Tissues, types and Intercellular Junctions

• Cells are organized into tissues in complex organisms.
• A tissue consists of similar cells performing a shared function.
• Histology is the study of tissues.
• The body has four major tissue types: epithelial, connective, muscle, and nervous.

Four Major Tissue Types:

• Epithelial tissue protects, secretes, absorbs, and excretes; it covers body surfaces, lines internal organs, and forms glands.
• Connective tissue binds, supports, and protects, fills spaces, stores fat, and produces blood cells. It is distributed widely.
• Muscle tissue facilitates movement and is present in bones, hollow organ walls, and the heart.
• Nervous tissue conducts impulses for coordination, regulation, integration, and sensory reception and is present in the brain, spinal cord, and nerves.

Intercellular Junctions:

• Tight junctions close the space between cells by fusing cell membranes; an example is cells lining the small intestine.
• Desmosomes bind cells by forming "spot welds" between cell membranes; an example is cells of the outer skin layer.
• Gap junctions form tubular channels between cells for substance exchange; an example is muscle cells of the heart and digestive tract.

General Characteristics of Epithelial Tissues:

• Epithelial tissues cover organs and body surfaces.
• Epithelial tissues line cavities and hollow organs.
• Epithelial tissues comprise glands.
• Epithelial tissues have a free surface on the outside and a basement membrane on the inside.
• Epithelial tissues lack blood vessels (are avascular).
• Nutrients diffuse to from underlying connective tissue to epithelial tissue
• Epithelial tissue cells divide readily, allowing injuries to heal rapidly.
• Cells are tightly packed in epithelial tissues.
• Epithelial tissues are classified by cell shape and number of cell layers.
  • Shapes consist of squamous (flat), cuboidal (cube-shaped), and columnar (tall).
  • Layers consist of simple (one layer), stratified (two or more layers), or pseudostratified (appears layered but is not).

Simple Squamous Epithelium:

• Single layer of thin, flat cells
• Allows substances to pass easily through air sacs (alveoli) and capillaries.
• Is thin, delicate, and easily damaged.
• Found in diffusion and filtration sites.
• Makes up walls of air sacs (alveoli) and capillaries.
• Lines blood and lymphatic vessels.

Simple Cuboidal Epithelium:

• Single layer of cube-shaped cells
• Involved in secretion and absorption.
• Lines kidney tubules, thyroid follicles, and ducts of some glands.
• Covers ovaries.

Simple Columnar Epithelium:

• Single layer of elongated cells.
• Nuclei usually at the same level, near the bottom of the basement membrane.
• Sometimes possesses microvilli, cilia, or goblet cells (which secrete mucus).
• Involved in secretion and absorption.
• Lines the uterus, stomach, and intestines.

Pseudostratified Columnar Epithelium:

• Single layer, but appears layered.
• Nuclei are at two or more levels.
• Cells vary in shape, but all touch the basement membrane.
• Often possesses cilia and goblet cells.
• Provides protection from infection.
• Lines respiratory passageways.

Stratified Squamous Epithelium:

• Many cell layers; thick.
• Protective layer.
• Outermost cells are squamous, deeper cells are cuboidal.
• New cells produced in deep layers push older cells toward free surface.
• Outer layer of skin, epidermis, is keratinized.
• Lines the oral cavity, vagina, and anal canal.

Stratified Cuboidal Epithelium:

• Two to three layers of cube-shaped cells.
• Provides more protection than one layer.
• Lines ducts of mammary glands, sweat glands, salivary glands, and the pancreas.

Stratified Columnar Epithelium:

• Several layers of cells.
• The top layer made of elongated cells.
• Cube-shaped cells in deeper layers.
• Lines part of the male urethra and ducts of exocrine glands.

Transitional Epithelium (Uroepithelium):

• Many cell layers.
• Composed of cube-shaped and elongated cells.
• Changes shape and appearance with increased tension and stretches.
• Forms a barrier and expansible lining.
• Lines the urinary bladder, ureters, and part of the urethra.

