Tissue Histology

Tissues

•      Groups of cells similar in structure and function

•      The four types of tissues

    •    Epithelial

    •    Connective

    •    Muscle

    •    Nerve

Epithelial Tissue

•      Cellularity – composed almost entirely of cells

•      Special contacts – form continuous sheets held together by tight junctions and desmosomes

•      Polarity – apical and basal surfaces

•      Supported by connective tissue – reticular and basal laminae

•      Avascular but innervated – contains no blood vessels but supplied by nerve fibers

•      Regenerative – rapidly replaces lost cells by cell division

Classification of Epithelia

•      Simple or stratified

•      Squamous, cuboidal, or columnar

Epithelia: Simple Squamous

•      Single layer of flattened cells with disc-shaped nuclei and sparse cytoplasm

•      Functions 

    •    Diffusion and filtration

    •    Provide a slick, friction-reducing lining in lymphatic and cardiovascular systems

    •      Present in the kidney glomeruli, lining of heart, blood vessels, lymphatic vessels, and serosae

Epithelia: Simple Cuboidal

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

•      Function in secretion and absorption

•      Present in kidney tubules, ducts and secretory portions of small glands, and ovary surface

Epithelia: Simple Columnar

•      Single layer of tall cells with oval nuclei; many contain cilia

•      Goblet cells are often found in this layer

•      Function in absorption and secretion

•      Nonciliated forms line digestive tract and gallbladder

•      Ciliated forms line small bronchi, uterine tubes, and some regions of the uterus

Epithelia: Pseudostratified Columnar

•      Single layer of cells with different heights; some do not reach the free surface

•      Nuclei are seen at different layers

•      Function in secretion and propulsion of mucus

•      Present in the epididymis (nonciliated) and trachea (ciliated)

Epithelia: Stratified Squamous

•      Thick membrane composed of several layers of cells

•      Function in protection of underlying areas subjected to abrasion

•      Forms the external part of the skin’s epidermis (keratinized cells), and linings of the esophagus, mouth, and vagina (nonkeratinized cells)

Epithelia: Stratified Columnar

•      Several cell layers with cuboidal basal cells and columnar superficial cells

•      Functions in protection and secretion

•      Present in large ducts of some glands (sweat glands), and in portions of the male urethra

Epithelia: Transitional

•      Several cell layers, basal cells are cuboidal, surface cells are dome-shaped

•      Stretches to permit the distension of the urinary bladder

•      Lines the urinary bladder, ureters, and part of the urethra

Glandular Epithelia

•      A gland is one or more cells that makes and secretes an aqueous fluid

•      Classified by:

    •    Site of product release – endocrine or exocrine

    •    Relative number of cells forming the gland – unicellular or multicellular

Endocrine Glands

•      Ductless glands that produce hormones

•      Secretions include amino acids, proteins, glycoproteins, and steroids

Exocrine Glands

•      More numerous than endocrine glands

•      Secrete their products onto body surfaces (skin) or into body cavities

•      Examples include mucous, sweat, sebaceous, and salivary glands

•      The only important unicellular gland is the goblet cell

•      Multicellular exocrine glands are composed of a duct and secretory unit

Multicellular Exocrine Glands

•      Classified according to:

    •    Simple or compound duct type

    •    Structure of their secretory units

Modes of Secretion

•      Merocrine – products are secreted by exocytosis (e.g., pancreas, sweat, and salivary glands)

•      Holocrine – products are secreted by the rupture of gland cells (e.g., sebaceous glands)

Connective Tissue (CT)

•      Found throughout the body; most abundant and widely distributed in primary tissues

Functions of Connective Tissue

•      Binding and support

•      Protection

•      Insulation

•      Transportation

Characteristics of Connective Tissue

•      Connective tissues have:

    •    Mesenchyme as their common tissue of origin

    •    Varying degrees of vascularity

    •    Nonliving extracellular matrix, consisting of ground substance and fibers

Structural Elements of Connective Tissue

•      Ground substance – unstructured material that fills the space between cells

•      Fibers – collagen, elastic, or reticular

•      Cells – fibroblasts, chondroblasts, osteoblasts, and hematopoietic stem cells

Ground Substance

•      Interstitial (tissue) fluid

•      Adhesion proteins – fibronectin and laminin

•      Proteoglycans – glycosaminoglycans (GAGs)

•      Functions as a molecular sieve through which nutrients diffuse between blood capillaries and cells

Fibers

•      Collagen – tough; provides high tensile strength

•      Elastic – long, thin fibers that allow for stretch

•      Reticular – branched collagenous fibers that form delicate networks

Connective Tissue: Fundamental Cell Types

•      Fibroblasts – connective tissue proper

•      Chondroblasts – cartilage

•      Osteoblasts – bone

•      Hematopoietic stem cells – blood

    •      White blood cells (WBCs), plasma cells, macrophages, and mast cells

Connective Tissue: Embryonic

•      Mesenchyme – embryonic CT

•    Gel-like ground substance with fibers and star-shaped mesenchymal cells

•    Gives rise to all other connective tissues

•    Found in the embryo

Connective Tissue Proper: Loose (areolar)

