con-
siderably larger than that of the glycogen granules. Ribosomes (answer e)are
found on the rough endoplasmic reticulum or as free structures, in which
case they are not found in clusters like glycogen. Mitochondria (answer c)
contain distinctive cristae and are much larger (0.5–1.0 μm in diameter) than
glycogen. Chylomicra (answer a)are located at the basal surface of the hepa-
tocytes and are less dense than glycogen. Secretory granules (answer d)
would also show polarity in their location.
210.The answer is c.(Junqueira, pp 325, 328–329, 331.)The bile canali-
culi are labeled with arrows in the scanning electron micrograph. They
comprise the space between the lateral surfaces of adjacent hepatocytes.
Microvilli line the bile canaliculi and are visible protruding into the lumen.
The membranes between the cells are connected by tight (zonula occlu-
dentes) and gap junctions, neither of which are visible in the photomicro-
graph. The zonula occludentes prevent material from passing between the
hepatocytes and desmosomes, and when they are present between cells,
function as spot welds.
211.The answer is e. (Young, pp 283–284. Junqueira, pp 321–323,
407–411.)The organ in the photomicrograph is the pancreas, and the cells
labeled are the islets of Langerhans. The pancreas functions as both an
exocrine (secretion of pancreatic juice) and endocrine (secretion of insulin
and glucagon) gland. The islets (A)have a heterogeneous distribution
within the pancreas (i.e., they decrease from the tail to the head of the
gland) and may be used to distinguish the pancreas from the parotid gland.
The submandibular and sublingual glands can be ruled out because of the
purely serous nature of the acini within the exocrine portion of the gland.
The centroacinar cells (B)are modified intralobular duct cells, specifically
from the intercalated duct, and are present in the lumen of each acinus.
The duct (C)can be distinguished by the presence of a cuboidal epithe-
lium, the absence of blood and blood cells from the lumen, and the absence
of a characteristic vascular wall. A pancreatic artery (D)and a vein (E)are
shown within the interlobular connective tissue (F).
212.The answer is a.(Junqueira, pp 281–282, 311.)Photomicrographs A
and B show two distinctly different types of epithelium: stratified squamous
epithelium of the anus (top panel) and crypts (without villi) of the rectum
Gastrointestinal Tract and Glands Answers 333