Tissues

Dr. Nabil Khouri

The Tissue Level of

Organization

Learning Objectives

• Identify the four major tissue types and describe their functions.

• Describe the relationship between form and function for each tissue type.

• Discuss the types and functions of epithelial tissues.

• Tissues are:

• Collections of specialized cells and cell products organized to perform a limited number of functions

• The four tissue types are:

• Epithelial

• Connective

• Muscular

• Nervous

Tissues of the Body: An Introduction

Tissues and tissue types

4

Embryonic Tissues

1. Embryo begins as a single cell

• divides into many cells that form layers (strata)

2. Three primary germ layers

A. Ectoderm (outer) gives rise to: epidermis & nervous system

B. Endoderm (inner): mucous membranes: GI tract and respiratory linings; digestive glands.

C. Mesoderm (middle) forms mesenchyme (gelatinuous tissue) and then give rise to muscle, bone, and blood

Tissues = groups of cells that are similar in structure and function.

Epithelium

Coverings

Linings of surfaces

Connective

Support

Bone, ligaments, fat

Muscle

Movement

Nervous

Control

Brain, nerves, spinal cord

Function of Epithelial Tissue

Protection

Skin protects from sunlight & bacteria & physical damage.

Absorption

Lining of small intestine, absorbing nutrients into blood

Filtration

Lining of Kidney tubules filtering wastes from blood plasma

Secretion

Different glands produce perspiration, oil, digestive enzymes and mucus

Characteristics of Epithelial Tissue

Form continuous sheets (fit like tiles)

Apical Surface

All epithelial cells have a top surface that borders an open space – known as a lumen

Basement Membrane

Underside of all epithelial cells which anchors them to connective tissue

Avascularity (a = without)

Lacks blood vessels

Nourished by connective tissue

Regenerate & repair quickly

• Includes epithelium & glands

• Glands are secretory epithelium lining organs

• The epithelium :

• Is avascular

• Forms a protective barrier that regulates permeability

• Cells may show polarity

Epithelial tissue

• Cells attach via cell adhesion molecules (CAM)

• Cells are attach at specialized cell junctions

• Tight junctions

• Desmosomes

• Gap junctions

Maintaining the integrity of epithelium

Intercellular connections

• Basal lamina attaches to underlying surface

• Lamina lucida

• Lamina densa

• Germinative cells replace short-lived epithelial cells

Structure of typical epithelium

The Polarity of Epithelial Cells

Classification of epithelia

Cell Shape

Squamous – flattened like fish scales

Cuboidal - cubes

Columnar - columns

Cell Layers (NUMBER)

Simple (one layer)

Stratified (many layers)

It is then named for the type of cell at the apical surface.

Simple Squamous Epithelium

Mesothelia or Endothelia

Structure Single Layer of

flattened cells

Function Absorption, secretion

permeability and filtration

Reduce frection

Not effective

protection – single layer of cells.

Location Walls of capillaries, air sacs in

lungs

Form serous membranes in body cavity

Stratified Squamous Epithelium

Structure

Many layers (usually cubodial/columnar at bottom and squamous at top)

Function

Physical Protection

Keratin (protein) is accumulated in older cells near the surface – waterproofs and toughens skin.

Location

Skin (keratinized), mouth & throat

Simple Cuboidal Epithelia

Simple Cuboidal Epithelium

Structure

Single layer of cube shaped cells

Function

Secretion and transportation in glands, filtration in kidneys

Location

Glands and ducts (pancreas & salivary), kidney tubules, covers ovaries

Stratified Cuboidal Epithelia

Stratified Cubodial (layers of

cubodial only)

Columnar Epithelia

Simple Columnar Epithelium

Structure

Elongated layer of cells with nuclei at same level

Function

Absorption, Protection & Secretion

When open to body cavities called mucous membranes

Special Features

Microvilli, bumpy extension of apical surface, increase surface area and absorption rate.

Goblet cells, single cell glands, produce protective mucus.

Location

Linings of entire digestive tract

Columnar Epithelia

Stratified columnar epithelium

• Is a rare type of epithelial tissue composed of column shaped cells arranged in multiple layers.

• Stratified columnar epithelia are found in the ocular conjunctiva of the eye, in parts of the pharynx and anus, the female's uterus, the male urethra and vas deferens. Also found in Lobar ducts in salivary glands.

• The cells function in secretion and protection.

Pseudostratified ciliated Columnar Epithelia

Pseudostratified Epithelium

Structure

Irregularly shaped cells with nuclei at different levels – appear stratified, but It is not.

All cells reach basement membrane

Function

Absorption and Secretion

Goblet cells produce mucus

Cilia (larger than microvilli) sweep mucus

Location

Respiratory Linings & Reproductive tract

Transitional Epithelium

Transitional Epithelium

Structure

Many layers

Very specialized – cells at base are cuboidal or columnar, at surface will vary.

