ANSWERS
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Anatomy
13
47 E. Although endocytosis and exocytosis may be
considered
opposites, they have many similarities. 80th
require the fusion of initially separate areas of the lipid
bilayer,
which
involves apposition followed by fusion
of the bilayers to form a vesicle (endocytosis) or to
fuse
a vesicle with the plasma membrane (exocytosis).
In both cases there i$ sequestration~of secretory
product in exocytosis and of ingested material in endo-
cytosis. There is also selected fusion of membranes
in
that
only certain membranous structures will fuse with
the next compartment of the endocytic or exocytic
pathways. For example, in the case of the exocytic
pathway, transition vesicles from the endoplasmic
reticulum fuse with the cisternae of the cis-Golgi.
Clathrin is involved in both the exocytic pathway
(conversion of prohormones to the active form of the
hormones-e.g., proinsulin to insulin) and the endo-
cytic pathway
in the formation of coated pits and
vesicles. In exocytosis, two cytoplasmic-side mono-
layers of the plasma membrane adhere; in endocytosis,
two external surface~side monolayers of the plasma
membrane adhere.
48 C. Microtubules are labile cytoplasmic structures
capable of very rapid assembly and disassembly. There
are a number of drugs that inhibit mitosis. Colchicine
inhibits
the addition of tubulin molecules to micro-
tubules and therefore induces depolymerization of
microtubules. Vinblastine also causes microtubule
depolymerization through the formation of paracry-
stalline aggregates of tubulin. Taxol stabilizes micro-
tubules by binding tightly to them and induces tubulin
to form
microtubules. Microtubules differ from
microfilaments,
which are often found in bundles with
extensive
cross-linking.
In
comparison, microtubules
are often
found singly in the cytoplasm.
49 D. Phagocytosis
is the process of ingestion of large
structures
by the cell. The process is triggered by the
presence of certain
particles at the cell surface; it is
not a constitutive mechanism and only occurs in regions
of the cell in contact with the substance to be ingested.
Agents that disrupt actin polymerization, such as
cytochalasin, block phagocytosis by prevention of the
formation of macrophage pseudopods and indicate the
importance of the cytoskeleton in phagocytosis.
Antibodies are the best example of a stimulus for the
phagocytic process. Antibodies bind to the surface of
antigens and target them for ingestion by macrophages
and phagocytic cells. Adhesion in this case is the
recognition and binding of IgG to Fc receptors on the
macrophage. The binding of ligand to receptor is
probably coupled with the initiation of changes in the
cytoskeleton that lead to phagocytosis. "Membrane-
zipping" is involved in phagocytosis since continuous
contact of the antibodies with the Fc receptors is
required for the pseudopod to surround the particle to
be ingested. If the IgG molecules are limited to a speci-
fic region of the antigen, then there is contact but no
ingestion. Inert as well as biologic materials are ingested
by-macrophages. Therefore, latex beads, India ink,
and trypan blue are phagocytosed by macrophages
when these substances are injected into the body or
provided in the medium surrounding cells in vitro.
50 B.
Mitochondria typically exist in cell areas that use
substantial amounts of ATP. They are abundant in the
apices of ciliated cells because the beating action of
cilia consumes ATP. They also exist in apices of cells
that have a microvillous brush border (e.g., certain
kidney cells), because solute transportation and pino-
cytosis of proteins in the glomerular filtrate consume
energy and, therefore, require ATP. Mitochondria are
distributed evenly throughout the cytoplasm of smooth
muscle cells, steroid-secreting cells, skeletal muscle
cells, and liver parenchymal cells rather than existing
in apical concentration.
51
C. Lymph nodes are filters for antigens in lymph. They
contain
8
cells in primary and secondary nodules and
T cells in the deep tertiary cortex. Secondary nodules
contain germinal centers, which are sites where
8
cells
are activated and differentiated into immunoglobulin
secreting plasma cells. Thus, secondary nodules con-
tain 8 cells, helper T cells, suppressor T cells, and
macrophages. Reticular givers exist throughout lymph
nodes, including around the subcapsular sinuses and
in nodules.
52 B.
8 cells are derived from bone marrow stem cells
and commonly differentiate into antibody-synthesizing
plasmacells after interacting with helper T cells and
macrophages. However memory 8 cells can differen-
tiate into plasma cells without interacting with
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