Four Major Tissue Types

• Tissues are the organization of similar cells with a common function.
• Histology studies tissues.
• The body has four major tissue types: epithelial, connective, muscle, and nervous.

Four Major Tissue Types

• Epithelial tissue provides protection, secretion, absorption, and excretion.
• Epithelial tissue covers body surfaces, lines internal organs and composes glands.
• Epithelial tissue lacks blood vessels, readily divides, and is tightly packed.
• Connective tissue binds, supports, protects, fills spaces, stores fat, and produces blood cells.
• Connective tissue is widely distributed throughout the body and mostly has a good blood supply.
• Cells in connective tissue are farther apart than in epithelial tissue, with an extracellular matrix in between.
• Muscle tissue is responsible for movement and is attached to bones, walls of hollow internal organs, and the heart.
• Muscle tissue can contract in response to specific stimuli.
• Nervous tissue conducts impulses for coordination, regulation, integration, and sensory reception.
• Nervous tissue is located in the brain, spinal cord, and nerves.
• Nervous tissues enable cells to communicate with each other and other body parts.

Intercellular Junctions

• Tight junctions create close spaces between cells by fusing cell membranes and can be found in cells lining the small intestine.
• Desmosomes bind cells by forming "spot welds" between cell membranes and are located in cells of the outer skin layer.
• Gap junctions form tubular channels between cells that allow the exchange of substances in muscle cells of the heart and digestive tract.

Nanotechnology and the Blood-Brain Barrier

• Nanotechnology aids drug delivery across the blood-brain barrier.
• The blood-brain barrier selects chemicals allowed to cross and protects against toxins and chemical fluctuations.
• Nanotechnology employs structures smaller than 100 nm in one direction to assist medication crossing the blood-brain barrier.
• Anesthetics or chemotherapeutics are combined with liposomes (phospholipid bubbles) to mask drug portions that cannot cross the barrier.
• Insulin can be inhaled in nanoparticles instead of injected.

General Characteristics of Epithelial Tissues

• Epithelial tissues cover organs and body surfaces.
• Epithelial tissues line cavities and hollow organs.
• Epithelial tissues make up glands.
• Epithelial tissues possess a free surface on the outside and a basement membrane on the inside.
• Epithelial tissues lack blood vessels, nutrients diffuse from underlying connective tissue (avascular).
• Epithelial tissues readily divide and heal rapidly.
• Cells in epithelial tissues are tightly packed.
• Epithelial tissues are classified by cell shape and number of cell layers: squamous, cuboidal, columnar, simple, stratified, or pseudostratified.

Simple Squamous Epithelium

• It is a single layer of thin, flat cells.
• Substances pass easily through air sacs (alveoli) and capillaries.
• Because it is thin and delicate, it can be easily damaged.
• It is located in diffusion and filtration sites.
• It makes up walls of air sacs (alveoli) and capillaries.
• It also lines blood and lymphatic vessels.

Simple Cuboidal Epithelium

• It has a single layer of cube-shaped cells.
• It functions with secretion and absorption.
• It lines kidney tubules, thyroid follicles, and ducts of some glands.
• It also covers ovaries.

Simple Columnar Epithelium

• It is composed of a single layer of elongated cells.
• Nuclei are usually at the same level, near the basement membrane.
• It sometimes has microvilli, cilia, and goblet cells, which secrete mucus.
• It functions with secretion and absorption.
• It lines the uterus, stomach, and intestines.

Pseudostratified Columnar Epithelium

• It consists of a single layer of cells, but appears layered.
• Nuclei are at two or more levels.
• Cells vary in shape, but all reach the basement membrane.
• It often has cilia and goblet cells.
• It functions with protection from infection.
• It lines respiratory passageways.

Stratified Squamous Epithelium

• It consists of many cell layers and is thick.
• Its primary function is protection.
• It has outermost cells that are squamous and deeper cells that are cuboidal.
• New cells are produced in deep layers, pushing older cells toward the free surface.
• The outer layer of skin, called the epidermis, is keratinized.
• It lines the oral cavity, vagina, and anal canal.

Stratified Cuboidal Epithelium

• It has two to three layers of cube-shaped cells.
• It offers more protection than a single layer.
• It also lines ducts of mammary, sweat, and salivary glands, and the pancreas.

Stratified Columnar Epithelium

• It has several layers of cells.
• The top layer has elongated cells.
• The deeper layers contain cube-shaped cells.
• It lines part of the male urethra and ducts of exocrine glands.