Epithelial Tissues:

• Simple squamous epithelium consists of a single layer of flattened cells where filtration, diffusion, and osmosis occur, covering surfaces in locations such as air sacs of the lungs and walls of capillaries.
• Simple cuboidal epithelium, made of a single layer of cube-shaped cells, is involved in protection, secretion, and absorption such as occurring on the surface of ovaries and lining kidney tubules.
• Simple columnar epithelium features a single layer of elongated cells for protection, secretion, and absorption, lining the uterus, stomach, and intestines
• Pseudostratified columnar epithelium consists of a single layer of elongated cells involved in protection, secretion, and mucus movement with locations in the lining of respiratory passages.
• Stratified squamous epithelium has many layers in the skin and lines the oral cavity featuring protection on top flattened cells
• Stratified cuboidal epithelium with 2 - 3 layers of cube-shaped cells provides protection by lining ducts such as ducts of mammary glands
• Stratified columnar epithelium with the top layer of elongated cells and lower cube-shaped cells protects and secretes in larger ducts of excretory glands
• Transitional epithelium which includes many cube-shaped and elongated cells creates a stretchable and protective inner lining of the urinary bladder, ureters, and part of the urethra

Glandular Epithelium:

• Contains cells that produce and secrete substances into ducts or body fluids.

Two types of glands:

• Endocrine glands secrete into tissue fluid or blood.
• Exocrine glands secrete into ducts that open onto a surface.

Two structural types of exocrine glands:

• Unicellular glands consist of one cell, such as a goblet cell (secreting mucus).
• Multicellular glands consist of many cells and can be simple or compound.

Structural types of exocrine glands:

• Simple glands features that duct does not branch
• Compound glands features duct that branches before it reaches secretory portion
• Tubular glands consist of epithelial-lined tubes
• Alveolar glands features its terminal portions in sac-like dilations

Types of glandular secretion:

• Merocrine glands secrete fluid products by exocytosis such as salivary and sweat glands, and the pancreas.
• Apocrine glands lose a small part of their cells during secretion such as mammary and ceruminous glands.
• Holocrine glands release entire cells filled with product such as sebaceous glands.

Types of Exocrine Glands:

• Unicellular glands are single secretory mucus-secreting cells.
• Multicellular glands consist of many cells.
• Simple Glands communicate with the surface through unbranched ducts.
  • The simple tubular gland is a straight tube that opens directly onto the surface which makes up intestinal glands within the small intestine.
  • The simple branched tubular gland is a branched tube gland with a duct being short or absent and is apart of the gastric glands.
  • The simple coiled tubular gland is a long coiled tube-like gland with a long duct and its the merocrine sweat glands in the skin.
  • The simple branched alveolar gland consists of the secretory portions of gland expanding into sac like compartments the integumentary system.
• Compound glands communicate with the surface via repeatedly branching ducts.
  • The compound tubular gland is where Secretory portions are tubules extending from branches of branches that combine into one duct. An example is the bulbourethral glands of the male (see Figure 22.4) An example is the sebaceous gland of skin
  • The compound alveolar gland is where secretory portions are irregularly branched tubules with numerous saclike outgrowths. Mammary glands (see Figure 23.28)

General characteristics of connective tissues:

• Most abundant tissue type by weight.
• Cells are farther apart than epithelial cells.
• Contain abundant extracellular matrix between cells made of protein fibers and a ground substance.
• Consistency varies from fluid to solid.

Categories of connective tissues:

• Connective tissue proper.
• Specialized connective tissues.
• Most have a good blood supply, so are well-nourished.
• Most cells can divide.
• Contain fixed cells, which remain in one place, and wandering cells move throughout tissues.

Functions of connective tissues:

• Bind structures together.
• Provide support and protection.
• Serve as frameworks.
• Fill spaces.
• Store fat.
• Produce blood cells.
• Protect against infections.
• Help repair tissue damage.

Major Cell Types of Connective Tissue:

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

Connective Tissue Fibers:

• Fibroblasts produce three types of fibers in connective tissue.
• Collagen Fibers are thick threads of collagen which give the body its main structural protein. They are strong and flexible.
• Elastic Fibers are composed of elastin protein, stretch and return to original shape, as well as being found in vocal cords and respiratory air passages.
• Recticular Fibers are thin, branching fibers of collagen. They form delicate supporting networks and are found in the spleen and liver.

