Section 1, Chapter 5

Introduction to Tissues& Epithelial Tissues

Tissues

A Tissue is to a group of similar cells that carries out a specialized function.

Histology is the science of tissues.

examples of tissues

Muscular tissue – specialized to contract

Nervous tissue – conducts, senses, and stores information

Epithelial tissue – forms coverings (skin) & linings (blood vessels)

Connective tissues – support, transport, protect

There are four broad categories of tissues

Intercellular junctions

1. Tight Junction

A tight junction fuses the cell membranes of adjacent cells together so molecules cannot move between the cells. Instead, they must move through the cells.

Tight junctions prevent the passage of unwanted & harmful substances

Examples: Blood Brain Barrier & Digestive Tract

Interlocking membranes form tight junction between two cells

Cell 1

Cell 2

Cells may be separated by a matrix or connected together by cellular junctions.

2. Desmosomedesmosomes form “spot welds” between cellsProvides structural supportExamples of desmosomes are in the epidermis of the skin

Cellular Junctions

Cells connected by desmosomes

3. Gap JunctionGap Junctions connect cells by protein ion channels that allow for cell-to-cell diffusion of ions

Example of gap junctions are within the intercalated discs of cardiac muscle

Intercalated discs of cardiac muscle are one type of gap junction.

Cellular Junctions

Types of Tissues: Epithelial Tissue

Epithelial tissue lines cavities, such as the digestive & respiratory tracts

It also forms coverings, such as the skin

Functions include protection, secretion, absorption, excretion

Epithelial Tissue:

Epithelium: structureEpithelium is anchored to a layer of connective tissue, called the basement membrane.

Epithelium has a basal surface that faces the basement membrane

The free (apical) surface is exposed to the open space (lumen)

apical surfa

ce

basal surfa

ce

Basement membrane

Open space = lumen

Characteristics of Epithelial Tissue

Epithelium consists of tightly packed cells

They lack a blood supply, instead they receive nutrients by diffusion

Epithelium readily divide, so they are continually replaced = rapid healing

1. Simple = Single layer of epithelial cells

Classifications of Epithelial Tissues

2. Stratified = two or more layers of cells

Epithelium are classified by the layer of cells

And they’re classified by the shape of cells

1. squamous = scale-like

2. cuboidal = cube-like shape

3. Columnar = tall cells

Simple Squamous Epithelium

Single layer of flattened cells

Functions include diffusion, filtration, and secretion

Locations include:air sacs of lungs (alveoli)

lining of capillaries and the walls of blood vessels

glomerulus of kidneys.

surface view of simple squamous ep. alveoli of lung

squa

mou

s ce

lls

Simple Cuboidal Epithelium

Single layer of cube-shaped cells

Functions include secretion and absorption

Locations includethe lining and the ducts of some glands

tubules within kidneys

lumen

cross-section of tubules in kidneys

Simple Columnar Epithelium

Single layer of elongated cells and their nuclei appear as a single row

Functions include absorption, secretion, or transport

Often contain goblet cells Goblet cells secrete mucus onto apical surface

may be ciliated, or have microvilli, or have no modifications on apical surface.

goblet cell

columnar cells

microvilli “brush border”

Cilia beat constantly in a rhythmic motion to propel substances across the cell surface.

Cilia propels the egg through uterine tube

Simple Columnar Epithelium

Some simple columnar epithelium are covered with cilia on their free surface.

Microvilli are extensions of the cell membrane that increases the surface area of a cell.

Example of microvilli include the intestinal epithelium, where they increase the amount of contact each cell makes with food.

Microvilli greatly enhance absorption of nutrients.

Simple Columnar Epithelium

Some simple columnar epithelium are covered with microvili on their free surface.

Pseudostratified Columnar Epithelium (PSCE)

PSCE appear striated, but each cell touches the basement membrane.

Usually ciliated

Most PSCE contain goblet cells that secrete mucus

Locations of PSCE include the lining of the respiratory tract: nasal cavity, pharynx, trachea, bronchi

Functions: Mucus from goblet cells traps debris & cilia moves the mucus (and trapped debris) away from the lungs

Stratified Squamous Epithelium

Several layers of cells = thick tissue

Primary function is to protect underlying tissues from abrasion

Superficial cells are squamous but the deeper cells are cuboidal or columnar

May be keratinized or non-keratinized

Keratinized epithelium is found in the epidermis (outer layer) of skin

Keratin is an insoluble protein that hardens and forms a protective coating.

Cells accumulate keratin over time, so the older outermost cells of the epidermis are keratinized

Stratified squamous epithelium of epidermis

Keratinized Stratified Squamous Epithelium

Non-keratinized (moist) stratified squamous epithelium lines the esophagus, oral cavity, vagina, and anal canal.

Apical surface

Basal surfaceStratified squamous epithelium of vagina

Non-keratinized Stratified Squamous Epithelium

Transitional Epithelium

Transitional epithelium forms an expandable lining (Function: permit expansion)

Locations include the inner layer of the urinary bladder & ureters

The lining is several layers thick and the cells appear cuboidal when the urinary bladder is contracted, but only a few layers thick when the urinary bladder is distended and the cells appear squamous.

empty urinary bladder distended urinary bladder

Glandular Epithelium Glandular epithelium is specialized to produce & secrete substances

Glandular epithelium form glands

Exocrine GlandsExocrine glands secretes substances through ducts onto an open surface Examples includes goblet cells, sweat glands, and mammary glands

Duct of a sweat gland

Endocrine GlandsEndocrine glands secretes substances (hormones) directly into the blood stream.

Examples includes the thyroid gland, pituitary gland, adrenal glands, ect.

Follicles of thyroid gland are composed of simple cuboidal epithelium

Glandular Epithelium

End of Section 1, Chapter 5