Cells are the fundamental units of life. Textbooks may describe a ‘typical’ cell, but such a cell does not exist. Most cells are, to some extent, specialised in terms of their structure and function. Accordingly, the structural appearance of cells can provide information about their function. The term used to describe how cells are specialised is ‘differentiation’. Most cells have a nucleus which contains molecular programmes encoded in DNA (in chromosomes) that direct how a cell differentiates and what function(s) it performs. Many types of cell, even some which are differentiated, also have the ability to replicate themselves by a process of cell division known as mitosis (see below). Mitotic activity may continue in many types of cell throughout the life of the individ-ual. Cells undergoing successive rounds of mitosis are described as passing through a series of events known as the cell cycle (see below). Other cells may reach an end stage of differentiation and become unable to undergo mitosis and replicate themselves further. For example, shortly after birth nerve cells (neurons) cease division and replication.
Despite there being no ‘typical’ cell, cells share certain characteristics. All have a cell membrane which encloses cytoplasm; in most cells, the cytoplasm surrounds a nucleus. (Mature red blood cells in humans do not have a nucleus, nevertheless they live for about 100days.) Although the nucleus and cytoplasm may be clearly distinguished using a light microscope to examine cells in slices of tissue (Chapter 1), to resolve finer detail an electron microscope may be used. As the wavelength of electrons is much shorter than light waves, electron microscopy can resolve structures about 1000 times smaller than the light microscope can. Within cytoplasm, electron microscopy reveals a range of membrane-bound organelles and other structures (Fig. 2.1) which carry out a variety of functions during the life of a cell. The term ‘ultra-structure’ is used to describe structures revealed by electron microscopy. A brief survey of the ultrastructure of cells in relation to their function is given below.
Ultrastructure of cells and extracellular matrix
Cell membrane
The cell membrane is semi-permeable and allows inward and outward passage of selected substances. It also forms an essential barrier to the exterior and a boundary for the internal structure of the cell. It may be involved in attachments to adjacent cells (see below) and in recognition and communication within and outside the cell. Within, it may communicate with its cytoplasm and some signals may pass to the nucleus. Outside, it may communicate with other ‘self’ cells, both normal and abnormal (e.g. tumour cells or virally infected cells). Many cells interact, via their cell membrane, with microbes, and various molecules foreign to the body and a variety of cellular activities are stimulated or inhibited as a result.
Nucleus and nucleolus
The nucleus contains the vast majority of the DNA of the cell. The DNA is the hereditary material that has the genetic code expressed in a double strand of DNA in each chromosome. (Chromosomes contain proteins as well as DNA.) The number of chromosomes in a typical cell is species specific (humans have 46). Human chromosomes comprise 22 homologous pairs and a pair of sex chromosomes (two X chromosomes in females and an X and a Y chromosome in males). One of each pair of chromosomes is derived from the mother’s oocyte and the other from the father’s spermatozoon (see, respectively, Chapters 16 and 15). The nucleus may also contain the structural and molecular mechanisms for the synthesis of RNA in one or more nucleoli.
The nucleus is enclosed by a nuclear membrane which is formed by two plasma membranes. In places, the nuclear membrane is perforated by pores which allow transport of material to and from the nucleus. The outer nuclear membrane is continuous in places with some membranes in the cytoplasm, and molecules (e.g. proteins and RNA) travel between nucleus and cytoplasm by this route.
The appearance of nuclei varies in relation to their function. Individual chromosomes are not apparent in cells unless the cell is dividing. The nuclear material, comprising DNA, proteins and RNA, is known as chromatin and two types are described, euchromatin and heterochromatin. Euchromatin appears less dense than heterochromatin (Figs 2.1 and 2.2). The DNA molecules in euchromatin are uncoiled and are being used as coding for RNA synthesis which, in turn, directs protein synthesis in the cytoplasm. A large amount of euchromatin in a nucleus (Fig. 2.1) indicates that a wide variety of RNA molecules are being made (and types of protein produced as a result). In contrast, in heterochromatin the DNA molecules are coiled and condensed and appear dense. The DNA in heterochromatin is mostly inactive, i.e. not directing the synthesis of RNA.
One or more nucleoli also appear as densely stained regions in the nuclei of cells actively synthesising proteins: their position may appear central or peripheral (Fig. 2.1). RNA molecules synthesised in the nucleolus leave the nucleus, via pores in the nuclear membrane, and are involved in organising protein synthesis in the cytoplasm.
Cytoplasm
Cytoplasm comprises a fluid matrix, a cytoskeleton, various membrane-bound organelles and may include stored molecules.
Cytoskeleton
The cytoskeleton of cells has several types of structure which affect the shape of the cells, e.g. tubular structures (centrosomes and microtubules) and protein filaments (intermediate and thin).
■ Centrosomes (Fig. 2.3). These contain paired units, known as centrioles, which are set at right angles to each other and which contain nine triplets of microtubules (see below). Centriole tubules anchor other microtubules.
■ Microtubules. Some microtubules are straight and long and allow the passage of substances within the cell. Many microtubules are polarised, i.e. they have specific ends with one end usually attached to a centrosome. Microtubules in some nerve cells transport molecules in a cytoplasmic process which extends several centimetres (Chapter 6). Other microtubules assist in maintaining cell shape, compartmentalising the cytoplasm or facilitating movement of organelles within the cytoplasm. Microtubules are present in cilia, and in cells undergoing division (see below).
■ Intermediate filaments. The main role of this group of filaments involves resisting external stresses on the cytoplasm by their attachment to specific internal cell structures. They include the protein keratin in the epithelium of skin (Chapter 7).
■ Thin (micro) filaments. These filaments are formed from the protein actin. Actin filaments are present in most cells and are involved in moving organelles within cells, in cell movement, and exocytosis and endocytosis (see below). One specific role of actin in muscle cells involves interaction with thicker protein filaments of myosin, which results in muscle contraction (Chapter 5).
Cytoplasmic organelles
The main cytoplasmic organelles are mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles and lysosomes. These organelles are involved in provision of energy for cellular processes, synthesis and secretion of protein, carbohydrate and lipid-based molecules, storage of proteins and degradation of waste material.
Mitochondria
Mitochondria vary in shape but many are ovoid (Figs 2.4 and 2.5). They are surrounded by an outer and inner membrane and provide most of the energy needs of the cell. The outer membrane is a typical plasma membrane, but the inner membrane has numerous in-foldings known as cristae. These folds increase the surface area inside each mitochondrion and provide a matrix in which metabolic processes occur. The matrix comprises a viscous fluid containing enzymes associated with the tricarboxylic acid (TCA) cycle. Mitochondria are involved in oxidative phosphorylation, which results in the production of adenosine triphosphate (ATP). ATP acts as a store of energy that is used for various cell activities. Mitochondria contain very small amounts of DNA in the matrix which code for some mitochondrial proteins.
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