Chapter 3 - Tissues
 

Tissues
  Tissues are collections of specialized cells and cell products that perform a limited number of functions.
  Histology is the study of tissues.
    There are only four primary tissue types: epithelial, connective, muscular and neural. For now we will only concentrate on epithelial and connective tissues.

Epithelial Tissue
Epithelial tissue covers exposed surfaces or lines internal cavities or passageways and forms glands.
Characteristics:
1. Cellularity
   Epithelial cells are always contiguous cells bound closely together by cell junctions. In other words, there is little intercellular space.
2. Polarity
   Epithelial cells have exposed surfaces and surfaces for attachment with neighboring cells and tissues. These surfaces are called: 
Apical surface for the side that faces the external or internal space lined by the epithelium.
Basolateral surface for the side attached to the underlying tissue and the sides attached to neighboring epithelial cells.
3. Attachment
   The basal surface (part of the basolateral surface) is attached to the underlying tissue by a basal lamina (a.k.a. basement membrane).
4. Avascularity
   Refers to the absence of blood vessels.
   Nutrients are obtained by diffusion or absorption across the apical and basal surfaces. 
5. Regeneration
   Cells that become damaged, worn out or abraded are replaced by division of stem cells.
6. Arranged into Sheets or Layers
   Epithelial cells are arranged in layers one or more cells thick. 
Functions:
1. Physical Protection
   Protects tissue from abrasion, dehydration, and damaging chemical or biological agents.
2. Control Permeability
   Controls the passage of materials into (absorption) and out of (secretion) tissue.
3. Provide Sensation
   Epithelium has nerve endings and specialized epithelial cells that detect environmental changes. A neuroepithelium is specialized sensory epithelium.
4. Produce Specialized Secretions
   Glands are derived from epithelial cells.
Single cells modified for secretion are called gland cells.
The epithelium is glandular epithelium when most or all epithelial cells produce secretions.
Exocrine and endocrine glands are derived from epithelial tissue.
Specializations of Epithelial Cells
   Because epithelial cells produce secretions, absorb materials or move fluid over its surface the epithelial cells show a distinct polarity. The apical surface in particular often display specializations that facilitate these functions. For example:
     Epithelial cells specialized for absorption or secretion have microvilli on the apical surface that increase the surface area for exchange of materials.
        Stereocilia are very long microvilli that are found along portions of the male reproductive tract and on receptor cells of the inner ear.  
       Ciliated epithelium has cilia on the apical surface to move fluids. Ciliated epithelium is typical of the upper respiratory tract. Cilia are also found on the epithelial cells lining the oviducts of the female.
Maintaining the Integrity of Epithelium
   1. Intercellular Connections. Connections between cells are maintained by cell junctions and the interlocking folds between cell membranes.
   2. Attachment to Basal Lamina (Basement Membrane). The epithelium adheres to the underlying tissue by means of a basal lamina which consists of:
a. lamina lucida
   A layer next to the epithelium containing glycoproteins and a network of microfilaments.
b. lamina densa  
   A layer next to the connective tissue containing protein fibers produced by the connective tissue cells.
   3. Maintenance and Repair. The integrity of the epithelium is also maintained by cell renewal. Germinative (stem) cells continually divide to replace cells lost due to wear and tear.
Basis for Classification:
1. Layers
   Simple Epithelium
     Only one cell layer thick, this epithelium is found in protected areas such as the ventral body cavity, heart chambers and lining of blood vessels. This type of epithelium is often involved with secretion, absorption and filtration.
   Stratified Epithelium
     This epithelium has more than one layer of cells resting on the basal lamina and is subjected to mechanical or chemical stresses.
2. Shape of Cells
   Classification is also based on the shape of the cells on the surface. Cell shapes are squamous, cuboidal and columnar.

