Tissues, Injury, Repair
Tissues are groups of similarly specialized cells that work together to perform a similar function.
Histology is the study of tissues.
There are 4 basic types of tissues: connective, muscular, epithelial, and nervous.
The tissues of the body develop from three primary germ layers: endoderm, mesoderm, and ectoderm.
Nervous tissue arises from ectoderm.
Muscle and connective tissues arise from mesoderm.
Epithelial tissues arise from all three germ layers.
Connective tissue protects, supports, and binds organs; stores energy as fat, and provides immunity.
Muscular tissue produces movement via contraction and generates body heat.
Epithelial tissue covers body surfaces, lines hollow organs and body cavities, and forms glands.
Nervous tissue detects changes in the body and responds by generating nerve impulses.
In some tissue types, adjacent cells are connected using a variety of intercellular junctions.
Occluding junctions include tight junctions.
Anchoring junctions include adherens junctions/belts, desmosomes, and hemi-desmosomes.
Communication junctions include gap junctions.
Tight junctions are found in epithelial tissue.
Tight junctions seal plasma membranes together and prevent materials from moving between cells and leaking out of organs.
Desmosomes are spot welds that connect the cytoskeletons of adjacent cells together.
Hemidesmosomes are spot welds that connect the cytoskeletons of a cell to the extracellular matrix.
Both types of junctions are widely distributed in tissues, especially those subjected to severe mechanical stress.
Gap junctions are found in all tissues where cells are in direct contact with one another.
Gap junctions allow various molecules, ions, and electrical impulses to directly pass through a regulated gate between cells.
Gap junctions allow rapid communication between cells.
Connective tissues are the most abundant and widely distributed tissues in the body.
Connective tissues perform numerous functions, including binding tissues together, supporting and strengthening tissue, protecting and insulating internal organs, compartmentalizing and transporting, and serving as energy reserves and immune responses.
Connective tissues are usually highly vascular and supplied with many nerves.
Connective tissues contain sparse cells separated by a non-living extracellular matrix.
The extracellular matrix is composed of a ground substance and fibers.
The ground substance is mostly water along with adhesion proteins and polysaccharide molecules.
The protein fibers include collagen (white, tensile strength), elastic (yellow, stretch), and reticular (form fine meshwork).
Connective tissue cell types include "blasts" (mitotically active secretory cells that lay down tissue) and "cytes" (mature cells).
Fibroblasts are the most numerous cell of connective tissues and secrete protein fibers.
Other cell types include chondroblasts and chondrocytes in cartilage, osteoblasts and osteocytes in bone, hematopoietic stem cells in bone marrow, and adipocytes, white blood cells, mast cells, and macrophages.
Connective tissue types include loose connective tissue (areolar, adipose, reticular), dense connective tissue (regular, irregular, elastic), cartilage (hyaline, fibrocartilage, elastic), bone (compact, spongy), and liquid connective tissue (blood, lymph).
Areolar tissue is the most widely distributed in the body and is a soft pliable tissue that wraps and cushions organs and soaks up excess fluid.
Adipose tissue is basically areolar tissue with lots of adipocytes and functions to insulate, support, protect, and act as an energy reserve.
Reticular connective tissue is a delicate network of interlacing reticular fibers and cells and forms a scaffolding used by cells of lymphoid tissues such as the lymph nodes, spleen, and bone marrow.
Areolar connective tissue is a soft packaging tissue of the body
It contains mucosa epithelium, lamina propria, fibers of matrix, and nuclei of fibroblasts
Adipose tissue is found in the subcutaneous layer beneath the skin
It contains vacuoles containing fat droplets and nuclei of fat cells
Reticular connective tissue forms a dark-staining network
It contains reticular cells, reticular fibers, and blood cells
Dense connective tissues contain very few cells and numerous thick, dense fibers
There are three types: dense regular connective tissue, dense irregular connective tissue, and elastic connective tissue
Dense regular connective tissue has collagen fibers arranged in parallel patterns and is found in tendons and ligaments
Dense irregular connective tissue has randomly arranged collagen fibers and is found in the dermis, ligaments, and tendons
Elastic connective tissue has freely branching elastic fibers and is found in arteries and lungs
Dense fibrous connective tissue is found in tendons
It contains collagen fibers and nuclei of fibroblasts
Dense irregular connective tissue is found in the reticular region of the dermis
It contains collagen fibers, fibroblasts, skin, and blood vessels
Elastic connective tissue is found in the aorta
It contains elastic fibers, the nucleus of fibroblasts, and the heart
Cartilage consists of a dense network of collagen and elastic fibers embedded in a gel-like ground substance
It contains few cells, such as chondrocytes, and has a poor blood supply
There are three types of cartilage tissue: hyaline cartilage, fibrocartilage, and elastic cartilage
Hyaline cartilage is the most abundant type and provides a smooth surface for joint movement
Fibrocartilage is a very strong, tough cartilage that absorbs shock
Elastic cartilage provides strength and elasticity
Hyaline cartilage is found in the trachea and has a matrix and chondrocytes in lacunae
Fibrocartilage is found in intervertebral discs and has collagen fibers and chondrocytes in lacunae
Elastic cartilage is found in the auricle of the ear and has perichondrium, the nucleus of chondrocytes, and elastic fibers in the ground substance
Bone tissue is composed of bone cells (osteocytes) in lacunae and a calcified ground substance
It is well vascularized and functions to support and protect body structures
There are two types of bone tissue: spongy bone and compact bone
Spongy bone consists of bone arranged in an irregular network of trabeculae and is found in the ends of long bones
Compact bone consists of a solid matrix of calcium and phosphate salts arranged in osteons and makes up the external layer of all bones
Bone tissue has lamellae, lacunae, and a central canal
Blood is composed of blood cells in a liquid matrix (plasma) and is found within the heart and blood vessels
