Four Types of Tissue

Four Types of Tissue

• The human body contains trillions of cells that can be categorized into roughly 200 different types
• Cells work together in groups called tissues to carry out similar functions
• The four basic tissue types are: epithelial, connective, muscle, and nerve

Epithelial Tissue

• Covers exterior surfaces of the body
• Lines internal cavities and passageways
• Forms certain glands

Connective Tissue

• Binds cells and organs together
• Functions in protection, support, and integration

Muscle Tissue

• Excitable, responding to stimulation
• Contracts to provide movement

Nervous Tissue

• Excitable, allowing propagation of electrochemical signals
• Communicates between different regions of the body

Kilo Practice

• Kilo means to watch closely, examine, or observe
• Necessary to study the human organism on the tissue level with equipment such as a microscope
• Connects a histology slide of the skin to the gross cutaneous membrane (epidermis of the skin)
• Relative location has ties to a tissue’s function

Epithelial Tissues

• Divided into tissue that covers and lines body surfaces and hollow spaces in the body, and glandular tissue
• Composed of closely joined cells with minimal intercellular material
• Have a high mitotic rate
• Innervated but avascular; no blood supply permeates throughout the cells
• Closely bordered by connective tissue and adhered to it by way of a basement membrane

Basement Membrane

• Also called the basal lamina
• Consists of a layer of protein and glycoprotein fibers
• Anchors epithelial tissue to the underlying connective tissue
• Allows living cells to get their demands met by diffusion of substances in and out of the cell via nearby blood vessels

Apical Surface

• Exposed to the body cavity, lumen, internal organ, or exterior of the body

Basal Surface

• Faces the border with the connective tissue

Microvilli

• Tiny finger-like projections that increase the surface area of the apical cell membrane
• Promotes efficient diffusion or transport and the movement of substances across the apical membrane into or out of the cell

Cilia

• Mobile as they contain contractile proteins throughout their structure
• Beat back and forth in a rhythmic manner to promote the movement of substances along the apical surface
• respiratory epithelium lines the trachea and bronchi

The Mucociliary Escalator

• Combination of mucus(slippery substance secreted by goblet cells or larger glands) and cilia moving particles up the airways

Goblet Cells

• Unicellular glands that are dispersed among the other cells in epithelial tissue
• Found lining the gastrointestinal and respiratory tracts in simple columnar and pseudostratified epithelial tissue

Classification

• Identified and named by the number of cell layers (strata) and cell shape

Simple Epithelium

• Composed of a single layer

Simple Squamous

• Single layer of flattened epithelial cells
• Ideal for promoting diffusion, excretion, and absorption
• Found in the alveoli (air sacs) of the lungs and capillaries
• Part of the visceral and parietal serosal membranes of the body cavities, responsible for secreting serous fluid
• cells borders are all irregular in shape

Simple Cuboidal

• Single layer of cube-shaped cells
• Found where excretion and absorption take place
• Makes up various glands, ducts, and portions of the kidney tubules

Simple Columnar

• Column-like or rectangular shape
• Cells’ nuclei are elongated and closer to the basement membrane
• Lines the luminal surface of the gastrointestinal (GI) tract
• Involved with excretion and absorption
• Contains numerous microvilli on the apical surface to increase surface area promoting absorption

Stratified Epithelium

• Composed of more than one layer

Stratified Squamous

• May form several layers thick
• Found in areas where abrasion takes place and there are mechanical or chemical stresses on the tissue
• Functions as a protective barrier
• Makes up the surface of the skin, lining the inside of the mouth, esophagus, and anus
• Named based on the shapes of the cells on the apical surface of the tissue

Keratinized Stratified Squamous Epithelium

• Contains keratin, a water-resistant protein that minimizes water loss
• Found on the body surface (skin)

Non-Keratinized Stratified Squamous Epithelium

• Lacks keratin
• Found in moist areas inside the body where abrasion is a potential hazard, such as the esophagus, the vagina, the anal canal, and the urethra

Stratified Cuboidal

• Not common
• Found in ducts of various sweat glands permeating through various layers of the skin and portions of mammary gland ducts

