Introduction to cells and tissues
Histology (‘histo’ meaning ‘tissue’ or ‘web’ in Greek) is the study of normal cells and tissues, mainly with the use of a microscope. It involves all aspects of tissue biology, focusing on how cells’ structure and arrangement optimize functions specific for each organ. The sound knowledge of these normal histologic structures is essential for understanding the histopathology or pathology of any disease, which often cause specific changes in cells and tissues.
Cells are the building blocks of all living organisms. They are the smallest units in the living body and contain all the fundamental molecules that permit life. The simplest organisms, like bacteria, for example, are formed of a single cell. More complex organisms, such as humans, consist of a vast array of highly specialized cells. Cells perform various functions, depending on their genetic blueprint. They can acquire and utilize energy, perform a variety of chemical reactions, engage in mechanical activities, and overall maintain proper body homeostasis. It should be noted that cells can only arise from the division of preexisting cells.
In general, the cell can be divided into two compartments: the cytoplasm and nucleus (N). The cytoplasm is located outside the nucleus, and contains various organelles and a cytoplasmic matrix. This matrix consists of solutes that include inorganic ions, like sodium, potassium, and calcium, and organic molecules, like carbohydrates, proteins, and ribonucleic acid (RNAs).
The nucleus, on the other hand, is considered the largest organelle within a cell. It contains the genetic material in the form of deoxyribonucleic acid (DNA), along with the enzymes necessary for DNA replication and RNA transcription.
The cytoplasm also contains various other organelles that are classified as being either membranous or nonmembranous. Some of the major membranous organelles include the endoplasmic reticulum (ER), Golgi apparatus (G), the mitochondria (M), intracellular transport vesicles (V), and lysosomes (L). Organelles that are not enclosed by a membrane include ribosomes, cytoskeletal structures, centrioles, cilia, and flagella. Both the cytoplasm and nucleus have their own distinct functional roles; however, they work together in concert to maintain the cell’s viability.
Tissues are formed of cells with similar morphology, function and an extracellular matrix. Examples of tissues include cartilage, bone, and muscles. The extracellular matrix is a collection of extracellular molecules secreted by the cells that impart great structural and biochemical support for the tissue cells. It provides mechanical support, transports nutrients and carries away catabolites and secretory products.
Organs are groups of tissues that act together to perform specific functions. Tissues and organs can comprise integrated functional systems that form major anatomical entities - e.g. the gastrointestinal tract. Within tissues, cells interact with each other in various ways during embryological development, growth maintenance, and response to injury. They also control tissue and organ function, and the maintenance of their overall structural and metabolic integrity. Intercellular junctions serve as channels for information exchange between cells in the form of electrical excitation or chemical messengers. Inside the tissue, cellular functions are integrated by a variety of local chemical mediators or by direct contact between the cells.