Eukaryotic Tissues PDF

**Eukaryotic Tissues** Introduction The evolution of multicellular organisms provided an opportunity for cells, the structural and functional units of all living organisms, to associate in specialized groups that perform distinct functions. These functional groups of cells are called **tissues**....

**Eukaryotic Tissues** Introduction The evolution of multicellular organisms provided an opportunity for cells, the structural and functional units of all living organisms, to associate in specialized groups that perform distinct functions. These functional groups of cells are called **tissues**. This lesson covers the structure and function of the four tissue types found in animal cells: epithelial, connective, muscle, and nervous. Different types of tissues can be organized to form organs and organ systems. For example, in the intestinal tract, epithelial cells are specialized for the passage of nutrients and water, connective tissue cells provide support, smooth muscle cells contract to aid in the propulsion and absorption of nutrients, and enteric nerves regulate motor functions and gastrointestinal enzyme secretion. 5.5.01 Types of Animal Tissues In animals, there are four major types of tissues: epithelial, connective, muscle, and nervous (Table 5.3). The organization of each of these tissue types within specific organs and organ systems is discussed in more detail in subsequent chapters. Because nervous and muscle tissue are covered in detail in Chapters 12 and 17, this concept focuses on the characteristics and functions of epithelial and connective tissues. **Table 5.3** Types of animal tissues. A diagram of human organs Description automatically generated Chapter 5: Eukaryotic Cells 209 **Epithelial tissues** make up outer and inner body surface linings such as the skin and inner surfaces of the digestive, respiratory, and reproductive tracts (Figure 5.66). The epithelial layer of skin is primarily protective in function, providing a barrier against physical, chemical, and microbial threats. Epithelial cells that line inner body surfaces are collectively called **mucous membranes** because these cells secrete a lubricating substance called **mucus**. Mucus serves to coat and protect mucous membranes, while also participating in trapping pathogenic organisms and other debris. Epithelial cells lining inner body surfaces also participate in the secretion and absorption of various substances, as well as the excretion of wastes. Epithelial cells are tightly joined to one another and to an underlying layer of connective tissue (ie, basement membrane). In general, epithelial tissues lack their own blood supply and must gain nutrients from underlying vascularized connective tissue. Rapid proliferation of epithelial cells facilitates replenishment of damaged or senescent cells. **Figure 5.66** Classification and functions of epithelial tissues. Classification of epithelial tissues is based on cell shape and number of cell layers present (Figure 5.66). While it is unlikely that an exam question would require memorization of epithelial cell classification, the terms used to describe the organization of epithelial tissues have been referenced in exam passages and questions. Tissue containing epithelial cells organized in a single layer is called **simple** epithelium. **Squamous** epithelium consists of thin and flat cells, while cube-shaped cells are called **cuboidal** epithelium. Tall, elongated cells are known as **columnar** epithelium.  Chapter 5: Eukaryotic Cells 210 Epithelial tissue consisting of two or more layers is called **stratified** epithelium. **Pseudostratified** epithelial cells appear to be organized in multiple layers due to differences in height, but this tissue type is actually composed of a single cell layer. **Transitional** epithelium is a stratified tissue that can appear to consist of cuboidal or squamous cells, depending on the degree to which the tissue is stretched. **Connective tissues** are a diverse group of tissues that most often serve to bind and connect structures, thereby providing support and protection. These tissues may also provide structural frameworks, fill spaces, store energy in the form of fat, protect against infections, and help repair tissue damage. Cells comprising connective tissues are less tightly associated with one another than epithelial cells; in addition, connective tissue cells are typically surrounded by a more extensive extracellular matrix than epithelial cells. Most connective tissues can proliferate and contain a blood supply to varying degrees, depending on the cell types involved. Connective tissues may be either loosely or densely organized, and some connective tissues have specialized functions. Due to these characteristics, connective tissue is categorized as loose, dense, or specialized (Figure 5.67). **Loose connective tissues** include areolar, adipose, and reticular connective tissues, which support various organs. **Dense connective tissues** may be categorized as regular, irregular, or elastic, and are more fibrous in nature (eg, tendons, ligaments, walls of arteries). **Specialized connective tissues** include cartilage, bone, and blood. **Figure 5.67** Types of connective tissue.