Chapter 4: Introduction to Histology (written notes)

Introduction

Histology

• Study of tissues under a microscope

Why Study Histology

• Understand how tissues function

• Understand diseases

• Understand organ system better

Four Basic Tissues

1. epithelial tissue

2. Connective tissue

3. Muscle tissue

4. Nerve tissue

Epithelial Tissue

Features:

1. High cellularity

• Made of tightly packed cells

• Very little extracellular matrix 

2. Polarity

• Apical surface → top (faces outside or a body cavity)

• Basal surface → (attached to basement membrane)

3. Avascular

• Epithelial tissues do not have their own blood supply

• Gets nutrients by diffusion from underlying connective tissue

4. Innervated

• Usually contains nerve ending

• That's why skin is sensitive to touch, pain, etc

5. Regenerative

• Cells divide frequently

• Heals quickly

6. Attachments

1. Tight Junctions

• form a seal between cells

• prevents substances from leaking between them

2. Desmosomes

• Act like anchors; keeps cells from pulling apart

• Hold cells together during stress

• Common in skin

3. Gap Junctions

• Allow communication between cells

• Let small molecules and ions pass directly through cells

Classification of Epithelium

1. Simple Squamous 

Characteristics:

• Single layer

• Flat

• Nucleus is flatted or oval

• Thin cells so substances can pass through easily

Found in

• Lining of blood vessels

• Alveoli of lungs

2. Simple Cuboidal

Characteristics:

• Single layer of cube shaped cells

• Contain round central nucleus

• Slightly taller than wide

• Often found lining facts or tubes

Found in:

• Tubules of Kidney 

• endocrine glands

3. Simple Columnar

Characteristics:

• Single layer

• Tall rectangular cells

• Nucleus near the basal side of cell

• Can contain microvilli (for absorption) or cilia (for moving substances)

• Sometimes contains goblet cells

Found in:

• Lining of intestines

4. Pseudostratified Columnar

Characteristics:

• Single layer of cells or different heights

• Nuclei appear at different levels

• Often ciliated

• Contain goblet cells

Found in:

• Trachea

5. Stratified Squamous

Characteristics:

• Multiple layer of cells

• Basal layer → cuboidal or columnar cells that divide

• Apical layer → flat squamous cells

• Keratinized → surface cells are dead and filled with keratin (touch,waterproof)

• Non-keratinized → surfaces cells remain alive and moist

Found in

• Epidermis of skin (keratinized)

• Lining of esophagus (non-keratinized)

6. Stratified Cuboidal

Characteristics:

• Multiple layer of cube-shaped cells

• Usually 2-3 layers thick

• Rare compared to other epithelial types

Found in:

• Sweat glands

7. Stratified Columnar

Characteristics:

• Multiple layers of cells

• Basal layers → usually cuboidal

• Apical layer → colum-shaped

• Rare in body

Found in:

• Submandibular gland (the major salivary gland)

8. Transitional Epithelial

Characteristics:

• Multiple layer of cells

• Basal layer → cuboidal or columnar

• Apical layer → dome-shaped when relaxed, flattened when stretched

• Can stretch and recoil without tearing

Found in:

• bladder

Glands and Membranes

Types of Glands

1. Exocrine Glands

• Secrete substances onto a surface or into a duct

2. Endocrine Glands

• Lack ducts

• Uses exocytosis to secrete hormones into blood

Methods of Glandular Secretion

Merocrine

• Secrete through exocytosis → cells stay intact, only product is releases

• Cells remain alive and continue to produce

• Most glands

Holocrine

• Secrete through cell disintegration 

• Cells die and are replaced by new cells from basal layer

• Oil glands

Membranes

• Epithelial layer sitting on a thin layer of loose fibrous connective tissue

• Types of membranes: cutaneous, mucous, serous, 

1. Cutaneous

• Skin

• Dry membrane exposed to air

Two main layers

1. Epidermis (outer layer)

• Made of stratified squamous epithelium

• Protects against abrasion, pathogens and water loss

2. Dermis (inner layer)

• Made of dense connective tissue

• Provides strength elasticity and support

• Contains blood vessels, nerve, glands and hair follicles

     2. Mucous 

• Wet membrane that lines body cavities and tubes that open to the outside

Structure:

• Epithelium layer → varies by location (simple or stratified)

• Lamina propria → underlying connective tissue

• Often contains goblet cells that secrete mucus

   3. Serous 

• Thin, double layered membrane that lines closed body cavities and covers the organs inside

• Unlike mucous membranes eros does not open to outside

Structure:

• Parietal layer → lines the wall off the cavity

• Visceral layer → covers the organ itself

• Serous fluid → thin fluid between the layers that reduces friction

Classification of Connective Tissues

Connective Tissue Characteristics

1. Mesenchyme Origin

• All connective tissues develop from mesenchyme and embryonic tissue

2. Cells and usually widely spaced

• Cells and not tightly packed

• A lot of extracellular matrix between cells

3. Extracellular matrix

• Consists of fibers, (collagen, elastic, reticular) and ground substance (gel-like material)

4. Vascularity

1. Varies by type

Bone → highly vascular

Cartilage → avascular (heals slowly)

