Types of Tissues and their Functions
The Four Types of Tissues
Epithelial tissue, commonly referred to as epithelium, comprises sheets of cells that cover the exterior surfaces of the body, line internal cavities and passageways, and form various glands. It plays a critical role in protection, secretion, and absorption.
Overview of Tissue Types
Connective tissue binds the cells and organs of the body together, providing protection, support, and integration of all parts. It contains a diverse range of cell types and is characterized by a significant amount of extracellular matrix.
Muscle tissue is excitable and responds to stimulation, contracting to create movement. There are three primary types: skeletal muscle (voluntary muscle responsible for locomotion), smooth muscle (involuntary muscle found in internal organs), and cardiac muscle (involuntary muscle found in the heart).
Nervous tissue, also excitable, is essential for propagating electrochemical signals in the form of nerve impulses, facilitating communication between different regions of the body (as illustrated in Figure 4.2).
Tissue Organization and Function
The next level of biological organization is the organ, which is formed by the collaboration of multiple tissue types. Understanding the structure and function of cells aids in comprehending the workings of tissues, which in turn informs how organs function. In this chapter, a detailed examination of epithelial and connective tissues is provided, whereas muscle and nervous tissues will only be briefly addressed.
Embryonic Origin of Tissues
The zygote, or fertilized egg, is a single cell resulting from the union of an egg and sperm. This zygote undergoes rapid mitotic divisions to generate many cells that make up the embryo. The initial cells produced are totipotent, having the capacity to divide and differentiate into any cell type in the body.
As development proceeds, three primary lineages of embryonic cells give rise to specific germ layers: the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer) (Figure 4.3). Notably, epithelial tissues arise from all three layers, while nervous tissue mainly derives from ectoderm and muscle tissue primarily from mesoderm.
Tissue Membranes
A tissue membrane is a thin layer or sheet of cells covering outer body surfaces, lining internal cavities, and covering organs. There are two primary types of tissue membranes: connective tissue membranes and epithelial membranes (Figure 4.4).
Connective Tissue Membranes
These membranes consist solely of connective tissue, encapsulating organs (e.g., kidneys) and lining movable joints. The synovial membrane, a specific type of connective tissue membrane, lines the cavity of freely movable joints like the shoulder and knee. Fibroblasts within the inner layer secrete hyaluronan, trapping water and forming synovial fluid, a lubricant that facilitates smooth movement of the bones in a joint. This synovial fluid engages in nutrient exchange with blood through all body fluids.
Epithelial Membranes
These are made up of epithelium attached to a layer of connective tissue. Examples include mucous membranes, which line open body cavities and passageways, and serous membranes, which line closed body cavities. Mucous membranes are sometimes referred to as mucosae and are crucial for secretion and protection.
Serous membranes consist of mesothelium, a type of epithelium derived from mesoderm, supported by connective tissue. They cover organs in body cavities and secrete serous fluid to lubricate and minimize friction between organs. Major serous membranes include the pleura (lungs), pericardium (heart), and peritoneum (abdominal organs).
Epithelial Tissue Characteristics
Epithelial tissues are extensive cellular sheets that line body surfaces exposed to the external environment and internal organs. These tissues are derived from all three embryonic germ layers: ectoderm, endoderm, and mesoderm.
Structural Features
High Cellular Density: Epithelial tissues are highly cellular with minimal extracellular material.
Polarity: Epithelial cells exhibit distinct polarity, with an apical surface facing the external environment and a basal surface attached to underlying tissues.
Basement Membrane: The basal lamina anchors epithelium to underlying connective tissue, comprising glycoproteins and collagen.
Avascular Nature: Epithelial tissues are largely avascular, relying on diffusion for nutrient acquisition from underlying tissues.
Regenerative Capacity: These tissues can rapidly regenerate, with capacity for replacing lost or damaged cells. The sloughing off of dead cells is a distinctive characteristic, allowing rapid renewal, especially in areas like the airways and digestive tract.
General Functions of Epithelial Tissue
Epithelial tissues serve as the body's first line of defense against physical, chemical, and biological insults. They regulate permeability, controlling the entry and exit of substances. Additionally, many epithelial cells possess secretory capabilities, producing mucus and various hormonal substances. For instance, the epithelium of the small intestine secretes digestive enzymes, while respiratory epithelium produces mucus to trap and eliminate pathogens.
