Gray’s Anatomy (Sec. 1, Chap. 1)

Section 1: Cells, Tissues and Systems
Chapter 1: Basic structure and function of cellsCell StructureGeneral Characteristics of CellsThe shapes of mammalian cells vary widely depending on their interactions with each other, their extracellular environment and internal structures. Their surfaces are often highly folded when absorptive or transport functions take place across their boundaries. Cell size is limited by rates of diffusion, either that of material entering or leaving cells, or of diffusion within them. Movement of macromolecules can be much accelerate and also directed by processes of active transport across membranes and by transport mechanisms within the cell. According to the location of the absorptive or transport functions, apical microvilli (Fig. 1.1) or basolateral infoldings create a large surface area for transport or diffusion.

Fig. 1.1 The main structural components and internal organization of a general cell.

Motility is a characteristic of most cells, in the form of movements of cytoplasm or specific organelles from one part of the cell to another. It also includes: the extension of parts of the cell surface such as psuedopodia, lamellipodia, filopodia and microvilli; locomotion of entire cells as in the amoeboid migration of tissue macrophages; the beating of flagella or cilia to move the cell (e.g. in spermatozoa) or fluids overlying it (e.g. in respiratory epithelium); cell division and muscle contraction. Cell movements are also involved in the uptake of materials from their environment (endocytosis, phagocytosis) and the passage of large molecular complexes out of cells (exocytosis, secretion).

Cells rarely operate independently of each other and commonly form aggregates by adhesion, often assisted by specialized intercellular junctions. They may also communicate with each other either by generating and detecting molecular signals that diffuse across intercellular spaces, or more rapidly by membrane contact, which may involve small, transient, transmembrane channels or interactions between membrane-bound signalling molecules. Cohesive groups of cells constitute issues and more complex assemblies of tissues form functional systems or organs.

Most cells are between 5-50 μm diameter: e.g. resting lymphocytes are 6 μm across, red blood cells 7.5 μm and columnar epithelial cells are 20 μm tall and 10 μm wide (all measurements are approximate). Some cells are much larger than this: e.g. megakaryocytes of the bone marrow are more than 200 μm in diameter. Large neurones and skeletal muscle cells have relatively enormous volumes because of their extended shapes, some of the former being over 1 metre in length.

Cellular OrganizationEach cell is contained within its limiting plasma, or surface, membrane which encloses the cyotoplasm. All cells except mature red blood cells also contain a nucleus that is surrounded by a nuclear membrane or envelope (Fig. 1.1, Fig. 1.2). The nucleus includes the genome of the cell contained within the chromosomes, the nucleolus, and other sub-nuclear organelles. The cytoplasm contains several systems of organelles. These include a series of membrane-bound structures that form separate compartments within the cytoplasm, such as rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria and vesicles for transport, secretion and storage of cellular components. There are also structures that lie free in non-membranous, cytosolic compartment. They include ribosomes and several filamentous protein networks known collectively as the cytoskeleton. The cytoskeleton determines general cell shape and supports specialized extensions of the cell surface (microvilli, cilia, flagella). It is involved in the assembly of new filamentous organelles (e.g. centrioles) and controls internal movements of the cytoplasm and cytoplasmic vesicles. The cytosol contains many soluble proteins, ions and metabolites.

Fig. 1.2 The structural organization and some principal organelles of a typical cell. This example is a ciliated columnar epithelial cell from human nasal mucosa. The central cell, which occupies most of the field of view, is closely apposed to its neighbours along their lateral plasma membranes. Within the apical junctional complex, these membranes form a tightly sealed zone (tight junction) that isolates underlying tissues from, in this instance, the nasal cavity. AJC, apical junctional complex; APM, apical plasma membrae; C, cilia; CY, cytoplasm; EN, euchromatic nucleus; LPM, lateral plasma membrane; M. mitochondria; MV, microvilli; N, nucleolus.

Cell polarity and domainsEpithelia (including endothelia and mesothelia) are organized into sheets or more complex structures (see Ch. 2) with very different environments on either side. These cells actively transfer macromolecules and ions between the two surfaces and are thus polarized in structure and function (Fig. 1.3). In polarized cells, particularly in epithelia, the cell is generally subdivided into domains that reflect the polarization of activities within it. The free surface, e.g. that facing the intestinal lumen or airway, is the apical surface, and its adjacent cytoplasm is the apical cell domain. This is where the cell interfaces with a specific body compartment (or, in the case of the epidermis, with the outside world). The apical surface is specialized to act as a barrier, restricting access of substances from this compartment to the rest of the body. Specific components are selectively absorbed from, or added to, the external compartment by the active processes, respectively, of active transport and endocytosis inwardly or exocytosis and secretion outwardly. The apical surface is often covered with small protrusions of the cell surface, microvilli, which increase the surface area, particularly for absorption.

Fig. 1.3 Absorptive epithelium from human colon, immunolabelled to show the functional division of cell surface domains. CD66 antigen expression (red) is concentrated in the apical domain facing the gut lumen (top), on striated border microvilli; diffuse labelling is present throughout the cytoplasm but is excluded from the basal zone occupied by nuclei. The intestinal cell antigen A33 (green) is a member of the immunoglobulin superfamily of transmembrane proteins, expressed here on basolateral cell surfaces.

The surface of the cell opposite to the apical surface is the basal surface, with its associated basal cell domain. In a single-layered epithelium, this surface is apposed to the basal lamina. The remaining surfaces are known as the lateral cell surfaces. In many instances the lateral and basal surfaces perform similar functions and the cellular domain is termed the basolateral domain. Cells actively transport substances, such as digested nutrients from the intestinal lumen or endocrine secretions, across their basal (or basolateral) surfaces into the subjacent connective tissue matrix and the blood capillaries within it. Dissolved non-polar gases (oxygen and carbon dioxide) diffuse freely between the cell and the bloodstream across the basolateral surface. Apical and basolateral surfaces are separated by a tight intercellular seal, the tight junction, which prevents the passage of even small ions through the space between adjacent cells and thus maintains the difference between environments either side of the epithelium.