Lymph is similar to blood plasma and contains lymph cells (lymphocytes) and is found in lymph vessels
Blood contains neutrophils, red blood cells, and monocytes
It functions to transport materials around the body and help fight disease
Loose connective tissue includes areolar, reticular, and adipose tissues
Dense connective tissue includes dense regular, dense irregular, and elastic tissues
Cartilage includes hyaline, fibrocartilage, and elastic cartilage
Bone tissue includes compact bone and spongy bone
Blood includes erythrocytes, leukocytes, and plasma
Both bone and blood tissues have specific functions and compositions
Epithelial tissues
Locations: body coverings, body linings, glandular tissue
Functions: protection, absorption, filtration, secretion
Structure:
Cells fit closely together and often form sheets; cells are held together by intercellular junctions
Have a free apical surface and a basal surface attached to a basement membrane and supported by a connective tissue reticular lamina
Good nerve supply
Avascular; blood vessels in the underlying connective tissue bring in nutrients and eliminate wastes
Can regenerate and heal easily if well nourished
Classification of Epithelial Tissues
Classified according to cell shape and the arrangement of layers
Cell shapes:
Squamous – flattened, tile-like
Cuboidal - cube-shaped
Columnar - column-like
Arrangement of layers:
Simple - one layer
Stratified - more than one layer
Simple Epithelia
Composed of a single thin layer of cells
Concerned primarily with movement/transport of substances from one body compartment to another by diffusion
Include:
Simple squamous epithelium
Simple cuboidal epithelium
Simple columnar epithelium
Pseudostratified columnar epithelium
Simple Squamous Epithelia
Composed of a single layer of flat cells
Locations:
Usually form membranes
Lines body cavities (forms part of serous membranes)
Lines heart, blood vessels, and lymphatic vessels (endothelium)
Lines alveoli in the lungs and the glomerular capsule of kidneys
Functions: very thin, so allows rapid diffusion, osmosis and filtration
Simple Cuboidal Epithelia
Composed of a single layer of cube-shaped cells
Locations:
Cubes make tubes
Common in glands and their ducts
Forms walls of kidney tubules
Covers the ovaries
Functions: secretion and absorption, ciliated types propel mucus or reproductive cells
Simple Columnar Epithelia
A single layer of column-like cells
Often interspersed with goblet cells (single-celled glands that produce mucus)
May have cilia or microvilli
Location: lines the digestive tract
Functions: secretion and absorption, with cilia - move mucus and other substances across the cell surface, with microvilli - involved in absorption (e.g. small intestine)
Pseudostratified Columnar Epithelia
Appears to have several layers but all cells are attached to the basement membrane
May contain cilia and goblet cells
Location: ciliated - upper respiratory tract, non-ciliated – sperm carrying ducts, ducts of large glands
Function: function in absorption or secretion (particularly of mucus), mucus traps inhaled particles & cilia move mucus up to the mouth where it can be swallowed or spat out
Stratified Epithelia
Contain 2 or more layers of cells
Are thicker and stronger than simple epithelia and typically act as a protective covering
Are named according to the shape of cells in the apical layer
Include:
Stratified squamous epithelium (widespread)
Stratified cuboidal epithelia (rare)
Stratified columnar epithelia (rare)
Stratified transitional epithelium (bladder)
Transitional Epithelium
Cells change shape depending on the state of stretch in the tissue
Location: found in the urinary system
Function: allow hollow structures (e.g. urinary bladder) to expand without causing damage to tissues
A gland is one or more cells that makes and secretes a product that contains protein molecules in an aqueous fluid.
Glands can be classified as endocrine or exocrine, and as unicellular or multicellular.
Endocrine glands are ductless and secrete hormones into the blood vessels.
Exocrine glands secrete their products through ducts to the epithelial surface or into the lumen of a hollow organ.
Endocrine glands secrete hormones into the blood.
Exocrine glands secrete their products through ducts to the epithelial surface.
Simple epithelia are a single thin layer of cells and are primarily concerned with the movement/transport of substances.
Different types of simple epithelia include simple squamous, simple cuboidal, and simple columnar.
Pseudostratified columnar epithelium appears to have several layers but all cells are attached to the basement membrane.
Stratified epithelia contain 2 or more layers of cells and act as a protective covering.
Different types of stratified epithelia include stratified squamous, stratified cuboidal, and stratified transitional.
Types of epithelial tissues include basement membrane, connective tissue, pseudostratified columnar, glandular, simple squamous, simple cuboidal, transitional, simple columnar, stratified columnar, and stratified squamous.
Muscular tissue consists of contractile cells called myocytes or muscle fibers.
There are 3 types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.
Skeletal muscle tissue contracts to pull on bones or skin and produces gross body movements or facial expressions.
Characteristics of skeletal muscle cells include long, cylindrical cells called muscle fibers, striations, multinucleate, and voluntary control.
Skeletal muscle tissue is represented by a diagram and photomicrograph.
Cardiac muscle tissue is found only in the myocardium of the heart and functions to pump blood.
Characteristics of cardiac muscle cells include being branched, attached to other cardiac muscle cells at intercalated disks, striations, uninucleate, and involuntary control.
Cardiac muscle tissue is represented by a diagram and photomicrograph.
Smooth muscle tissue is found in the walls of hollow organs such as the stomach, intestines, uterus, and blood vessels.
Characteristics of smooth muscle cells include spindle-shaped cells, no visible striations, uninucleate, and involuntary control.
Smooth muscle tissue is usually found in two layers: circular and longitudinal.
Smooth muscle tissue is represented by a diagram and photomicrograph.
Skeletal muscle is voluntary and contracts to pull on bones or skin.
Cardiac muscle is involuntary and found in the myocardium of the heart.
Smooth muscle is involuntary and found in the walls of hollow organs.
Summary: Muscular Tissue
Types of muscular tissue include cardiac muscle cells, skeletal muscle cells, and smooth muscle cells.
