Tissues

Histology

• The study of tissues
• Trillions of cells in the body, but only ~200 types
• Tissues, composed of cells, are visualized using stains:

Planes and Sections

• 3D objects can look very different when viewed in different sections!

​

• Tissues can be cut in cross section, longitudinal section or somewhere in between.

​

Features of Primary Tissue Types

• Tissues are a group of similar cells and cell products that arose from the same embryonic region
• Main differences between primary tissues types:
• Types and functions of cells
• Characteristics of the Extracellular Matrix (ECM; when present)
• Fibrous proteins
• Ground substance
• Clear gel, mostly water in most tissues
• Known as ECF and interstitial fluid most often
• Rubbery or stony in cartilage/bone
• Amount of space occupied by cells versus matrix

Embryonic Tissues

• When a sperm fertilizes an egg, a zygote is formed
• Divides into many cells that form layers in the early pre-embryo
• 3 Primary Germ Layers
• Ectoderm (outer)
• Forms epidermis and nervous system
• Mesoderm (middle)
• Forms mesenchyme that gives rise to muscle, bone, blood, and all other CTs
• Endoderm (inner)
• Forms mucous membrane lining GI tract, respiratory tract, digestive glands, and reproductive tract

Primary Tissue Types

Epithelial Tissue

• Creates a continuous covering
• Lines hollow organs, cavities, and ducts
• Forms glands

​

​

​

Connective Tissue

• Supports and binds structures
• Fills internal spaces
• Stores energy

Primary Tissue Types

Muscle Tissue

• Contracts to produce movement

Nervous Tissue

• Propagates electrical impulses
• Carries information

Epithelial Tissue Characteristics

• Closely packed cells forming continuous sheets
• Basal layer with cells adhered to basement membrane
• Apical free surface
• Avascular
• Well innervated
• Rapid cell division
• Can be either covering/lining or glandular

​

Epithelial Tissue Functions

• Protection—protect deeper tissues from injury and infection
• Secretion—produce and release mucus, sweat, enzymes, hormones, and other substances
• Excretion—void wastes from the tissues
• Absorption—absorb chemicals, such as nutrients
• Filtration—all substances leaving the body are selectively filtered by an epithelium
• Sensation—nerve endings in epithelia detect stimuli

Epithelial Tissue – Basement Membrane

• Basal lamina
• From epithelial cells
• Collagen fibers
• Reticular lamina
• Secreted by connective tissue cells
• Reticular fibers
• Holds cells to connective tissue
• Guide for cell migration during development
• Assists in filtration of leaked capillary fluids

​

Cell Junctions

• Occluding Junctions
• Tight Junctions
• Anchoring Junctions
• Adherens
• Anchor cells to one another and ECM
• Desmosomes
• Anchpr cells to other cells
• Hemidesmosomes
• Anchor cells to ECM
• Communicating Junctions
• Gap Junctions

​

Cell Junctions - Occluding

• Tight Junction
• Watertight seal between cells
• Plasma membranes fused with a zipper-like strip of proteins
• Common where substances must be kept in one space (stomach, intestines, blood brain barrier, parts of kidney)

Cell Junctions - Anchoring

• Adherens
• Holds epithelial cell together with plaque of proteins inside cell membrane
• Microfilaments form a band/belt around cell
• Integral membrane proteins connect to membrane of another cell
• Seem to give rise to complex 3D structure of some epithelial like in the GI tract
• Desmosomes
• Rivet-like structure anchored to intermediate filaments of cytoskeleton
• Strong cell to cell connection i.e. cardiac muscle
• Hemidesmosomes
• Half a desmosome
• Connects cell to ECM like basement membrane

​

​

Cell Junctions - Communicating

• Gap Junctions
• Channel between cells formed by ringlike connexon
• Connexon consists of transmembrane proteins arranged like segments of an orange around water-filled channel
• Ions, nutrients, and other small solutes pass between cells
• Located in cardiac and smooth muscle, embryonic tissue, lens, and cornea

