Chapter 5.1-5.3

Chapter 5: Tissues

5.1 Cells are Organized into Tissues

  • Cells are organized into tissues; all cells have a common function.

  • Four major types of tissues:

    • Epithelial

    • Connective

    • Muscle

    • Nervous

Intercellular Junctions

  • Tight Junctions:

    • Allow membranes to converge and fuse.

  • Desmosomes:

    • Bind cells by forming "spot welds" between cell membranes.

  • Gap Junctions:

    • Link the cytoplasm of adjacent cells and allow ions and nutrients to move through.

5.2 Epithelial Tissues

  • Epithelial tissues cover organs and body surfaces, line cavities and hollow organs, and make up glands.

  • Features of epithelial tissues:

    • Have a free surface (apical) on the outside, and the underside (basal) is anchored by connective tissue through a thin, nonliving layer called the basement membrane.

    • Lack blood vessels (avascular); nutrients diffuse from underlying connective tissue.

    • Cells readily divide, allowing for rapid healing of injuries.

    • Cells are tightly packed, acting as a barrier.

  • Classified according to cell shape and number of layers:

Classifications

  • Shapes:

    • Squamous: flat

    • Cuboidal: cube-shaped

    • Columnar: tall

  • Layers:

    • Simple: one layer of cells

    • Stratified: two or more layers of cells

    • Pseudostratified: appears layered, but is not.

Types of Epithelial Tissues

  • Simple Squamous Epithelium:

    • Fit tightly together like floor tiles; nuclei are broad and thin.

    • Thin and delicate.

    • Found in diffusion and filtration sites.

    • Composes walls of air sacs (alveoli) and capillaries.

    • Lines blood and lymphatic vessels.

  • Simple Cuboidal Epithelium:

    • Single layer of cube-shaped cells with centrally located nuclei.

    • Functions in secretion and absorption.

    • Lines kidney tubules, thyroid follicles, and ducts of some glands; free layer faces the hollow channel or lumen.

  • Simple Columnar Epithelium:

    • Nuclei usually at the same level, near the basement membrane.

    • Functions in secretion, absorption, and protection.

    • Contains goblet cells, which secrete a protective fluid called mucus on the free surface.

    • Microvilli amplify surface area of the cell membrane—specialized for absorption.

    • Examples include lining of the uterus, stomach, and intestines.

  • Pseudostratified Columnar Epithelium:

    • Nuclei at two or more levels.

    • Cells vary in shape, but all reach the basement membrane.

    • Often has cilia and goblet cells.

    • Provides protection from infection.

    • Lines respiratory passageways.

  • Stratified Squamous Epithelium:

    • Composed of many layers of cells; thick and protective.

    • Outermost cells are squamous, while deeper cells are cuboidal.

    • New cells produced in deep layers push older cells towards the free surface, causing them to die.

    • Epidermis is keratinized to allow for a dry, tough, protective material.

    • Lines oral cavity, vagina, anal canal, and esophagus; tissue in these regions is not keratinized, and cells remain alive.

  • Stratified Cuboidal Epithelium:

    • Composed of 2 to 3 layers of cube-shaped cells forming the lining of the lumen.

    • Provides more protection than a single layer.

    • Lines ducts of mammary, sweat, and salivary glands, as well as the pancreas.

  • Stratified Columnar Epithelium:

    • Composed of several layers of cells; the top layer consists of elongated cells, while deeper layers contain cube-shaped cells.

    • Functions in protection, absorption, and secretion.

    • Lines parts of the male urethra (vas deferens) and portions of the pharynx.

  • Transitional Epithelium:

    • Composed of many cell layers; features both cube-shaped and elongated cells.

    • Changes shape and appearance with increased tension.

    • Acts as a barrier and lining that can expand.

    • Lines the urinary bladder, ureters, and part of the urethra.

Glandular Epithelium

  • Composed of cells that produce and secrete substances into ducts or body fluids.

  • Two types of glands in the body:

    • Endocrine glands: secrete into tissue fluid or blood.

    • Exocrine glands: secrete into ducts that open onto an internal or external surface.

  • Exocrine Glands:

    • Structural Types:

    • Unicellular: Composed of one epithelial cell, e.g., a goblet cell (secretes mucus).

    • Multicellular: Composed of many cells; these can be simple or compound; examples include sweat and salivary glands.

Exocrine Gland Structures

  • Various structural types of exocrine glands:

    • Simple tubular: single gland with an unbranched duct.

    • Simple branched tubular: multiple tubular glands, but with a single duct.

    • Simple coiled tubular: coiled tubular glands with an unbranched duct.

    • Simple branched alveolar: gland with a branched duct and terminal saclike structures.

    • Compound tubular: glands with branched ducts leading to secretory portions.

    • Compound alveolar: similar to compound tubular but with sac-like secretory cavities.

Mechanisms of Secretion in Glandular Epithelium

  • Merocrine Glands:

    • Secrete fluid products by exocytosis; examples include salivary and sweat glands, and pancreas (which releases digestive juices into the duodenum).

  • Apocrine Glands:

    • Lose a small part of the cell during secretion; examples include mammary and ceruminous glands (which produce earwax).

  • Holocrine Glands:

    • Release entire cells filled with the product; an example is the sebaceous gland, which secretes sebum, an oily substance on our skin, associated with hair follicles.

5.3 Connective Tissues

  • Connective tissues are the most abundant tissue type by weight.

  • Cells are farther apart than epithelial cells; they can be either fixed or wandering cells.

