Chapter 4: Tissues – Human Anatomy & Physiology

Tissue Overview

Groups of cells that are similar in structure and work together to perform common or related functions are known as tissues. The study of tissues is called histology. The human body is composed of four basic tissue types:

• Epithelial

• Connective

• Muscle

• Nervous

Each individual body cell is specialized to perform specific functions, all contributing to the maintenance of overall homeostasis within the organism.

Epithelial Tissue

Epithelial tissue consists of sheets of cells that cover body surfaces or line cavities. It also forms the secretory portions of glands. This tissue primarily exists in two main forms:

• Covering & Lining Epithelium: Found, for example, on the skin or lining the digestive tract.

• Glandular Epithelium: Specialized for secretion, such as in salivary glands.

Epithelial tissue serves a variety of crucial functions, including protection, absorption, filtration, excretion, secretion, and sensory reception.

Special Characteristics ("P.A.S.A.R.")

The unique characteristics of epithelial tissue can be summarized by the acronym P.A.S.A.R.

Polarity: Epithelial cells exhibit polarity, meaning they have distinct top and bottom surfaces. The apical surface is the free or exposed side, often facing the exterior or a cavity, and may feature microvilli or cilia. The basal surface is attached to underlying connective tissue via a thin, adhesive sheet called the basal lamina.

Specialized Contacts: Epithelial cells fit closely together to form continuous sheets, except in glands. This close proximity is maintained by specialized cell junctions. Tight junctions create seals between cells, preventing leakage of substances through the extracellular space. Desmosomes are anchoring junctions that provide structural integrity, resisting mechanical stress and preventing cells from being pulled apart.

Supported by Connective Tissue: All epithelial sheets are supported by an underlying layer of connective tissue. The basement membrane, which resists stretching and tearing and defines the epithelial boundary, is formed by the basal lamina (originating from the epithelial cells) and the reticular lamina (originating from the connective tissue).

Avascular but Innervated: A distinguishing feature of epithelial tissue is that it is avascular, meaning it contains no blood vessels. Nutrients and oxygen diffuse into the epithelial cells from the blood vessels located in the underlying connective tissue. Despite being avascular, epithelial tissue is richly supplied with nerve endings, making it innervated.

Regeneration: Epithelial tissue possesses a high capacity for regeneration. This rapid replacement of lost cells is stimulated by factors such as the loss of cell polarity and broken cell contacts, provided there are adequate nutrients for cell division.

Classification of Epithelia

Epithelial tissues are classified based on two main criteria: the number of cell layers and the shape of the cells.

By Layers: Simple epithelia consist of a single layer of cells. This thinness makes them ideal for processes like absorption and filtration, where a barrier needs to be minimal. Stratified epithelia, conversely, are composed of two or more layers of cells, providing robust protection against abrasion in areas subject to significant wear and tear, such as the skin.

By Cell Shape: The shapes of epithelial cells are categorized into three basic forms:

• Squamous – flat, scale-like.

• Cuboidal – cube-shaped.

• Columnar – tall, column-like.

It is important to note that in stratified epithelia, the name of the tissue is determined by the shape of the cells in the surface (apical) layer.

Representative Types & Details

Simple Squamous Epithelium: This tissue is remarkably thin, consisting of a single layer of flattened cells. Its primary functions are diffusion and filtration, due to the minimal barrier it presents. It also secretes lubricating substances in serosae. Locations where simple squamous epithelium is found include the kidney glomeruli, the air sacs of the lungs, the endothelium lining the heart and blood vessels, and the mesothelium lining the ventral body cavity.

Simple Cuboidal Epithelium: Composed of a single layer of cube-shaped cells, this epithelium is specialized for secretion and absorption. Key locations include kidney tubules, small gland ducts, and the surface of the ovary.

Simple Columnar Epithelium: This tissue consists of a single layer of tall, column-like cells. Its main roles are absorption and the secretion of mucus and enzymes. A ciliated version of simple columnar epithelium contains cilia that help propel mucus or reproductive cells. Non-ciliated simple columnar epithelium is found in the digestive tract and gallbladder, while the ciliated variety lines small bronchi and uterine tubes.