​

Epithelial Tissue - Classification

• Named according to numer of layers
• Simple: One layer
• Stratified: Multiple layers
• Named according to shape
• Squamous: Scale-like; thin and flat
• Cuboidal
• Columnar

Simple Squamous ET

• Single row of thin cells
• Permits rapid diffusion or transport of substances
• Secretes serous fluid
• Locations: alveoli, glomeruli, endothelium, and serosa

​

Simple Cuboidal ET

• Single layer of square or round cells
• Absorption and secretion, mucus production and movement
• Locations: liver, thyroid, mammary and salivary glands, bronchioles, and kidney tubules

Simple Columnar ET

• Single row of tall, narrow cells
• Oval nuclei in basal half of cell
• Brush border of microvilli, sometimes ciliated, may possess goblet cells
• Absorption and secretion; secretion of mucus
• Locations: lining of GI tract, uterus, kidney, and uterine tubes

Ciliated Psuedostratified Columnar ET

• Looks multilayered, but all cells touch basement membrane
• Nuclei at several layers
• Has cilia and goblet cells
• Secretes and propels mucus
• Locations: respiratory tract and portions of male urethra

Keratinized Stratified Squamous ET

• Multiple cell layers; cells become flat and scaly toward surface
• Resists abrasion; retards water loss through skin; resists penetration by pathogenic organisms
• Locations: epidermis; palms and soles heavily keratinized

(Nonkeratinized) Stratified Squamous ET

• Same as keratinized epithelium without surface layer of dead cells
• Resists abrasion and penetration of pathogens
• Locations: tongue, oral mucosa, esophagus, and vagina

Stratified Squamous ET - Pap Smear

• Collect sloughed off cells of uterus and vaginal walls
• Detect cellular changes (precancerous cells)
• Semi-annually for women over 18 or if sexually active
• “Depending on which vaccine they received, HPV infections fell by 76% to 98% over 17 years among vaccinated women.”

Stratified Squamous ET - Pap Smear

Stratified Cuboidal ET

• Two or more cell layers; surface cells square or round
• Secretes sweat; produces sperm, produces ovarian hormones
• Locations: sweat gland ducts; ovarian follicles and seminiferous tubules

Urothelium (Transitional ET)

• Multilayered epithelium with surface cells that change from round to flat when stretched
• Allows for filling of urinary tract
• Locations: ureter and bladder

Epithelial Tissue - Glands

• Glands are collections of epithelial cells that produce secretions
• Endocrine glands
• Release hormones that enter bloodstream
• No ducts
• Exocrine glands
• Produce exocrine secretions 
• Discharge secretions through ducts onto epithelial surfaces

Epithelial Tissue - Glands

• Gland structure
• Unicellular glands
• Multicellular glands
• Unicellular glands
• Goblet cells are unicellular exocrine glands

Epithelial Tissue - Glands

• Multicellular exocrine glands are classified by
• Structure of the duct
• Simple (undivided)
• Compound (divided)
• Shape of secretory portion of the gland
• Tubular (tube shaped)
• Alveolar or acinar (blind pockets)
• Relationship between ducts and glandular areas
• Branched (several secretory areas sharing one duct)

Epithelial Tissue - Glands

• Methods of exocrine secretion:
• Merocrine secretion
• Released by secretory vesicles (exocytosis) 
• Example: merocrine sweat glands
• Apocrine secretion
• Released by shedding cytoplasm
• Example: mammary glands
• Holocrine secretion
• Released by cells bursting, killing gland cells
• Gland cells replaced by stem cells
• Example: sebaceous glands

Connective Tissue

• Most abundant primary tissue type
• Most variable tissue type
• Widely (usually) spaced cells separated by protein fibers and ground substance

Connective Tissue - Functions

• Binding of organs to one another
• Support the body physically; forms internal framework of organs
• Physical protection by cushioning and protecting internal organs
• Immune protection provided by specific CT cells attacking pathogens and cancerous cells
• Movement by providings levers that muscle can utilize
• Storage of fat, calcium, phosphorous
• Production of heat by brown adipose tissue
• Transport of gases, fluids, nutrients, wastes, hormones, and cells by blood

