CONTENTS
Main Page Dynamic Development
The Foundations of Developmental
Biology
Gametogenesis
From Sperm and Egg to Embryo
Genetic Regulation of Development
Organizing the Multicellular
Embryo
Generating Cell Diversity
Dynamic Development at a
Glance
Learning Resources
Research Resources
The Developmental Biology Journal
Club
Developmental Biology Tutorial |
Determination of Cell Fate: Introduction
by Dr. Caren Helbing
Department of Medical Biochemistry, University of Calgary
and
Leon W. Browder, Department of Biological Sciences, University of Calgary
The following material is a guide to studying Chapter 11 of Browder
et al. (1991), Chapter 13 of Gilbert (1997), Chapter 8 of Kalthoff
(1996) and Chapter 14 of Shostak (1991). Reference is also made to relevant
material in Wolpert et al. (1998).
CELL DETERMINATION is the process by which portions of the genome are
selected for expression in different embryonic cells. This involves developmental
decisions that gradually restrict cell fate. Cells can progress from TOTIPOTENT
to PLURIPOTENT to DETERMINED.
Note the importance of EXTRINSIC versus INTRINSIC factors.
Methods of Studying Cell Determination
1. Fate Maps
A FATE MAP shows us what each part of the embryo becomes at later stages
of development and allows us to trace the embryonic origins of specific
cells (CELL LINEAGE) .
(See Browder et al., 1991, Figs. 11.1 and 11.2; Gilbert, 1997,
Figs. 6.14 and 13.3; Kalthoff, 1996, Figs. 6.2, 6.3; Wolpert et al.,
1998, pages 75-81)
Constructing a fate map involves a wide variety of techniques:
- following pigment granules in some cells
- vital dye / carbon particle / enzymatic (HRP)/fluorescent dye marking
- transplantation of genetically or radioactively labelled donors cells
to a host
- using antibody or nucleic acid probes to examine cell- and tissue-specific
gene products
The lineage of all cells of some simple organisms has been determined.
eg. C. elegans (959 cells in the adult hermaphorodite and 1031 in
the adult male)
2. How is Cell Determination Assayed?
- Kill or remove specific cells and see what is missing later on.
- Dissociate embryonic cells and culture them in isolation (e.g., trochoblast
cells in the limpet Patella : Browder et al., 1991, Fig.
11.4; Gilbert, 1997, p. 13.8).
If the isolated cells form only the cells or tissues expected from the
normal fate map or cell lineage, then it is DETERMINED. If it forms an
entire embryo or more cell types that expected, then the cell is not determined.
- Transplant blastomeres to different parts of the embryo (eg. Spemann's
experiments with neural ectoderm induction).
MOSAIC and REGULATIVE Embryos
The difference between mosaic and regulative embryos lies in the timing
of when fate restrictions become apparent in the embryo.
In mosaic embryos, the blastomeres become restricted during the first few
cleavages. Because cell fates are established early, they cannot compensate
for blastomeres that are removed or destroyed. E.g. in tunicates,
separated blastomeres from the two cell embryo will develop into half embryos.
In regulative embryos, this restriction occurs later, so regulative embryos
can compensate for blastomeres that are removed in early development.
Regulative embryos differ from mosaic embryos in the orientation of the
cleavage planes with respect to the distribution of ooplasmic substances
involved in cell determination
(See Browder et al., 1991, Fig. 11.5).
Regulative embryo example: Browder et al., 1991, Fig. 11.6; Gilbert,
1997, Fig. 15.5; Kalthoff, 1996, Fig. 8.2; Shostak, 1991, Fig. 14.19, Wolpert,
1998, Fig. 6.20.
Regulation of Cell Determination by Ooplasmic Determinants
History: The Roux-Weisman theory suggested that nuclear determinants
were partitioned during cleavage, but Driesch showed that nuclei do not
lose their genetic information. Therefore nuclei remain totipotent. (Note
the exception to this rule of CHROMATIN DIMINUTION in the process of germ
cell determination in some nematodes - see Browder et al., 1991,
Figs. 11.8 and 11.9; Kalthoff, 1996, Fig. 7.3).
The cytoplasm is critical in determining what genetic information can be
accessed. Herein lie ooplasmic determinants.
Germ Cell Determination (e.g., Drosophila)
This involves the segregation of primordial germ cells with distinct
cytoplasmic granules or GERM PLASM.
The pole plasm is incorporated into the pole cells at the posterior pole
of the insect embryo. There is considerable evidence that the pole plasm
contains ooplasmic determinants for germ cell development. See Germ plasm in insects from
Zygote.
Somatic Cell Determination
Ooplasmic determination in Tunicates
See The search for
the yellow crescent myogenic factor in Zygote
Spiralian embryos (e.g., Dentalium)
There are several examples in your text that illustrate the ubiquitous
nature of cytoplasmic determinants in cell fate determination.
In spirilians, a polar lobe forms before the first cleavage which results
in the unequal partitioning of the cytoplasm to the daughter cells.
(See Browder et al., 1991, Figs. 5.30, 11.29; Gilbert, 1997,
Fig. 13.9; Kalthoff, 1996, Figs. 8.3 and 8.4; Wolpert et al., 1998,
Fig. 6.13)
Polar lobe ablation and isolation experiments suggest that the polar lobe
contains substances important in directing the formation of mesodermal structures
and other structures that are induced by mesoderm (e.g., velum, heart, intestine,
etc.).
(See Browder et al., 1991, Fig. 11.28, Kalthoff, 1996, Fig.
8.6)
If the formation of the polar lobe during the first cleavage is suppressed
with low doses of cytochalasin B (disrupts microfilament formation), the
cytoplasm is distributed to both AB and CD blastomeres. The end result is
the formation of a double embryo which develops into a trochophore larva
with two shells.
(See Browder et al., 1991, Fig. 11.30; Gilbert, 1997, Fig.
13.13)
Sequential ablation of the D macromere illustrates the partitioning of cytoplasmic
information during cleavage divisions and the concept of recipient cell
cytoplasm programming, which yields progeny cells that adopt specific fates
and gives the cells the capacity to influence the fate of other cells in
the embryo.
(See Browder et al., 1991, Fig. 11.32 and Table 11-1)
Learning Objectives
- Define cell determination
- How are fate maps used to study cell determination? How are fate maps
produced?
- Compare and contrast mosaic and regualtive development. Describe examples
of both forms of development.
- What is chromatin diminution, and how has it been studied?
- Discuss the experimental evidence that demonstrates that the polar
plasm in insects functions to determine germ cells.
- Discuss the experimental evidence that the myoplasm in tunicate eggs
contains muscle determinants.
- Describe the polar lobe of Dentalium and discuss the experiments
designed to demonstrate its role in cell determination.
Digging Deeper:
Links to Related Material
The organization
and cell-lineage of the ascidian egg by E.G. Conklin (from Zygote)
The molecular basis
of trochoblast differentiation from Zygote
Modification of spiralian
specification from Zygote
Nematode gut determination
from Zygote
See related material prepared by Dr. Richard Carthew, University of Pittsburgh:
Cell Determination
I and Cell
Determination II.
References
Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental
Biology. Third edition. Saunders College Pub. Philadelphia.
Gilbert, S.F. 1997. Developmental Biology. Fifth Edition. Sinauer.
Sunderland, MA.
Kalthoff, K. 1996. Analysis of Biological Development. McGraw-Hill. New
York.
Shostak, S. 1991. Embryology. An Introduction to Developmental Biology.
HarperCollins. New York.
Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and
Meyerowitz, E. 1998. Principles of Development. Current Biology.
London. |