Classification of Epithelium
Simple Squamous
   This type of epithelium is very delicate and the thinness of the cells readily allow molecules to pass through it. It is found in regions where rapid exchange of molecules is necessary such as the alveoli of lungs and endothelia of capillaries. It also provides a slick, slippery surface for the serous (watery) membranes of the ventral body cavities.
   Two types of simple squamous epithelia have special names:
a. Mesothelium refers to the simple squamous epithelium that lines the ventral body cavity.
b. Endothelium is the simple squamous epithelium that lines the interior of the heart chambers and blood vessels.
Stratified Squamous
   This type of epithelium typically consists of many layers of cells and is found where the mechanical stresses are severe. Stratified squamous epithelium can be further subcategorized into:
a. Keratinized - The apical layers of epithelial cells become packed with intermediate filaments of keratin. Keratin gives the epithelium toughness and water-resistance that helps the skin to conserve body water.  
b. Nonkeratinized - The apical layers lack water resistance and must be kept moist by glandular secretions. This type of epithelium is found lining the oral cavity, esophagus, rectum, anus and vagina.
Simple Cuboidal
   The cells of the this type of epithelium appear square in cross section (when viewed at a right angle to the basal lamina) and the nuclei appear round and near the center. 
   Simple cuboidal epithelium provides limited protection and occurs where secretion and absorption is required. 
Stratified Cuboidal
   This type of epithelium sometimes lines the ducts of sweat glands and the larger ducts of exocrine glands.
Transitional Epithelium
   Transitional epithelium is stratified with plump cuboidal cells on the surface.
   This type of epithelium is designed to accommodate the stretching that occurs in the urinary bladder and ureters while maintaining the impermeable barrier these locations require.
Simple Columnar
   This type of epithelium is similar to simple cuboidal except that the cells appear taller than wide.
   Simple columnar provides some protection and is found in regions where active absorption or secretion occurs including the stomach, intestinal tract, uterine tubes, and excretory ducts.
   The cells show a distinct polarity with the nuclei crowded into a band near the basal lamina.
Pseudostratified Columnar
   In this epithelium there is only a single layer of cells but there appears to be a multiple layers because of the varying locations of the nuclei.
   This type of epithelium is typically found lining the upper respiratory tract as pseudostratified ciliated columnar epithelium (PCCE). PCCE is also called respiratory epithelium because it typically lines the upper respiratory passages. However, it also lines parts of the male reproductive tract.
Stratified Columnar
   This type of epithelium is rare and is found lining portions of the pharynx, urethra, anus and large ducts.
Glandular Epithelium
   Epithelial cells may develop into gland cells that produce secretions. There are a number of ways to classify glands:
Classification of Glands
1. Type of Secretion
  Exocrine secretions are discharged onto the epithelial surface. Glandular cells may move away from the epithelial surface but remain connected by tubes called ducts. Exocrine glands secrete mucins, enzymes, water and waste products. Examples include sweat glands, intestinal glands and mammary glands. Exocrine glands are further subclassified according to nature of secretion:
  a. Serous glands secrete a watery secretion containing enzymes.
  b. Mucous glands secrete mucins that absorb water to form mucus.
  c. Mixed glands contain a mixture of cells secreting both serous and mucous secretions.
Endocrine cells secrete by exocytosis into the fluid surrounding the cell. Endocrine cells may either be single cells or aggregations of cells and secrete hormones that serve as chemical messengers. Aggregations of endocrine cells, such as the thyroid, adrenal or pituitary glands, are  ductless.
2. Gland Structure
Unicellular glands are single secretory epithelial cells. The only unicellular glands are goblet cells that secrete mucins.
Multicellular glands include
   a. Glandular epithelium in which all the surface cells are secretory. The epithelium that lines the lumen of the stomach is an example.
   b. Aggregations of glandular cells that move away from the surface and form either exocrine or endocrine glands. 
     Multicellular exocrine glands have two components that results in further subclassification, the secretory portion and the duct.
    The secretory portion may either be:
Tubular when the cells are arranged in tubes.
Alveolar or acinar when the cells are arranged in blind pockets.
Tubuloalveolar when both configurations are present.
     Ducts may either be:
Simple when it is unbranched.
Compound when it is branched.
3. Modes of Secretion
a. Merocrine glands are the most common and release their secretions by exocytosis.
b. Apocrine glands release their secretions by detachment of an apical portion of the cytoplasm.
c. Holocrine glands release their secretions with the disintegration of the entire cell.

Connective Tissue
This tissue is found throughout the body but is never exposed to the environment.
Three Basic Components:
1. Specialized Cells and a
Matrix that surrounds the cells with:
2. Extracellular Protein Fibers
3. Ground Substance
  