Change between stratified & simple as tissue is stretched out.

Function

Allows stretching (change size)

Location

Urinary bladder, ureters & urethra

• Unicellular

• Individual secretory cells

• Multicellular

• Organs containing glandular epithelium

• Classified according to structure

Glands

Glands

One or more cells that make and secrete a product.

Secretion = protein in aqueous solution: hormones, acids, oils.

Endocrine glands

No duct, release secretion into blood vessels

Often hormones

Thyroid, adrenal and pituitary glands

Exocrine glands

Contain ducts, empty onto epithelial surface

Sweat, Oil glands, Salivary glands, Mammary glands.

a. Sweet Glands

There are 2 types of sweat glands:

•Eccrine sweat glands, all over body

except lips and part of external genitalia;

•Apocrine sweat glands, only in axilla, areola, nipple

of mammary gland, and circumanal region and

the external genitalia. The ceruminous glands of

ear and glands of Moll of eyelid are also apocrine.

•Both the Eccrine and the Apocrine sweat

glands are innervated by the sympathetic nervous

system.

•Eccrine glands respond differently

to heat and nervous state.

•The apocrine glands respond

to emotional and sensory stimuli but not heat.

•Are simple coiled glands that regulate body

temperature.

•The secretory segment is deep in

the dermis or upper hypodermis.

•Its duct leads to surface.

•In the secretory region there are clear cells that

produce the watery

component of sweat and dark cells that

produce a protein secretion.

•Duct cells form the walls from the secretory

portion to the area near the surface where the

epidermal cells form the wall.

•The duct is stratified cuboidal epithelium .

Eccrine Sweat Glands

Eccrine Sweat Glands

b. Sebaceous Glands or Oil Glands

Found in the skin of mammals and these glands

secrete sebum.

Sebum is made of fat (lipids) and the debris of

dead fat-producing cells.

•These glands exist in humans throughout the

skin except in the palms of the hands and soles of

the feet.

•Sebum acts to protect and waterproof hair and

skin, and keep them from becoming dry, brittle,

and cracked. It can also inhibit the growth of

microorganisms on skin.

Sebaceous Glands or Oil Glands

•Are involved in skin problems such as acne and

keratosis pilaris. A blocked sebaceous gland can

result in a sebaceous cyst.

•Earwax, also known by the medical term cerumen, is

a yellowish, waxy substance secreted in the ear canal

of humans and many other mammals. It plays a vital

role in the human ear canal, assisting in cleaning and

lubrication, and also provides some protection from

bacteria, fungus, and insects

c. Ceruminous glands

Are the organs that, in the female mammal, produce milk for the

sustenance of the young. These Exocrine glands are enlarged and

modified sweat glands and are the characteristic of mammals

which gave the class its name.

D. Mammary glands

•The human mammary glands are modified sweat glands and are

developed from two sources --the parenchyma (alveoli and ductules )

from the surface ectoderm ,the fibrofatty stroma from the underlying

endoderm.

•At birth mammary glands of both sexes remain in infantile form .This

condition persists throughout life in normal male.

•First change is seen at puberty in females ,in the form of deposition of

fat and increase in size and attain hemispherical outlines. With the start

of reproductive cycle after puberty ,glandular tissue show changes with

the alteration in concentration of oestrogen and progesterone in each

cycle.

•Ultimately during pregnancy final maturation of the glands takes place

and they are ready for milk secretion under the influence of oestrogen

,progesterone ,prolactin and probably hCG..

•Some milk is secreted into the ducts as early as 5 months but the

amount is less compared to large amount secreted at child birth.Milk is

secreted within an hour of child birth and first formed milk is

called colostrum which is yellowish in colour and rich in protein and

antibodies and provide immunity to the baby.Normal milk production

starts 2-3 days after child birth.

• Merocrine (product released through exocytosis)

• Apocrine (involves the loss of both product and cytoplasm)

• Holocrine (destroys the cell)

Glandular secretions can be:

Apocrine Sweat Glands

•The secretory product from the gland.

•The duct has a narrow lumen.

•Apocrine

secretions contain protein, carbohydrate, ammonia and l

ipid.

Merocrine gland

• Cells that secrete products via the merocrine method form membrane-bound secretory vesicles internal to the cell.

• These are moved to the apical surface where the vesicles coalesce with the membrane on the apical surface to release the product. Most glands release their products in way.

Holocrine Glands

• The third type of secretory release, Holocrine, involves death of the cell. The secretory cell is released and as it breaks apart, the contents of the cell become the secretory product. This mode of secretion results in the most complex secretory product. Some sweat glands located in the axillae, pubic areas, and around the areoli of the breasts release their products in this manner. Sebaceous glands also are of this type.

Mechanisms of Glandular Secretion

A Structural Classification of Exocrine Glands