Chapter 5

Histology

Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Histology

• Study of Tissues

• Epithelial Tissue

• Connective Tissue

• Nervous and Muscular Tissue

• Intercellular Junctions, Glands and Membranes

• Tissue Growth, Development, Death and Repair

The Study of Tissues

• 200 Different cell types• Four primary tissue classes

– epithelial tissue– connective tissue– muscular tissue– nervous tissue

• Histology (microscopic anatomy)– study of tissues organ formation

• Organ = structure with discrete boundaries– composed of 2 or more tissue types

Features of Tissue Classes• Tissue = similar cells and cell products

– arose from same region of embryo

• Differences between tissue classes– types and functions of cells– characteristics of matrix (extracellular material)

• fibrous proteins • ground substance

– clear gels (ECF, tissue fluid, interstitial fluid, tissue gel)– rubbery or stony in cartilage or bone

– space occupied by cells versus matrix• connective tissue cells are widely separated • little matrix between epithelial and muscle cells

Embryonic Tissues

• Embryo begins as single cell– divides into many cells and layers (strata)

• 3 Primary germ layers– ectoderm (outer)

• forms epidermis and nervous system

– endoderm (inner) • forms mucous membrane lining GI tract and respiratory system

and digestive glands

– mesoderm (middle) becomes mesenchyme• wispy collagen fibers and fibroblasts in gel matrix• gives rise to muscle, bone, blood

Tissue Techniques and Sectioning

• Preparation of histological specimens– fixative prevents decay (formalin)– sliced into thin sections 1 or 2 cells thick– mounted on slides and colored with

histological stain• stains bind to different cellular components

• Sectioning reduces 3-dimensional structure to 2-dimensional slice

Sectioning Solid Objects

• Sectioning a cell with a centrally located nucleus

• Some slices miss the cell nucleus

• In some the nucleus is smaller

Sectioning Hollow Structures

• Cross section of blood vessel, gut, or other tubular organ.

• Longitudinal section of a sweat gland. Notice what a single slice could look like.

Types of Tissue Sections

• Longitudinal section– tissue cut along

longest direction of organ

• Cross section– tissue cut

perpendicular to length of organ

• Oblique section– tissue cut at angle

between cross and longitudinal section

Four primary tissue classes

• Epithelial tissue

• Connective tissue

• Muscular tissue

• Nervous tissue

Epithelial Tissue

• Layers of closely adhering cells• Flat sheet with upper surface exposed to the

environment or an internal body cavity• No blood vessels

– underlying connective tissue supplies oxygen

• Rests on basement membrane– thin layer of collagen and adhesive proteins– anchors epithelium to connective tissue

Simple Versus Stratified Epithelia

• Simple epithelium– contains one layer of cells– named by shape of cells

• Stratified epithelium– contains more than one layer– named by shape of apical cells

Simple Squamous Epithelium

• Single row of flat cells• Permits diffusion of substances• Secretes serous fluid• Alveoli, glomeruli, endothelium, and serosa

Simple Cuboidal Epithelium

• Single row cube-shaped cells with microvilli

• Absorption and secretion, mucus production

• Liver, thyroid, mammary and salivary glands, bronchioles, and kidney tubules

Simple Columnar Epithelium

• Single row tall, narrow cells– oval nuclei in basal

half of cell• Absorption and

secretion; mucus secretion

• Lining of GI tract, uterus, kidney and uterine tubes

Pseudostratified Epithelium

• Single row of cells some not reaching free surface– nuclei give layer

stratified look• Secretes and

propels respiratory mucus

Stratified Epithelia

• More than one layer of cells • Named for shape of surface cells

– exception is transitional epithelium

• Deepest cells on basement membrane• Variations

– keratinized epithelium has surface layer of dead cells

– nonkeratinized epithelium lacks the layer of dead cells

Keratinized Stratified Squamous

• Multilayered epithelium covered with dead squamous cells, packed with keratin– epidermal layer of

skin• Retards water loss and

barrier to organisms

Nonkeratinized Stratified Squamous

• Multilayered surface epithelium forming moist, slippery layer

• Tongue, oral mucosa, esophagus and vagina

Stratified Cuboidal Epithelium

• Two or more cell layers; surface cells square

• Secretes sweat; produces sperm and hormones

• Sweat gland ducts; ovarian follicles and seminiferous tubules

Transitional Epithelium

• Multilayered epithelium surface cells that change from round to flat when stretched– allows for filling of

urinary tract– ureter and bladder

Four Types of Connective Tissue

1. Fibrous– Loose– Dense

2. Cartilage

3. Bone– Spongy– Compact

4. Blood

Connective Tissue

• Widely spaced cells separated by fibers and ground substance

• Most abundant and variable tissue type

• Functions– connects organs– gives support and protection (physical and

immune)– stores energy and produces heat– movement and transport of materials

Cells of Connective Tissue

• Fibroblasts produce fibers and ground substance• Macrophages phagocytize foreign material and

activate immune system– arise from monocytes (WBCs)

• Neutrophils wander in search of bacteria• Plasma cells synthesize antibodies

– arise from WBCs

• Mast cells secrete – heparin inhibits clotting– histamine that dilates blood vessels