Connective Tissue - Cells

• Fibroblasts produce fibers and ground substance of matrix
• Macrophages arise from monocytes; phagocytize foreign material and activate immune system when they sense foreign matter (antigens)
• Leukocytes, or white blood cells (WBCs) function in immune defense
• Examples: Neutrophils attack bacteria; lymphocytes react against bacteria, toxins, and other foreign agents
• Plasma cells arise from lymphocytes and synthesize antibodies
• Mast cells secrete heparin to inhibit clotting and histamine to dilate blood vessels
• Adipocytes (fat cells) clustered in some fibrous tissues; where they dominate, called adipose tissue

​

Connective Tissue – Fibers

• Collagenous fibers made of collagen
• Tough, flexible, and stretch-resistant
• Called white fibers due to appearance in fresh tissue
• Tendons, ligaments, and deep layer of the skin are mostly collagen
• Less visible in the matrix of cartilage and bone
• Reticular fibers
• Thin collagen fibers coated with glycoprotein
• Form framework of spleen and lymph nodes; part of basement membranes under epithelia
• Elastic fibers
• Thinner than collagenous fibers; made of protein elastin
• Allows stretch and recoil like a rubber band

​

Connective Tissue - Types

• Two broad categories: Loose and dense connective tissue
• Loose connective tissue—mostly ground substance in space surrounding cells
• Types: areolar, adipose, and reticular
• Dense connective tissue—mostly fibers in space surrounding cells
• Types: dense regular and dense irregular

Loose CT - Areolar

• Loosely organized fibers, abundant blood vessels
• Possesses all six cell types and fiber types
• Underlies epithelia, in serous membranes, between muscles, passageways for nerves and blood vessels

Loose CT - Reticular

• Mesh of reticular fibers and fibroblasts
• Forms supportive stroma (framework) for lymphatic organs
• Found in lymph nodes, spleen, thymus, and bone marrow

Loose CT - Adipose

• Empty-looking cells with thin margins; nucleus pressed against cell membrane
• Energy storage, insulation, cushioning
• Brown fat produces heat

Loose CT - Brown and White Adipose

• Two types in humans: white (or yellow) fat and brown fat
• White adipose tissue (WAT, or white fat) is the most abundant and significant type in adults
• Provides thermal insulation
• Cushions organs such as eyeballs, kidneys
• Contributes to body contours—female breasts and hips
• Secretes hormones that regulate metabolism
• Brown adipose tissue (BAT, or brown fat) is found mainly in fetuses, infants, children
• Adults have smaller deposits of it
• Color comes from blood vessels, mitochondria, and mitochondrial enzymes
• Functions as a heat-generating tissue

Dense CT – Dense Regular

• Densely packed, parallel collagen fibers
• Compressed fibroblast nuclei
• Elastic tissue forms wavy sheets in some locations
• Tendons attach muscles to bones and ligaments hold bones together

Dense CT – Dense Irregular

• Densely packed, randomly arranged, collagen fibers and few visible cells
• Withstands unpredictable stresses
• Locations: deeper layer of skin; capsules around organs

Dense CT - Elastic

• Densely packed, layered collagen and elastin fibers
• Resists stress and provides the ability to stretch and recoil to much greater extent than other CTs
• Locations: Aorta, larger arteries, stroma of lung

Supporting Connective Tissue - Cartilage

• Functions
• Shock absorption
• Protection
• Cells
• Chondroblasts – Build cartilage
• Chondrocytes – Maintain cartilage
• Features
• Lacunae
• Avascular
• Perichondrium
• Outer fibrous layer (protection and support)
• Inner cellular layer (growth and maintenance)
• Matrix rich in collagen, hyaluronic acid, glycosoaminoglycans

Cartilage - Hyaline

• Clear, glassy appearance because of fineness of collagen fibers
• Eases joint movement, holds airway open, moves vocal cords, growth of juvenile long bones
• Locations: articular cartilage, costal cartilage, trachea, larynx, fetal skeleton