Transitional Epithelium (Uroepithelium)

• It has many cell layers, with cube-shaped and elongated cells.
• It changes shape and appearance with increased tension and stretches.
• It forms a barrier and lining that can expand.
• It lines the urinary bladder, ureters, and part of the urethra.

Glandular Epithelium Characteristics

• Glandular epithelium is composed of cells that produce and secrete substances into ducts or body fluids.
• There are two types of glands in the body: endocrine and exocrine.
• Endocrine glands secrete into tissue fluid or blood.
• Exocrine glands secrete into ducts that open onto a surface.
• The two structural types of exocrine glands are unicellular and multicellular.
• Unicellular glands are composed of one cell, such as a goblet cell (secretes mucus).
• Multicellular glands are composed of many cells and can be simple or compound, e.g., sweat and salivary glands.

Features of Exocrine Glands

• Simple exocrine glands do not have branching ducts.
• Compound exocrine glands have ducts that branch before reaching the secretory portion.
• Tubular exocrine glands consist of epithelial-lined tubes.
• Alveolar exocrine glands have terminal portions that form sac-like dilations.
• Merocrine glands secrete fluid products by exocytosis, such as salivary and sweat glands.
• Apocrine glands lose a small part of the cell during secretion, including mammary and ceruminous glands.
• Holocrine glands release entire cells filled with product, e.g., sebaceous glands.

Connective Tissues 1

• Connective tissue is the most abundant tissue type by weight.
• Cells in connective tissues are farther apart than epithelial cells.
• Connective tissues contain an abundant extracellular matrix between cells, consisting of protein fibers and a ground substance.
• Connective tissue consistency varies from fluid to solid.
• Connective tissue can be divided into two categories: connective tissue proper and specialized connective tissues.
• Most connective tissues have a good blood supply, so they are well-nourished.
• Most connective tissue cells have the ability to divide.
• Connective tissues contain fixed cells (remain in one place) and wandering cells, such as macrophages, which move throughout the tissues.

Connective Tissues 2

• Connective tissues bind structures together, provide support and protection, and serve as frameworks.
• Connective tissues fill spaces, store fat, and produce blood cells.
• Connective tissues protect against infections and help repair tissue damage.

The Body’s Glue: The Extracellular Matrix

• Normal ECM functions as scaffolding, organizing and anchoring cells into tissues and relaying chemical signals.
• In cancer, fibroblasts convert into myofibroblasts, loosening connections, and spreading cancer.
• In liver fibrosis, collagen deposition increases, damaging liver cells and possibly causing cirrhosis.
• Heart failure and atherosclerosis sometimes involve excess collagen deposition, which can stiffen the heart or block blood flow.

Major Cell Types of Connective Tissue

• Fibroblasts are the most common fixed cell in connective tissue.
• Fibroblasts are large, star-shaped cells that secrete fibers into the extracellular matrix.
• Macrophages (histiocytes) are usually attached to fibers but can detach to wander.
• Macrophages conduct phagocytosis and defend against infection.
• Mast cells are large, widely distributed cells in connective tissue.
• Mast cells release heparin to prevent blood clotting and histamine, which causes inflammatory response.

Connective Tissue Fibers

• Fibroblasts produce collagen, elastic, and reticular fibers.
• Collagen fibers are thick threads of collagen, which are the body's main structural protein.
• They have great tensile strength, are flexible, slightly elastic, can be found in ligaments and tendons.
• Elastic fibers are composed of elastin protein, branching and can stretch and return to their original shape.
• They are not as strong as collagen fibers, are found in vocal cords and respiratory air passages.
• Reticular fibers are thin, branching fibers of collagen.
• Reticular fibers form delicate, supporting networks and are found in the spleen and liver.

Components of Connective Tissue

• Fibroblasts secrete proteins that become fibers.
• Macrophages clear foreign particles from tissues by phagocytosis.
• Mast cells release substances that help prevent blood clotting (heparin) and promote inflammation (histamine).
• Collagen fibers hold structures together.
• Elastic fibers provide elasticity to parts that stretch.
• Reticular fibers form delicate, supportive networks within tissues.
• Ground substance fills in spaces around cells and fibers.

Categories of Connective Tissue

• Connective Tissue Proper includes loose and dense connective tissues.
• Loose connective tissues have fewer collagen fibers than dense tissues.
• Types of loose connective tissue consist of areolar, adipose, and reticular.
• Dense connective tissues have abundant collagen fibers.
• Types of dense connective tissue consist of dense regular, dense irregular, and elastic.
• Specialized connective tissues include cartilage, bone, and blood.

