Chapter 4: Tissues - Condensed Reviewer

Fundamental Overview of Tissues and Histology

A tissue is defined as a group of cells possessing similar structure and function, combined with the surrounding extracellular substance. The scientific study of these tissues is known as histology. All tissues within the human body are categorized into four primary types: epithelial, connective, muscle, and nervous. Epithelial tissue acts as a covering or lining for various structures. Connective tissue is highly diverse and serves as a constituent part of every organ. Muscle tissue is specialized to contract or shorten, thereby producing movement. Nervous tissue is responsible for coordinating and controlling various body activities.

Structural and Functional Characteristics of Epithelial Tissue

Epithelial tissue is characterized by being mostly cellular with very little extracellular matrix. it covers body surfaces both internally and externally. These tissues possess a distinct free surface and are attached to a basement membrane at their base. Connections within this tissue type occur both cell-to-cell and cell-to-matrix. Notably, epithelial tissue is avascular, meaning it lacks a direct blood supply, and it possesses a high capacity for regeneration. This category includes both exocrine and endocrine glands.

The functions of epithelial tissue are multifaceted. It provides protection to underlying structures and acts as a barrier. It permits the passage of specific substances while facilitating secretion and absorption across its surfaces.

Classification and Varieties of Epithelia

Epithelia are classified based on the number of cell layers and the shape of the cells. In terms of layers, simple epithelium consists of a single layer where every cell touches both the basement membrane and the free surface. Stratified epithelium consists of more than one layer, with only the basal layer making contact with the basement membrane. Pseudostratified epithelium appears to be layered but is actually a single layer of cells with varying heights; nevertheless, all cells in this type touch the basement membrane.

Cell shapes are categorized as squamous (flat), cuboidal (cube-shaped), or columnar (tall). Transitional epithelium is a specialized type where the shape varies depending on the degree of stretch. Simple squamous epithelium consists of a single layer of thin, flat cells specialized for gas exchange in the lungs, filtration in the kidneys, and active transport. Simple cuboidal epithelium is a single layer of cube-shaped cells with a high secretory capacity, often involved in secretion. Simple columnar epithelium consists of a single layer of tall, thin cells functioning in the secretion of mucus or enzymes, such as in the small intestine. Pseudostratified columnar epithelium looks stratified but is one layer; it often features cilia to move mucus and debris toward the throat in the airways.

Stratified squamous epithelium can be keratinized or nonkeratinized. The keratinized variety forms the outer layer of the skin, where keratin reduces water loss. The nonkeratinized variety provides a moist lining in areas like the mouth, protecting against abrasion while allowing water to pass more readily than through the skin. Stratified cuboidal epithelium is rare, found in sweat gland ducts, ovarian follicles, and salivary glands, functioning in absorption, secretion, and protection. Stratified columnar epithelium is also rare, located in mammary ducts, the larynx, and parts of the male urethra, providing secretion, protection, and some absorption. Transitional epithelium is uniquely stretchy; cells appear cuboidal when the tissue is relaxed and squamous when it is stretched. This type lines the urinary bladder and protects against caustic urine.

Cellular Specializations and Junctional Complexes

The free surface of an epithelial cell is the side not touching other cells. This surface can be smooth, or it may possess microvilli to increase surface area for absorption, or cilia to move materials across the surface. Cells are held together by specific structures: desmosomes mechanically bind cells to one another, while hemidesmosomes (or "half desmosomes") anchor cells specifically to the basement membrane.

Tight junctions form a belt-like seal around cells to prevent the passage of materials between them. These are commonly found in the intestinal lining, forcing materials to pass through the cells in a regulated manner. Gap junctions are small channels that allowed ions and small molecules to pass directly from cell to cell, serving as critical communication signals.

Glandular Anatomy and Secretory Mechanisms

Glands are secretory organs composed of epithelium and supporting connective tissue. Exocrine glands possess ducts and secrete substances onto a surface or into a cavity. Endocrine glands are ductless and secrete hormones directly into the bloodstream. Exocrine glands are further classified by cellularity into unicellular glands (such as goblet cells) and multicellular glands. They are also classified by duct branching into simple (unbranched) or compound (branched) types, and by the shape of their secretory regions as tubular, acinar/alveolar (saclike), or tubuloacinar/tubuloalveolar (a combination of both).

Secretory mechanisms define how substances are released. Merocrine secretion involves exocytosis, where vesicles are released while the cell remains intact. Apocrine secretion involves the release of pinched-off fragments of the cell. Holocrine secretion occurs when the entire cell is shed during the process.

Connective Tissue: Composition and Specialized Cell Types

Connective tissue is a diverse group of tissues that form part of every organ, distinguished by having cells separated by an abundant extracellular matrix. Its functions include enclosing and separating tissues, connecting tissues, providing support and movement, storage, cushioning and insulating, transport, and protection. The extracellular matrix consists of three components: protein fibers, ground substance, and fluid.

Specialized cells within connective tissue serve specific roles. Osteoblasts form bone, osteocytes maintain bone, and osteoclasts break down bone. Fibroblasts form fibrous connective tissue while fibrocytes maintain it. Chondroblasts form cartilage and chondrocytes maintain it. Macrophages are mobile cells that ingest foreign substances and microorganisms. Mast cells release histamine to trigger inflammation.

The protein fibers in the matrix include collagen, which is rope-like, flexible, and resists stretching; reticular fibers, which are fine, short, branching collagen fibers forming a supporting network; and elastic fibers, which can stretch and recoil to their original shape. The ground substance is a non-fibrous, shapeless material made of proteoglycans (a protein core plus polysaccharides) that trap water, allowing the tissue to spring back after compression.

