Tissues and Cell Junctions

• Tissues are groups of cells with a common embryonic origin and function, working together for specialized activities.
• The consistency of tissues can be hard, semisolid, or liquid, as seen in bone, fat, and blood, respectively.
• Tissues serve to protect, support, facilitate communication among cells and resist disease.

Types of Tissues

• The four basic tissue types are epithelial, connective, muscular, and nervous tissue.
• Epithelial tissue's role is in covering and lining surfaces, lining hollow organs to form glands.
• Connective tissue protects and supports the body and its organs, binds organs together, stores energy reserves as fat, and bolsters immunity.
• Muscular tissue generates physical force for body structures to move, as well as generating body heat.
• Nervous tissue detects changes inside and outside the body and responds by generating action potentials that activate muscular contractions and glandular secretions.

Cell Junctions

• Epithelial, muscle, and nerve cells are often tightly joined into functional units.
• Cell junctions serve as contact points between the plasma membranes of tissue cells.
• Five main types of cell junctions include: tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions.

Epithelial Tissue

• Epithelial tissue consists of cells arranged in continuous sheets of single or multiple layers, closely packed with minimal intercellular space due to many cell junctions.
• Epithelial tissue always has a free surface, as it forms coverings & linings throughout the body and is never covered by another tissue.
• Three main functions include selective barriers for substance transfer, secretory surfaces for product release, and protective surfaces against environmental abrasion.

Basement Membrane

• The basement membrane, supporting epithelial tissue, comprises two layers: the basal lamina and the reticular lamina.
• The basal lamina (thin layer) which contains laminin and collagen proteins, is secreted by epithelial cells.
• Laminin molecules in the basal lamina attach epithelial cells to the basement membrane via integrins in hemidesmosomes.
• The reticular lamina located closer to the underlying connective tissue, contains collagen produced by connective tissue cells, known as fibroblasts.
• Epithelial tissue has its own nerve supply and is avascular.
• Nutrients reach the epithelium through diffusion because the blood vessels are in the adjacent connective tissue.
• Epithelial tissue functions significantly in protection, filtration, secretion, absorption, and excretion.
• In combination with nervous tissue it can for special organs for smell, vision, touch and hearing.

Types of Epithelial Tissue

• Epithelial tissue has two types: covering and lining epithelium, and glandular epithelium.
• Covering and lining epithelium forms coverings of skin, the inner linings of blood vessels, ducts, body cavities, and interior of the respiratory, reproductive, digestive, and urinary systems.
• Glandular epithelium forms the secreting portion of glands like the thyroid, adrenal, and sweat glands.
• Glandular epithelium includes endocrine and exocrine glands.

Covering and Lining Epithelium

• Covering and lining epithelium are classified according to their arrangement of cells into layers and the shapes of the cells.
• Cell arrangement can be simple (single layer), pseudostratified, or stratified (multiple layers).
• Cell shapes can be squamous, cuboidal, columnar, or transitional.

Types of Covering and Lining Epithelia

• Simple epithelium includes:
  • Simple squamous epithelium which consists of a single layer of flat cells with a central nucleus; location is in air sacs of lungs, glomerular capsule of kidneys, and serous membranes; functions in filtration, diffusion, and osmosis.
  • Simple cuboidal epithelium is made up of cube-shaped cells with a round, central nucleus and it facilitates secretion and absorption: it is is found in the anterior surface of the lens of the eye, kidney tubules, and smaller gland ducts.
  • Nonciliated simple columnar epithelium which features a single layer of nonciliated column-like cells with nuclei near the base, goblet cells, and microvilli; lines the gastrointestinal tract and gallbladder; functions in secretion and absorption.
  • Ciliated simple columnar epithelium consists of a single layer of ciliated column-like cells with nuclei near base; lines bronchioles, uterine tubes, uterus, sinuses, spinal cord's central canal, and brain ventricles; functions in moving mucus and other substances.
  • Pseudostratified columnar epithelium isn't truly stratified; it features multiple levels of nuclei and possesses cells which are connected to the basement membrane but don't all reach the apical surface lines airways of upper respiratory tract, many large gland ducts, the epididymis, and male urethra and facilitates secretion and movement of mucus by ciliary action.
• Stratified epithelium includes:
  • Stratified squamous epithelium which has multiple layers with cuboidal to columnar shapes in deep layers and squamous cells in the apical layer: keratinized forms superficial skin and the nonkeratinized lines wet surfaces; used for protection.
  • Stratified cuboidal epithelium features two or more layers of cube-shaped cells in the apical layer and it is used for protection; found in adult sweat gland ducts, esophageal glands, and male urethra.
  • Stratified columnar epithelium contains multiple layers of irregularly shaped cells and only the apical layer has columnar cells; located in part of urethra, in large excretory gland ducts, in the anal mucous membrane and conjunctiva of the eye; for protection and secretion.
  • Transitional epithelium features a variable appearance related to its transitional ability. Cell shape in the apical layer is squamous when stretched and cuboidal when relaxed; located in the urethra, ureters and the urinary bladder; for distension.