• Adipocytes store triglycerides

Fibers of Connective Tissue• Collagen fibers (white fibers)

– tough, stretch resistant, yet flexible– tendons, ligaments and deep layer of the skin

• Reticular fibers– thin, collagen fibers coated with glycoprotein– framework in spleen and lymph nodes

• Elastic fibers (yellow fibers)– thin branching fibers of elastin protein– stretch and recoil like rubberband (elasticity)– skin, lungs and arteries stretch and recoil

Connective Tissue Ground Substance

• Gelatinous material between cells – absorbs compressive forces

• Consists of 3 classes of large molecules– glycosaminoglycans – chondroitin sulfate

• disaccharides that attract sodium and hold water• role in regulating water and electrolyte balance

– Proteoglycan (bottlebrush-shaped molecule) • create bonds with cells or extracellular

macromolecules

– adhesive glycoproteins• protein-carbohydrate complexes bind cell membrane

to collagen outside the cells

Fibrous Connective Tissue Types

• Loose connective tissue– gel-like ground substance between cells– types

• areolar• reticular• adipose

• Dense connective tissue– fibers fill spaces between cells– types vary in fiber orientation

• dense regular connective tissue• dense irregular connective tissue

Loose Connective: Areolar Tissue

• Loose arrangement of fibers and cells in abundant ground substance

• Underlies all epithelia, between muscles, passageways for nerves and blood vessels

Loose Connective: Reticular Tissue

• Loose network of reticular fibers and cells• Forms supportive stroma (framework) for

lymphatic organs• Found in lymph nodes, spleen, thymus and bone

marrow

Loose Connective: Adipose Tissue

• Empty-looking cells with thin margins; nucleus pressed against cell membrane

• Energy storage, insulation, cushioning– subcutaneous fat and organ packing– brown fat (hibernating animals) produces heat

Dense Regular Connective Tissue

• Densely, packed, parallel collagen fibers

– compressed fibroblast nuclei

• Tendons and ligaments hold bones together and attach muscles to bones

Dense Irregular Connective Tissue

• Densely packed, randomly arranged, collagen fibers and few visible cells– withstands stresses applied in different directions – deeper layer of skin; capsules around organs

Connective: Cartilage

• Supportive connective tissue with rubbery matrix

• Chondroblasts produce matrix– called chondrocytes once surrounded

• No blood vessels– diffusion brings nutrients and removes wastes– heals slowly

• Types of cartilage vary with fiber types– hyaline, fibrocartilage and elastic cartilage

Hyaline Cartilage

• Rubbery matrix; dispersed collagen fibers; clustered chondrocytes in lacunae– supports airway, eases joint movements

• Ends of bones at movable joints; sternal ends of ribs; supportive material in larynx, trachea, bronchi and fetal skeleton

Elastic Cartilage

• Hyaline cartilage with elastic fibers • Provides flexible, elastic support

– external ear and epiglottis

Fibrocartilage

• Hyaline cartilage with extensive collagen fibers (never has perichondrium)

• Resists compression and absorbs shock– pubic symphysis, meniscus and intervertebral discs

Connective: Bone

• Spongy bone - spongy in appearance– delicate struts of bone– covered by compact bone – found in heads of long bones

• Compact bone - solid in appearance– more complex arrangement– cells and matrix surround vertically oriented

blood vessels in long bones

Compact Bone

Bone Tissue (compact bone)

• Calcified matrix in lamellae around central canal• Osteocytes in lacunae between lamellae • Skeletal support; leverage for muscles; mineral

storage

• Variety of cells and cell fragments; some with nuclei and some without

• Nonnucleated pale pink cells or nucleated white blood cells

• Found in heart and blood vessels

Connective: Blood

Nerve Tissue

• Large cells with long cell processes– surrounded by smaller glial cells lacking processes

• Internal communication between cells– in brain, spinal cord, nerves and ganglia

Muscle Tissue

• Elongated cells stimulated to contract

• Exert physical force on other tissues– move limbs– push blood through a vessel– expel urine

• Source of body heat

• 3 histological types of muscle– skeletal, cardiac and smooth

Skeletal Muscle

• Long, cylindrical, unbranched cells with striations and multiple peripheral nuclei– movement, facial expression, posture, breathing,

speech, swallowing and excretion

Cardiac Muscle• Short branched cells

with striations and intercalated discs– one central nuclei per

cell

• Pumping of blood by cardiac (heart) muscle

Smooth Muscle

• Short fusiform cells; nonstriated with only one central nucleus– sheets of muscle in viscera; iris; hair follicles and

sphincters – swallowing, GI tract functions, labor contractions, control of

airflow, erection of hairs and control of pupil

• All cells (except blood) anchored to each other or their matrix by intercellular junctions

Intercellular Junctions

Tight Junctions• Encircle the cell joining it to surrounding cells

– zipperlike complementary grooves and ridges• Prevents passage between cells

– GI and urinary tracts

Desmosomes• Patch between cells holding them together

– cells spanned by filaments terminating on protein plaque• cytoplasmic intermediate filaments also attach to plaque