Nervous tissue is composed of neurons and nerve support cells called neuroglia.
The function of nervous tissue is to send impulses to other areas of the body.
Neurons conduct nerve impulses, analyze information, store memories, and direct the body's responses.
Neuroglia insulate, protect, and support neurons.
Nervous tissue is represented by a diagram and photomicrograph.
Summary: Tissue Types
Connective tissue has 5 types and includes loose CT, dense/fibrous CT, cartilage, bone, and liquid CT.
Muscular tissue has 3 types: skeletal, cardiac, and smooth muscle.
Epithelial tissue has many types and forms body coverings, linings, and glandular tissue.
Nervous tissue is composed of neurons and supporting cells.
Summary: Tissue Functions
Connective tissue protects, supports, binds organs, stores energy, and provides immunity.
Muscular tissue produces movement via contraction and generates body heat.
Epithelial tissue forms boundaries between different environments, protects, secretes, absorbs, filters.
Nervous tissue detects changes in the body and responds by generating nerve impulses.
Nervous tissue is responsible for internal communication in the brain, spinal cord, and nerves.
Muscular tissue contracts to cause movement in muscles attached to bones, muscles of the heart, and muscles of the walls of hollow organs.
Epithelial tissue forms boundaries between different environments, protects, secretes, absorbs, and filters in the skin surface, lining of GI tract organs, and other hollow organs.
Connective tissue supports, protects, and binds other tissues together in bones, tendons, fat, and other soft padding tissue.
There are 2 types of tissue repair: regeneration and fibrosis.
Regeneration is the replacement of destroyed tissue by the same kind of cells without scarring.
Fibrosis is the replacement of destroyed tissue by dense (fibrous) connective tissue, forming scar tissue.
Whether regeneration or fibrosis occurs depends on the type of tissue damaged and the severity of the injury.
Tissues that regenerate easily:
Epithelial tissue (skin and mucous membranes)
Loose connective tissues, bone and blood
In these tissues, parenchymal cells divide to replace damaged tissue with new tissue of the same type
Tissues that regenerate poorly and have limited capacity for tissue repair:
Nervous, muscle, dense connective tissue, and cartilage
In these tissues, damaged tissue is replaced largely with scar tissue reducing functionality
Carried out by fibroblasts
Fibroblasts are the most common cells in the body that maintain connective tissue and repair tissue damage
When an injury (or infection) occurs, fibroblasts:
Stop making collagen, change shape and travel to the area of injury
Release inflammatory products, destroying damaged tissue for phagocytosis
Then start making collagen and repair the area of damage with scar tissue
When finished, they migrate back to where they came from and return to their normal shape and function
Fibroblasts are the most common cells in connective tissue
Some fibroblasts are able to transform into any of the other types of connective tissue cells (regeneration)
Some fibroblasts make scar tissue (fibrosis)
Mesenchymal cells (in bone marrow and the periosteum) are multipotent adult stem cells that can differentiate into any type of connective tissue cells needed for regeneration to repair damaged tissue
Repair of dense connective and cartilage tissues in adults is limited by:
Intrinsic hypocellularity (lack of cells)
Dense extracellular matrix that limits cellular migration and local proliferation at an injury site
Haematopoietic stem cells found in bone marrow make blood cells
Skeletal muscle:
Cells/fibers cannot divide but can lay down new protein and enlarge (hypertrophy)
Contains stem cells called satellite cells found underneath the basal lamina
Capable of repairing limited damage
Cardiac muscle lacks stem cells for tissue regeneration
Muscle tissue has a relatively poor capacity for the repair of dead or damaged cells
Smooth muscle regenerates from stem cells called pericytes found in some blood vessels
Capable of slow and limited repair
Regenerates and repairs much more readily than skeletal and cardiac muscle tissue
Myofibrosis is the replacement of muscle tissue by connective tissues (scar tissue)
Epithelial covering and linings are often under constant heavy wear and tear and therefore must be highly regenerative
This occurs either by division and differentiation of stem cells (e.g., in the epidermis) or division of parenchymal cells (e.g., endothelial cells)
Glandular tissue:
Many exocrine glands have a continuous loss of cells which have to be constantly replaced by new ones (regeneration) e.g., the liver, sebaceous glands
Stem cells have been identified in some endocrine glands e.g., pituitary, adrenal, pancreas
Nerve cells are amitotic, therefore do not divide and cannot replace damaged cells
The PNS has the capacity for repair and regeneration
Axons are able to regrow as long as the cell body is intact and they have contact with the Schwann cells
The CNS is largely incapable of self-repair and regeneration
Damaged CNS tissue undergoes gliosis, the formation of scar tissue composed of glial cells
Certain areas of the adult brain possess neural stem cells, but their capability of repairing damage to neurons or neuroglia is still uncertain
Causes of tissue damage and injury include trauma, disease (homeostatic imbalance), or simple wear and tear
Physical trauma is an injury to living tissue caused by an extrinsic agent
Two main types of physical trauma are:
Blunt force trauma—when an object or force strikes the body, often causing hematoma and/or broken bones
Penetrating trauma—when an object pierces the skin or body, usually creating an open wound e.g., a needle or knife
Strains and sprains are caused by wear and tear
Blood vessels are often damaged as a result of wear and tear and physical trauma
Haemostasis is the stoppage of bleeding resulting from a break in a blood vessel in order to maintain blood volume
Haemostasis involves three phases:
Vascular spasms
Platelet plug formation
Coagulation (blood clotting)
Vascular spasms:
Vasoconstriction causes blood vessels to spasm and decreases blood loss
Platelet plug formation:
Collagen fibers are exposed by a break in a blood vessel
Platelets become "sticky" and cling to fibers
Anchored platelets release chemicals to attract more platelets (positive feedback)
Platelets pile up to form a platelet plug
Coagulation:
The blood is transformed from a liquid to a gel
Injured tissues release chemicals and calcium ions that trigger a clotting cascade