Varieties of Connective Tissue

Loose connective tissue contains few fibers and a lacy network with significant ground substance and fluid. Areolar tissue consists mostly of collagen with some elastic fibers, with fibroblasts being the most common cell type. Adipose tissue is composed of adipocytes (fat cells) used for energy storage, padding, and insulation. Reticular tissue forms the framework for bone marrow, the liver, and lymphatic tissues like the spleen and lymph nodes.

Dense connective tissue contains many fibers in thick bundles that fill the matrix. Dense regular collagenous tissue features collagen fibers oriented in the same direction, typical of tendons and ligaments. Dense irregular collagenous tissue has collagen fibers oriented in multiple directions, found in the dermis and organ capsules. Dense elastic tissue contains abundant elastic fibers and collagen, allowing for stretch and recoil in structures like the vocal cords. A defect in elastic fibers is associated with Marfan syndrome.

Cartilage consists of chondrocytes located in spaces called lacunae. Collagen provides flexibility and strength, while proteoglycans trap water for resilience. Hyaline cartilage is the most abundant type, covering bone ends at joints. Fibrocartilage contains more collagen to resist compression and tearing, found in the intervertebral disks, the knee, and the TMJ. Elastic cartilage contains collagen and coiled elastic fibers, found in the external ear, epiglottis, and auditory tube.

Bone is a living tissue with a mineralized matrix, containing osteocytes in lacunae. Spongy bone has spaces between trabeculae, giving it a sponge-like appearance. Compact bone is solid with minimal space and thin mineralized layers. Blood is a liquid connective tissue where the matrix is plasma. Its formed elements include erythrocytes, leukocytes, and platelets, and its primary function is the transport of food, $O_2$, waste, and hormones.

Muscle and Nervous Tissue Systems

Muscle tissue functions by contracting or shortening to produce movement via contractile proteins. Skeletal muscle attaches to the skeleton, consisting of long, cylindrical, striated cells with multiple nuclei. Cardiac muscle is found in the heart; its cells are shorter, cylindrical, often branched, striated, and connected by intercalated disks, usually containing a single nucleus. Smooth muscle is found in the walls of hollow organs, skin, and eyes, consisting of non-striated, tapered cells with a single nucleus.

Nervous tissue forms the brain, spinal cord, and nerves to coordinate and control body activities. It is composed of neurons, which conduct action potentials, and glial cells, which provide support. The parts of a neuron include the cell body, dendrites, and the axon.

Classification and Functions of Tissue Membranes

Tissue membranes are thin sheets or layers that cover structures or line cavities, usually consisting of epithelium and underlying connective tissue. The cutaneous membrane is the external body surface (skin), composed of the epidermis and dermis for protection. Mucous membranes line cavities open to the exterior, such as the digestive, respiratory, and reproductive tracts. They consist of epithelium, a basement membrane, and loose connective tissue, providing protection, absorption, and secretion (often mucus).

Serous membranes line cavities not open to the exterior, including the pericardial, pleural, and peritoneal cavities. They are composed of simple squamous epithelium, a basement membrane, and loose connective tissue, and they secrete serous fluid for lubrication without the use of glands. Synovial membranes line the cavities of freely movable joints and consist only of modified connective tissue cells that produce synovial fluid to reduce friction.

Pathophysiological Responses: Inflammation and Tissue Repair

Tissue inflammation is generally a beneficial response to damage that occurs in stages to mobilize defenses and isolate or destroy pathogens and damaged cells to enable repair. The five major symptoms are redness, heat, swelling, pain, and disturbance of function.

Tissue repair replaces dead cells with viable cells through two mechanisms. Regeneration results in new cells of the same type as those destroyed, restoring normal function. This process is driven by stem cells, which are self-renewing, undifferentiated cells. Fibrosis, or replacement, involves the formation of a new tissue type, resulting in scar tissue and some loss of function. The skin and intestinal mucosa are capable of full regeneration via stem cells. Repair always occurs in sequential steps.

Questions & Discussion

1. List the 4 primary tissue types and their main functions. Epithelial tissue functions as a covering and lining. Connective tissue is diverse and found in every organ. Muscle tissue contracts and shortens to produce movement. Nervous tissue coordinates and controls body activities.

2. Differentiate simple, stratified, and pseudostratified epithelium. Simple epithelium is one layer thick where every cell touches the basement membrane and the free surface. Stratified epithelium is more than one layer thick, and only the basal layer touches the basement membrane. Pseudostratified epithelium appears to be multiple layers but is actually one layer where all cells touch the basement membrane but have varying heights.

3. Compare merocrine, apocrine, and holocrine secretion. Merocrine secretion uses exocytosis to release vesicles while keeping the cell intact. Apocrine secretion involves releasing pinched-off fragments of the cell. Holocrine secretion involves the shedding of the entire cell.

4. What are the 3 components of the extracellular matrix, and the 3 fiber types? The three components of the extracellular matrix are protein fibers, ground substance, and fluid. The three types of protein fibers are collagen (flexible and rope-like), reticular (short, branching supporting networks), and elastic (capable of stretch and recoil).

5. Differentiate the 4 tissue membranes (cutaneous, mucous, serous, synovial) by location and secretion. The cutaneous membrane is the skin (external surface) and does not have a specific listed secretion in this context. Mucous membranes line cavities open to the exterior and often secrete mucus. Serous membranes line internal cavities not open to the exterior and secrete serous fluid for lubrication. Synovial membranes line movable joint cavities and produce synovial fluid to reduce friction.

6. Compare regeneration vs. fibrosis in tissue repair. Regeneration results in the replacement of destroyed cells with the same cell type, restoring normal function. Fibrosis replaces destroyed cells with a different tissue type, creating a scar and resulting in some loss of function.