A viscous, clear substance with a slippery feel and a high water content and very large macromolecules including proteoglycans and glycosaminoglycans.
Functions:
1. Establishes structural framework for the body.
    e.g. bone, cartilage.
2. Transports fluids and materials throughout the body.
   e.g. blood, lymph.
3. Provides protection for delicate organs.
   e.g. bone of skull, rib cage, spinal column.
4. Supports, surrounds and interconnects other tissue types.
   e.g. fascia, tendons, organ capsules.
5. Stores energy.
   e.g. adipose tissue.
6. Defends body from invasion.
   e.g. inflammatory cells reside and migrate through connective tissue.
Classifications:
   Connective tissue can be placed in one of three broad categories:
1. Connective Tissue Proper
   This type of connective tissue is literally connecting or binding of other tissue and hence is "proper"ly speaking connective tissue.
2. Fluid Connective Tissue
   Fluid connective tissue is connective in a broader sense in that it allows substances to move from one part of the body to another, e.g. blood and lymph. Matrix is essentially a fluid rather than a gel or solid.
3. Supporting Connective Tissue
   This connective tissue supports the body against the forces of gravity but also provides physical protection and functions as levers upon which muscle can exert force, e.g. cartilage and bone.
Connective Tissue Proper
1. Cells
   Fixed Cells:
a. Fibroblasts/Fibrocytes
   These cells are always present in connective tissue proper and is the most abundant cell found there.
   Fibroblasts secrete protein subunits that form extracellular fibers (collagen and elastic fibers) and molecules that form the ground substance. Fibrocytes differentiate from fibroblasts and maintain the connective tissue fibers of the tissue.   
   Hyaluronan is secreted and gives the ground substance is viscous consistency.
b. Fixed Macrophages (Histiocytes)
   Defensive cells that engulf damaged cells and pathogens.
   Fixed macrophages are not as abundant as fibroblasts but when mobilized for defense or repair attracts other cells of the immune system.
c. Adipocytes (Fat Cells)
   These cells contain lipid inclusions that push other organelles to the edge of the cell.
   The number of adipocytes will vary in different regions of the body and in different individuals.
d. Mesenchymal Cells
   These are stem cells that can divide and differentiate into the other specialized cells when needed.
e. Mast Cells
   Mast cells secrete histamine and heparin in response to injury and infection. This results in local inflammation.
f. Melanocytes
   These cells synthesize the dark pigment, melanin.
   Melanocytes play a major role in determining skin color.
   Wandering Cells:
a. Free Macrophages
   Phagocytic cells that circulate in the blood as monocytes and migrate rapidly through connective tissue.
   These cells are recruited to areas of infection or tissue damage by fixed macrophages.
b. Lymphocytes
   These cells migrate throughout the body and provide the body with specific immunity.
   Lymphocytes that actively produce antibodies are called plasma cells.
c. Neutrophils and Eosinophils
   These are phagocytic cells that are smaller than macrophages and are attracted to areas of infection and injury by chemicals released by cells already at the site. 
2. Connective Tissue Fibers
   All tissue fibers are assembled from protein subunits secreted by fibroblasts.
a. Collagen
   Collagen fibers are the most common and strongest fibers.
   These fibers combine tensile strength with flexibility.
b. Reticular
   These are a type of collagen fiber (Type III) that form thin interweaving fibers.
   The mesh, or network, that these fibers form help the organs that they support resist pull from many directions.
   This network is referred to as a stroma, and the functional cells that are supported are together called the parenchyma.
c. Elastic
   These fibers are formed from the protein, elastin.
   Elastic fibers are branching and wavy and can be stretched to 150% their length and recoil to their original length.
3. Ground Substance
   Ground substance is clear, colorless and of a thick consistency (like maple syrup).
   This is the substance that surrounds the cells and fibers. It contains hyaluronan and proteoglycans and glycoproteins.
Types of Connective Tissue Proper:
1. Embryonic Tissues
a. Mesenchyme
   Connective tissue found in the embryo with mesenchymal (unspecialized) cells in a matrix with fine fibers.
   Mesenchymal cells differentiate into other kinds of tissue including adult connective tissue cells. 
b. Mucous Connective Tissue
   Embryonic connective tissue found in the umbilical cord.
   The extracellular matrix is like gelatin in consistency and is often referred to as Wharton's jelly.
2. Loose Connective Tissue ("packing material")
a. Areolar Connective Tissue
   Areolar connective tissue is the least specialized and has all the cells and fibers found in connective tissue proper.
   This tissue is loosely organized and easily distorted but is also resilient. This permits movement between connected tissues.
   It is found immediately under the dermis of the skin and other epithelia, between muscles, around blood vessels and nerves.
   This tissue is highly vascular and provides the vascular support for epithelial tissue.
   The loose, open nature of this tissue also provides the major pathway for cells of the immune system to reach sites of damage or invasion.
b. Adipose Tissue
   The distinction between adipose tissue and areolar tissue is arbitrary as areolar tissue is considered adipose tissue when there is a high concentration of adipocytes.
   Adipose tissue pads, cushions, insulates, and packs and fills spaces between other tissues.
c. Reticular Tissue
   This tissue is characterized by the predominance of reticular fibers.
   It forms the stroma in various organs including liver, spleen, lymph nodes and bone marrow.
3. Dense Connective Tissue
   Connective tissue with a high concentration of collagen fibers. 
a. Dense Regular Connective Tissue
   Connective tissue in which the collagen fibers are arrayed in parallel.
   Examples:
1. Tendons
   The parallel arrays of fibers form cords that attach muscle to bone.
2. Aponeuroses
   Parallel arrays form flat sheets of connective tissue that connect flat muscles to their attachments.
3. Elastic Tissue
   Elastic fibers predominate and give the tissue greater resilience.
   Found in the wall of large blood vessels, underlying transitional epithelium and surrounding respiratory passages. 
4. Ligaments
   Similar to tendons but attaching bone to bone.
   Ligaments often contain a significant number of elastic fibers in order to better tolerate stretching. 
   Elastic ligaments have a predominance of elastic fibers.
b. Dense Irregular Connective Tissue
   Bundles of collagen fibers are oriented in various directions to form a meshwork.
   The orientation of fibers enables tissue to resist stresses from many directions.
   This tissue is found in the dermis of the skin and forms fibrous coverings on the surface of cartilage and bone.
   Dense irregular connective tissue also forms tough capsules around organs and joints.
 
