LAB 4: Epithelial tissues

Epithelial tissues

• Composition: Tightly packed cells with minimila extracelluar fluid

• Functions: Covers body/organ surfaces, lines body cavities and organ cavities, forms glands

• Subtypes: Simple and Stratified epithelium

  • Simple Epithelium

    • simple squamous

    • simple cuboidal

    • simple columnar

    • pseudostratified columnar

  • Stratified Epithelium

    • stratified squamous

    • stratified cuboidal

    • stratified columnar

    • transitional

Characteristics of epithelial tissue

• Cellularity:

  • composed almost entirely of tightly packed cells.

  • There is a minimal amount of extracellular matrix between the cells.

• Polarity:

  • An epithelium has an apical surface (free, or superficial), which is exposed either to the external environment or to some internal body space.

  • The apical surface may have either microvilli or cilia.

  • Cilia are numerous, slightly longer, membranous projections that move fluid, mucus, and materials past the cell surface

  • microvilli are small, membranous projections on the apical surface of the cell that increase its surface area for secretion and absorption.

  • The lateral surfaces may contain membrane (intercellular) junctions

  • Additionally, each epithelium has a basal surface (a fixed or deep surface), where the epithelium is attached to a basement membrane with underlying connective tissue.

• Attachment to a basement membrane.

  • The epithelial layer is bound at its basal surface to a thin basement membrane.

  • It may be seen as a single noncellular (or molecular) layer using light microscopy-however, in reality it consists of three molecular layers that can be viewed using an electron microscope.

  • These molecular layers are formed by secretions of both the epithelium and the underlying connective tissue, and are composed of collagen, glycoproteins, and proteoglycans .

  • Together, these molecular structures of the basement membrane components act like "glue" to strengthen the attachment and form a selective molecular barrier between the epithelium and the underlying connective tissue.

• Avascularity.

  • All epithelial tissues lack blood vessels.

  • Nutrients for epithelial cells are obtained either directly across the apical surface or by diffusion across the basal surface from blood vessels within the underlying connective tissue.

• Extensive innervation.

  • Epithelia are richly innervated (supplied with nerves) to detect changes in the environment at that body or organ region.

• High regeneration capacity.

  • Epithelial cells undergo cell division frequently

  • This characteristic allows this tissue to regenerate itself at a high rate, a necessary condition for a tissue that is often exposed to the environment and lost by abrasion and damage.

  • The continual replacement occurs through cell division of the deepest epithelial cells (called stem cells), which are adjacent to the basement membrane.

Functions of Epithelial Tissue

Epithelia have several primary functions, although no one epithelium performs all of them:

• Physical protection:

  • Epithelial tissues protect both external and internal surfaces from dehydration, abrasion, and destruction by physical, chemical, or biological agents.

• Selective permeability:

  • An epithelium typically exhibits a range of permeability; it may be relatively non-permeable to some substances, while promoting and assisting the passage of other ions and molecules.

  • All substances that enter or leave the body must pass through an epithelium, and thus epithelial tissues act as the body's "gatekeepers."

• Secretions:

  • Some epithelial cells are specialized to produce and release secretions.

  • These cells form glands.

  • Glands may be individual cells scattered among other cell types in an epithelium (e.g., goblet cells) or arranged in small, organized clusters within a multicellular gland

• Sensations:

  • Epithelial tissues are innervated by sensory nerve endings to detect or respond to a stimulus.

  • These nerve endings-and those in the underlying connective tissue-continuously relay sensory input to the central nervous system (i.e., brain and spinal cord) concerning touch, pressure, temperature, and pain.

Classification of Epithelial Tissue

• The body contains many different types of epithelia, and the classification of each type is typically indicated by a two-part name.

• The first part of the name refers to the number of epithelial cell layers

• second part describes the shape of cells at the apical (superficial) surface of the epithelium.

• Epithelia are classified as either simple or stratified

Simple Epithelium classification

• A simple epithelium is one is layer of epithelial cells and all of the epithelial cells are in direct contact with the basement membrane.