Prothrombin activator converts prothrombin to thrombin (an enzyme)
Thrombin joins fibrinogen proteins into insoluble fibrin
Fibrin forms a meshwork (the basis for a blood clot) which traps RBCs
Blood usually clots within 3 to 6 minutes
The clot remains as the endothelium regenerates
The clot is broken down after tissue repair by fibrinolysis
A collection of coagulated blood outside a blood vessel but within the body
A haemorrhage is active, ongoing bleeding
Can be seen under the skin or nails as bruises (aka contusions)
Can also happen deep inside the body where they may not be visible
During the healing process, oxygen-rich blood loses oxygen (red → purple/blue), then the RBCs degrade and haemoglobin breaks down biliverdin (green) → bilirubin (yellow) → haemosiderin (brown)
Phagocytosis clears the breakdown products from the area
There are two kinds of wound healing:
Epidermal wound healing occurs following superficial wounds that affect only the epidermis
Usually return to normal function
Deep wound healing occurs when an injury extends to the dermis and subcutaneous layer
Usually loss of some function and development of scar tissue
Inflammation and Haemostasis
Injured blood vessels bleed
Inflammatory chemicals are released
Haemostasis occurs in injured blood vessels
Uninjured capillaries become very permeable
Clotting proteins migrate into the area
A clot walls off the injured area
Organisation and blood supply restored
Growth of new capillaries
The blood clot is replaced with granulation tissue
Epithelium begins to regenerate
Fibroblasts produce collagen fibres to bridge the gap
Debris is phagocytized
Inflammation sets the stage:
Severed blood vessels bleed and inflammatory chemicals are released
Local blood vessels become more permeable, allowing white blood cells, fluid, clotting proteins, and other plasma proteins to seep into the injured area
Clotting occurs; surface dries and forms a scab
Organization restores the blood supply:
The clot is replaced by granulation tissue, which restores the vascular supply
Fibroblasts produce collagen fibers that bridge the gap
Macrophages phagocytize cell debris
Surface epithelial cells multiply and migrate over the granulation tissue
Regeneration and fibrosis
Regeneration of surface epithelium
Scab detaches
Fibrous tissue matures; epithelium thickens and begins to resemble adjacent tissue
Results in a fully regenerated epithelium with underlying scar tissue
Regeneration and fibrosis effect permanent repair:
The fibrosed area matures and contracts; the epithelium thickens
A fully regenerated epithelium with an underlying area of scar tissue results
Inflammation sets the stage:
Severed blood vessels bleed and inflammatory chemicals are released
Local blood vessels become more permeable, allowing white blood cells, fluid, clotting proteins, and other plasma proteins to seep into the injured area
Clotting occurs; surface dries and forms a scab
Organization restores the blood supply:
The clot is replaced by granulation tissue, which restores the vascular supply
Fibroblasts produce collagen fibers that bridge the gap
Macrophages phagocytize cell debris
Surface epithelial cells multiply and migrate over the granulation tissue
Regeneration and fibrosis effect permanent repair:
The fibrosed area matures and contracts; the epithelium thickens
A fully regenerated epithelium with an underlying area of scar tissue results
Fracture—break in a bone
Types of bone fractures
Closed (simple) fracture—break that does not penetrate the skin
Open (compound) fracture—broken bone penetrates through the skin
Bone fractures are treated by reduction and immobilization
Fracture type
Comminuted
Bone breaks into many fragments
Particularly common in older people, whose bones are more brittle
Compression
Bone is crushed
Common in porous bones (i.e., osteoporotic bones of older people)
Depressed
Broken bone portion is pressed inward
Typical of skull fracture
Impacted
Broken bone ends are forced into each other
Commonly occurs when one attempts to break a fall with outstretched arms
Spiral
Ragged break occurs when excessive twisting forces are applied to a bone
Common sports fracture
Greenstick
Bone breaks incompletely, much in the way a green twig breaks
Common in children, whose bones are more flexible than those of adults
Haematoma (blood-filled swelling) is formed
The break is splinted by fibrocartilage to form a callus
Phagocytes remove cellular debris and fibroblasts deposit collagen to form the callus
Fibrocartilage callus is replaced by a bony callus of spongy bone
Bony callus is remodeled to form a permanent patch
The spongy bone is replaced by compact bone
Hematoma
Fibrocartilage callus forms
Bony callus forms
Bone remodeling occurs
Tissue damage caused by excessive heat, electricity, radioactivity, or corrosive chemicals that denature the proteins in the skin cells
Immediate threat = Dehydration and electrolyte imbalance, leading to renal shutdown and circulatory shock
Burns are graded according to their severity
A first-degree burn involves only the epidermis
A second-degree burn destroys the epidermis and part of the dermis
A third-degree burn is a full-thickness burn (epidermis, dermis, and subcutaneous layer)
Critical if:
25% of the body has second-degree burns
10% of the body has third-degree burns
Third-degree burns on the face, hands, feet, or perineum
When the burn area >70%, more than half the victims die
The rule of nines is used to quickly estimate the surface area affected by a burn
Stretching or tearing of skeletal or cardiac muscle fibers
Classified depending on the severity of muscle fiber damage:
Grade I – mild: only a few muscle fibers are stretched or torn. Muscle is intact and has normal strength
Grade II – moderate: with a greater number of injured fibers. There is inflammation, loss of strength, and may be bruising (due to blood vessel damage)
Grade III - tears the muscle all the way through. Complete loss of muscle function. There is inflammation and bruising. May require surgery.
The stretch or tear of ligaments
Grade 1 - the ligament is stretched but not torn
Grade 2 - the ligament is partially torn. Can be inflammation and bruising
Grade 3 – the ligament is completely torn or ruptured. There is inflammation and bruising
In severe cases, joints can become unstable
Bones can move out of alignment
Joint may extend beyond its normal range of motion
Severe sprains sometimes require surgery to repair torn ligaments
Tissues are groups of similarly specialized cells that work together to perform a similar function.