Stratified Columnar

• Quite rare
• Found in limited areas of the pharynx (throat), epiglottis, and ducts of the parotid salivary glands
• May be 2 or multiple cell layers thick
• Only the cells facing the apical surface are columnar

Pseudostratified Epithelium

• Has the appearance of stratification because the nuclei are found at multiple levels
• All of the columnar cells are attached to the basement membrane and are therefore not truly stratified
• Typically has cilia
• Lines most portions of the respiratory tract
• Involved with protection, secretion of mucous, and movement of substances across its apical surface

Transitional Epithelium

• Tolerates stretch and recoil in repeated cycles
• When stretched, the apical cells appear flattened
• With recoil, the apical cells transition into a more cuboidal shape, often bulging out into the lumen
• Found in areas of stretching such as the urinary bladder and ureters

Intercellular Connections

• Cells are physically connected through various junctions

Gap Junctions

• Provide gaps between two touching cells
• Allow ions and molecules to move directly from one cell to another
• Made up of connexin and other proteins
• Found in cardiac muscles, and the epithelium of the skin

Desmosomes

• Act as molecular tethers that anchor epithelial cells to adjacent epithelial cells and the basement membrane
• Act as molecular tethers that anchor epithelial cells to adjacent epithelial cells and the basement membrane
• Made up of intermediate filaments and other fibrous and anchoring proteins
• Spot desmosomes tie epithelial cells to one another
• Hemidesmosomes anchor the epithelium to the underlying basement membrane
• Give epithelial tissue integrity and some resilience
• Found in cardiac muscle tissue.

Tight Junctions

• Prevent the paracellular movement of substances by forming a tight seal between cells
• Prevent any diffusion between cells or can provide some selective permeability
• Made up of several types of integral proteins tied to the cytoskeleton
• Occluding junctions are on the apical side of cell-cell junctions
• Adherens junctions are on the basal side of cell-cell junctions

Glandular Tissues

• Glands can be either single cells or multicellular organs
• Primarily composed of epithelial tissues that secrete substances

Endocrine Glands

• Secrete endocrines (hormones) into the blood to influence tissues in another part of the body
• Examples include the thyroid gland and the adrenal glands

Exocrine Glands

• An invaginated epithelium with supporting connective tissue
• Connects to the epithelial surface via a duct (epithelial-lined tube)
• Secrete into a body compartment or out of the body
• Examples include sweat glands and the glands that secrete digestive enzymes into the intestines

Merocrine Glands

• Package secretions into vesicles and release them through exocytosis
• Examples include lacrimal glands of the eyes, salivary glands of the oral cavity, pancreatic and gastric glands of the digestive system, and sweat glands
• Most common type of exocrine gland

Apocrine Glands

• Secrete by pinching off a piece of the plasma membrane
• Secretion contains cytoplasm and plasma membrane; more oily than merocrine secretions
• Found in the axillary and pelvic areas but also make up the mammary (breast) and ceruminous (ear) glands
• Some apocrine glands secrete using the merocrine process

Holocrine Glands

• Epithelial cells completely disintegrate and are replaced by other mitotically active cells
• Secretions consist of the cell and all of the material inside of it
• Oil-producing glands called Sebaceous glands are connected to hair follicles
• Secrete sebum, allowing hairs to grow through the follicle without ingrowing

Connective Tissue

• Specialized functions such as support, binding, and attachment of other tissues, protection of organs, energy storage, and body defenses against possible pathogens

Connective Tissue Components

• Specialized connective tissue cells
• Extracellular protein fibers
• Ground substances

Specialized Cells of Connective Tissue

• Surrounded by and suspended in the matrix (the environment in which the cell lives)

Matrix

• Combination of the ground substance (a viscous, gel-like solution of water, ions, glycoproteins, and proteoglycans) and protein fibers
• Specialized cells within the connective tissue secrete the ground substance and protein fibers

Naming Connective Tissue Cells

• Named for the connective tissue substance they produce
• Immature connective tissue cells have blasts in their suffix
• Mature, less active maintenance cells have cyte in their suffix (fibroblasts and fibrocytes)

Fibroblasts

• In abundance to secrete proteins and hyaluronan which combine to form the viscous proteoglycan
• Actively produce collagen, elastic, or reticular fibers, or a combination of the three

