Chapter 2 The mammalian body is composed of an array of organs, tissues and individual cells which function in a specialised and highly coordinated manner. Although these cells, tissues and organs exhibit considerable diversity in both structure and function, they all derive from a single cell, a fertilised oocyte. The fertilised oocyte is the product of the fusion of two specialised reproductive cells, gametes, of male and female origin. Following fertilisation, the zygote undergoes a series of mitotic divisions which ultimately lead to the formation of totipotent stem cells, from which all cells, tissues and organs of the body arise. Cells associated with tissue formation and regeneration are described as somatic cells. Specialised reproductive cells, referred to as germ cells, include gametes and their precursors of male and female origin. Coordinated and regulated cell division is essential for embryonic development. Somatic cell division consists of nuclear division, mitosis, followed by cytoplasmic division, cytokinesis. In mitotic division of somatic cells, the daughter cells produced are genetically identical. A form of cell division distinctly different from mitosis occurs in germ cells. In this form of cell division, referred to as meiosis, the cells produced contain half the number of chromosomes of the progenitor germ cell and are not genetically identical. Somatic cell division combined with other cellular processes such as progressive differentiation, migration, adhesion, hypertrophy and apoptosis are prerequisites for embryonic development. As part of the cell cycle, somatic cells undergo a series of molecular and morphological changes. These changes occur in four sequential phases, namely G1, S, G2 and M, and also a quiescent phase, termed G0 (Fig 2.1). The G1 and G2 phases are termed resting phases. In these phases, the cell is metabolically active, fulfilling its specialised function preparatory to the next phase of the cycle, but DNA replication does not take place. During the S phase, DNA synthesis takes place prior to chromosomal replication. This is followed by mitosis which occurs during the M phase. Collectively, the G1, S and G2 phases constitute the interphase (Fig 2.1). Cells which enter a G0 state may remain transiently or permanently in that state. Certain fully differentiated cells, such as neurons, do not divide and continue to function permanently in a G0 state. Other cell types, such as epithelial cells and hepatocytes, can re‐enter the cell cycle from G0 and proceed to mitotic division in response to appropriate stimuli. Figure 2.1 Stages in somatic cell division indicating the major phases of the cell cycle. A number of stimuli such as growth factors, mitogens and signals from other cells and from the extracellular matrix can induce cells in a G0 state to re‐enter the cell cycle near the end of the G1 phase. Growth factors which bind to cell surface receptors activate intracellular signalling pathways. In most mammalian cells, the activation of genes encoding cyclins and cyclin‐dependent kinases (CDKs) specific to the G1 phase regulate the cell cycle and commit the cell to enter the S phase. This process is initiated at the restriction point, a stage at which mammalian cells become committed to entering the S phase and are then capable of completing the cell cycle independent of extracellular influences. The rate of cell division varies in different cell types and at different stages of differentiation. Variations in cell cycle length are largely attributed to differences in the length of the G1 phase, which can range from six hours to several days. Early embryonic development is characterised by rapid cell division but, as cells become more differentiated during organ development, the rate of cell division generally decreases. The nuclei of somatic cells of each mammalian species have a defined number of chromosomes (Table 2.1). A somatic cell with a full complement of chromosomes is referred to as diploid and given the designation 2n. The term mitosis is used to describe nuclear division of somatic cells, a process which usually results in the production of two cells with the same chromosome complement as the progenitor cell from which they derived. Mitosis is essential for embryonic growth and development and for repair and replacement of tissue throughout life. The stages of mitosis occur as a distinct sequence of cytological events which are part of the cell cycle. Table 2.1 The number of chromosomes in human and animal diploid cells. In preparation for mitosis, the chromosomes are replicated in the S phase of the cell cycle, forming sister chromatids. Within the nuclear envelope, sister chromatids remain attached at a constricted region of the chromosome called a centromere. Following the G2 phase (Fig 2.2A), mitosis, which can be divided into four stages, prophase (Fig 2.2B), metaphase (Fig 2.2C), anaphase (Fig 2.2D) and, finally, telophase (Fig 2.2E), begins. The stages of mitosis are usually followed by cytoplasmic division or cytokinesis (Fig 2.2 F). Figure 2.2 An outline of the sequential stages in mitosis (A to G). After the G2 phase, prophase commences, followed by metaphase, anaphase, telophase and cytokinesis, leading to the formation of two daughter cells.
Division, growth and differentiation of cells
The cell cycle
Mitosis
Species
Number of chromosomes (2n)
Humans
46
Cats
38
Cattle
60
Chickens
78
Dogs
78
Donkeys
62
Goats
60
Horses
64
Pigs
38
Rabbits
44
Rats
42
Sheep
54
Stages of mitosis
Prophase
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