Pseudostratified Columnar Epithelium: Despite appearing to have multiple layers due to differing cell heights and nuclei at various levels, this epithelium is actually a single layer of cells. It often contains cilia and goblet cells. Its functions include the secretion and propulsion of mucus. The ciliated form is common in the trachea and upper respiratory tract, whereas the non-ciliated form is found in male sperm ducts and large gland ducts.

Stratified Squamous Epithelium: This is a thick protective membrane designed to withstand abrasion. It can be keratinized, meaning it contains the tough protein keratin, which is characteristic of dry skin, or non-keratinized, which forms moist linings of the mouth, esophagus, and vagina.

Transitional Epithelium: This unique stratified epithelium features basal cells that are cuboidal or columnar, while the apical cells vary in shape (from dome-like to flattened) depending on the degree of stretch. Its crucial function is to permit the distension of urinary organs without tearing. It is found exclusively in the ureters, bladder, and part of the urethra.

Glandular Epithelium

A gland is defined as one or more cells that produce and secrete an aqueous fluid called secretion. Glands are classified based on two main criteria:

Site of Product Release: Glands are either endocrine or exocrine.

• Endocrine Glands: These are ductless glands that secrete their products, typically hormones, directly into the blood or lymph system for internal distribution. They exhibit varied structures and secretions.

• Exocrine Glands: In contrast, these glands release their secretions onto a body surface or into a body cavity through ducts. Examples include sweat, oil, saliva, and mucous glands.

Number of Cells: Glands can be either unicellular or multicellular.

• Unicellular Glands: Consist of a single cell that functions as a gland, such as goblet cells which produce mucus.

• Multicellular Glands: Composed of many cells and include structures like salivary glands or submucosal glands.

Goblet cells are unicellular mucous glands, while multicellular mucous cells, along with club cells, contribute to the mucous bilayer found in various lining tissues.

Extracellular Matrix (ECM)

The extracellular matrix (ECM) is a non-living material found between cells, and its composition varies significantly among different tissue types. Generally, it consists of a jelly-like ground substance and protein fibers. For example, blood has a thin, watery matrix with few fibers, except during clotting when fibers increase to form a gel. Epithelium is characterized by densely packed cells with minimal extracellular matrix, while connective tissue, by contrast, has fewer cells and a much more abundant matrix, which is a key characteristic of its type.

Connective Tissue (CT)

Connective tissue is the most abundant and widely distributed primary tissue in the body, found in plentiful amounts in the skin but minimally in the brain. Its major functions include:

• Binding and support

• Protection

• Insulation

• Energy storage

• Transport (as seen in blood)

The four main classes of connective tissue are:

• CT Proper

• Cartilage

• Bone (Osseous Tissue)

• Blood

Common Characteristics

All connective tissues share two defining characteristics:

ECM Dominance: Unlike other tissue types, connective tissue is largely composed of nonliving extracellular matrix. This dominant ECM is responsible for the tissue's ability to bear weight, withstand tension, and tolerate various forms of abuse.

Common Origin: A key shared characteristic is that all connective tissues originate from an embryonic tissue called mesenchyme.

Structural Elements

The structural elements of connective tissue include ground substance, fibers, and cells.

1. Ground Substance: This is the unstructured gel-like material that fills the space between the cells and contains the fibers. Its components include interstitial fluid, cell adhesion proteins (which act as "glue" to hold cells and fibers together), and proteoglycans (protein core + GAGs), which trap water, providing the ground substance with its viscosity and ability to act as a medium for nutrient diffusion.

2. Fibers: These provide support and strength to the connective tissue:

   • Collagen fibers: The strongest and most abundant type, providing high tensile strength, meaning they resist stretching.

   • Elastic fibers: Long, thin fibers made of elastin, which allow for stretch and recoil, helping tissues to return to their original shape after deformation.

   • Reticular fibers: Short, fine, highly branched collagenous fibers that form delicate networks, offering more give than coarser collagen fibers and supporting soft tissue organs.

3. Cells: Connective tissues are home to various cell types, each with specific roles:

   • "Blast" cells: These are immature, actively mitotic cells that secrete the ground substance and fibers characteristic of their respective matrices:

     • Fibroblasts (in CT proper)

     • Chondroblasts (in cartilage)

     • Osteoblasts (in bone)

     • Hematopoietic stem cells (in blood)

   • "Cyte" cells: These are mature, less active forms of the "blast" cells, primarily involved in maintaining the health of the matrix:

     • Fibrocytes

     • Chondrocytes

     • Osteocytes

   • Other cells: Additional cell types found in connective tissues include:

     • Adipocytes (fat cells) for energy storage.