Abnormalities of Collagen

• Collagen makes up 60% of the protein in bone and cartilage, a large percentage of dry weight of skin, tendons, and ligaments.
• Precise structure is vulnerable to disruption.
• Chondrodysplasia is the asymmetric and too wide collagen chains causing stunted growth.
• Marfan syndrome is a deficiency of the protein fibrillin, which leads to long limbs, spindly fingers, sunken chest, weak aorta, and dislocation of the lens of the eye.

Areolar Connective Tissue

• It forms thin and delicate membranes.
• Cells in this tissue are mainly fibroblasts in a gel-like ground substance.
• It contains collagenous and elastic fibers.
• It is located in the subcutaneous layer and beneath most epithelia.
• It also nourishes nearby epithelial cells.

Adipose Tissue (Fat)

• Adipocytes, or fat cells, store fat.
• Adipocytes push their nuclei to one side and crowd out other cell types.
• It cushions and insulates.
• It is located beneath the skin (subcutaneous layer), behind eyeballs, around the heart and kidneys, and in spaces between muscles.

Reticular Connective Tissue

• It is composed of a network of thin reticular fibers.
• It supports the walls of internal organs.
• These walls can be found in the liver and spleen.

Dense Regular Connective Tissue

• It contains closely packed collagenous fibers.
• It features a fine network of elastic fibers.
• Most cells in this tissue are fibroblasts.
• It is very strong and withstands pulling forces.
• It binds body parts together.
• It is found in tendons, ligaments, and the dermis.
• It also has a limited blood supply and heals slowly.

Dense Irregular Connective Tissue

• It is characterized by randomly organized, thick, interwoven collagenous fibers.
• It can withstand tension exerted from different directions.
• It is found in the dermis of the skin and around skeletal muscles.

Elastic Connective Tissue

• It contains abundant yellow elastic fibers and some collagenous fibers.
• It features fibroblasts.
• It is located in attachments between bones of the spinal column.
• It is also located in the walls of hollow organs, such as large arteries and airways, and parts of the heart.
• It has elastic quality and stretches.

Specialized Connective Tissues - Cartilage

• Specialized connective tissues include cartilage, bone, and blood.
• They feature a unique matrix with specialized cells.
• Cartilage is a rigid, specialized connective tissue.
• It provides support, framework, and attachments.
• It functions in the protection of underlying tissue.
• It's a model for developing bone.
• The cartilage matrix contains collagen in a gel-like ground substance.
• Chondrocytes (cartilage cells) are contained in lacunae (chambers), surrounded by matrix.
• Cartilage lacks a blood supply; therefore, it heals slowly.
• It is covered by perichondrium (connective tissue), which provides some nutrients.
• There are three types of cartilage: hyaline, elastic, and fibrocartilage.

Three Types of Cartilage

• Hyaline cartilage is the most common type, contains fine collagen fibers in their matrix.
• Hyaline cartilage is located at the ends of bones, in joints, the nose, respiratory passages, and in the embryonic skeleton.
• Elastic cartilage is flexible due to elastic fibers in its matrix.
• Elastic cartilage is located in the external ear and larynx.
• Fibrocartilage is very tough due to many collagen fibers
• Fibrocartilage are shock absorbers found in intervertebral discs, pads (menisci) of the knee, and the pelvic girdle.

Specialized Connective Tissues - Bone (Osseous Tissue)

• Bone is the most rigid connective tissue.
• It has a solid matrix composed of mineral (Ca) salts and collagen.
• It supports structures and protects vital structures.
• Bone produces blood cells.
• The solid matrix stores and releases Ca and P.
• It provides attachment sites for muscles and forms the skeleton.
• Bone contains osteocytes (bone cells) in lacunae.
• Bone comes in two types: compact and spongy.

Bone Tissue

• Osteoblasts deposit matrix in lamellae (layers) in compact bone
• Lamellae occur in rings around central canals.
• Osteocytes, matrix, and central canals form cylindrical units called osteons.
• Osteocytes send out processes into canaliculi and share a blood supply.
• Osteons are cemented together to form compact bone.
• Central canals contain blood vessels and bones, well-nourished and heals more quickly than cartilage.
• Spongy bone makes up the interior part of bone and contains osteocytes, with space between them for marrow, and is lighter in weight than compact bone.

Specialized Connective Tissues - Blood

• Blood consists of formed elements (cells and fragments) suspended in a fluid matrix called plasma.
• There are three types of formed elements: red blood cells, white blood cells, and platelets. -Red blood cells transport gases. -White blood cells defend against infection. -Platelets help in blood clotting.
• Blood transports substances around the body.