• Uterus, heart and epidermis

Gap Junctions

• Ring of transmembrane proteins form a water-filled channel

– small solutes pass directly from cell to cell

– in embryos, cardiac and smooth muscle

Glands• A gland is a cell or organ that secretes substances

in the body or releases them for elimination.• Secrete substances

– composed of epithelial tissue

• Exocrine glands connect to surface with a duct (epithelial tube)

• Endocrine glands secrete (hormones) directly into bloodstream

• Mixed organs do both – liver, gonads, pancreas

• Unicellular glands – endo or exocrine– goblet or intrinsic cells of stomach wall

Exocrine Gland Structure

• Stroma = capsule and septa divide gland into lobes and lobules

• Parenchyma = cells that secrete

• Acinus = cluster of cells surrounding the duct draining those cells

Types of Exocrine Glands

• Simple glands - unbranched duct• Compound glands - branched duct• Shape of gland

– acinar - secretory cells form dilated sac – tubuloacinar - both tube and sacs

Types of Secretions

• Serous glands– produce thin, watery secretions

• sweat, milk, tears and digestive juices

• Mucous glands– produce mucin that absorbs water to form a sticky

secretion called mucus

• Mixed glands contain both cell types

• Cytogenic glands release whole cells – sperm and egg cells

Holocrine Gland

• Secretory cells disintegrate to deliver their accumulated product– oil-producing glands of the scalp

• Merocrine glands release their product by exocytosis– tears, gastric glands,

pancreas, etc.• Apocrine glands are merocrine

glands with confusing appearance (apical cytoplasm not lost)– mammary and armpit sweat

glands

Merocrine and Apocrine Secretion

Mucous Membranes

• Epithelium, lamina propria and muscularis mucosae• Lines passageways that open to the exterior: reproductive,

respiratory, urinary and digestive– Mucous (movement of cilia) trap and remove foreign particles and

bacteria from internal body surfaces

Membrane Types

• Cutaneous membrane = skin– stratified squamous epithelium over connective tissue– relatively dry layer serves protective function

• Synovial membrane lines joint cavities– connective tissue layer only, secretes synovial fluid

• Serous membrane (serosa) –internal membrane– simple squamous epithelium over areolar tissue,

produces serous fluid– covers organs and lines walls of body cavities

Membranes

Synovial Membrane

Cutaneous Membrane

Serous Membrane

Tissue Growth

• Hyperplasia = tissue growth through cell multiplication

• Hypertrophy = enlargement of preexisting cells– muscle grow through exercise

• Neoplasia = growth of a tumor (benign or malignant) through growth of abnormal tissue

Changes in Tissue Types• Tissues can change types

• Differentiation– unspecialized tissues of embryo become

specialized mature types• mesenchyme to muscle

• Metaplasia– changing from one type of mature tissue to

another• simple cuboidal tissue before puberty changes to

stratified squamous after puberty

Stem Cells• Undifferentiated cells with developmental

plasticity• Embryonic stem cells

– totipotent (any cell type possible)• source = cells of very early embryo

– Pluripotent (tissue types only possible)• source = cells of inner cell mass of embryo

• Adult stem cells (undifferentiated cells in tissues of adults)– multipotent (bone marrow producing several blood

cell types)– unipotent (only epidermal cells produced)

Tissue Shrinkage and Death

• Atrophy = loss of cell size or number– disuse atrophy from lack of use (leg in a cast)

• Necrosis = pathological death of tissue– gangrene - insufficient blood supply– gas gangrene - anaerobic bacterial infection– infarction - death of tissue from lack of blood– decubitus ulcer - bed sore or pressure sore

• Apoptosis = programmed cell death– cells shrink and are phagocytized (no

inflammation)

Tissue Repair

• Regeneration– replacement of damaged cells with original cells– skin injuries and liver regenerate

• Fibrosis– replacement of damaged cells with scar tissue

• function is not restored– healing muscle injuries, scarring of lung tissue in TB or

healing of severe cuts and burns of the skin

– keloid is healing with excessive fibrosis (raised shiny scars)

Tissue Engineering

• Production of tissues and organs in the lab– framework of collagen or biodegradable polyester fibers– seeded with human cells– grown in “bioreactor” (inside of mouse)

• supplies nutrients and oxygen to growing tissue

• Skin grafts already available– research in progress on heart valves, coronary arteries,

bone, liver, tendons

Wound Healing of a Laceration

• Damaged vessels leak blood• Damaged cells and mast

cells leak histamine– dilates blood vessels– increases blood flow– increases capillary

permeability• Plasma carries antibodies,

clotting factors and WBCs into wound

Wound Healing of a Laceration

• Clot forms• Scab forms on

surface• Macrophages

start to clean up debris

Wound Healing of a Laceration

• New capillaries grow into wound

• Fibroblasts deposit new collagen to replace old material

• Fibroblastic phase begins in 3-4 days and lasts up to 2 weeks

Wound Healing of a Laceration

• Epithelial cells multiply and spread beneath scab

• Scab falls off• Epithelium thickens• Connective tissue

forms only scar tissue (fibrosis)

• Remodeling phase may last 2 years