Egg and Sperm Cells
In plants and animals, a whole organism is sexually reproduced from a zygote, a product of fusion between egg and sperm. Zygotes are totipotent. A zygote in the seed of a plant or in the uterus of a mammal divides by mitosis and has the potential to produce more cells, called embryonic cells, before developing into an adult individual. During embryonic cell division, the cells begin to differentiate. Once differentiated, these specialized cells still possess all the genetic materials inherited from the zygote. Differentiated cells express or use some of their genes (not all) to produce their own specific proteins. For example, epidermal cells in human beings produce fibrous proteins called keratin to protect the skin, and red blood cells produce hemoglobin to help transport oxygen. Due to the differences in gene expression, differentiated cells have their own distinct structures and functions, and some differentiated cells are totipotent.
A whole organism can be asexually reproduced from a single egg without the sperm by a process called parthenogenesis. This occurs naturally in some insects, snakes, lizards, and amphibians, as well as in some plants. In this type of reproduction, the haploid chromosomes within an unfertilized egg duplicate, and the embryo develops as if the egg had been fertilized. The pseudo-fertilized eggs are totipotent and generate all female individuals. The females can reproduce under favorable environmental conditions without waiting for a mate. Like in vitro fertilization, parthenogenesis is used as a technique to create an embryo in the laboratory. Chromosomal duplication is induced in the egg cell to reproduce female individuals.
It may become possible to produce males through a process called androgenesis. In the laboratory, the haploid chromosomes from one sperm may be induced to duplicate. As in animal cloning, the duplicated chromosomes, which are diploid, can be implanted into an enucleated egg cell (a cell from which the nucleus has been removed). Although androgenesis holds some promise, so far it has not produced normal embryos.
Cell Differentiation
Cell differentiation is a process whereby genetically identical cells become different or specialized for their specific functions. During differentiation, enzymes and other polypeptides, including other large molecules, are synthesized. Ribosomes and other cell structures are assembled. Differentiated cells express only some of their genes to make enzymes and other proteins.
Tissue differentiation is usually triggered by mitosis, followed by cytokinesis. Then differentiation occurs in the daughter cells. Often the two daughter cells have different structures and functions, but both retain the same genes. For example, the epidermal cell mitotically divides to produce one large and one small cell on the root surface; the large one maintains the role of epidermal cell as a root covering, whereas the small one becomes the root hair.
Totipotent Cells in Plant-Tissue Culture
Cuttings of plants and tissue-culture techniques have proven that many plant cells are totipotent. Tissue culture, however, helps to identify what specific type of cell is totipotent, because the technique uses a very small piece of known tissue. For example, if pith tissues from tobacco (Nicotiana tobaccum), soybean (Glycine max), and other dicot stems are cut off and cultured aseptically on an agar medium with proper nutrients and hormones, a clump of unspecialized and loosely arranged cells, called a callus, is formed. Each cell from the callus begins to divide and differentiate, forming a multicellular embryoid. One test tube can accommodate thousands of cells, and each embryoid has the potential to become a complete plantlet. Plantlets can be transplanted into the soil to develop into adult plants.
The phloem tissues from the roots of carrots (Daucus species) also exhibit totipotency. Cells in pollen grains of tobacco are totipotent, and they produce haploid plants. Using meristem tissues of shoot and root tips, the cells regenerate new plants that are free of viruses, bacteria, and fungi. Pathogen elimination is possible because vascular tissues (xylem and phloem), in which viruses move, do not reach the root or shoot apex. The protoplasts (cells without cell walls) from mesophyll cells of the leaf regenerate new plants.
Plant Hormones
Totipotency of plant cells is enhanced by the presence of hormones, such as auxins and cytokinins, in the culture media. Addition of auxins influences the expression of genes and causes physiological and morphological changes in plants. Addition of cytokinins promotes cell division, cytokinesis, and organ formation. If these are present in the proper ratio, callus from many plant species can be made to develop into an entire new plant. If the cytokinin-to-auxin ratio is high, cells in the callus divide and give rise to the development of buds, stems, and leaves. If the cytokinin-to-auxin ratio is low, root formation is favored. Totipotency of some plant cells is promoted by the addition of coconut water to the culture media—an indication that coconut water has the right proportion of cytokinin and auxin to regenerate an entire plant.