Components of Connective Tissue:

• Fibroblasts are widely distributed, large, star-shaped cells that secrete proteins that become fibers.
• Macrophages are mobile cells that clear foreign particles from tissues by phagocytosis.
• Mast cells are large cells near blood vessels that release substances to prevent blood clotting (heparin) and to promote inflammation (histamine).
• Collagen fibers (white fibers) are thick, threadlike fibers of collagen with great tensile strength, holding structures together.
• Elastic fibers (yellow fibers) consist of bundles of microfibrils embedded in proteins, which provide elastic quality to parts that stretch.
• Reticular fibers are thin fibers of collagen that form supportive networks within tissues.
• Ground substance consists of a nonfibrous protein and other molecules, which fills space around cells and fibers.

Categories of Connective Tissue:

• Connective Tissue Proper has loose and dense connective tissues.
  • Loose connective tissue has fewer collagen fibers than dense tissues.
    • Types: Areolar, Adipose, Reticular. Example: Beneath most epithelia, where it nourishes nearby epithelial cells.
  • Dense connective tissues contain abundant collagen fibers.
    • Types: Dense regular, Dense irregular, Elastic. Example: Tendons, ligaments, dermis
• Specialized connective tissues have Cartilage, Bone, and Blood. Example: The inner most lining of Bone (Spongy)

Clinical Application of Connective Tissue Proper:

• Collagen makes up >60% of protein in bone and cartilage.
• Collage chains are vulnerable to disruption.
• Chondrodysplasia where collagen chains are asymmetric cause stunted growth and deformed joints
• Marfan syndrome a deficiency of fibrillin which leads to long limbs, sinkin chest and and weak aorta

Areolar Connective Tissue:

• Forms thin, delicate membranes.
• Cells mainly are fibroblasts in a gel-like ground substance.
• Composed of collagenous and elastic fibers.
• Exists in the subcutaneous layer.
• Located beneath most epithelia, where it nourishes nearby epithelial cells.

Adipose Tissue (Fat):

• Has adipocytes, which store fat.
• Pushes nuclei to one side.
• Crowds out other cell types.
• Cushions and Insulates.
• Located beneath skin (subcutaneous layer), behind eyeballs, around the heart and kidneys, and in spaces between muscles.

Reticular Connective Tissue:

• Consists of a network of thin reticular fibers.
• Supports walls of internal organs.
• Walls of the liver and spleen.

Dense Regular Connective Tissue:

• Closely packed collagenous fibers.
• A fine network of elastic fibers.
• Most cells are fibroblasts.
• Very strong; withstands pulling and binds body parts together.
• Found in tendons, ligaments, and the dermis.
• Has poor blood supply; slow to heal.

Dense Irregular Connective Tissue:

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

Elastic Connective Tissue:

• Has abundant yellow elastic fibers and some collagenous fibers.
• Contains fibroblasts.
• Found in attachments between bones of the spinal column.
• Exists in walls of hollow organs, such as large arteries and airways.
• Found in parts of the heart.
• Has elastic quality and stretches.

Specialized Connective Tissues:

• Contain cartilage, bone, and blood
• They have unique matrices with specialized cells.

Cartilage:

• A rigid, specialized connective tissue
• Provides support, framework, and attachments.
• Offers protection of underlying tissue.
• Is a model for developing bone.
• The matrix contains collagen in gel-like ground substance.
• Has chondrocytes (cartilage cells) in lacunae (chambers), surrounded by matrix.
• Lacks a blood supply; heals slowly.
• Covered by perichondrium, to provide nutrients to the cartilage.
• Hyaline, Elastic, and Fibrocartilage are 3 types of cartilage.

Types of Cartilage:

• Hyaline cartilage is the most common type with fine collagen fibers which makes up ends of bones in joints, nose, respiratory passages, and the embryonic skeleton.
• Elastic cartilage is flexible which is found in the external ear, larynx
• Fibrocartilage, which is very tough due to many collagen fibers makes up the shock absorber, intervertebral disc and menisci of the knee.

Bone (Osseous Tissue):

• The most rigid connective tissue
• Solid matrix, composed of mineral (calcium) salts and collagen.
• This is important for Supports structures and Protects vital structures.
• One key responsibility is Producing blood cells along with storing and releasing Ca and P
• Bone is an important Attachment site for muscles and bone is important.
• Forms the skeleton.
• Bone contains osteocytes (bone cells) in lacunae with 2 types being compact and spongy bone

Bone Tissue:

• Compact Bone features osteoblasts depositing matrix alongisde lamella.
  • Osteocytes within matrix form cylindrical units called osteoms which are cemented together.
  • Central canals well nourish osteons and are heal faster
• Spongy Bone forms bony plates containing osteocytes with space between for marrow. Spongy Bone is also lighter than compact bone.

