Epithelium and Cell Structure: Tissue Types, Junctions, and Microscopy

Epithelium

Tissue upon or on top of.

Tissues

Many cells make up a tissue.

Apical

Top.

Avascular

No blood vessels in epithelium.

High regenerative capacity

Cells die and are replaced by new cells.

Polarity

Cells exhibit apical (top), basal, and lateral surfaces.

Basement membrane (BM)

Divided into a basal lamina and reticular lamina.

Basal lamina

Beneath the epithelial cells, produced by epithelial cells, functions in attachment of epithelial cells.

Reticular lamina

Produced by underlying connective tissue cells.

Cell junctions

Function in cell attachment to each other and the basal lamina.

Cell adhesion molecules (CAMs)

Important cell surface proteins found on most types of cells, function in cell recognition and attachment.

Occluding (tight) junctions (zonula occludens)

Prevent passage of substances between adjacent cells.

Anchoring (attachment) junctions

Attach cells to each other or the extracellular matrix.

Gap junctions

Function in intercellular communication.

Connexins

Proteins common in embryonic tissues and found in adult cardiac and smooth muscle, mediating electrical coupling of cells.

Microvilli

Finger-like projections of the plasma membrane with a core of actin filaments that increase surface area of the cell for absorption.

Stereocilia

Long microvilli.

Cilia

Cell organelles that insert into the apical surface of epithelial cells, with a core of microtubules, functioning in movement of substances across the epithelial cell surface.

Intercellular interdigitations

Specializations located between cells that regulate passage of substances through an epithelium.

Basal infoldings

Basal plasma membrane folded with many mitochondria, characteristic of ion-pumping cells.

Simple epithelium

Composed of 1 cell layer, with all cells resting on the basal lamina.

Simple squamous epithelium

Very thin epithelium with selective permeability, lining all blood vessels, air interface in the lung, and visceral covering of organs.

Simple cuboidal epithelium

Found in ducts and kidney tubules.

Simple columnar epithelium

Found in the gastrointestinal tract and female reproductive tract.

Pseudostratified columnar epithelium

A simple epithelium that appears stratified due to varying cell heights, found in the upper respiratory tract and male reproductive tract.

Stratified epithelium

Composed of multiple layers, named for the shape of the surface cells.

Stratified squamous epithelium

Multiple layers of thick epithelium specialized for protection.

Nonkeratinized stratified squamous epithelium

Found in the esophagus, lining mucosa of oral cavity, and vagina.

Keratinized stratified squamous epithelium

Has a thick surface layer of dead cells, found in skin and parts of the oral cavity.

Stratified cuboidal epithelium

2 layers thick, found in ducts.

Stratified columnar epithelium

2 layers thick, found in ducts.

Transitional epithelium

Allows for distention (stretch), can fold or unfold, found in the urinary bladder, ureters, and part of the urethra.

Endocrine glands

Glands that are separate from surface epithelium, have no ducts, and secrete into the blood.

Exocrine glands

Glands that secrete onto a surface epithelium, which may occur within an epithelium or below it and secrete into ducts.

Classification of exocrine glands

Based on type of secretory cells, arrangement of secretory cells (units), branching of ducts, and modes of secretion.

Modes of secretion

Includes merocrine (eccrine), apocrine, and holocrine.

Merocrine secretion

Secretions are exocytosed.

Apocrine secretion

Apical cytoplasm is shed with secretions.

Holocrine secretion

Whole cell filled with secretions is shed.

Functions of epithelia

Includes protection, absorption, secretion, selective barrier, and transport of substances (e.g., cilia movement).

Intracellular

inside the cell

Subcellular

below cellular level (another way of saying inside the cell)

Extracellular

outside of the cell

Intercellular

between cells

lamina

layer

lysis

to break down

"philic"

loving

"phobic"

fear of

"cyto"

cell

"endo"

within

"exo"

external

"macro"

large

"micro"

small

"phago"

to eat

Morphology

appearance, shape or form

"necro"

death

"poly"

many

Light microscopy (LM)

light transmitted through specimen to eyepiece.

Transmission electron microscopy (TEM or EM)

used to achieve higher magnifications, electron beam transmitted through specimen to a fluorescent screen, image viewed on screen or photographed. Images appear as shades of black and white (varying densities: electron-dense, electron-lucent)

Scanning electron microscopy (SEM)

electron beam deflects off metal-coated specimen creating a 3D surface image.