Table 5.7 - Connective Tissues

• Areolar connective tissue, in a fluid-gel matrix, binds organs, located beneath the skin, between muscles, beneath epithelial tissues.
• Adipose tissue, in a fluid-gel matrix, protects, insulates, and stores fat, located beneath the skin, around the kidneys, behind the eyeballs, on the surface of the heart.
• Reticular connective tissue, in a fluid-gel matrix, supports, located in walls of liver and spleen.
• Dense regular connective tissue, in a fluid-gel matrix, binds body parts, located in tendons and ligaments.
• Dense irregular connective tissue, in a fluid-gel matrix, sustains tissue tension, located in the deep layer of skin.
• Elastic connective tissue, in a fluid-gel matrix, provides elastic quality, located in connecting parts of the spinal column, in walls of arteries and airways.
• Hyaline cartilage, in a solid-gel matrix, supports, protects, and provides framework, located at the ends of bones, nose, and rings in walls of respiratory passages.
• Elastic cartilage, in a solid-gel matrix, supports, protects, and provides flexible framework, located in the framework of the external ear and part of the larynx.
• Fibrocartilage, in a solid-gel matrix, supports, protects, and absorbs shock, located between bony parts of the spinal column and parts of pelvic girdle, and knee.
• Bone, in a solid matrix, supports, protects, and provides framework, located in bones of skeleton and middle ear.
• Blood, cells and platelets in fluid matrix, transports gases, defends against disease, and aids in blood clotting, located throughout the body in a closed system of blood vessels and heart chambers.

Membranes

• Membranes are sheets of cells.
• Epithelial membranes are composed of epithelial and connective tissue
• Epithelial membranes cover body surfaces and line cavities.
• There are three types of epithelial membranes: serous, mucous, and cutaneous.
• Serous membranes line body cavities that do not open to the outside of the body, such as the inner linings of the thorax and abdomen, and cover organs. The are composed of simple squamous epithelium and areolar connective tissue, secreting serous fluid for lubrication and reducing friction.
• Mucous membranes line cavities and tubes that open to the outside of the body, e.g., the lining of digestive, respiratory, urinary, and reproductive tracts, they are composed of the epithelium + areolar connective tissue and Goblet cells secrete mucus.
• Cutaneous membranes cover the body surface, commonly called skin, and are part of the integumentary system.
• Synovial membranes differ from epithelial membranes, are composed entirely of connective tissue, and line joint cavities.

Muscle Tissues

• General Characteristics: Excitable and respond to nervous stimulation Muscle cells or fibers are contractile, and can shorten and thicken.
• There are 3 types of muscle tissue: skeletal, cardiac, and smooth.
• Skeletal muscle tissue: Attached to bones, striated, voluntary and its ling, cylindrical, multinucleated cells are stimulated by nerve cells.
• Smooth muscle tissue: Non-striated, composed of spindle-shaped fibers, found in the walls of hollow organs and vessels, and is involuntary.
• Cardiac muscle tissue: Exists only in the heart wall, branching cells, involuntary, striated, and contains intercalated discs and specialized intercellular junctions.

Nervous Tissue

• Nervous tissues are located in the brain, spinal cord, and peripheral nerves.
• Neurons are main cells specialized for communication via nerve impulses (sensory reception, motor control).
• Neurons coordinate, integrate, and regulate body functions.
• Neurons are composed of a cell body (soma), dendrites, and an axon. Neuroglia cells support and nourish neurons.

Tissue Engineering

• Donor organs are in short supply.
• Tissue engineering replaces by growing cells/matrix from a person's own cells/matrix on a scaffold.
• Tissue engineering has already been used to provide skin, cartilage, bone, blood vessels and has no rejection because of the immune system.
• Urinary bladders replaced via tissue donated from patient with progenitor cells for smooth muscle and uroepithelium, grown on a synthetic dome.
• After implant synthetic scaffold degenerates, leaving new bladder in place.

Muscle and Nervous Tissues

• Skeletal muscle tissue is long and threadlike with many nuclei and striations, used for voluntary movements of skeletal parts, located in muscles that are usually attached to bones.
• Smooth muscle tissue is shorter with one central nucleus, used for involuntary movements of internal organs, located in the walls of hollow internal organs.
• Cardiac muscle tissue is branched, striated with one nucleus, used for heart movements, located in heart muscle.
• Nervous tissue consists of cells with cytoplasmic extensions, used to process sensory reception, neurotransmitter release, and conduct electrical impulses, located in the brain, spinal cord, and peripheral nerves.