Importance of Totipotency in Plants
Clonal propagation of plants using tissue culturing is used commercially to mass-produce numerous ornamentals, vegetables, and forest trees. A major use of pathogen-free plants is for the storage of germ plasm and for transport of plant materials into different countries. It is also posssible to generate plants with desirable traits, such as resistance to herbicides and environmental stressors or tolerance of soil salinity, soil acidity, and heavy-metal toxicity. It is easier to select resistant or tolerant plants from a thousand cells than from a thousand plants.
Somatic Cells in Animal Cloning
Animals are more difficult to reproduce asexually than plants are. Somatic cells of animals become totipotent when used as donor cells in cloning. The first animal successfully cloned was a frog, Xenopus laevis. This cloning involved the use of a nucleus from the intestinal epithelial cells of a tadpole and an egg cell from a mature frog. In the laboratory, the nucleus from the egg cell was removed (enucleated) by micropipette. The tadpole’s nucleus (the donor cell) was inserted into the enucleated frog’s egg cell. The nuclei-injected egg cell underwent a series of embryonic developmental stages, including the blastula stage, developing into tadpoles that later died before becoming adults.
Cloning of Dolly the sheep (Ovis species) used the mammary cell of a six-year-old ewe as the donor cell. It was injected into the enucleated sheep’s egg cell. Cloning a mammal requires a surrogate mother. The blastula stage of the embryo was developed in vitro and was implanted into a surrogate mother. After five months, a lamb was born. The lamb was genetically identical to the sheep from which the mammary cell was taken. Cloning is now done by scientists to produce other animals, including cattle, pigs, monkeys, cats, and dogs. Cloning a human seems possible, but there are many ethical and moral questions about whether it should or should not be done.
Stem Cells in Animal Cloning
Stem cells
exhibit totipotency because they can generate new types of tissues. Some sources of stem cells are the blastocyst (the immature embryo), the fetus, the placenta, bone marrow, blood, skeletal muscle, and brain. Because there is no proof yet whether a single embryonic stem cell has the ability to regenerate into a complete individual, stem cells are generally only partially totipotent. A unipotent stem cell can form only one differentiated tissue. A multipotent stem cell can form multiple differentiated tissues. For example, stem cells from blood can form platelets, white blood cells, or red blood cells. The stem cells from skeletal muscle can form smooth muscle, cardiac muscle, bone, or cartilage. A pluripotent stem cell from embryo, brain, or bone marrow has the ability to develop all types of differentiated tissues of the body. For example, brain stem cells can be turned into all tissue types, including brain, muscles, blood cells, and nerves.
Key terms
multipotent cell
:
a stem cell capable of forming multiple differentiated tissues
parthenogenesis
:
asexual reproduction from a single egg without fertilization by sperm
pluripotent cell
:
a stem cell that forms all types of differentiated tissues
unipotent cell
:
a stem cell that forms only one differentiated tissue
Bibliography
Bhojwani, S. S., and Prem Kumar Dantu. Plant Tissue Culture: An Introductory Text. New Delhi: Springer, 2013. Print.
Condic, Maureen L. "Totipotency: What It Is and What It Is Not." Stem Cells & Development 23.8 (2014): 796–812. Print.
Galan, Amparo, et al. "Defining the Genomic Signature of Totipotency and Pluripotency During Early Human Development." PLoS ONE 8.4 (2013): 1–12. Print.
Gibbs, Melissa. “Plant Cell Totipotency.” A Practical Guide to Developmental Biology. New York: Oxford UP, 2003. Print.
Lanza, Robert, et al., eds. Essentials of Stem Cell Biology. 2d ed. San Diego: Academic Press, 2009. Print.
Prentice, David A. Stem Cells and Cloning. New York: Cummings, 2003. Print.
Russell, Peter J. Genetics. San Francisco: Cummings, 2002. Print.
Smith, Roberta H. Plant Tissue Culture Techniques and Experiments. New York: Academic Press, 2000. Print.
Stillman, Bruce, et al. Control and Regulation of Stem Cells. Woodbury: Cold Spring Harbor Laboratory, 2008. Print.
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