Tissues Living Communities Lecture Part 1 Study Notes

Introduction to Tissues

• Definition of Tissues: Tissues are clusters of similar cells that work together to perform a specific function, creating the organizational structure of the body. They represent a higher level of organization than individual cells but precede organs. These cells often have a common embryonic origin and intercellular substance.

• Hierarchy of Biological Organization: Living organisms are organized into a hierarchical structure, starting from the chemical level (atoms, molecules), progressing to cells.

  • Cellular Level: Individual cells (e.g., muscle cell, neuron).

  • Tissue Level: Groups of similar cells working together (e.g., muscle tissue, nervous tissue).

  • Organ Level: Two or more different tissues working together to perform a specific function (e.g., heart, stomach).

  • Organ System Level: Groups of organs that work together to perform major functions or meet physiological needs of the body (e.g., digestive system, circulatory system).

  • Organismal Level: The complete living being.

• Primary Types of Tissues: These four basic tissue types are the fundamental building blocks from which all body organs and structures are constructed.

  • Epithelial Tissue

  • Connective Tissue

  • Muscle Tissue

  • Nervous Tissue

Functions of Tissues

• Varied and Interdependent Functions: Each primary tissue type has specialized roles that are integrated to maintain homeostasis and enable complex bodily functions.

  • Epithelial: Forms covering for all body surfaces, lines body cavities and hollow organs, and is the major tissue in glands. Functions include protection, secretion, absorption, excretion, filtration, and sensory reception.

  • Connective: The most abundant and widely distributed tissue type. Provides support, connects, and binds other tissues together. Functions include binding and support, protection, insulation, storage of reserve fuel, and transport of substances (e.g., blood).

  • Muscle: Specialized for contraction. Produces movement in the body through the contraction of its cells, which contain contractile proteins (actin and myosin). There are three types: skeletal, cardiac, and smooth.

  • Nervous: Composed of neurons and glial cells, responsible for communication. Transmits electrical signals (nerve impulses) rapidly over long distances, integrating and controlling body functions, and processing information.

• Presence in Organs: All four tissue types are typically found in most organs, working in concert. For instance, the stomach contains:

  • Epithelial tissue lining the lumen for protection and secretion of digestive juices.

  • Connective tissue forming its wall to bind and support.

  • Muscle tissue (smooth muscle) for churning food.

  • Nervous tissue to regulate its activity.

Epithelial Tissue

• Overview: Epithelial tissue, or epithelium (epi = upon; thele = to cover), is a sheet of cells that covers body surfaces or lines body cavities. It forms boundaries between different environments, e.g., the outer layer of the skin, lining of the digestive tract.

• Example: Stratified squamous epithelial tissue is found in the skin (epidermis) and consists of multiple layers of cells, protecting underlying tissues from abrasion. Its deepest layers are cuboidal or columnar, which flatten as they push towards the surface, reminiscent of scrambled eggs in appearance.

• Characteristics: These unique features enable epithelium to perform its diverse functions.

  • Cellularity: Composed almost entirely of tightly packed sheets of cells with minimal extracellular material. Cells are bound together by various specialized junctions.

  • Polarity: Always has an apical (free, exposed to the exterior or cavity) and a basal (attached) surface. The apical surface may have microvilli or cilia.

  • Specialized Contacts: Adjacent epithelial cells are bound together by tight junctions and desmosomes, forming continuous sheets.

  • Avascular but Innervated: Contains nerve endings (innervated) but lacks its own direct blood supply (avascular). Nutrients are diffused from the underlying connective tissue.

  • Supported by Connective Tissue: Rests on and is supported by a basement membrane, which is anchored to underlying connective tissue.

  • Regeneration: High regenerative capacity; rapidly replaces lost or damaged cells by cell division.

Structural Features of Epithelial Tissue

• Basement Membrane (Basal Lamina and Reticular Lamina):

  • A thin, extracellular, nonliving, supportive sheet that epithelial tissue rests upon.

  • Composed of two layers:

    • Basal Lamina: An adhesive sheet secreted by the epithelial cells, consisting of glycoproteins and collagen fibers, acting as a selective filter.

    • Reticular Lamina: Underneath the basal lamina, secreted by the underlying connective tissue, composed of collagen fibers.

  • Functions include anchoring the epithelial tissue, defining the epithelial boundary, and resisting stretching and tearing. Its thickness varies with the amount of stress the tissue endures.

• Types of Junctions: Epithelial cells are held together by highly specialized intercellular junctions, crucial for tissue integrity and function.

