In multicellular animals, cells are organized into tissues, which perform specific functions. There are four primary types of tissues in animals: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.
1. Epithelial Tissue:
o Overview: This tissue forms the covering of all body surfaces, lines body cavities and hollow organs, and is the major tissue in glands. It plays roles in protection, absorption, secretion, and excretion.
o Characteristics:
§ Cells are tightly packed with minimal intercellular space.
§ They rest on a basement membrane.
§ Lacks blood vessels, so nutrients are received from the underlying connective tissue via diffusion.
o Functions:
§ Protection against mechanical injury, pathogens, and dehydration.
§ Secretion of hormones, enzymes, and mucus.
§ Absorption of nutrients (e.g., in the intestines).
§ Sensory reception (e.g., skin, taste buds).
o Types of Epithelial Tissue: This will be covered in more detail in the next section.
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2. Connective Tissue:
o Overview: This tissue supports, binds, or separates different tissues and organs. It is the most abundant tissue in animals.
o Characteristics:
§ Composed of cells embedded in an extracellular matrix (ECM).
§ The ECM consists of protein fibers (collagen, elastin) and ground substance (fluid, gelatinous, or solid).
§ Rich in blood vessels, except for cartilage.
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