Histology is the study of tissues.
There are 4 basic types of tissues: connective, muscular, epithelial, and nervous.
The tissues of the body develop from three primary germ layers: endoderm, mesoderm, and ectoderm.
Nervous tissue arises from ectoderm.
Muscle and connective tissues arise from mesoderm.
Epithelial tissues arise from all three germ layers.
Connective tissue protects, supports, and binds organs; stores energy as fat, and provides immunity.
Muscular tissue produces movement via contraction and generates body heat.
Epithelial tissue covers body surfaces, lines hollow organs and body cavities, and forms glands.
Nervous tissue detects changes in the body and responds by generating nerve impulses.
In some tissue types, adjacent cells are connected using a variety of intercellular junctions.
Occluding junctions include tight junctions.
Anchoring junctions include adherens junctions/belts, desmosomes, and hemi-desmosomes.
Communication junctions include gap junctions.
Tight junctions are found in epithelial tissue.
Tight junctions seal plasma membranes together and prevent materials from moving between cells and leaking out of organs.
Desmosomes are spot welds that connect the cytoskeletons of adjacent cells together.
Hemidesmosomes are spot welds that connect the cytoskeletons of a cell to the extracellular matrix.
Both types of junctions are widely distributed in tissues, especially those subjected to severe mechanical stress.
Gap junctions are found in all tissues where cells are in direct contact with one another.
Gap junctions allow various molecules, ions, and electrical impulses to directly pass through a regulated gate between cells.
Gap junctions allow rapid communication between cells.
Connective tissues are the most abundant and widely distributed tissues in the body.
Connective tissues perform numerous functions, including binding tissues together, supporting and strengthening tissue, protecting and insulating internal organs, compartmentalizing and transporting, and serving as energy reserves and immune responses.
Connective tissues are usually highly vascular and supplied with many nerves.
Connective tissues contain sparse cells separated by a non-living extracellular matrix.
The extracellular matrix is composed of a ground substance and fibers.
The ground substance is mostly water along with adhesion proteins and polysaccharide molecules.
The protein fibers include collagen (white, tensile strength), elastic (yellow, stretch), and reticular (form fine meshwork).
Connective tissue cell types include "blasts" (mitotically active secretory cells that lay down tissue) and "cytes" (mature cells).
Fibroblasts are the most numerous cell of connective tissues and secrete protein fibers.
Other cell types include chondroblasts and chondrocytes in cartilage, osteoblasts and osteocytes in bone, hematopoietic stem cells in bone marrow, and adipocytes, white blood cells, mast cells, and macrophages.
Connective tissue types include loose connective tissue (areolar, adipose, reticular), dense connective tissue (regular, irregular, elastic), cartilage (hyaline, fibrocartilage, elastic), bone (compact, spongy), and liquid connective tissue (blood, lymph).
Areolar tissue is the most widely distributed in the body and is a soft pliable tissue that wraps and cushions organs and soaks up excess fluid.
Adipose tissue is basically areolar tissue with lots of adipocytes and functions to insulate, support, protect, and act as an energy reserve.
Reticular connective tissue is a delicate network of interlacing reticular fibers and cells and forms a scaffolding used by cells of lymphoid tissues such as the lymph nodes, spleen, and bone marrow.
Areolar connective tissue is a soft packaging tissue of the body
It contains mucosa epithelium, lamina propria, fibers of matrix, and nuclei of fibroblasts
Adipose tissue is found in the subcutaneous layer beneath the skin
It contains vacuoles containing fat droplets and nuclei of fat cells
Reticular connective tissue forms a dark-staining network
It contains reticular cells, reticular fibers, and blood cells
Dense connective tissues contain very few cells and numerous thick, dense fibers
There are three types: dense regular connective tissue, dense irregular connective tissue, and elastic connective tissue
Dense regular connective tissue has collagen fibers arranged in parallel patterns and is found in tendons and ligaments
Dense irregular connective tissue has randomly arranged collagen fibers and is found in the dermis, ligaments, and tendons
Elastic connective tissue has freely branching elastic fibers and is found in arteries and lungs
Dense fibrous connective tissue is found in tendons
It contains collagen fibers and nuclei of fibroblasts
Dense irregular connective tissue is found in the reticular region of the dermis
It contains collagen fibers, fibroblasts, skin, and blood vessels
Elastic connective tissue is found in the aorta
It contains elastic fibers, the nucleus of fibroblasts, and the heart
Cartilage consists of a dense network of collagen and elastic fibers embedded in a gel-like ground substance
It contains few cells, such as chondrocytes, and has a poor blood supply
There are three types of cartilage tissue: hyaline cartilage, fibrocartilage, and elastic cartilage
Hyaline cartilage is the most abundant type and provides a smooth surface for joint movement
Fibrocartilage is a very strong, tough cartilage that absorbs shock
Elastic cartilage provides strength and elasticity
Hyaline cartilage is found in the trachea and has a matrix and chondrocytes in lacunae
Fibrocartilage is found in intervertebral discs and has collagen fibers and chondrocytes in lacunae
Elastic cartilage is found in the auricle of the ear and has perichondrium, the nucleus of chondrocytes, and elastic fibers in the ground substance
Bone tissue is composed of bone cells (osteocytes) in lacunae and a calcified ground substance
It is well vascularized and functions to support and protect body structures
There are two types of bone tissue: spongy bone and compact bone
Spongy bone consists of bone arranged in an irregular network of trabeculae and is found in the ends of long bones
Compact bone consists of a solid matrix of calcium and phosphate salts arranged in osteons and makes up the external layer of all bones
Bone tissue has lamellae, lacunae, and a central canal
Blood is composed of blood cells in a liquid matrix (plasma) and is found within the heart and blood vessels
Lymph is similar to blood plasma and contains lymph cells (lymphocytes) and is found in lymph vessels