Fibrocytes

• Less active matrix secreters
• Function to monitor and maintain the matrix including existing protein fibers
• Involved with tissue repair

Macrophages

• Derived from a white blood cell known as a monocyte
• Differentiate into larger cells known as macrophages, literally known as “large eaters”
• Maybe fixed (residing in one place in the connective tissue) or free to wander throughout the connective tissue searching for any debris to remove
• Phagocytose debris or foreign substances and release chemicals that activate other cells of the immune system

Lymphocytes

• Able to roam through connective tissue
• Involved with the body’s defense system and provide the body with different types of immunity

Mast Cells

• Mobile cells associated with the immune system
• Most numerous around blood vessels, especially capillaries
• Injured or infected sights trigger the local mast cells to “dump” their numerous vesicles filled with histamine and heparin
• Further stimulates the inflammatory response in tissue

Adipocytes

• Fat cells that are specialized to store large amounts of triglyceride
• Have an almost limitless ability to expand to store more and more lipids
• Found in different concentrations in various connective tissues

Mesenchymal Cells

• Embryonic connective tissue cells that can differentiate into various connective tissue cells
• In adults, the connective tissue will still contain mesenchymal stem cells that become active in tissue repair in the case of injury

Collagen Fiber

• Most abundant type of fiber found in connective tissue proper
• Result of three protein subunits called tropocollagen coiled together like a rope
• Type I collagen has a great deal of strength, high tensile strength
• Ideal where there may be tissue subjected to pulling and high tension such as a tendon or ligament

Elastic Fibers

• Contain the protein elastin, with properties compared to that of a bungee cord
• When pulled from end to end elastic fibers will stretch and elongate
• When released they will recoil
• Found in tissues that are subjected to stretch
• Assembled on a precursor scaffolding made of a protein called fibrillin
• In Marfan syndrome, there is a mutation in the gene that codes for this protein leading to weakened elastic fibers

Reticular Fibers

• Type III collagen
• Form a delicate branching three-dimensional or scaffolding framework
• Creates a three-dimensional framework holding functional cells or parenchyma in place, giving organization to the cells of certain organs
• Found in reticular tissue in the liver, bone marrow, lymph nodes, and spleen

Embryonic Connective Tissue

• Gives rise to the various tissues mentioned below, with mesenchymal cells present in the developing embryo harboring the ability to differentiate into specific types of connective tissue cells
• In adults, connective tissue will still contain mesenchymal stem cells that become active in tissue repair in the case of injury

Connective Tissue Proper

• Classification of many different tissue types that are all produced by fibroblasts and contain various tissue properties
• Classified into loose and dense connective tissue, the degree to how densely packed the protein fibers appear

Loose Connective Tissue

• Types that have “loosely” packed fibers

Areolar Tissue

• Ground substance occupies the majority of the volume of the tissue which is permeated with loosely packed collagen, elastic and reticular fibers throughout
• Allows the tissue to stretch and distort compensating for changes in volume and length
• Sandwiched between the top and underlying layers of the skin
• Provides an effective passageway and scaffolding for blood vessels
• Commonly observed bordering underneath the avascular epithelial tissue, referred to as the lamina propria

Adipose Tissue

• Contains the same protein fibers as areolar, however, the vast majority of the volume is adipocytes, or fat cells
• Stores energy in the form of lipids
• Used as a “packing” material, surrounding organs such as the kidneys that are susceptible to shock or jarring

Reticular Tissue

• Contains mostly the three-dimensional branching reticular fibers
• Forms a three-dimensional connective tissue framework or stroma for the functional cells, or parenchyma of an organ

Dense Connective Tissue

• Contains more collagen fibers than does loose connective tissue
• Has greater resistance to stretching

Dense Regular Connective Tissue

• Has densely packed fibers (collagen) that are arranged in a regular fashion running parallel with each other
• High tensile strength
• Tendons connect muscle to bone
• Ligaments connects bone to bone