Cartilage - Elastic

• Cartilage containing abundance of elastic fibers
• Covered with perichondrium
• Provides flexible, elastic support
• Locations: external ear and epiglottis

Cartilage - Fibrocartilage

• Cartilage containing large, coarse bundles of collagen fibers
• Resists compression and absorbs shock
• Locations: pubic symphysis, menisci, growing bone, bone-tendon/ligament connection, and intervertebral discs

Supporting Connective Tissue - Bone

• Bone (osseous tissue) is a calcified connective tissue that composes the skeleton
• Two forms of osseous tissue:
• Spongy bone—delicate slivers and plates give it a spongy appearance
• Found in heads of long bones and in middle of flat bones such as the sternum
• Compact (dense) bone—denser calcified tissue with no visible spaces
• Forms external surfaces of all bones

​

Dense Bone

• Arranged in cylinders called osteons
• Central (osteonic) canal—passage running longitudinally along bone shaft; contains blood vessels and nerves
• Concentric lamellae—ringlike layers of bone surrounding central canal
• Osteon—central canal and its surrounding lamellae
• Osteocytes—mature bone cells within lacunae between lamellae
• Canaliculi—delicate canals radiating from each lacuna to its neighbors, allowing osteocytes to contact each other
• Periosteum—tough fibrous connective tissue covering the whole bone

​

​

Fluid Connective Tissue - Blood

• Blood—fluid connective tissue that travels through tubular blood vessels
• Transports cells and dissolved matter from place to place
• Contains formed elements suspended in a liquid ground substance called blood plasma
• Formed elements include:
• Erythrocytes (red blood cells, RBCs)—transport O₂ and CO₂
• Leukocytes (white blood cells, WBCs)—defend against infection and disease
• Examples: neutrophils, eosinophils, basophils, lymphocytes, monocytes
• Platelets—cell fragments involved in clotting

​

Muscle

• Muscular tissue—specialized to contract when stimulated, exerting a physical force on other tissues, organs, or fluids
• Excitable
• Also an important source of body heat
• Three types:
• Skeletal
• Cardiac
• Smooth

​

Muscle – Skeletal MT

• Made of muscle fibers—long thin cells
• Most skeletal muscles attach to bone
• Contains multiple nuclei adjacent to plasma membrane
• Striations—alternating dark and light bands
• Voluntary—conscious control over skeletal muscles

Muscle – Cardiac MT

• Limited to the heart wall
• Cardiomyocytes are short and branched with one centrally located nucleus
• Intercalated discs join cardiomyocytes end to end
• Striated and involuntary (not under conscious control)

Muscle – Smooth MT

• Made of short, fusiform-shaped cells
• Cells have one central nucleus, no striations
• Involuntary function
• Most is visceral muscle—making up parts of walls of hollow organs

Nervous Tissue

• Excitability—ability to respond to stimuli by changing membrane potential
• Nervous tissue
• Specialized for conducting electrical impulses
• Concentrated in the brain and spinal cord
• Types of cells in nervous tissue
• Neurons
• ​Neuroglia (supporting cells)

​

Nervous Tissue

• Parts of a neuron
• Neurosoma (cell body)
• Contains the nucleus and nucleolus
• Dendrites
• Short branches extending from the cell body
• Receive incoming signals
• Axon (nerve fiber)
• Long, thin extension of the cell body
• Carries outgoing electrical signals to their destination

​

Membranes

• Tissue membranes = physical barriers that line or cover body surfaces
• Generally consist of an epithelium supported by connective tissue
• Four types of tissue membranes
• Mucous membranes
• Serous membranes
• Cutaneous membrane
• Synovial membranes

​

Mucus Membranes (Mucosae)

• Line passageways that have external connections
• In digestive, respiratory, urinary, and reproductive tracts
• Epithelial surfaces must be moist
• To reduce friction
• To facilitate absorption or secretion
• Lamina propria is areolar tissue in mucous membranes

​

Serous Membranes (Serosa)