Epithelial Cell Specializations
Epithelial cells have polarized organelle distribution, which corresponds to their functional roles. Structures like cilia (microscopic extensions) are found on some epithelial cells, which facilitate the movement of fluids and particles, such as in the respiratory tract where they form a mucociliary escalator. Cilia beat in unison to propel mucus and trapped particles towards the throat for clearance.
Cell-to-Cell Junctions in Epithelial Tissue
Adjacent epithelial cells are tightly connected via various junctions, facilitating cell communication and structural integrity.
Types of Junctions
Tight Junctions: Prevent the passage of materials between adjacent cells, creating distinct apical and basal compartments. This distinguishes epithelial tissues as selective barriers.
Anchoring Junctions: Provide structural stability and flexibility to epithelial tissues. These junctions include:
Desmosomes: Link cells together at specific points via cadherins, crucial for mechanical strength.
Hemidesmosomes: Anchor inward to the basal lamina using integrins, connecting cells to the extracellular matrix.
Adherens Junctions: Utilize either cadherins or integrins and involve contractile proteins like actin, influencing the shape of epithelial tissue.
Gap Junctions: Form channels between adjacent cells facilitating the transfer of small molecules and ions, supporting coordination of cell functions.
Classification of Epithelial Tissues
Epithelial tissues are classified by cell shape and number of layers.
Cell Shapes
Squamous: Flattened and thin cells.
Cuboidal: Boxy cells, equally wide and tall.
Columnar: Taller than they are wide cells.
Cell Layering
Simple Epithelium: Single layer of cells resting on the basal lamina.
Stratified Epithelium: Multiple layers of cells, with only the basal layer resting on the basal lamina.
Pseudostratified Epithelium: Appears stratified but consists of a single layer of irregularly shaped cells.
Transitional Epithelium: Specialized stratified epithelium where the shape of cells varies to accommodate stretching (e.g., bladder).
Types of Simple Epithelium
Simple Squamous Epithelium: Cells are thin and flattened, ideal for diffusion (e.g., alveoli in lungs, endothelium).
Simple Cuboidal Epithelium: Round nuclei in the center; involved in secretion and absorption (e.g., kidney tubules).
Simple Columnar Epithelium: Tall cells with nuclei at the base, functions in absorption and secretion found in the digestive tract and reproductive system.
Ciliated Columnar Epithelium: Similar to simple columnar but with cilia to assist in moving trapped particles (e.g., respiratory tract).
Pseudostratified Columnar Epithelium: Appears stratified due to varied cell heights, often ciliated, found in respiratory tracts with goblet cells for mucous secretion.
Types of Stratified Epithelium
Stratified Squamous Epithelium: Most common type, with apical layers being squamous; protects against abrasion (e.g., skin). Can be keratinized (dry) or non-keratinized (moist).
Stratified Cuboidal and Columnar Epithelium: Rare in the human body, found in larger ducts of glands.
Transitional Epithelium: Specialized for stretching in the bladder, with varying cell shapes depending on bladder fullness.
Glandular Epithelium
Glands consist of epithelial cells that synthesize and secrete substances. They can be classified into:
Endocrine Glands
Endocrine glands are ductless and secrete hormones directly into the bloodstream. Examples include the pituitary gland and adrenal glands.
Exocrine Glands
Exocrine glands release secretions through ducts to external surfaces. Examples include sweat, salivary, and mammary glands, with secretions including mucus and enzymes. Exocrine glands can be either unicellular (e.g., goblet cells) or multicellular with complex structures such as tubular or acinar glands.
Methods and Types of Secretion
The classification includes:
Merocrine Secretion: Release contents via exocytosis, e.g., salivary glands.
Apocrine Secretion: Portions of the cell pinches off to release secretion, e.g., mammary glands.
Holocrine Secretion: Entire cell disintegrates to release secretory products, e.g., sebaceous glands.
Glandular Structure
Exocrine glands are classified by the structure of their ducts—simple glands have a single duct, while compound glands have branched ducts.
Connective Tissues
Connective tissues are diverse and serve to connect, support and protect tissues and organs in the body. They contain three primary components: cells, ground substance, and protein fibers.
Functional Roles of Connective Tissues
Connective tissues perform numerous functions, primarily supporting and connecting other tissues, providing protection to organs (e.g., in bones and capsules), and facilitating transport of fluids such as blood and lymph. Adipose tissue stores energy and insulates the body.
Embryonic Connective Tissue
Derived primarily from mesoderm, the first connective tissue to develop is mesenchyme, which contains stem cells that can differentiate into various connective tissue types. Another type, mucous connective tissue (Wharton's jelly), is found in the umbilical cord.