• A simple epithelium is found in areas where stress is minimal and filtration, absorption, or secretion is the primary function.

Classification of Stratified Epithelium

• contains two or more layers of epithelial cells.

• Only the cells in the deepest (basal) layer are in direct contact with the basement membrane.

• resembles a brick wall, where the bricks in contact with the ground represent the basal layer and the bricks at the top of the wall represent the apical (superficial) layer.

• provides either more structural support or better protection for underlying tissue.

• Cells in the basal layer continuously regenerate as the cells in the apical layer are lost due to abrasion or stress.

Classification of Pseudostratified

• A pseudostratified (su'do-strat'i-fid; pseudo = false, stratum = layer) epithelium appears layered (stratified) because the cells' nuclei are distributed at different levels between the apical and basal surfaces.

• Although all of these epithelial cells are attached to the basement membrane, some of them do not reach its apical surface.

• For our purposes, we have classified pseudostratified epithelium as a type of simple epithelium, because all of the cells are attached to the basement membrane.

Classification of Pseudostratified

• A pseudostratified (su'do-strat'i-fid; pseudo = false, stratum = layer) epithelium appears layered (stratified) because the cells' nuclei are distributed at different levels between the apical and basal surfaces.

• Although all of these epithelial cells are attached to the basement membrane, some of them do not reach its apical surface.

• For our purposes, we have classified pseudostratified epithelium as a type of simple epithelium, because all of the cells are attached to the basement membrane.

Simple Squamous Epithelium

• consists of a single layer of flattened cells

• When viewed laterally, each cell can be seen to contain a centrally located nucleus that is relatively flat but bulges at its center.

• Each squamous cell resembles a fried egg, with the slightly bulging nucleus of the cell representing the yolk.

• This epithelium is extremely delicate and represents the thinnest possible barrier to allow rapid movement of molecules and ions across the epithelium by membrane transport processes .

• Simple squamous epithelium forms the lining of the air sacs (alveoli) of the lung

• where this thin epithelium is well suited for the exchange of oxygen and carbon dioxide between the blood and the inhaled air.

Endothelium

• (en-do-the'le-um; endon = within) is the name of the simple squamous epithelium that lines both blood vessels and lymph vessels

mesothelium

• (mez-o-the lē-um; mesos = middle) is the name given to the simple squamous epithelium that forms the serous membranes of body cavities

• Mesothelium gets its name from the embryonic primary germ layer called mesoderm, from which it is derived (see L' section

Simple Cuboidal Epithelium

• A simple cuboidal epithelium contains one layer of uniformly shaped cells that are about as tall as they are wide with a centrally located, spherical nucleus.

• This epithelium allows for both absorption and secretion. Its cells' uniformity in shape makes them ideal to form the structural components of glands.

• For example, a simple cuboidal epithelium forms the follicles (spherical structures) of the thyroid gland and covers each ovary. Simple cuboidal epithelium also composes the walls of small ducts (or tubules), including those of kidney

Simple Columnar Epithelium

• A simple columnar epithelium is composed of a single layer of cells that are taller than they are wide. The nucleus is oval, oriented lengthwise, and located in the basal region of the cell. This type of epithelium is ideal for both absorptive and secretory functions.

• Simple columnar epithelium has two forms:

  • One type has no cilia,

  • whereas the apical surface of the other type is covered with cilia.

Nonciliated simple columnar epithelium

• often contains microvilli and a scattering of unicellular glands called goblet cells

• Individual microvilli cannot be distinguished with light microscopy; rather, the microvilli collectively appear as a bright, fuzzy structure known as a brush border.

• Goblet cells secrete mucin, which is a glycoprotein that when hydrated (mixed with water) forms mucus.

• Nonciliated simple columnar epithelium lines most of the gastrointestinal (digestive) tract from the stomach to the upper portion of the anal canal.

Ciliated simple columnar epithelium

• has cilia that project from the apical surfaces of the cells

• Mucus covers the apical surface and is moved along by the beating of the cilia.

• Goblet cells typically are interspersed throughout this epithelium.

• Ciliated columnar epithelium lines the larger bronchioles (air passageways) in the lung.