• Line cavities that do not open to the outside
• Thin but strong
• Parietal portion lines inner surface of cavity
• Visceral portion covers the organs
• Serous fluid reduces friction
• Three kinds:
• Peritoneum
• Pleura
• Pericardium

Cutaneous and Synovial Membranes

• Cutaneous membrane
• Skin that covers the body
• Thick, relatively waterproof, and usually dry
• Synovial membranes
• Line synovial joint cavities
• Movement stimulates production of synovial fluid for lubrication
• Lack a true epithelium

Tissue Growth

• Tissues grow by increasing the number of cells or size of existing cells
• Hyperplasia—growth through cell multiplication
• Most embryonic and childhood growth
• Hypertrophy—enlargement of preexisting cells
• Muscle growth through exercise
• Accumulation of body fat
• Neoplasia—development of a tumor (neoplasm) composed of abnormal, nonfunctional tissue
• May be benign or malignant
• Tissues can change types within certain limits
• Differentiation—development of more specialized form and function by unspecialized tissue
• Example: embryonic mesenchyme becoming cartilage and bone
• Metaplasia—changing from one type of mature tissue to another
• Simple cuboidal tissue of vagina before puberty changes to stratified squamous after puberty
• Pseudostratified columnar epithelium of bronchi of smokers to stratified squamous epithelium

​

​

Tissue Repair

• Damaged tissues can be repaired in two ways:
• Regeneration—replacement of dead or damaged cells by the same type of cell as before
• Restores normal function
• Examples: repair of minor skin or liver injuries
• Fibrosis—replacement of damaged cells with scar tissue
• Scar holds organs together, but does not restore function
• Examples: repair of severe cuts and burns, scarring of lungs in tuberculosis

Tissue Repair – 1^st Stage of Healing

• Severed vessels bleed into cut
• Mast cells and damaged cells release histamine that dilates blood vessels and makes capillaries more permeable
• Blood plasma seeps into the wound carrying antibodies and clotting proteins

Tissue Repair – 2^nd Stage of Healing

• Blood clot forms
• Knits edges of cut together
• Inhibits spread of pathogens
• Forms scab that temporarily seals wound and blocks infection
• Macrophages phagocytize and digest tissue debris

Tissue Repair – 3^rd Stage of Healing

• New capillaries sprout from nearby vessels
• Deeper portions of clot become infiltrated by capillaries and fibroblasts
• Transform into soft mass called granulation tissue (which can become a keloid)
• Macrophages remove the blood clot
• Fibroblasts deposit new collagen—fibroblastic (reconstructive phase)
• Begins 3–4 days after injury and lasts up to 2 weeks

Tissue Repair – 4^th Stage of Healing

• Epithelial cells around wound multiply and migrate beneath scab; epithelium regenerates
• Underlying connective tissue undergoes fibrosis
• Scar tissue may or may not show through epithelium
• This remodeling (maturation) phase begins several weeks after injury and may last up to 2 years

Tissue Repair – Scars and Healing

• Cleavage lines in dermis, when known, can be used to minimize post-surgery scaring
• Langer’s lines
• More information on Langer’s lines, reconstructive surgery, and the integumentary system

Tissue Degeneration and Death

• Atrophy—shrinkage of a tissue through loss in cell size or number, due to aging (senile atrophy) or lack of use (disuse atrophy)
• Necrosis—pathological tissue death
• Infarction—sudden death of tissue when blood supply is cut off
• Gangrene—tissue necrosis due to insufficient blood supply (usually involves infection)
• Dry gangrene: common complication of diabetes
• Decubitus ulcer (bed sore or pressure sore)—form of dry gangrene from continual pressure on skin
• Wet gangrene—liquefaction of internal organs with infection
• Gas gangrene—usually from infection of soil bacterium Clostridium that results in hydrogen bubbles in tissues
• Apoptosis (programmed cell death)—normal death of cells that have completed their function and best serve the body by dying and getting out of the way
• Phagocytized by macrophages and other cells
• Billions of cells die by apoptosis
• Every cell has a built-in “self-destruct program”