Fluid Connective Tissue
   Includes blood and lymph.
   Will be covered later.
 
Supporting Connective Tissue
1. Cartilage
   The matrix contains numerous fibers and the ground substance has complex polysaccharides, including chondroitin sulfate and keratan sulfate, that forms links with protein molecules. The matrix as a result is firm, but pliable. Cartilage combines tensile strength with flexibility and resilience.
   The matrix is produced by both chondroblasts and chondrocytes and is maintained by chondrocytes.
   Chondrocytes exist in spaces of the matrix called lacunae (sing. lacuna).
   Separate pieces of cartilage are encased in dense connective tissue sheaths called perichondria (singular: perichondrium). The perichondrium has two layers, and outer fibrous layer for support, protection and attachment, and an inner cellular layer for growth and maintenance.
   Cartilage is avascular and all nutrient and waste product exchange occurs by diffusion through the matrix. 
   Formation and Growth
     Cartilage grows by two mechanisms:
     Appositional growth - Fibroblasts of the cellular layer of the perichondrium differentiate into chondroblasts that produce matrix and become chondrocytes when embedded in matrix.
     Interstitial growth - Chondrocytes within the matrix divide by mitosis and produce additional matrix that causes internal expansion of cartilage.
Types of Cartilage
a. Hyaline Cartilage (hyaline means glassy)
   This is the most common type of cartilage. It is the weakest type of cartilage but is still tough and flexible.
   The matrix contains collagen fibers that are thinner than the most common type of cartilage (Type I) and do not pick up stain. This gives this cartilage a glassy appearance.
   Hyaline cartilage connects ribs to the sternum as costal cartilage, supports the conducting passages of the respiratory tract (larynx, trachea and bronchi), and forms the articular surfaces of moveable joints.
b. Elastic Cartilage
   Elastic cartilage contains numerous elastic fibers that make it very resilient and flexible. 
   It is found in the pinnae of the outer ear, in the epiglottis and cuneiform cartilages of the larynx, and supporting the auditory canal.
c. Fibrocartilage
   Fibrocartilage is a combination of dense regular connective tissue and hyaline cartilage.
   The denser concentration and more regular arrangement of collagen fibers makes this composite tissue tougher and more durable
   Fibrocartilage is found in intervertebral discs, pubic symphysis, and in the menisci (sing. meniscus) of the knee and temporomandibular joints.
2. Bone
   Bone will be covered later.
 
Membranes
   Epithelia and their underlying connective tissue form membranes that cover and protect other structures and tissues. There are four types of membranes.
1. Mucous Membranes
   Mucous membranes line passageways into the body that are continuous with the exterior.
   These passageways include the digestive, urinary, reproductive and respiratory tracts.
   This lining is kept moist and lubricated at all times by secretions including the mucus secreted by mucous glands.
   The areolar connective tissue that attaches the epithelium to the underlying tissue is called lamina propria.
2. Serous Membranes
   Serous membranes are very thin and transparent and line compartments of the ventral body cavity:
a. Pleura
   The lining on the lungs and the pleural cavities in which the lungs are found.
b. Peritoneum
   The lining of the abdominopelvic cavity and all the organs situated there.
c. Pericardium
   The lining on the surface of the heart and the pericardial sac that surrounds the heart.
   Serous membranes consist of a simple squamous epithelium called mesothelium and a thin underlying layer of areolar connective tissue.
   The thinness of serous membranes allow fluid to seep through and form a transudate. The transudate contributes to the smooth and slippery nature of the lining.
3. Cutaneous Membrane
   Also known as skin. This membrane consists of keratinized stratified squamous epithelium attached by areolar connective tissue to a layer of dense connective tissue.
4. Synovial Membrane
   This membrane is found within joint capsules and is composed of areolar connective tissue incompletely lined by "epithelial" cells. These cells are not true epithelial cells because they, 1) do not have a basal lamina, 2) do not form a complete lining and 3) are derived from connective tissue cells. Some of these cells are phagocytic and others are secretory.
   Secretory cells secrete synovial fluid that lubricates and nourishes the hyaline cartilage of the joint.