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Basic Histology and Connective Tissue

Chapter 5

• Histology, the Study of Tissues

• Tissue Types

• Connective Tissues

Histology is the Study of Tissues

• 200 different types of cells in the human body. • A Tissue consist of two or more types of cells that

function together.

• Four basic types of tissues:
– epithelial tissue

– connective tissue

– muscular tissue – nervous tissue
• An Organ is a structure with discrete boundaries that is composed of 2 or more tissue types.

• Example: skin is an organ composed of

epidermal tissue and dermal tissue.

Distinguishing Features of Tissue Types

• Types of cells (shapes and functions) • Arrangement of cells • Characteristics of the Extracellular Matrix:
– proportion of water – types of fibrous proteins – composition of the ground substance

• ground substance is the gelatinous material between

cells in addition to the water and fibrous proteins
• ground substance consistency may be liquid (plasma),

rubbery (cartilage), stony (bone), elastic (tendon)

• Amount of space occupied by cells versus extracellular matrix distinguishes connective tissue from other tissues

– cells of connective tissues are widely separated by a large

amount of extracellular matrix

– very little extracellular matrix between the cells of epithelia, nerve, and muscle tissue

Embryonic Tissues

• An embryo begins as a single cell that divides into many cells that eventually forms 3 Primary

Layers:

– ectoderm (outer layer)
• forms epidermis and nervous system

– endoderm (inner layer)

• forms digestive glands and the mucous membrane lining digestive tract and respiratory system

– mesoderm (middle layer)
• Forms muscle, bone, blood and other organs.

Histotechnology

• Preparation of specimens for histology:
– preserve tissue in a fixative to prevent decay (formalin) – dehydrate in solvents like alcohol and xylene – embed in wax or plastic

– slice into very thin sections only 1 or 2 cells thick

– float slices on water and mount on slides and then add color with stains

• Sectioning an organ or tissue reduces a 3-dimensional structure to a 2-

dimensional slice.

Planes of Section

Longitudinal section

– tissue cut along the

longest direction of a structure

Cross section

– tissue cut perpendicular to the

length of a structure

Oblique section

– tissue cut at an

angle between a cross section and a longitudinal section

Two Dimensional Sections of Solid Three

Dimensional Objects

1 2 3 4 5

• Slicing through a

boiled egg is similar to sectioning a cell

and its nucleus.

1
2
5
4

• Slices 1 and 5 miss the yolk.

• Yolk appears larger

in section 3 than in sections 2 and 4.

3

Sections of Complex Hollow Structures

  • A
  • B

• Image A is a cross

section of a curved tubular structure like a

blood vessel or a

section of intestine.

• Image B is a

longitudinal section of a

spiraling, tubular structure like a sweat

gland.

• Notice what a single slice could look like.

Epithelial Tissue (Epithelia)

• One or more layers of closely adhering cells. • Forms a flat sheet with an unattached free surface (may be exposed to the environment or an internal body cavity) and a

basal surface attached to the basement membrane made of

collagen.
• Epithelia are avascular. Epithelial cells depend on diffusion of nutrients from capillaries in the underlying connective tissue or from the free surface.

• Epithelia are innervated by sensory neurons.

• Basement membrane is a is semi-permeable layer of collagen and adhesive proteins that anchors epithelial cells to underlying connective tissue.

• The connective tissue under an epithelium is called the lamina

propria.

Free Surface Basal Surface

Lamina Propria

Naming Epithelia

Epithelia are named for: • the number of layers of cells

– simple epithelium = one layer of cells

– stratified epithelium = more than one layer of cells

– pseudostratified

epithelium = simple that looks stratified

• the shape of cells at the surface

– squamous

– cuboidal – columnar – transitional

• surface modifications

– cilia – microvilli – keratinization

Simple

Squamous Epithelium

• Single row of squamous (flat)

cells.

• Can allow rapid diffusion of substances or secretion of fluid.