Blood:

• Consists of formed elements (cells and fragments) suspended in a fluid matrix called plasma.
• Contains Red blood cells that transport gases.
• Contains White blood cells that defend again infection.
• Contains Platelets that help in blood clotting.
• Transports substances throughout the body.

Connective Tissue Summary:

• Areolar connective tissue is in a fluid-gel matrix which binds organs together and is beneath the skin, muscles, and linings of epithelial tissues
• Adipose tissue cells are in fluid-gel matrix to store fat
• Reticular connective cells in fluid-gel matrix supports walls of the liver and spleen
• Dense regular connective tissue cells in fluid-gel matrix binds the body parts
• Elastic connective tissue cells in fluid-gel matrix provides stretch
• Hyaline cartilage cells are in solid-gel matrix protects in ends of bones nose and walls of respiratory
• Elastic cartilage cells in solid-gel matrix gives flexible suppor in the external ear and laarynx
• Fibrocartilage cells in solid-gel matrix gives stretchablity to spinal column adn pelvic area
• Bone cells are protective in skelton
• Blood cells as well as platelets in the fluid matrix assist against disease.

Membranes:

• Sheets of cells.
• Epithelial membranes consist of epithelial and connective tissue and cover body surfaces and line cavities;

Three types of epithelial membranes:

• Serous membranes line body cavities not open to the outside, specifically the inner linings of the thorax and abdomen, covers organs, features simple squamous epithelium + areolar connective tissue, and secretes serous fluid for lubrication
• Mucous membranes line cavities and tubes that open to the outside, are in the digestive, respiratory, urinary, and reproductive tracts, which feature the epithelium +areolar connective tissue and goblet cells secrete mucus.

Types of Membranes:

• Cutaneous membranes cover the body surface, are known as skin, and part of the integumentary system.
• Synovial membranes differ from epithelial membranes, composed entirely of connective tissue, and line joint cavities.

General Characteristics of Muscle Tissue:

• Muscle tissues are excitable and responds to nervous system stimulation along with being contractile
• The types skeletal (bones and striated), cardiac (involuntary as well striated), and smooth ( walls of hollow organs and involuntary).

Skeletal Muscle Tissue:

• Attached to bones.
• Striated.
• Voluntary.
• Has long, cylindrical, multinucleated cells.
• Stimulated by nerve cells.

Smooth Muscle Tissue:

• Non-striated.
• Composed of spindle-shaped fibers.
• Located in walls of hollow organs and blood vessels.
• Involuntary.

Cardiac Muscle Tissue:

• Exists only in the wall of the heart.
• Contains branching cells.
• Involuntary.
• Striated.
• Features intercalated discs, specialized intercellular junctions.

Types of Muscle Tissue:

• Skeletal muscle tissue is a Long, thread-like cells, striated, and comprised of many nuclei. Functions consist of being voluntary movement of skeletal parts and it is in the muscles attached to bones and cartilage
• Smooth muscle tissue consists of Shorter cells, single, and central nuclei. Its role is Involuntary movements of internal organs and it is the Walls of hollow internal organs
• Cardiac muscle tissue consists of Branched cells, and are striated, single nucleus to create Heart movements. It is found in The Heart.

Nervous Tissues:

• Found in the brain, spinal cord, and peripheral nerves.

Neurons:

• Main cells are specialized for communication, via conduction of nerve impulses.

• They coordinate, integrate, and regulate body functions.

• Composed of a cell body (soma), dendrites, and an axon.

• Neuroglia are cells that support and nourish neurons.

• Nervous tissue has cells with cytoplasmic extensions and help with sensory reception as well as a conduction of electrical impulses. It is foundin the core as well as the peripheral section

Explore the four major tissue types: epithelial, connective, muscle, and nervous. Learn how cells are organized into tissues in complex organisms, each with specific functions. Understand the roles of intercellular junctions like tight junctions and desmosomes.