• It also lines the uterine tubes, where it helps move an oocyte (egg) from the ovary to the uterus

Pseudostratified Columnar Epithelium

• named because upon first glance, it appears to consist of multiple layers of cells.

• However, this epithelium is not really stratified because all of its cells are in direct contact with the basement membrane, but the nuclei are scattered at different distances from the basal surface; not all of the cells reach the apical surface in this epithelium.

• Its columnar cells always reach the apical surface, and the shorter basal cells are stem cells that give rise to the columnar cells.

• consists of two forms: pseudostratified ciliated columnar epithelium, which contains cilia on its apical surface and pseudostratified

  nonciliated columnar epithelium, which lacks cilia

• Both types perform protect functions.

Pseudostratified ciliated columnar epithelium

• contains cilia on its apical surface

• houses goblet cells that secrete mucus which hydrates to become the mucus that traps foreign particles and is moved by the beating cilia.

• This type lines the larger air passageways of the respiratory system (e.g., the nasal cavity, part of the pharynx [throat], part of the larynx [voice box], trachea, and bronchi.

pseudostratified nonciliated columnar epithelium

• lacks goblet cells and cilia.

• It is rare and found primarily in the membranous part of the male urethra (the tube that conveys urine from the urinary bladder to outside the body) and epididymis (the structure that stores sperm within the testes).

Stratified Squamous Epithelium

• multiple cell layers, and only the deepest layer of cells is in direct contact with the basement membrane.

• The cells in the basal layers have a cuboidal or polyhedral shape, whereas the apical cells display a flattened, squamous shape.

• This epithelium is adapted to protect underlying tissues from damage caused by abrasion and friction.

• Stem cells in the basal layer continuously divide, to produce a new stem cell and a committed cell that is gradually displaced toward the surface to replace those cells that have been lost.

• This type of epithelium exists in two forms: keratinized and nonkeratinized.

keratinized stratified squamous epithelium

• the superficial layers are composed of cells that are dead.

• These cells lack nuclei and all organelles, and instead are filled with the protein keratin

• (ker'a-tin; keras = horn), which is a tough, protective protein that strengthens the tissue

• The epidermis (outer layer) of the skin consists of keratinized stratified squamous epithelium

nonkeratinized stratified squamous epithelium

• including those at the tissue's apical surface, lack keratin and remain alive.

• Because all of the cells are alive, the nuclei of squamous cells are visible throughout the tissue.

• This tissue is kept moist with secretions such as saliva or mucus.

• forms the surface tissue of mucous membranes that line the oral cavity (mouth), part of the pharynx (throat), part of the larynx (voice box), the esophagus, the vagina, and the anus.

stratified cuboidal epithelium

• contains two or more layers of cells, and the superficial cells tend to be cuboidal in shape.

• Stratified cuboidal epithelium, like simple cuboidal epithelium, forms tubes and ducts, and it functions in protection and secretion.

• This tissue forms the walls of the ducts of most exocrine glands such as the ducts of the sweat glands in the skin and the periphery of ovarian follicles

stratified columnar epithelium

• is relatively rare in the body.

• It consists of two or more layers of cells, but only the cells at the apical surface are columnar in shape.

• This type of epithelium protects and secretes. It is found in the large ducts of salivary glands, the conjunctiva covering the eye, and a segment of the male urethra (i.e., membranous urethra).

ciliated pseudostratified columnar epithelium

nonciliated pseudostratified columnar epithelium

transitional epithelium

• is limited to the urinary tract (urinary bladder, ureters, and part of the urethra

• It varies in appearance, depending upon whether it is in a relaxed state or a distended (stretched) state .

• In a relaxed state, the basal cells appear cuboidal or polyhedral, and the apical cells are large and rounded.

• When transitional epithelium stretches, it thins and the apical cells flatten and become almost squamous in shape.

• One distinguishing feature of transitional epithelium is the presence of some binucleated (containing two nuclei) cells.

• By being able to stretch as the bladder fills, this tissue ensures that urine does not seep into the underlying tissues of these organs.