Dense Irregular Connective Tissue

• Permeated with mostly collagen fibers are not parallel to one another, they are dispersed in the tissue in random directions
• Provides strength when the tissue is being pulled in various directions
• Comprises a major part of the dermis, surrounding portions of bones, cartilage, joint, and organ capsules

Elastic Connective Tissue

• Contains densely packed arrangements of elastic fibers
• Found in areas that require elasticity allowing for movement such as the elastic ligaments that are found running up and down the vertebral column

Fluid Connective Tissues

Blood

• Connects all the tissues together by distributing nutrients and removing waste products
• Oxygen is distributed throughout the body carried by RBCs (red blood cells, aka erythrocytes (“red cells”))
• Plays an important role in your immune system by helping to distribute WBCs (white blood cells, aka leukocytes (“white cells”))
• Platelets (thrombocytes (“clotting cells”)) help plug holes in damaged blood vessels (blood clotting)
• RBCs, WBCs, and platelets are collectively called the formed elements of the blood
• Major fluid component called plasma that contains dissolved proteins, nutrients, wastes, hormones, and ions

Lymph

• Acellular fluid connective tissue found in the lymphatic system, a vascular system separate from the blood’s cardiovascular system
• Returns plasma that moved from capillaries to the tissues but did not reenter capillaries at the venous end
• Gets filtered through a series of lymph nodes to remove foreign particles and mount immune responses before it is returned to the cardiovascular system as plasma

Supporting Connective Tissues

Osseous

• Provides a framework for the body and assists with weight bearing
• Provides anchor sites for muscles providing movement
• Contains osteocytes (bone cells) and the ground matrix is a dense, hardened material containing calcium-phosphate crystals covering a helical arrangement of collagen called osteoid
• Osteocytes live in lacuna (little pockets) within the hardened osteoid
• Spongy bone looks like a sponge; it is very porous
• Compact bone is very dense
• Spongy bone is found on the interior of bone organs, whereas compact bone is found on the exterior regions
• Compact bone is made of cylindrical structures called osteons which are made of many circular plate-like layers (lamellae)
• Protects vital organs, stores minerals and performs hemopoietic functions"

Cartilage

• Ground matrix is between that of blood and bone; it is a semisolid extracellular matrix
• Collagen and elastic protein fibers are present along with chondrocytes (cartilage cells) that live in pockets called lacunae
• Cartilage is sometimes surrounded by a dense irregular CT covering called the perichondrium
• Stronger and more resilient than other connective tissues except for bone and is more flexible than bone
• Fully mature cartilage is avascular and is therefore slow to heal from injury

Hyaline Cartilage

• Most common type
• Has a clear glassy appearance with scattered cells in their lacunae
• Found in the nose, trachea, larynx, costals (ribs) and the articular (joint-forming) ends of long bones

Fibrocartilage

• Dense weight- and shock-absorbing pads
• More numerous parallel type I collagen fibers within the ground matrix provide the necessary resilience to resist compression
• Found in intervertebral discs, the pubic symphysis, and menisci of the knee and other joints

Elastic Cartilage

• Flexible due to more elastic fibers and fewer collagen fibers
• Can recoil (return to its original shape) when stretched
• Chondrocytes are more numerous and the ground matrix appears a dark purple in microscope images
• Found in the external ear and the epiglottis

Muscle Tissue

• Mainly characterized by its ability to contract

Skeletal Muscle

• Muscle fibers are long slender, cylinder-shaped cells with striations
• Arise from multiple germ cells fusing together, hence these cells are multinucleate
• Cells are parallel (aligned along their broadsides), attach to and pull on bones to move the body
• Described as voluntary

Cardiac Muscle

• Only found in the heart
• Cells are also striated but instead of being long parallel cylinders, they are branched, uninucleated, encircling heart to allow it to contract
• Involuntary

Smooth Muscle

• Found in the walls of hollow organs G.I. (gastrointestinal), respiratory, and reproductive tracts and also in the pupils of the eye and the arrector pili muscles attached to the hairs in your skin
• Spindle-shaped (fusiform, pointed at both ends) and non-striated
• Involuntary and uninucleated

Nervous Tissues

• Respond to various stimuli, resulting in the stimulation of other nervous tissues, contraction of muscles, and/or gland secretion