Classification of Connective Tissues
Connective tissues are essentially identified by their ground substance and fiber types:
Connective Tissue Proper: Includes loose and dense connective tissues, differing in the arrangement and types of fibers.
Supportive Connective Tissue: Encompasses cartilage and bone, providing structural strength and protection.
Fluid Connective Tissue: Includes blood and lymph, characterizing the matrix as fluid-based.
Cell Types in Connective Tissue Proper
Fibroblasts: Main cells producing extracellular matrix components.
Adipocytes: Fat-storing cells, either white (energy storage) or brown (heat generation).
Macrophages: Immune cells responsible for engulfing pathogens and cellular debris; classified as fixed (permanent residents) or roaming.
Connective Tissue Fibers
Collagen Fibers: Provide strength and structure, predominantly found in tendons and ligaments.
Elastic Fibers: Allow for stretching and flexibility, returning to original shape; found in ligaments and skin.
Reticular Fibers: Form supportive networks in soft organs such as the liver and spleen.
Ground Substance
Secreted by fibroblasts, it consists of water, glycoproteins, and proteoglycans which help maintain tissue hydration and provide a medium for nutrient exchange.
Loose Connective Tissue
Loose connective tissues, like areolar tissue, fill spaces between organs and provide elasticity and cushioning. Adipose tissue contains many fat cells for energy storage and insulation. Reticular tissue supports internal organ structures with a mesh-like framework.
Dense Connective Tissue
Dense connective tissue, having higher collagen concentration, resists stretching and includes dense regular connective tissue (e.g., tendons) and dense irregular connective tissue (e.g., dermis of the skin).
Supportive Connective Tissues
Supportive connective tissues, composed mainly of cartilage and bone, provide stronger structural integrity and support for the body.
Cartilage
Cartilage is characterized by chondrocytes within a matrix of fibers and a gel-like ground substance, encased by the perichondrium. Types include:
Hyaline Cartilage: Smooth and flexible, found in joints and respiratory tract.
Fibrocartilage: Tough and resists compression, found in intervertebral discs and knee menisci.
Elastic Cartilage: Contains elastic fibers for flexibility, found in the ear and epiglottis.
Bone
Bone is the hardest connective tissue, classified into compact (dense structure) and cancellous (spongy structure). Composed of an organic matrix mainly of collagen and inorganic minerals such as hydroxyapatite. It is highly vascularized and capable of rapid healing.
Fluid Connective Tissue
Fluid connective tissues like blood and lymph circulate within liquid extracellular matrices containing various cells (erythrocytes, leukocytes, and platelets for blood).
Muscle Tissue
Muscle tissue is responsible for body movement, divided into:
Skeletal Muscle: Voluntary muscle; makes up about 40% of body mass, appearing striated and multinucleated.
Cardiac Muscle: Involuntary muscle; found in the heart and striated with intercalated discs facilitating synchronized contractions.
Smooth Muscle: Involuntary muscle found in organ walls, responsible for peristalsis and other involuntary motions; non-striated and spindle-shaped.
Nervous Tissue
Nervous tissue is essential for communication within the body, comprising:
Neurons: Responsible for transmitting electrochemical impulses (action potentials) and processing information.
Neuroglia: Support neurons, modulating functions and maintaining homeostasis in the neural environment.
Neuron Structure
Neurons consist of a cell body, dendrites (input), and an axon (output). They communicate via synapses using neurotransmitters. Classification includes unipolar, bipolar, and multipolar neurons based on their structure.
Tissue Injury and Repair
Tissue injury initiates an inflammatory response leading to repair mechanisms. Inflammation serves to limit further damage and promote healing. Key aspects of the inflammatory process include:
Release of chemical signals by damaged cells.
Increased blood flow and permeability of blood vessels allowing immune cell access.
Formation of granulation tissue, signifying healing and regeneration.
Healing Phases
Acute Inflammation: Immediate response involving vasodilation and recruitment of immune cells.
Repair Phase: Fibroblasts proliferate, producing new collagen and extracellular material, eventually leading to scab formation and tissue remodeling.
Tissue and Aging
Tissue aging impacts functionality and regeneration. Signs of aging include thinner skin, decreased elasticity, slower wound healing, and reduced immune response. Cellular aging involves shortened telomeres affecting cell division and functionality. However, lifestyle choices and exercise can mitigate some effects of aging and improve overall health.