  • Tight Junctions (Zonula Occludens): Form an impermeable barrier by fusing the outer layers of adjacent cell membranes, preventing paracellular transport (leakage) of fluids and solutes between cells. Found in areas requiring strict control over substance movement, like the lining of the bladder, stomach, intestines, and the blood-brain barrier.

  • Desmosomes (Macula Adherens): Anchoring junctions that bind adjacent cells together like "spot welds" or Velcro. They provide mechanical strength and stability, resisting mechanical stress. Intermediate filaments (keratin filaments) extend from the plaques across the cell to anchor to the opposite side, distributing tension. Abundant in tissues subjected to great mechanical stress, such as the heart muscle, skin, and uterus.

  • Hemidesmosomes: "Half-desmosomes," similar to desmosomes but anchor epithelial cells to the underlying basement membrane, not to other cells. Provide strong adhesion between the basal epithelial cells and the connective tissue, preventing separation.

  • Gap Junctions (Nexus): Intercellular channels formed by tubular channel proteins called connexons. These allow rapid communication and direct passage of small molecules (ions, simple sugars, small signaling molecules) between adjacent cells for coordinated activity. Essential in electrically excitable tissues like the heart (for synchronized contractions) and smooth muscle (in the intestines for coordinated peristalsis).

Classifications of Epithelial Tissue

• By Layer: Refers to the number of cell layers.

  • Simple Epithelium: Consists of a single layer of cells. Primarily involved in absorption, secretion, and filtration, as a thin barrier is optimal for these processes. Found where rapid diffusion or absorption occurs (e.g., lining of blood vessels and pulmonary alveoli).

  • Stratified Epithelium: Composed of two or more layers of cells. Its main function is protection in areas subjected to abrasion. The basal cells divide and push apically to replace older superficial cells. Found in areas of high wear-and-tear (e.g., skin, mouth lining, esophagus).

  • Transitional Epithelium: A specialized type of stratified epithelium. The superficial cells are dome-shaped (cuboidal/columnar) when the organ is relaxed and flatten when stretched. Found exclusively in the urinary system (bladder, ureters, part of urethra), allowing distension and recoil.

  • Pseudostratified Columnar Epithelium: Appears stratified because the nuclei are at different levels and not all cells reach the apical surface, but it is actually a single layer of cells, all attached to the basement membrane. Often ciliated and involved in secretion and propulsion of mucus. Common in the respiratory tract.

• By Cell Shape: Refers to the shape of the cells in the apical layer for stratified epithelia, or the single layer for simple epithelia.

  • Squamous Cells: Flattened and scale-like, with a flattened nucleus. Ideal for diffusion and filtration.

  • Cuboidal Cells: Cube-shaped, with a spherical central nucleus. Involved in secretion and absorption.

  • Columnar Cells: Tall and column-shaped, with an oval nucleus typically located near the basal surface. Specialized for absorption and secretion.

• Special Features: Specialized structures often found on the apical surface of epithelial cells to enhance specific functions.

  • Microvilli: Tiny, finger-like extensions of the plasma membrane that significantly increase the surface area for absorption and secretion. Found extensively in absorptive cells, such as those lining the small intestine and kidney tubules.

  • Cilia: Hair-like motile projections that sweep substances across the epithelial surface. Composed of microtubules, they move in a coordinated way to propel mucus or other substances. Abundant in the respiratory tract (to sweep dust and pathogens out) and the female reproductive tract (to move ova).

  • Keratin: A tough, fibrous protein providing waterproofing and protection against desiccation and mechanical stress. Found in the apical layers of keratinized stratified squamous epithelium (e.g., epidermis of the skin), making the tissue more durable and resistant to water loss. Non-keratinized stratified squamous epithelia (e.g., lining of the mouth, esophagus) lack this protein and remain moist.

Detailed Types of Epithelia

• Simple Squamous Epithelium:

  • Description: Single layer of flattened, scale-like cells with sparse cytoplasm and a disc-shaped central nucleus.

  • Function: Allows materials to pass by diffusion and filtration in sites where protection is not important; secretes lubricating substances in serosae.

  • Location: Found in the lining of the cardiovascular and lymphatic systems (endothelium), serous membranes (mesothelium, e.g., pericardium, pleura, peritoneum), air sacs of lungs (alveoli), and glomeruli of kidneys.

• Stratified Squamous Epithelium:

  • Description: Multiple layers of cells; basal cells are cuboidal or columnar and metabolically active; surface cells are flattened (squamous). Can be keratinized or non-keratinized.

