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Anatomy and Physiology - Tissues

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    Study Guide Anatomy and Physiology – Tissues 1. Epithelial Tissue W hat is Epithelial Tissue? Epithelial tissue (epithelium) is a type of tissue made of closely packed cells . It forms the coverings and linings of many parts of the body. These cells are usually arranged like a “sheet,” and they protect body surfaces and control what passes in and out. 1.1 Main Characteristics of Epithelium Epithelial tissue has some special features: 1. Cells are tightly packed The cells are close together with very little space between them. This means there is very little intercellular material . 2. Epithelium has no blood vessels (Avascular) Epithelium is avascular , which means it does not contain blood vessels . So how does it get nutrients and remove wastes? By diffusion from nearby connective tissue . 3. It has two surfaces Epithelium has: • a free (apical) surface → faces outside the body or inside a body cavity • a basal surface → rests on connective tissue Between the epithelium and connective tissue, there is a thin supportive layer called the basement membrane .

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    Study Guide 1.2 Types of Epithelial Tissue There are two major types of epithelium: 1. Covering and lining epithelium This type: • covers the outside surface of the body • lines internal organs and body cavities 2. Glandular epithelium This type forms glands and secretes substances like hormones or enzymes. Covering and Lining Epithelium Covering/lining epithelium is classified using two important features : 1) Cell Shape 2) Number of Cell Layers 1) Classification by Cell Shape A. Squamous cells • Flat and thin • Their nucleus is near the upper surface • They often look like a fried egg (flat with a bulge in the middle) B. Cuboidal cells • Cube - shaped or hexagon - shaped • Have a central, round nucleus • Main functions: o secretion (example: sweat) o absorption (example: digested food)

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    Study Guide C. Columnar cells • Tall cells • Oval nucleus located near the basement membrane • Functions: o protect underlying tissue o absorption Some columnar cells may have: • microvilli → tiny surface extensions that increase surface area for absorption • cilia → help move substances like mucus (example: in respiratory tract) D. Transitional cells These cells can change shape: • may look flat or tall • they stretch or compress depending on body movement 2) Classification by Number of Cell Layers A. Simple epithelium • Single layer of cells B. Stratified epithelium • Many layers of cells • Usually provides strong protection C. Pseudostratified epithelium • Actually one layer , but cells have different heights • This makes it look like it has multiple layers How Epithelial Tissues are Named Epithelial tissues are named using:

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    Study Guide 1. number of layers 2. cell shape (and sometimes cilia) Example: • Simple squamous epithelium = one layer of flat cells • Pseudostratified columnar ciliated epithelium = one layer of columnar cells, different sizes, with cilia I mportant point: Stratified epithelium is named by the shape of the top (outermost) layer only. So even if inner layers are cuboidal/columnar, the name depends on the outer layer.

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    Study Guide Figure 1. Types of epithelial tissues. Glandular Epithelium Glandular epithelium forms glands that make and release useful substances.

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    Study Guide There are two main types of glands : 1. Endocrine Glands (No Ducts) Endocrine glands release hormones directly into the bloodstream . Example: • The thyroid gland releases thyroxin • Thyroxin travels through the blood and increases the metabolic rate of body cells 2. Exocrine Glands (Have Ducts) Exocrine glands release their products into ducts (tubes) , which carry the secretion to the epithelial surface. Examples of exocrine secretions: • sweat • saliva • milk • stomach acid • digestive enzymes 1.3 Exocrine Glands: Classification by Structure Exocrine glands can be classified based on how they are built.

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    Study Guide Figure 2. Exocrine glands can be classified as simple or compound with either a tubular or alveolar structure. Structural classification terms 1. Unicellular vs Multicellular • Unicellular gland = single - celled gland • Multicellular gland = many cells A multicellular gland includes: • a group of secretory cells • a duct for carrying secretions out 2. Branched “Branched” means the secretory cells are arranged in branches. 3. Simple vs Compound This depends on the duct , not the secretory part: • Simple gland → duct does not branch • Compound gland → duct branches

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    Study Guide 4. Tubular vs Alveolar (Acinar) This describes the shape of the secretory part: • Tubular gland → secretory portion forms a tube • Alveolar (acinar) gland → secretory portion forms a round, sac - like shape 1.4 Exocrine Glands: Classification by Function Exocrine glands are also classified by how they release their secretions . Figure 3. Exocrine glands can be classified according to their function. 1. Merocrine Glands In merocrine glands : • secretions leave the cell by exocytosis • the cell stays intact Example: • Goblet cells in the trachea release mucus by exocytosis

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    Study Guide 2. Apocrine Glands In apocrine glands : • a portion of the cell (the top/apical part) breaks off • this portion contains the secretion • the rest of the cell remains and continues working Example: • Lactiferous glands release milk this way 3. Holocrine Glands In holocrine glands : • the entire secretory cell fills with product • the whole cell disintegrates • cell contents + cell debris are released together Example: • Sebaceous glands release sebum • sebum helps lubricate the skin Key Takeaways Epithelium = tightly packed cells that cover surfaces and form glands Avascular = no blood vessels (diffusion from connective tissue) Basement membrane = between epithelium and connective tissue Classified by: • cell shape (squamous, cuboidal, columnar, transitional) • layers (simple, stratified, pseudostratified)

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    Study Guide Glands: • endocrine → hormones into blood • exocrine → secretions into ducts Exocrine glands: • structure (simple/compound, tubular/alveolar) • function (merocrine, apocrine, holocrine) 2. Quiz: Epithelial Tissue 1. Question A group of organized __________ working together forms __________. An organized group of the latter work together to form __________. Answer Choices • tissues; organs; cells • organs; cells; tissue • cells; tissue; organs Correct Answer cells; tissue; organs Why This Is Correct The body is organized in levels: 1. Cells are the smallest living units. 2. Groups of similar cells form tissues . 3. Different tissues work together to form organs . Example: Muscle cells → muscle tissue → heart (organ)

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