     • White Blood Cells (WBCs) for defense, such as neutrophils, eosinophils, and lymphocytes.

     • Mast cells, which detect foreign substances and initiate local inflammatory responses by releasing chemicals like heparin (an anticoagulant), histamine (increases permeability), and proteases.

     • Macrophages, large phagocytic cells that engulf foreign materials and dead cells.

Connective Tissue Proper

Connective tissue proper is subdivided into loose and dense types.

Loose CT: These tissues have fewer fibers and more ground substance, making them softer and more flexible.

• Areolar CT: This is a widely distributed, loosely arranged connective tissue that wraps and cushions organs, plays a key role in inflammation, and holds body fluids. It is considered the prototype CT due to its content of all three fiber types – collagen, elastic, and reticular.

• Adipose CT: Commonly known as fat tissue, it primarily consists of adipocytes. Its main functions are energy reservoir, insulation against heat loss, and support/protection for organs. White fat is the dominant type in adults.

• Reticular CT: This tissue forms a delicate internal skeleton, or stroma, that supports various cell types, including white blood cells, mast cells, and macrophages within organs like lymph nodes, the spleen, and bone marrow.

Dense CT: These tissues have a higher density of fibers, particularly collagen, making them strong and resistant to tension.

• Dense Regular CT: Characterized by parallel collagen fibers, this tissue provides high tensile strength in one direction. It is found in structures that require strong, unidirectional resistance to tension, such as tendons (attaching muscles to bones or other muscles), ligaments (attaching bones to bones), and aponeuroses (sheet-like tendons).

• Dense Irregular CT: In contrast to dense regular, this tissue has irregularly arranged collagen bundles, allowing it to withstand tension exerted from multiple directions. It is found in the dermis of the skin and in fibrous joint capsules.

• Elastic CT: This dense connective tissue is rich in elastic fibers, enabling it to recoil after stretching. It is crucial for maintaining pulsatile blood flow in large arteries and aiding the recoil of lung tissue during breathing.

Cartilage

General Features: Cartilage is a tough, flexible connective tissue that can resist both tension and compression. It is notable for being avascular (lacking blood vessels) and innervated (lacking nerves). Nutrients must diffuse to chondrocytes from the perichondrium, a dense irregular CT sheath surrounding most cartilage. The cells of cartilage are chondroblasts (immature, matrix-secreting) and chondrocytes (mature, found within small cavities called lacunae).

Types: There are three main types of cartilage:

• Hyaline Cartilage: The most abundant type, providing support and cushioning. It is found at the ends of long bones (articular cartilage), in costal cartilages of the ribs, the nose, trachea, and larynx.

• Elastic Cartilage: Similar to hyaline but with abundant elastic fibers, allowing it to maintain shape while being very flexible. It is found in the external ear and the epiglottis.

• Fibrocartilage: This type contains thick collagen fibers, giving it great tensile strength and the ability to absorb shock. Locations include intervertebral discs, the pubic symphysis, and the menisci of the knee.

Bone (Osseous Tissue)

Bone is a highly vascularized connective tissue containing more collagen than cartilage, and critically, a significant amount of inorganic calcium salts that confer its characteristic hardness. Its functions include providing support for the body, protecting internal organs, serving as levers for muscle action, storing minerals (like calcium) and fat, and being the site of blood cell formation (hematopoiesis) in the marrow. The cells of bone are osteoblasts (matrix-secreting) and osteocytes (mature, maintenance cells found in lacunae). The structural units of compact bone are osteons.

Blood

Blood is an atypical connective tissue, distinguished by its fluid matrix called plasma, which is derived from mesenchyme. Its cellular components include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. Unlike other CTs, blood's fibers are soluble protein molecules that only become visible and precipitate during blood clotting. The primary function of blood is transport of substances throughout the body, including oxygen (O2), carbon dioxide (CO2), nutrients, wastes, and hormones.

Muscle Tissue

Muscle tissue is highly vascularized and is responsible for body movement. This movement is achieved through the contraction of specialized proteins called myofilaments (actin and myosin).

