Epithelium and Glands

BIO 408

Histology

Dr. D. L. Daley

Epithelial Tissue

n Present in two forms:

n Sheets of continuous cells that cover the external body surface and lines the body on its internal surface

n As glands - originate from invaginated epithelial cells

Functions of Epithelial Tissues

1. Protection of underlying tissues from abrasion and injury

2. Transcellular transport of molecules across epithelial layers

3. Secretion of mucus, hormones, enzymes and etc. from various glands

Functions of Epithelial Tissues

4. Absorption of material from a lumen (intestinal tract or certain kidney tubules)

5. Selective permeability of intercellular junctions between epithelial cells controls the movement of materials between body compartments

6. Detection of sensations via taste buds, retina of the eye and specialized hair cells in the ear

Epithelium

n Sheets of continuous cells

n Bound together by junctional complexes

n Display little intercellular space and little extracellular matrix

n Separated from the underlying tissue by extracellular matrix and basal lamina that is secreted by the epithelial cells

n Avascular - adjacent connective tissue supplies nourishment and oxygen via diffusion through the basal lamina

Classification of Epithelial Membranes

n Classification according to the number of cell layers between the basal lamina and the free surface

n Also by the morphology of the epithelial cells

n Simple epithelium - single cell layer

n Stratified epithelium - more than one cell layer

Simple Squamous Epithelium

n Single layer of tightly packed, thin , low-profile polygonal cells, with centrally placed bulging nucleus

n Appear as tile floor when viewed from above

n Line pulmonary alveoli, compose the loop of Henle and the parietal layer of Bowman’s capsule in the kidney, and form the endothelial lining of blood and lymph vessels as well as the mesothelium of the pleural and peritoneal cavities

Simple Squamous Epithelium

Simple Cuboidal Epithelium

n Single layer of polygon-shaped cells

n When viewed in a section cut perpendicular to the surface, the cells appear as a square with a central round nucleus

n They are found in many glands of the body, form the covering of the ovary and compose some kidney tubules

Simple Cuboidal Epithelium

Simple Columnar Epithelium

n Single layer of tall rectangular cells whose ovoid nuclei are usually located at the same level in the basal half of the cell

n Found in the lining of much of the digestive tract, gall bladder, and large ducts of glands

n May exhibit a striated boarder or microvilli

n The simple columnar epithelium that lines the uterus, oviducts, ductuli efferentes and small bronchi is ciliated

Simple Columnar Epithelium

Stratified Squamous (Nonkeratinized) Epithelium

n Composed of many layers of cells

n Deepest layer in contact with the basal lamina

n The basal or deepest cells are cuboidal in shape, those in the middle layers are polymorphous and the cells on the free surface are flattened (squamous)

n Cell at the surface are nucleated and usually wet - nonkeratinized

n Found in the mouth, oral pharynx, true vocal cords, esophagus and vagina

Stratified Squamous (Nonkeratinized) Epithelium

Stratified Squamous (Keratinized) Epithelium

n Similar to nonkeratinized stratified squamous epithelium except that the superficial layers are composed of dead cells filled with the protein keratin

n The epithelium of the skin, a tough layer that resists friction and is impermeable to water

Stratified Squamous (Keratinized) Epithelium

Stratified Cuboidal Epithelium

n Contains only two layers of cuboidal cells

n Lines the ducts of sweat glands

Stratified Cuboidal Epithelium

Stratified Columnar Epithelium

n Composed of low polyhedral to cuboidal deeper layer in contact with the basal lamina and a superficial layer of columnar cells

n Found only in the conjunctiva of the eye and some large excretory ducts and regions of the male urethra

Stratified Columnar Epithelium

Transitional Epithelium

n Named because it erroneously was believed to be in transition between stratified columnar and stratified squamous

n However it is distinct type located exclusively in the urinary system

n Lines the urinary tract from the renal calyces to the urethra

Transitional Epithelium

n Composed of many layers of cells

n Basal cells are either low columnar or cuboidal cells

n Polyhedral cells come next

n The most superficial cells of the empty bladder are large, sometimes binucleate & have bulging dome shaped tops

