Page 43 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
ing. In other words, an increased hormone speeds of up to around 2000 µm/min. Other
supply down-regulates the receptor density. cells also migrate, but at much slower rates. Fi-
Exocytosis (! p. 13) is a method for selec- broblasts, for example, move at a rate of around
tive export of macromolecules out of the cell 1.2 µm/min. When an injury occurs, fibroblasts
(e.g., pancreatic enzymes; ! p. 246 ff.) and for migrate to the wound and aid in the formation
release of many hormones (e.g., posterior of scar tissue. Cell migration also plays a role in
pituitary hormone; ! p. 280) or neu- embryonal development. Chemotactically at-
rotransmitters (! p. 50 ff.). These substances tracted neutrophil granulocytes and macro-
phages can even migrate through vessel walls
are kept “packed” and readily available in
Fundamentals and Cell Physiology (increase in cytosolic Ca ). The “packing mate- sues of the body or metastasize, thereby
to attack invading bacteria (! p. 94ff.). Cells of
(clathrin-coated) secretory vesicles, waiting to
be released when a certain signal is received
some tumors can also migrate to various tis-
2+
rial” (vesicle membrane) is later re-endocy-
spreading their harmful effects.
Cells migrate by “crawling” on a stable sur-
tosed and recycled. Exocytotic membrane fu-
sion also helps to insert vesicle-bound pro-
face (! E1). The following activities occur
teins into the plasma membrane (! p. 13).The
during cell migration:
! Back end of the cell: (a) Depolymerization of
liquid contents of the vesicle then are auto-
matically emptied in a process called constitu-
actin and tubulin in the cytoskeleton; (b) en-
tive exocytosis (! D).
docytosis of parts of the cell membrane, which
icles to the front of the cell, and (c) release of
coatomer (coat assembly protomer) takes on the role
ions and fluids from the cell.
of clathrin (see above). Within the Golgi membrane,
1 In constitutive exocytosis, the protein complex are then propelled forward as endocytotic ves-
GNRP (guanine nucleotide-releasing protein) ! Front end of the cell (lamellipodia): (a) Po-
phosphorylates the GDP of the ADP-ribosylation fac- lymerization of actin monomers is achieved
tor (ARF) to GTP ! D1), resulting in the dispatch of with the aid of profilin (! E2). The monomers
vesicles from the trans-Golgi network. ARF-GTP are propelled forward with the help of plasma
complexes then anchor on the membrane and bind membrane-based myosin I (fueled by ATP);
with coatomer (! D2), thereby producing (b) reinsertion of the vesicles in the cell mem-
coatomer-coated vesicles (! D3). The membranes
of the vesicles contain v-SNAREs (vesicle synapto- brane; (c) uptake of ions and fluids from the
some-associated protein receptors), which recognize environment.
t-SNAREs (target-SNAREs) in the target membrane Parts of the cell membrane that are not in-
(the plasma membrane, in this case). This results in volved in cytosis are conveyed from front to
cleavage of ARF-GTP, dissociation of ARF-GDP and back, as on a track chain. Since the cell mem-
coatomer molecules and, ultimately, to membrane brane is attached to the stable surface (pri-
fusion and exocytosis (! D4, D5) to the extracellular marily fibronectin of the extracellular matrix
space (ECS).
in the case of fibroblasts), the cell moves for-
Transcytosis is the uptake of macromolecules ward relative to the surface. This is achieved
such as proteins and hormones by endocytosis with the aid of specific receptors, such as fi-
on one side of the cell, and their release on the bronectin receptors in the case of fibroblasts.
opposite side. This is useful for transcellular
transport of the macromolecules across cell lay-
ers such as endothelia.
Cell Migration
Most cells in the body are theoretically able to
move from one place to another or migrate
(! E), but only a few cell species actually do so.
The sperm are probably the only cells with a
special propulsion mechanism. By waving
30 their whip-like tail, the sperm can travel at
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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