Essential Cell Biology 5th edition

Stem Cells and Tissue Renewal
709
cytoplasm
smooth endoplasmic
reticulum
cytoplasm
CELL 1
plasma
membrane
middle
lamella
cell wall
CELL 2
desmotubule
plasmodesmata
(B)
100 nm
(A)
plasma membrane lining a plasmodesma
connecting two adjacent cells
(C)
100 nm
Figure 20–31 The cytoplasms of adjacent plant cells are connected via plasmodesmata. (A) The intercytoplasmic
channels of plasmodesmata pierce the plant cell walls and connect the interiors of all cells in a plant. (B) Each
plasmodesma is lined with plasma membrane common to the two connected cells. It usually also contains a fine tubular
structure, the desmotubule, derived from smooth endoplasmic reticulum. (C) Micrograph of plasmodesmata. (C, courtesy
of L.G. Tilney.)
STEM CELLS AND TISSUE RENEWAL
ECB5 n20.101-20.32
One cannot contemplate the organization of tissues without wondering
how these astonishingly patterned structures come into being. This question
raises an even more challenging one—a puzzle that is one of the
most ancient and fundamental in all of biology: how is a complex multicellular
organism generated from a single fertilized egg?
In the process of development, the fertilized egg cell divides repeatedly to
give a clone of cells—about 10,000,000,000,000 for a human—essentially
all containing the same genome but specialized in different ways. This
clone has a remarkable structure. It may take the form of a daisy or an
oak tree, a sea urchin, a whale, or a mouse. The structure is determined
by the genome that the fertilized egg contains (Figure 20–32). The linear
sequence of A, G, C, and T nucleotides in the DNA directs the production
(A)
100 µm
(C)
50 µm
(B)
(D)
Figure 20–32 The genome of the
fertilized egg determines the ultimate
structure of the clone of cells that will
develop from it. (A and B) A sea-urchin
egg gives rise to a sea urchin; (C and D)
a mouse egg gives rise to a mouse.
(A, courtesy of David McClay; B, courtesy
of Alaska Department of Fish and Game;
C, courtesy of Patricia Calarco; D, US
Department of Agriculture, Agricultural
Research Service.)