• Example: lining of blood vessels or lining of lung alveoli

Simple Cuboidal Epithelium

• Single row of cube-shaped cells

• Functions include absorption, secretion, conduction

• Example: most kidney tubules

Simple Columnar Epithelium

Microvilli
Absorptive Cell

Mucus

Goblet Cell

Nucleus
• Single row of tall, narrow cells

• Free Surface may have microvilli or cilia

• Layer of microvilli is called the brush border • Functions: absorption, secretion (of mucus) • Example: Lines the intestines

Pseudostratified Epithelium

Cilia Goblet Cells

Basal

Cells

• Single row of cells all attached to basement membrane • Not all cells reach the free surface

– nuclei of basal cells give a stratified appearance

• Secretes and propels respiratory mucus • Example: lining of trachea

Mucous Membranes

• Consists of a mucous-producing epithelium and underlying layers of

connective tissue (lamina propria) and smooth muscle (muscularis mucosae).

• Lines passageways that open to the exterior: digestive, respiratory, urinary and reproductive tracts.

• Mucous forms a barrier and traps foreign particles or pathogens.

• Epithelia of upper respiratory tract and parts of the reproductive tract
(oviducts) are ciliated to sweep the mucous out of the body.

Stratified Epithelia

• Composed of more than one layer of cells.

• Always named for shape of surface cells.

• Deepest cells sit on basement membrane and are the source of replacement cells for the epithelium.

• Keratinization:

– keratinized epithelium has surface layer of dead cells that contain abundant protein and are

surrounded by lipids

– nonkeratinized epithelium has living cells with nuclei in all layers

Nonkeratinized

Stratified
Squamous

• Stratified epithelium of living cells forms an

abrasion-resistant,

moist, slippery layer.
• Examples: lining of the

mouth, esophagus,

vagina

Keratinized Stratified Squamous Epithelium

dead, keratinized epithelial cells

living epithelial

cells

connective tissue

• Surface layer of dead squamous cells surrounded by lipids and packed with granules of keratin protein.

• Dead layer is “keratinized” or “cornified”.

• Retards water loss and prevents penetration of microorganisms.

• Example: skin

Stratified Cuboidal and Columnar Epithelium

  • sweat gland duct
  • kidney collecting duct

• In certain ducts, stratified columnar and cuboidal

epithelia can occur. As epithelial types, both are

uncommon. Basal cells are typically cuboidal with surface cells either columnar or cuboidal.

• Example: large ducts of salivary glands

Stratified Columnar Epithelium

Transitional

Epithelium

• Stratified

epithelium with rounded (domed)

surface cells.

• Stretches to allow storage of urine.

• Example: urinary

bladder.

Quiz is on material up to this

point.

Intercellular Junctions

• All cells except blood cells are anchored to each other or to the matrix surrounding them by intercellular junctions.

Tight Junctions

• Tight junctions completely encircle the cell (like a

sweat band around a person’s head)

• Tight Junctions form a zipper-like pattern of

complementary grooves and ridges that prevent

substances and bacteria from passing between

cells.

Tight Junctions

Desmosomes

• Attachment between cells that holds them together against mechanical stress (shearing

forces).

• A mesh of protein filaments connects integral membrane proteins and cytoskeletal proteins.

• Abundant in muscle and skin

• Hemidesmosomes attach cells to the basement membrane.

Desmosome
Hemidesmosome

Gap Junctions

• Also called communicating junctions. • Cluster of tube-shaped transmembrane proteins that make channels between cells.

• Small solutes and electrical signals pass directly from cell to cell and can synchronize the activity of groups of cells.

• Found in embryos, cardiac

muscle and smooth muscle.

Gap Junction

Glands

• Glands secrete substances for elimination or for use elsewhere in the body

• Glands are composed predominantly of epithelial tissue • Exocrine glands maintain connection to the surface through a duct (examples: sweat glands, salivary glands)

• Endocrine glands have no ducts but secrete their

products (hormones) onto capillaries for absorption directly into bloodstream (pituitary, adrenal) or into

interstitial fluid

• Mixed organs have both types of glands:
– pancreas secretes digestive enzymes into ducts and

hormones into blood

– gonads release gametes into ducts and secrete hormones into blood

Types of Glandular Secretions

• Serous
– thin, watery secretions such as sweat, milk, tears

and digestive juices.

• Mucus
– the sticky secretion called mucus is a glycoprotein,

mucin, that absorbs water

• Mixed Glands secrete both serous fluid and mucus • Note: Mucus is a noun. Mucous is an adjective.

“Mucus is secreted by mucous glands.”

• Cellular mechanisms of glandular secretion include:
1) merocrine 2) apocrine 3) holocrine

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