Blood contains neutrophils, red blood cells, and monocytes
It functions to transport materials around the body and help fight disease
Loose connective tissue includes areolar, reticular, and adipose tissues
Dense connective tissue includes dense regular, dense irregular, and elastic tissues
Cartilage includes hyaline, fibrocartilage, and elastic cartilage
Bone tissue includes compact bone and spongy bone
Blood includes erythrocytes, leukocytes, and plasma
Both bone and blood tissues have specific functions and compositions
Epithelial tissues
Locations: body coverings, body linings, glandular tissue
Functions: protection, absorption, filtration, secretion
Structure:
Cells fit closely together and often form sheets; cells are held together by intercellular junctions
Have a free apical surface and a basal surface attached to a basement membrane and supported by a connective tissue reticular lamina
Good nerve supply
Avascular; blood vessels in the underlying connective tissue bring in nutrients and eliminate wastes
Can regenerate and heal easily if well nourished
Classification of Epithelial Tissues
Classified according to cell shape and the arrangement of layers
Cell shapes:
Squamous – flattened, tile-like
Cuboidal - cube-shaped
Columnar - column-like
Arrangement of layers:
Simple - one layer
Stratified - more than one layer
Simple Epithelia
Composed of a single thin layer of cells
Concerned primarily with movement/transport of substances from one body compartment to another by diffusion
Include:
Simple squamous epithelium
Simple cuboidal epithelium
Simple columnar epithelium
Pseudostratified columnar epithelium
Simple Squamous Epithelia
Composed of a single layer of flat cells
Locations:
Usually form membranes
Lines body cavities (forms part of serous membranes)
Lines heart, blood vessels, and lymphatic vessels (endothelium)
Lines alveoli in the lungs and the glomerular capsule of kidneys
Functions: very thin, so allows rapid diffusion, osmosis and filtration
Simple Cuboidal Epithelia
Composed of a single layer of cube-shaped cells
Locations:
Cubes make tubes
Common in glands and their ducts
Forms walls of kidney tubules
Covers the ovaries
Functions: secretion and absorption, ciliated types propel mucus or reproductive cells
Simple Columnar Epithelia
A single layer of column-like cells
Often interspersed with goblet cells (single-celled glands that produce mucus)
May have cilia or microvilli
Location: lines the digestive tract
Functions: secretion and absorption, with cilia - move mucus and other substances across the cell surface, with microvilli - involved in absorption (e.g. small intestine)
Pseudostratified Columnar Epithelia
Appears to have several layers but all cells are attached to the basement membrane
May contain cilia and goblet cells
Location: ciliated - upper respiratory tract, non-ciliated – sperm carrying ducts, ducts of large glands
Function: function in absorption or secretion (particularly of mucus), mucus traps inhaled particles & cilia move mucus up to the mouth where it can be swallowed or spat out
Stratified Epithelia
Contain 2 or more layers of cells
Are thicker and stronger than simple epithelia and typically act as a protective covering
Are named according to the shape of cells in the apical layer
Include:
Stratified squamous epithelium (widespread)
Stratified cuboidal epithelia (rare)
Stratified columnar epithelia (rare)
Stratified transitional epithelium (bladder)
Transitional Epithelium
Cells change shape depending on the state of stretch in the tissue
Location: found in the urinary system
Function: allow hollow structures (e.g. urinary bladder) to expand without causing damage to tissues
A gland is one or more cells that makes and secretes a product that contains protein molecules in an aqueous fluid.
Glands can be classified as endocrine or exocrine, and as unicellular or multicellular.
Endocrine glands are ductless and secrete hormones into the blood vessels.
Exocrine glands secrete their products through ducts to the epithelial surface or into the lumen of a hollow organ.
Endocrine glands secrete hormones into the blood.
Exocrine glands secrete their products through ducts to the epithelial surface.
Simple epithelia are a single thin layer of cells and are primarily concerned with the movement/transport of substances.
Different types of simple epithelia include simple squamous, simple cuboidal, and simple columnar.
Pseudostratified columnar epithelium appears to have several layers but all cells are attached to the basement membrane.
Stratified epithelia contain 2 or more layers of cells and act as a protective covering.
Different types of stratified epithelia include stratified squamous, stratified cuboidal, and stratified transitional.
Types of epithelial tissues include basement membrane, connective tissue, pseudostratified columnar, glandular, simple squamous, simple cuboidal, transitional, simple columnar, stratified columnar, and stratified squamous.
Muscular tissue consists of contractile cells called myocytes or muscle fibers.
There are 3 types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.
Skeletal muscle tissue contracts to pull on bones or skin and produces gross body movements or facial expressions.
Characteristics of skeletal muscle cells include long, cylindrical cells called muscle fibers, striations, multinucleate, and voluntary control.
Skeletal muscle tissue is represented by a diagram and photomicrograph.
Cardiac muscle tissue is found only in the myocardium of the heart and functions to pump blood.
Characteristics of cardiac muscle cells include being branched, attached to other cardiac muscle cells at intercalated disks, striations, uninucleate, and involuntary control.
Cardiac muscle tissue is represented by a diagram and photomicrograph.
Smooth muscle tissue is found in the walls of hollow organs such as the stomach, intestines, uterus, and blood vessels.
Characteristics of smooth muscle cells include spindle-shaped cells, no visible striations, uninucleate, and involuntary control.
Smooth muscle tissue is usually found in two layers: circular and longitudinal.
Smooth muscle tissue is represented by a diagram and photomicrograph.
Skeletal muscle is voluntary and contracts to pull on bones or skin.
Cardiac muscle is involuntary and found in the myocardium of the heart.
Smooth muscle is involuntary and found in the walls of hollow organs.
Summary: Muscular Tissue
Types of muscular tissue include cardiac muscle cells, skeletal muscle cells, and smooth muscle cells.
Nervous tissue is composed of neurons and nerve support cells called neuroglia.