•    Gel-like matrix with all three CT fibers

•    Fibroblasts, macrophages, mast cells, and some WBCs

•    Wraps and cushions organs

•    Widely distributed throughout the body

Connective Tissue Proper: Adipose (fat)

•    Matrix similar to areolar CT with closely packed adipocytes

•    Reserve food stores, insulates against heat loss, and supports and protects

•    Found under skin, around kidneys, within abdomen, and in breasts

Connective Tissue Proper: Reticular

•    Loose ground substance with reticular fibers

•    Reticular cells lie in a fiber network

•    Forms a soft internal skeleton, or stroma, that supports other cell types

•    Found in lymph nodes, bone marrow, and the spleen

Connective Tissue Proper: Dense Regular

•      Parallel collagen fibers with a few elastic fibers

•      Major cell type is fibroblasts

•      Attaches muscles to bone or to other muscles, and bone to bone

•      Found in tendons, ligaments, and aponeuroses

Connective Tissue Proper: Dense Irregular

•      Irregularly arranged collagen fibers with some elastic fibers

•      Major cell type is fibroblasts

•      Withstands tension in many directions providing structural strength

•      Found in the dermis, submucosa of the digestive tract, and fibrous organ capsules

Supportive Connective Tissue: Cartilage

•      Hyaline cartilage

    •    Amorphous, firm matrix with imperceptible network of collagen fibers

    •    Chondrocytes lie in lacunae

    •    Supports, reinforces, cushions, and resists compression

    •    Forms the costal cartilage

    •    Found in:  embryonic skeleton, the end of long bones, nose, trachea, and larynx

•      Elastic cartilage

    •    Similar to hyaline cartilage but with more elastic fibers

    •    Maintains shape and structure while allowing flexibility

    •    Supports external ear (pinna) and the epiglottis

•      Fibrocartilage

    •    Matrix similar to hyaline cartilage but less firm with thick collagen fibers

    •    Provides tensile strength and absorbs compression shock

    •    Found in intervertebral discs, the pubic symphysis, and in discs of the knee joint

Supportive Connective Tissue: Bone (Osseous Tissue)

•      Osseous tissue

•    Hard, calcified matrix with collagen fibers found in bone

•    Osteocytes are found in lacunae and are well vascularized

•    Supports, protects, and provides levers for muscular action

•    Stores calcium, minerals, and fat

•    Marrow inside bones is the site of hematopoiesis

Connective Tissue: Blood

•      Blood

•    Red and white cells in a fluid matrix (plasma)

•    Contained within blood vessels

•    Functions in the transport of respiratory gases, nutrients, and wastes

Epithelial Membranes

•      Cutaneous – skin

•      Mucous – lines body cavities open to the exterior (e.g., digestive and respiratory tracts)

•      Serous – moist membranes found in closed ventral body cavity

Nervous Tissue

•      Branched neurons with long cellular processes and support cells

•      Transmits electrical signals from sensory receptors to effectors

•      Found in the brain, spinal cord, and peripheral nerves

•      Most include glial components (astrocytes, oligodendrocytes, and microglia)

Muscle Tissue: Skeletal

•      Long, cylindrical, multinucleate cells with obvious striations

•      Initiates and controls voluntary movement

•      Found in skeletal muscles that attach to bones or skin

Muscle Tissue: Cardiac

•      Branching, striated, uninucleate cells interdigitating at intercalated discs

•      Propels blood into the circulation

•      Found in the walls of the heart

Muscle Tissue: Smooth

•      Sheets of spindle-shaped cells with central nuclei that have no striations

•      Propels substances along internal passageways (i.e., peristalsis)

•      Found in the walls of hollow organs

Tissue Trauma

•      Causes inflammation, characterized by:

    •    Dilation of blood vessels

    •    Increase in vessel permeability

    •    Redness, heat, swelling, and pain

Tissue Repair

•      Organization and restored blood supply

•    The blood clot is replaced with granulation tissue

•      Regeneration and fibrosis

•    Surface epithelium regenerates and the scab detaches

•    Fibrous tissue matures and begins to resemble the adjacent tissue

•    Results in a fully regenerated epithelium with underlying scar tissue

Developmental Aspects

•      Primary germ layers: ectoderm, mesoderm, and endoderm

•    Three layers of cells formed early in embryonic development

•    Specialize to form the four primary tissues

    •      Nerve tissue arises from ectoderm

    •      Muscle, connective tissue, endothelium, and mesothelium arise from mesoderm

    •      Most mucosae arise from endoderm

    •      Epithelial tissues arise from all three germ layers