  • Function: Protects underlying tissues in areas subjected to abrasion.

  • Location: Non-keratinized type forms the moist linings of the esophagus, mouth, and vagina. Keratinized type forms the epidermis of the skin, a dry membrane.

• Simple Cuboidal Epithelium:

  • Description: Single layer of cube-like cells with large, spherical central nuclei.

  • Function: Secretion and absorption.

  • Location: Kidney tubules, ducts and secretory portions of small glands, ovary surface.

• Stratified Cuboidal Epithelium:

  • Description: Generally two layers of cubelike cells.

  • Function: Protection and secretion.

  • Location: Rare in the body; found in the ducts of larger glands (e.g., sweat glands, mammary glands).

• Simple Columnar Epithelium:

  • Description: Single layer of tall, column-shaped cells with round to oval nuclei usually near the basal surface. Some cells may bear microvilli; some may contain goblet cells.

  • Function: Absorption; secretion of mucus, enzymes, and other substances; ciliated type propels mucus (or ova) by ciliary action.

  • Location: Non-ciliated type lines most of the digestive tract (stomach to rectum), gallbladder, and excretory ducts of some glands. Ciliated type lines small bronchi, uterine tubes, and some regions of the uterus.

• Stratified Columnar Epithelium:

  • Description: Multiple layers; superficial cells are columnar; basal cells are often cuboidal.

  • Function: Protection and secretion.

  • Location: Rare in the body; small amounts in the pharynx, male urethra, and lining of some glandular ducts; also found at transition areas between two other types of epithelia.

• Pseudostratified Columnar Epithelium:

  • Description: Single layer of cells of differing heights, some not reaching the free surface; nuclei seen at different levels; may contain mucus-secreting goblet cells and bear cilia.

  • Function: Secrete substances, particularly mucus; propulsion of mucus by ciliary action.

  • Location: Non-ciliated type in male's sperm-carrying ducts and ducts of large glands. Ciliated type lines the trachea and most of the upper respiratory tract.

• Transitional Epithelium:

  • Description: Resembles both stratified squamous and stratified cuboidal; basal cells are cuboidal or columnar; surface cells are dome-shaped or squamous-like, depending on the degree of organ stretch.

  • Function: Stretches readily and permits distension of urinary organ by contained urine.

  • Location: Lines the ureters, bladder, and part of the urethra.

Glands and Their Functions

• Definition: A gland is one or more cells that make and secrete a particular product, known as a secretion. Secretions are usually water-based fluids containing proteins, lipids, or steroids.

• Types of Glands: Classified by their site of product release (exocrine vs. endocrine) and number of cells (unicellular vs. multicellular).

  • Exocrine Glands:

    • Mechanism: Secrete their products into ducts, which then carry the secretions to the body surface or into body cavities.

    • Examples: Include sebaceous (oil) glands, sweat glands, salivary glands, mammary glands, liver (bile), pancreas (digestive enzymes).

    • Diversity: Can be unicellular (like goblet cells secreting mucus directly onto surface) or multicellular (forming complex structures with ducts and secretory units).

  • Endocrine Glands:

    • Mechanism: Ductless glands that produce hormones, which are regulatory chemicals. These hormones are released directly into the interstitial fluid and then diffuse into the bloodstream, traveling to target organs to exert their effects.

    • Examples: Adrenal glands, pituitary gland, thyroid gland, pancreas (insulin and glucagon-secreting parts), ovaries (estrogen/progesterone), and testes (testosterone).

• Glandular Secretions: These secretions are vital for maintaining homeostasis and performing diverse physiological processes.

  • Saliva: Produced by salivary glands, contains enzymes like amylase for carbohydrate digestion, and antimicrobial agents to protect the mouth.

  • Sweat: Produced by sweat glands, helps regulate body temperature through evaporative cooling and eliminates some waste products.

  • Hormones: A wide array of chemical messengers regulating growth, metabolism, reproduction, mood, and many other bodily functions.

Conclusion

• Tissues are integral to the structure and function of the body, with distinct characteristics and specializations that enable the various functions necessary for life, from maintaining structural integrity to facilitating complex physiological processes. Understanding the different types of tissues, their cellular compositions, structural features (like junctions and basement membrane), and their specific functions is crucial for comprehending everything from basic human physiology to the development and progression of complex pathologies. The integration of these tissues forms organs and organ systems, allowing the body to function as a coordinated whole. Further discussions will delve into additional details about connective, muscle, and nervous tissue functions and morphology.