The function of nervous tissue is to send impulses to other areas of the body.
Neurons conduct nerve impulses, analyze information, store memories, and direct the body's responses.
Neuroglia insulate, protect, and support neurons.
Nervous tissue is represented by a diagram and photomicrograph.
Summary: Tissue Types
Connective tissue has 5 types and includes loose CT, dense/fibrous CT, cartilage, bone, and liquid CT.
Muscular tissue has 3 types: skeletal, cardiac, and smooth muscle.
Epithelial tissue has many types and forms body coverings, linings, and glandular tissue.
Nervous tissue is composed of neurons and supporting cells.
Summary: Tissue Functions
Connective tissue protects, supports, binds organs, stores energy, and provides immunity.
Muscular tissue produces movement via contraction and generates body heat.
Epithelial tissue forms boundaries between different environments, protects, secretes, absorbs, filters.
Nervous tissue detects changes in the body and responds by generating nerve impulses.
Nervous tissue is responsible for internal communication in the brain, spinal cord, and nerves.
Muscular tissue contracts to cause movement in muscles attached to bones, muscles of the heart, and muscles of the walls of hollow organs.
Epithelial tissue forms boundaries between different environments, protects, secretes, absorbs, and filters in the skin surface, lining of GI tract organs, and other hollow organs.
Connective tissue supports, protects, and binds other tissues together in bones, tendons, fat, and other soft padding tissue.
There are 2 types of tissue repair: regeneration and fibrosis.
Regeneration is the replacement of destroyed tissue by the same kind of cells without scarring.
Fibrosis is the replacement of destroyed tissue by dense (fibrous) connective tissue, forming scar tissue.
Whether regeneration or fibrosis occurs depends on the type of tissue damaged and the severity of the injury.
Tissues that regenerate easily:
Epithelial tissue (skin and mucous membranes)
Loose connective tissues, bone and blood
In these tissues, parenchymal cells divide to replace damaged tissue with new tissue of the same type
Tissues that regenerate poorly and have limited capacity for tissue repair:
Nervous, muscle, dense connective tissue, and cartilage
In these tissues, damaged tissue is replaced largely with scar tissue reducing functionality
Carried out by fibroblasts
Fibroblasts are the most common cells in the body that maintain connective tissue and repair tissue damage
When an injury (or infection) occurs, fibroblasts:
Stop making collagen, change shape and travel to the area of injury
Release inflammatory products, destroying damaged tissue for phagocytosis
Then start making collagen and repair the area of damage with scar tissue
When finished, they migrate back to where they came from and return to their normal shape and function
Fibroblasts are the most common cells in connective tissue
Some fibroblasts are able to transform into any of the other types of connective tissue cells (regeneration)
Some fibroblasts make scar tissue (fibrosis)
Mesenchymal cells (in bone marrow and the periosteum) are multipotent adult stem cells that can differentiate into any type of connective tissue cells needed for regeneration to repair damaged tissue
Repair of dense connective and cartilage tissues in adults is limited by:
Intrinsic hypocellularity (lack of cells)
Dense extracellular matrix that limits cellular migration and local proliferation at an injury site
Haematopoietic stem cells found in bone marrow make blood cells
Skeletal muscle:
Cells/fibers cannot divide but can lay down new protein and enlarge (hypertrophy)
Contains stem cells called satellite cells found underneath the basal lamina
Capable of repairing limited damage
Cardiac muscle lacks stem cells for tissue regeneration
Muscle tissue has a relatively poor capacity for the repair of dead or damaged cells
Smooth muscle regenerates from stem cells called pericytes found in some blood vessels
Capable of slow and limited repair
Regenerates and repairs much more readily than skeletal and cardiac muscle tissue
Myofibrosis is the replacement of muscle tissue by connective tissues (scar tissue)
Epithelial covering and linings are often under constant heavy wear and tear and therefore must be highly regenerative
This occurs either by division and differentiation of stem cells (e.g., in the epidermis) or division of parenchymal cells (e.g., endothelial cells)
Glandular tissue:
Many exocrine glands have a continuous loss of cells which have to be constantly replaced by new ones (regeneration) e.g., the liver, sebaceous glands
Stem cells have been identified in some endocrine glands e.g., pituitary, adrenal, pancreas
Nerve cells are amitotic, therefore do not divide and cannot replace damaged cells
The PNS has the capacity for repair and regeneration
Axons are able to regrow as long as the cell body is intact and they have contact with the Schwann cells
The CNS is largely incapable of self-repair and regeneration
Damaged CNS tissue undergoes gliosis, the formation of scar tissue composed of glial cells
Certain areas of the adult brain possess neural stem cells, but their capability of repairing damage to neurons or neuroglia is still uncertain
Causes of tissue damage and injury include trauma, disease (homeostatic imbalance), or simple wear and tear
Physical trauma is an injury to living tissue caused by an extrinsic agent
Two main types of physical trauma are:
Blunt force trauma—when an object or force strikes the body, often causing hematoma and/or broken bones
Penetrating trauma—when an object pierces the skin or body, usually creating an open wound e.g., a needle or knife
Strains and sprains are caused by wear and tear
Blood vessels are often damaged as a result of wear and tear and physical trauma
Haemostasis is the stoppage of bleeding resulting from a break in a blood vessel in order to maintain blood volume
Haemostasis involves three phases:
Vascular spasms
Platelet plug formation
Coagulation (blood clotting)
Vascular spasms:
Vasoconstriction causes blood vessels to spasm and decreases blood loss
Platelet plug formation:
Collagen fibers are exposed by a break in a blood vessel
Platelets become "sticky" and cling to fibers
Anchored platelets release chemicals to attract more platelets (positive feedback)
Platelets pile up to form a platelet plug
Coagulation:
The blood is transformed from a liquid to a gel