Glandular Epithelium

• Glandular epithelium performs the function of secretion, which is accomplished by glandular cells that often lie in clusters deep to the covering and lining epithelium.
• A gland may consist of a single cell or a group of cells that secrete substances into ducts, onto a surface, or into the blood.
• Glands are classified into endocrine and exocrine types.

Endocrine Glands

• Endocrine gland's secretory products hormones which diffuse into blood after passing through interstitial fluid in the brain, thyroid gland, larynx, and testes; producing hormones to regulate various body activities

Exocrine Glands

• Exocrine gland's secretory products are released into ducts in the sweat, oil and earwax glands of the skin, and digestive such as salivary glands or pancreas; used for producing substances.

Connective Tissue

• Connective tissues most abundant and widely distributed tissues in the body, with a variety of functions including binding, support, strengthening, protecting and insulating organs, compartmentalizing structures, transport, energy storage, and immune responses.

General Features of Connective Tissue

• Composed of two basic elements: extracellular matrix and cells.
• The extracellular matrix is the material located between widely spaced cells and consists of protein fibers and ground substance which is secreted by the connective tissue cells and determines tissue qualities.
• Connective tissues don't usually occur on body surfaces and are highly vascular.

Connective Tissue Cells

• Types of connective tissue cells include fibroblasts, adipocytes, mast cells, white blood cells, macrophages, and plasma cells.

Extracellular Matrix

• Consists of two major components: ground substance and fibers.
• Ground substance is the component between the connective tissue cells and fibers and it may be fluid, semi-fluid, gelatinous, or calcified.
• Ground substance supports cells, stores water, binds cells and provides a medium thru which substances are exchanged between the blood and cells and influences tissue development, migration, proliferation, and shape.

Fibers

• Collagen fibers, elastic fibers, and reticular fibers are three types of fibers which are embedded in the extracellular matrix.

Classification of Connective Tissues

• Connective tissues are classified into embryonic and mature connective tissue.
• Embryonic connective tissue includes mesenchyme and mucous connective tissue.
• Mature connective tissue includes loose connective tissue, dense connective tissue, cartilage, bone tissue, and liquid connective tissue.

Characteristics of Cartilage

• Cartilage can be hyaline, fibrocartilage or elastic cartilage.
• Hyaline carriage feature a bluish-white and shiny ground substance, thin and fine collagen fibers, found on the ends of long bones, anterior ends of ribs, nose, and provide for movement at joints, flexibility and support.
• Fibrocartilage is made up of chondrocytes among thick bundles of collagen fibers inside of the Pubic and intervertebral which provide fusion, support and fusion.
• Elastic cartilage feature chondrocytes located in a threadlike network of elastic fibers in the epiglottis, external ear and auditory tubes which help maintain shape and structure.

Bone Tissue

• Bone tissue or compact bone consists of osteons that contain lamellae, lacunae, osteocytes, canaliculi, and central canals.
• Bone Tissue is for support, protection, storage; houses blood-forming tissue; serves as levers, and is spongy and makes up the body's bones.

Liquid Connective Tissue - Blood

• Liquid connective tissue contains blood plasma and red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
• Liquid connective tissue is within blood vessels and heart chambers.
• Liquid connective tissue facilitates oxygen and carbon dioxide transport, phagocytosis, immunity, allergenic reactions, and blood clotting.

Muscular Tissue

• Muscular tissue consists of elongated cells known as muscle fibers or myocytes that use ATP to generate force.
• Muscular tissue gives movements, maintains posture, helps with the protection and also produces heat

Types of Muscle TIssue

• Muscular tissue is classified into three types: skeletal, cardiac, and smooth muscle tissues.
• Skeletal muscle tissue is for motion, posture, heat production, and protection and it is attached to the bones by tendons.
• The cardiac muscle tissue is known to control with blood pressure.
• Smooth muscle tissue contracts blood vessels, airways, etc, and involuntary.

Nervous Tissue

• Nervous tissue consists of only two principAl types of cells known as neurons and neuroglia (glial cells).
• Neurons, or nerve cells, are sensitive to various stimuli they turn stimuli into electrical signals called action potentials.
• Most neurons consist of three basic parts: a cell body and two kinds of cell processes- dendrites and axons.

Glial Cells

• Glial cells (non-Neuronal cells) maintains homeostasis, forms myelin, protects the neurons in central and peripheral nervous systems.
• The types of glial cells found in the are oligodendrocytes, astrocytes, ependymal cells, microglia, schwann cells and also satellite cells.