Transitional Epithelium

Pseudostratified Columnar Epithelium

n Appears to be stratified but actually only one cell layer thick

n All cells are in contact with the basal lamina, but only some cells reach the top

n Cells that do not reach the surface have a broad bas and narrow apical surface

n Taller cells that reach the surface tend have a narrow base and wider apical surface

n Nuclei are thus found in different levels - giving the impression of a stratified epithelium

Pseudostratified Columnar Epithelium

n Found in part of the male urethra, epididymis and larger excretory ducts of glands

n Most common type is ciliated - apical cells have cilia

n Also fond in the lining of the trachea and primary bronchi

n Part of the tympanic cavity, nasal cavity and the lacrimal sac are lined with ciliated PCE

Pseudostratified Columnar Epithelium

Polarity & Cell Surface Specializations

Most have an apical domain - faces a lumen and a basolateral domain - basal component is in contact with the basal lamina

Apical Domain

n Free surface of the cells

n Commonly rich in ion channels, carrier proteins, H⁺ - ATPase, glycoproteins, hydrolytic enzymes and aquaporins (water channels)

n Also may exhibit microvilli or other cell surface specializations (cilia, stereocilia and flagella)

Microvilli

n Cylindrical membrane projections - appear as a striated boarder on intestinal epithelia and called the brush boarder of the cells of the proximal convoluted tubule of the kidney

n 1-2 mm in length - core of 25 to 30 actin filaments cross-linked with villin

n Terminal web - complex of actin & spectrin molecules & intermediate filaments located at the cortex of the epithelial cells

n At regular intervals myosin-I and calmodulin connect the actin to the plasma membrane of the microvillus for support

Microvilli

Stereocilia

n Long microvilli found in the epididymis and sensory cells of the cochlea

Cilia and Flagella

n Cilia

n Long motile, hair-like structures arising from the apical surface - core composed of complex arrangement of microtubules

n Flagella

n Only in sperm

Basolateral Domain - Lateral Membrane

Specializations

Light microscopy - terminal bars - most notable in the columnar epithelium of the gut - EM shows them to be junctional complexes - hold cells together

Basolateral Domain - Lateral Membrane

Specializations

n Zonulae Occludentes

n Belt-like intercellular seal between cells

n Prevent movement of membrane proteins and function to prevent intercellular movement of water-soluble molecules

Basolateral Domain - Lateral Membrane Specializations

n Zonulae Adherentes

n Assist adjoining cells to adhere to one another

n Maculae Adherentes (desmosomes)

n Weld-like junctions along lateral membranes - resist shearing forces

n Gap junctions (nexus)

n Communicating junctions - intercellular channels

n composed of the protein - connexin

Overview of Junctional Areas

Basolateral Domain - Basal Surface Specializations

n Basal lamina

n Plasma membrane enfoldings - increase surface area

n Hemidesmosomes - attach the basal membrane to the underlying basal lamina

Glands

n Originate from epithelial cells

n Secretory units along the ducts are called parenchyma

n Stroma - elements of connective tissue that support the parenchyma

n Types

n Exocrine - secrete products via ducts

n Endocrine - secrete products into the blood or lymph

Exocrine Glands

n Classification according to:

n Nature of the secretion

n Mode of secretion

n Number of cells

Exocrine Glands

n Mucous glands - secrete mucinogens (large glycosylated proteins) - swell on hydration

n Goblet cells & minor salivary glands of the tongue and palate

n Serous glands - enzyme-rich watery fluid

n pancreas

n Mixed glands - contain acini (secretory units) - may produce both mucous and serous secretions

n Sublingual and submandibular glands

Release Mechanisms

n Merocrine glands - products released via exocytosis

n Parotid glands

n Apocrine glands - product secreted with parts of the apical cytoplasm

n Lactating mammary gland

n Holocrine glands - product of secretion is shed with the whole cell

n Sebaceous glands

Unicellular Exocrine Glands

n Simplest form

n Goblet cells

Multicellular Exocrine Glands

n Organized clusters of secretory units

n Classification

n Simple ducts do not branch

n Compound - ducts branch

n Duct shape - Tubular (e.g.. colon), acinar (grape-like)(e.g.. Sebaceous glands) and tubuloalveolar (e.g. submandibular gland)

n Myoepithelial cells - sweat glands and major salivary glands - epithelial origin with contractile properties