Injured tissues release chemicals and calcium ions that trigger a clotting cascade
Prothrombin activator converts prothrombin to thrombin (an enzyme)
Thrombin joins fibrinogen proteins into insoluble fibrin
Fibrin forms a meshwork (the basis for a blood clot) which traps RBCs
Blood usually clots within 3 to 6 minutes
The clot remains as the endothelium regenerates
The clot is broken down after tissue repair by fibrinolysis
A collection of coagulated blood outside a blood vessel but within the body
A haemorrhage is active, ongoing bleeding
Can be seen under the skin or nails as bruises (aka contusions)
Can also happen deep inside the body where they may not be visible
During the healing process, oxygen-rich blood loses oxygen (red → purple/blue), then the RBCs degrade and haemoglobin breaks down biliverdin (green) → bilirubin (yellow) → haemosiderin (brown)
Phagocytosis clears the breakdown products from the area
There are two kinds of wound healing:
Epidermal wound healing occurs following superficial wounds that affect only the epidermis
Usually return to normal function
Deep wound healing occurs when an injury extends to the dermis and subcutaneous layer
Usually loss of some function and development of scar tissue
Inflammation and Haemostasis
Injured blood vessels bleed
Inflammatory chemicals are released
Haemostasis occurs in injured blood vessels
Uninjured capillaries become very permeable
Clotting proteins migrate into the area
A clot walls off the injured area
Organisation and blood supply restored
Growth of new capillaries
The blood clot is replaced with granulation tissue
Epithelium begins to regenerate
Fibroblasts produce collagen fibres to bridge the gap
Debris is phagocytized
Inflammation sets the stage:
Severed blood vessels bleed and inflammatory chemicals are released
Local blood vessels become more permeable, allowing white blood cells, fluid, clotting proteins, and other plasma proteins to seep into the injured area
Clotting occurs; surface dries and forms a scab
Organization restores the blood supply:
The clot is replaced by granulation tissue, which restores the vascular supply
Fibroblasts produce collagen fibers that bridge the gap
Macrophages phagocytize cell debris
Surface epithelial cells multiply and migrate over the granulation tissue
Regeneration and fibrosis
Regeneration of surface epithelium
Scab detaches
Fibrous tissue matures; epithelium thickens and begins to resemble adjacent tissue
Results in a fully regenerated epithelium with underlying scar tissue
Regeneration and fibrosis effect permanent repair:
The fibrosed area matures and contracts; the epithelium thickens
A fully regenerated epithelium with an underlying area of scar tissue results
Inflammation sets the stage:
Severed blood vessels bleed and inflammatory chemicals are released
Local blood vessels become more permeable, allowing white blood cells, fluid, clotting proteins, and other plasma proteins to seep into the injured area
Clotting occurs; surface dries and forms a scab
Organization restores the blood supply:
The clot is replaced by granulation tissue, which restores the vascular supply
Fibroblasts produce collagen fibers that bridge the gap
Macrophages phagocytize cell debris
Surface epithelial cells multiply and migrate over the granulation tissue
Regeneration and fibrosis effect permanent repair:
The fibrosed area matures and contracts; the epithelium thickens
A fully regenerated epithelium with an underlying area of scar tissue results
Fracture—break in a bone
Types of bone fractures
Closed (simple) fracture—break that does not penetrate the skin
Open (compound) fracture—broken bone penetrates through the skin
Bone fractures are treated by reduction and immobilization
Fracture type
Comminuted
Bone breaks into many fragments
Particularly common in older people, whose bones are more brittle
Compression
Bone is crushed
Common in porous bones (i.e., osteoporotic bones of older people)
Depressed
Broken bone portion is pressed inward
Typical of skull fracture
Impacted
Broken bone ends are forced into each other
Commonly occurs when one attempts to break a fall with outstretched arms
Spiral
Ragged break occurs when excessive twisting forces are applied to a bone
Common sports fracture
Greenstick
Bone breaks incompletely, much in the way a green twig breaks
Common in children, whose bones are more flexible than those of adults
Haematoma (blood-filled swelling) is formed
The break is splinted by fibrocartilage to form a callus
Phagocytes remove cellular debris and fibroblasts deposit collagen to form the callus
Fibrocartilage callus is replaced by a bony callus of spongy bone
Bony callus is remodeled to form a permanent patch
The spongy bone is replaced by compact bone
Hematoma
Fibrocartilage callus forms
Bony callus forms
Bone remodeling occurs
Tissue damage caused by excessive heat, electricity, radioactivity, or corrosive chemicals that denature the proteins in the skin cells
Immediate threat = Dehydration and electrolyte imbalance, leading to renal shutdown and circulatory shock
Burns are graded according to their severity
A first-degree burn involves only the epidermis
A second-degree burn destroys the epidermis and part of the dermis
A third-degree burn is a full-thickness burn (epidermis, dermis, and subcutaneous layer)
Critical if:
25% of the body has second-degree burns
10% of the body has third-degree burns
Third-degree burns on the face, hands, feet, or perineum
When the burn area >70%, more than half the victims die
The rule of nines is used to quickly estimate the surface area affected by a burn
Stretching or tearing of skeletal or cardiac muscle fibers
Classified depending on the severity of muscle fiber damage:
Grade I – mild: only a few muscle fibers are stretched or torn. Muscle is intact and has normal strength
Grade II – moderate: with a greater number of injured fibers. There is inflammation, loss of strength, and may be bruising (due to blood vessel damage)
Grade III - tears the muscle all the way through. Complete loss of muscle function. There is inflammation and bruising. May require surgery.
The stretch or tear of ligaments
Grade 1 - the ligament is stretched but not torn
Grade 2 - the ligament is partially torn. Can be inflammation and bruising
Grade 3 – the ligament is completely torn or ruptured. There is inflammation and bruising
In severe cases, joints can become unstable
Bones can move out of alignment
Joint may extend beyond its normal range of motion
Severe sprains sometimes require surgery to repair torn ligaments