Neurons

• Functional cell type of nervous tissues
• Distinct soma (body) with projecting dendrites (finger-like processes) and a long thin axon
• Soma and dendrites react to stimuli by initiating electrical signals that spread down the axon causing the release of a chemical called a neurotransmitter from the terminus
• Neurotransmitters transmit signals to other neurons, muscle tissue, or glands

Neuroglia

• Glial cells that support, nourish, and bind neurons together

Body Membranes

• Membranes line or cover body cavities, viscera (organs), and the external surface

Mucous Membranes

• Line compartments that are exposed to the outside environment such as the digestive, respiratory, urinary, and reproductive tracts
• Perform adsorption and secretory functions in addition to protecting the underlying tissues
• The connective tissue component is referred to as the lamina propria
• The epithelial tissues include mucus-secreting goblet cells scattered amongst the epithelial cells and/or multicellular mucus-secreting glands in the submucosal connective tissue attached to the surface by ducts
• Mucin is a glycoprotein that combines with water to produce mucus which makes the surface of these mucous membranes slippery and slimy and protects the tissues from abrasion and dangerous substances or microorganisms

Serous Membranes

• Line body cavities not exposed to the outside environment such as the abdominopelvic or peritoneal cavity (peritoneal serous membranes), pleural cavity (pleural serous membranes), and pericardial cavity (pericardial serous membranes)
• Fold so that two layers are covering the organ; the layer closest to the organ or viscera is called the visceral layer, the layer on the outside that lines the cavity is called the parietal layer
• Made of a simple squamous epithelial layer called mesothelium that produces serous fluid that is secreted into the serous cavity- reduces friction between the organs

Synovial Membranes

• Line synovial joint cavities of the body
• Composed of areolar connective tissue
• Produce synovial fluid which helps to nourish the avascular articular cartilage covering the ends of the long bones and also creating a slippery surface for the bones to glide over one another during movements

Cutaneous Membranes

• Skin or integument that covers the surface of the human body
• Referred to as keratinized stratified squamous epithelium
• Generates a lot of the protein keratin
• Includes an underlying connective tissue layer (dermis)
• Protects against infection, loss of water and is also the body’s largest sensory organ

Tissue Development

Primary Germ Layers

• Ectoderm
• Mesoderm
• Endoderm
• Differentiate into all the various tissues of the body

Stem Cells

• Early developed cells that can differentiate into other cell types
• The zygote is omnipotent or totipotent (total potency) can divide to form any other cell type

Stem Cells:

- Pluripotent stem cells change into any cell type other than the placenta
- Multipotent stem cells are limited in their capacity to differentiate but can change into many other cell types
- Unipotent stem cells can divide to form one other cell type
- Terminally differentiated cells can form one cell type and cannot differentiate to become others

Stem Cells Differentiation

• Ectoderm forms nervous tissue and the epithelium of the skin
• Endoderm forms the epithelium lining the gastrointestinal and respiratory tracts
• Mesoderm forms all of the connective tissue, the lining of the blood vessels and muscles of the body

Tissue Change

• Can occur due to normal developmental processes or in response to insult or injury

Hypertrophy

• Tissue cells increase in size but not in number

Hyperplasia

• Cells increase in size

Atrophy

• Shrinkage of tissue either due to loss of cell numbers or size

Metaplasia

• Tissue morphology (appearance, shape) changes in response to a damaging stimulus and the transition is a precursor to cancer

Dysplasia

• There is a risk for cells changing in a way that is a precursor to cancer

Neoplasia

• When tissue has changed its characteristics and growth becomes unregulated
• Benign tumor (localized growth, not spreading)
• Malignant cancers are dangerous because the rapidly multiplying cells begin to metastasize (spread around the body to other tissues)

Necrosis

• Tissue death due to irreparable damage as a result of insult or injury or inflammation

Gangrene

• Necrosis of a tissue due to loss of blood supply to a tissue
  • Dry gangrene desiccated, shriveled tissue often caused by extreme cold
  • Wet gangrene is often caused by bacterial infection
  • Gas gangrene is gas produced by invading bacteria

Apoptosis

• Programmed cell death, an orderly form of cell death that does not result in inflammation