  • Contain abundant extracellular matrix between cells, which consists of protein fibers and a ground substance.

    • The consistency of this matrix can vary from fluid to solid.

  • Cells usually divide, and most connective tissues have a good blood supply.

Functions of Connective Tissues

  • Bind structures together.

  • Provide support and protection.

  • Serve as frameworks.

  • Fill spaces.

  • Store fat.

  • Produce blood cells.

  • Protect against infections.

  • Help repair tissue damage.

Major Cell Types in Connective Tissues

  • Fibroblasts (Fixed):

    • The most common fixed cell type.

    • Large star-shaped cells that produce fibers by secreting proteins into the matrix.

    • Rapidly respond to injury by increasing in number to repair tissues that have limited ability to regenerate.

  • Mast Cells (Fixed):

    • Usually located near blood vessels and are derived from white blood cells.

    • Covered in granules; they release heparin (which prevents blood clotting) and histamine (which stimulates inflammation and acts as a vasodilator).

  • Macrophages (Wandering):

    • Originate as white blood cells and are very numerous.

    • Typically attached to fibers but can also detach and wander.

    • Conduct phagocytosis and function as scavenger cells, removing foreign particles from tissues and defending against infection.

Connective Tissue Fibers

  • Fibroblasts produce three types of connective tissue fibers:

    • Collagen:

    • Grouped in long, parallel bundles.

    • Provides great tensile strength; flexible, but only slightly elastic.

    • Found in ligaments (which connect bone to bone) and tendons (which connect muscles to bones).

    • Elastic:

    • Composed of bundles of myofibrils embedded in elastin.

    • Can stretch and return to original shape; not as strong as collagen fibers.

    • Found in vocal cords and respiratory air passages.

    • Reticular:

    • Thin, branching fibers of collagen.

    • Form delicate, supporting networks; found in the spleen and liver.

Types of Connective Tissue Proper

  • Loose connective tissue (areolar tissue):

    • Forms thin, delicate membranes.

    • Mainly composed of fibroblasts in a gel-like ground substance.

    • Found in the subcutaneous layer; binds skin to underlying organs and fills spaces between muscles.

  • Adipose (Fat) Tissue:

    • Contains adipocytes that store fat droplets in the cytoplasm, crowding out other cell types to form adipose tissue.

    • Functions include cushioning joints and organs (like the kidneys), and insulating beneath the skin.

    • White adipose tissue stores nutrients for nearby cells to use in energy production, while brown adipose tissue contains many mitochondria that break down nutrients to generate heat to warm the body.

  • Reticular Connective Tissue:

    • Composed of a network of thin reticular fibers, functioning as a filter.

    • Supports the walls of internal organs, such as the liver and spleen.

  • Dense Regular Connective Tissue:

    • Consists of closely packed, thick, collagenous fibers organized in a regular arrangement.

    • Very strong and binds body parts together, as seen in tendons and ligaments.

    • Has a poor blood supply.

  • Dense Irregular Connective Tissue:

    • Consists of thicker, interwoven, and more randomly organized fibers.

    • Found in the dermis, periosteum (the covering of bones), perichondrium (the covering of cartilage), and encasing organs—providing nourishment and protection.

  • Elastic Connective Tissue:

    • Composed of elastic fibers arranged in parallel strands or in a branching network.

    • Includes collagenous fibers and fibroblasts between elastic fibers.

    • Found in attachments between vertebrae and within the walls of some internal hollow organs, such as large arteries and some areas of the heart (like the aorta) and larger airways.

Specialized Connective Tissue

  • Cartilage:

    • Provides support, frameworks, and attachments, protects underlying tissues, and forms structural models for developing bones.

    • Matrix is composed of collagen fibers in a gel-like ground substance rich in chondromucoprotein, containing high water content.

    • Cells called chondrocytes occupy lacunae; these cells do not divide frequently.

    • Surrounded by perichondrium, a cartilaginous structure enclosed in dense irregular tissue.

  • Types of Cartilage:

    • Hyaline Cartilage:

    • Most common type; it is slippery and smooth, allowing bones to move seamlessly.

    • Contains fine collagen fibers in its matrix.

    • Found in the ends of bones in joints, nose, respiratory passages, and between ribs.

    • Elastic Cartilage:

    • Most flexible due to elastic fibers in its matrix.

    • Found in the external ear and larynx.

    • Fibrocartilage:

    • Very tough due to numerous collagen fibers.

    • Acts as a shock absorber found in intervertebral discs, pads (menisci) of the knee, and pelvic girdle.

  • Bone:

    • The most rigid connective tissue due to the presence of mineral salts.

    • Supports and protects vital structures (like those found in cranial and thoracic cavities) and produces blood cells.

    • Serves as attachment sites for muscles and forms the skeleton.

    • Contains osteocytes (bone cells) in lacunae, which deposit matrix in thin layers called lamellae, forming concentric circles around capillaries located in Haversian canals.

  • Types of Bone:

    • Compact Bone:

    • Composed of osteocytes, matrix, and Haversian canals that form cylindrical units called osteons.

    • The osteocytes extend processes into canaliculi and share a blood supply.

    • Spongy Bone:

    • Comprises the interior part of bones, consisting of bony plates containing osteocytes with spaces in between that house marrow.

    • Lighter than compact bone.

  • Blood:

    • Composed of red blood cells, white blood cells, and cellular fragments known as platelets suspended in plasma.

    • Most blood cells form in hematopoietic tissues located in red marrow.