Dense connective tissue → poorly vascularized

5. innervation

• Most connective tissues have nerve supply

Types of Connective Tissue Fibers

1. Collagen Fibers

1. Structure: long, thick and strong protein fibers

2. Properties: very strong, resists tension, not very stretchy

3. Function: provide strength and support

4. Location: tendons, ligaments, germane of skin, bones

2. Elastic Fibers 

1. Structure: thin, branching fibers made of elastin

2. Properties: recoil back to original shape

3. Function: allows tissues to stretch and return to shape 

4. Location: walls of large arteries, lungs, some ligaments skin

3. Reticular FIbers

1. Structure: thin, delicate fibers made of collagen

2. Properties: form mesh or scaffolding

3. Function: provide support for soft organs and cells

4. Location: lymph nodes, spleen, liver, bone marrow

Cells Types in Connective Tissue

Defensive Cells

1. Macrophage - consume pathogens + cellular debris

2. Microphage - consume pathogens/produce “antibodies”

3. Mast Cells - regular inflammation

4. Lymphocytes - manage immune response

Blast Cells

• Create connective tissue

1. Fibroblats

2. Osteoblast

3. Chondroblat

4. Hemocytoblast

5. adipocytes

Types of Connective Tissue 

1. Adipose Tissue

Characteristics:

• Large flat filled cells (adipocytes)

• Each contain a large lipid droplet

• Nucleus is pushed to side

• Looks like a signet ring under microscope

• Very little extracellular matrix

• Few fibers

• Cells are tightly packed

• Many blood vessels needed to store and release energy

• Soft and lightweight

Functions:

• Stores energy as triglycerides

• Releases energy when body needs it

• Helps maintain body temperature

• Protects organs

2. Areolar Tissue

• Contains all 3 fibers: collagen, elastic, reticular

• No bundles

• Numerous scattered cells

• Binds tissues together

• Wraps and cushions organs

3. Reticular Tissue

• Reticular (net like) fibers and cells

• Soft structure

• Supports immune system cells

• Filters blood and lymph

4. Dense Regular Connective Tissue

• Densely packed collagen fibers

• Few cells

• Poor blood supply

• Resists tension in one direction

5. Dense Irregular Connective Tissue

• Densely packed collagen fibers

• Few fibroblasts

• Little ground substance

• Better blood supply than dense regular

6. Elastic Connective Tissue

• Types of dense connective tissue

• Dense but flexible 

• Stretches and recoils

• Contains: fibroblasts and many elastic fibers

Connective Tissue Proper

• A category of connective tissue that mainly functions to bind, support protect and connect other tissues

• Most common type

• Contains: cells, fibers, ground substance and cells are widely spaced in extracellular matrix

• Functions: binds tissues, supporting organs, protection, immune defense

Dividing into Two Main types

1. Loose Connective tissue

• Fewer Fibers, more ground substance

• More flexible

Types:

1. Areolar

2. Adipose

3. Reticular

2. Dense connective Tissue

• More collagen fibers

• Stronger and less flexible

Types:

• Dense regular

• Dense irregular 

• Elastic 

Specialized Connective Tissue

• Have unique structures and functions

• Contains a specialized extracellular matrix

• Cells are often unique to tissue

• Provide: support structure, and specialized functions

• Types: Cartilage, Bone, Blood

1. Cartilage

• Characteristics: cells, extracellular matrix, avascular, flexible and resilient

• Functions: support, cushioning, smooth surfaces for joints, growth

• Types: hyaline, elastic, fibrocartilage

2. Hyaline Cartilage

• Most common type of cartilage

• Glassy, smooth, semi-transperant

• Matrix contains fine collagen fibers

• Cells are chondrocytes in lacunae

• Avascular → Nutrition via diffusion

3. Bone

• Characteristics: cells, extracellular matrix, vascularized, rigid and strong

• Cells: osteocytes, osteoblasts, osteoclasts

• Functions: support, protection, movement, mineral storage, energy storage, blood cell formation

4. Blood

• Characteristics: cells, extracellular matrix, vascularized

• Functions: transport, protection, regulation

Muscles and Nervous Tissues

Muscle Tissue

• Type of tissue that is specialized for contraction which allows body to move and generate force

• Characteristics: excitable, contractile, elongated cells, vascular

• Functions: movement, posture and body support, heat production, protection

• Types: skeletal, cardiac, smooth

1. Skeletal

• Characteristics: cell shaped, voluntary control, vascular

• Functions: body movement, posture and stability, heat production, protection

• Locations: attached to bones by tendons

2. Cardiac

• Characteristics: cell shape, involuntary control, intercalated discs, vascular

• Functions: pumping blood, maintains heart rhythm, involuntary movement

• Locations: heat wall

3. Smooth

• Characteristics: cells shape, involuntary control, slow and sustained contractions, vascular

• Functions: moves substances through organs, regulates organ diameter, involuntary movement

• Location: walls of digestive organs, blood vessels, urinary bladder, uterus

1. Neurons

• Functional cells of the nervous system that transmit electrical signals

• Characteristics: excitable cells, long-lived, high metabolic rate, specialized structure

• Functions: transmit signals, integration, response

• Locations: brain, spinal cord, peripheral nerves