Skeletal Muscle: This is voluntary muscle, meaning its contractions are consciously controlled. It is primarily attached to bones, enabling body movement. Skeletal muscle cells are long, cylindrical, multinucleate, and striated (have a striped appearance).

Cardiac Muscle: Found only in the walls of the heart, cardiac muscle is involuntary, meaning its contractions are not consciously controlled. Its cells are striated, typically branched, and contain a single nucleus. Unique features include intercalated discs, which are specialized junctions containing gap junctions (allowing rapid electrical communication) and desmosomes (providing strong adhesion).

Smooth Muscle: This involuntary muscle is characterized by spindle-shaped cells that lack striations. It forms the walls of most hollow organs in the body, such as those of the digestive and urinary tracts, blood vessels, and the uterus, where it propels substances through internal passageways.

Nervous Tissue

Components: Nervous tissue is primarily composed of two main cell types:

• Neurons: These are specialized cells that generate and conduct electrical impulses, transmitting information throughout the body.

• Neuroglia: These are supporting cells that insulate, protect, and provide metabolic support to neurons, ensuring their optimal functioning.

Locations: Nervous tissue serves as the main component of the brain, spinal cord, and nerves. Its primary role is in the regulation and control of all bodily functions.

Covering & Lining Membranes

Cutaneous Membrane: More commonly known as the skin, this membrane covers the external surface of the body. It consists of a keratinized stratified squamous epithelium (epidermis) attached to a thick layer of connective tissue (dermis). It is a dry membrane.

Mucous Membranes (Mucosae): These membranes line all body cavities that open to the exterior, such as those of the digestive, respiratory, and urogenital tracts. They are typically moist membranes, and often secrete mucus, which serves protective and lubricating functions.

Serous Membranes (Serosae): These are moist membranes found in closed ventral body cavities. They consist of a simple squamous epithelium called a mesothelium, resting on a thin layer of areolar connective tissue. Serous membranes form two layers: a parietal layer which lines the internal body cavity wall, and a visceral layer, which covers the external surface of the organs within that cavity. A thin layer of serous fluid between these layers reduces friction.

Tissue Repair

Tissue repair is a vital process that is triggered when the body's protective barriers are compromised, often initiated by an inflammatory response. The body employs two main mechanisms for tissue repair:

• Regeneration: In this process, the damaged tissue is replaced by the same type of tissue, restoring the original function and structure.

• Fibrosis: In contrast, fibrosis involves the replacement of damaged tissue with non-functional connective (scar) tissue, resulting in a loss of the original tissue's function. The outcome of tissue repair (regeneration vs. fibrosis) depends on both the specific type of tissue damaged and the severity of the injury.

Steps of Repair

Tissue repair generally proceeds through three main steps:

1. Step 1: Inflammation: Upon injury, damaged cells and mast cells in the vicinity release various chemicals. These chemicals trigger vasodilation (widening of blood vessels) and an increase in vascular permeability, allowing fluid, clotting proteins, and other immune cells to leak into the injured area. A blood clot then forms, which stops bleeding and isolates the injured site to prevent the spread of harmful agents.

2. Step 2: Organization (Restores Blood Supply): During this phase, the blood clot is replaced by granulation tissue, which is a delicate, pink tissue composed of new capillaries (restoring blood supply) and fibroblasts. The fibroblasts begin to synthesize collagen fibers, bridging the gap created by the injury. Macrophages actively phagocytize (engulf) debris and dead cells, clearing the area for repair. Simultaneously, the overlying epithelium begins to regenerate.

3. Step 3: Regeneration/Fibrosis (Permanent Repair): As repair progresses, the scab detaches, and the regenerated epithelium thickens underneath, completely covering the wound. The underlying fibrous tissue continues to mature and contract. The result is permanent repair, where the epithelium is fully regenerated, but the underlying fibrous tissue forms a scar. This scar can be visible or invisible depending on the extent of the fibrosis.

Regenerative Capacity by Tissue

Different tissues have varying capacities for regeneration:

• High Regeneration: Epithelia, bone, areolar connective tissue, dense irregular connective tissue, and blood-forming tissue.

• Moderate Regeneration: Smooth muscle and dense regular connective tissue.

• Poor/None Regeneration: Cardiac muscle and nervous tissue (especially in the brain and spinal cord). Research is ongoing to find ways to enhance regeneration in these critical tissues.