Fixation

aldehydes, stops tissue degradation and hardens tissue

Washing

Dehydration and clearing

in graded alcohols and xylene. The dehydration dissolves out fats and glycogen in tissues.

Infiltration and embedding

paraffin or plastic

Sectioning

Acidic dyes

will stain basic or positively charged components in the cell. Something that attracts an acid dye is said to be acidophilic (acid-loving)

Basic dyes

will stain acidic or negatively charged components in the cell. Something that attracts a basic dye is basophilic (base-loving).

Hematoxylin and eosin (H & E)

most common staining method in LM.

Hematoxylin

blue to purple dye, acts as a basic dye so stains acidic cell components such as nucleic acids, complex carbs

Eosin

pink to red dye, acid dye which stains basic components in cells (protein filaments such as collagen, muscle filaments)

Cytoplasm

gelatinous fluid composed of water, salts, organic constituents

Plasma membrane

Selective barrier, fluid and dynamic (always changing). The plasma membrane is 1 Unit membrane. A unit membrane is 2 layers of lipid molecules with proteins embedded in the membrane or spanning through the whole membrane.

Transport of substance across the membrane (transmembrane transport)

continual process

Passive diffusion

high to low concentration

Active transport

against concentration gradient, requires energy

Transport/carrier proteins

bind to molecules to assist transport across plasma membrane.

Channel proteins (pores)

create channels through the plasma membrane allowing for passive diffusion of large amounts of ions and small molecules.

Cystic Fibrosis

is caused by a genetic defect in channel proteins that transport chloride ions across epithelial cells in the gastrointestinal tract, respiratory tract, reproductive tract and sweat glands. This results in more concentrated sweat and thicker mucous secretions. Symptoms include frequent respiratory tract infections, gastrointestinal disturbances and infertility.

Endocytosis

changes in plasma membrane structure form invaginations that allow the taking in of fluid, molecules and particles to form membrane bound vesicles called endosomes.

Pinocytosis

Small buds of plasma membrane pinch off to allow for 'drinking in' of fluid and small molecules.

Macropinocytosis

Crescent-shaped extensions of plasma membrane encapsulate fluid pools, allowing for 'drinking in' of fluid and small molecules.

Phagocytosis

Cytoplasmic processes extend from the cell and engulf large particles.

Receptor-mediated endocytosis

Involves specific plasma membrane proteins that coat endocytosed substances.

Endosome transport

Membrane-bound vesicles are utilized for intracellular transport of substances through the cell and between organelles.

Exocytosis

Cellular secretion.

Nucleus

Contains the genes (DNA).

Nuclear membrane (envelope)

Consists of 2 unit membranes: inner membrane and outer membrane, which is continuous with the endoplasmic reticulum (ER).

Nuclear pores

Protein-lined pores that regulate passage of substances in and out of the nucleus.

Chromatin

Filaments formed by DNA wound around proteins.

Euchromatin

Pale chromatin found in active protein-synthesizing cells where DNA is unwound for transcription.

Heterochromatin

Dense chromatin found in inactive cells where DNA is tightly coiled.

Nucleolus

Small dense clump in the nucleus, prominent in active cells, site of ribosome production.

Cellular organelles

Subcellular structures that perform a variety of functions, with continual turnover of organelles.

Ribosomes

Made of RNA and specific proteins, found in clusters (polyribosomes) or along granular endoplasmic reticulum.

Polyribosomes (polysomes)

Site of initiation of all protein synthesis where ribosomes bind to mRNA strand and begin translation.

Endoplasmic reticulum (ER)

Sheet-like stacks or arrays of membranes, consisting of granular (rough) and smooth types.

Granular (Rough) endoplasmic reticulum (GER, RER)

Stacks of membranes studded with ribosomes where membrane proteins and secretory proteins complete synthesis.

Smooth endoplasmic reticulum (SER)

Tubular arrays of membrane involved in lipid and steroid synthesis, detoxification, and calcium storage in muscle cells.

Golgi apparatus

Stacked membranes with tubular extensions where protein-containing vesicles are modified and packaged.

Lysosomes

Organelles involved in digestion of wastes, containing hydrolytic enzymes synthesized in the RER and Golgi.