Introduction to Human Tissues

Introduction to Tissues

• A tissue constitutes a group of similar cells performing a shared function.
• Tissues can range in thickness from a single cell to hundreds of layers.
• Tissues commonly exist within an extracellular matrix.
• Histology involves the biological study of tissues.

Principle Tissue Types

• Epithelial tissue protects the body's surface, specializes in secretion, excretion, absorption, and forms glands.
• Connective tissue supports the body and its parts, connects body parts together, transports substances, and protects from foreign invasion.
• Connective tissue cells are usually far apart and are separated by large amounts of matrix.
• Muscle tissue produces movement for locomotion. Muscle cells produce movement by shortening contractile units in the cytoplasm.
• Nervous tissue is the body's most complex tissue, specializing in communication between different body parts.

Epithelial Tissue

• Epithelium can be simple, consisting of a single layer of cells, or stratified, with multiple layers.
• Simple epithelium lines body cavities, ducts, and tubes, facilitating absorption and diffusion.
• Stratified epithelium primarily functions in protection.
• Epithelium is subdivided based on the cell's shape at the tissue's free surface.

Glandular and Membranous Epithelium

• Glandular epithelium groups into cords or follicles, which form secretory units of glands.
• Membranous epithelium covers the body and lines body cavities, vessels, and tracts.
• Membranous epithelium lines the pleural, pericardial, and peritoneal cavities.
• Membranous epithelium lines blood and lymphatic vessels.
• Membranous epithelium lines the respiratory, digestive, and genitourinary tracts.

Epithelial Tissue Functions

• Protection includes a tough, impermeable skin covering to protect the body from mechanical, chemical, and invasive injuries.
• Sensory functions are present in the skin, nose, eye, and ear.
• Secretion involves the release of mucus, hormones, digestive juices, and sweat.
• Absorption occurs with nutrient absorption in the gut and gas exchange in the lungs.
• Excretion occurs in the lining of kidney tubules.

Epithelial Cell Shape and Layering

• Squamous cells are flat and plate-like.
• Cuboidal cells are cube-shaped.
• Columnar cells are taller than they are wide, appearing narrow and cylindrical.
• Pseudostratified columnar epithelium has only one layer of oddly shaped columnar cells.
• Simple epithelium consists of a single layer of cells.
• Stratified epithelium consists of multiple layers of cells stacked on each other.
• Transitional epithelium is an arrangement of differing cell shapes in a stratified epithelial sheet.

Epithelial Types and Locations

• Simple squamous epithelium is located in alveoli in the lungs, lining blood and lymphatic vessels, and surfaces of pleura, pericardium, and peritoneum.
• Simple cuboidal epithelium is located in glands, gland ducts, and kidney tubules.
• Simple columnar epithelium is located on the surface of mucous membranes lining the stomach, intestine, uterus, uterine tubes, and parts of the respiratory tract.
• Modifications of simple columnar epithelium include goblet cells, cilia, and microvilli.
• Pseudostratified columnar epithelium is located in the the lining of air passages and segments of the male reproductive system.
• Stratified squamous (keratinized) epithelium has keratin that contributes to protective qualities of the skin surface.
• Stratified squamous epithelium is located in the lining of the vagina, mouth, and esophagus.
• Stratified columnar epithelium is located in segments of the male urethra and the mucous layer near the anus.
• Transitional epithelium is located in areas subjected to stress and tension changes, like the wall of the urinary bladder and other distensible structures.

Connective Tissue

• Connective tissue commonly exists in or around nearly all organs.
• Connective tissue consists primarily of extracellular matrix (ECM) with relatively few cells.
• The ECM of connective tissues consists of various fibers, fluid, and additional materials.
• Fibers and components determine the structural characteristics for each connective tissue type.
• Blood's matrix is fluid plasma.

Connective ECM Fibers

• Collagenous fibers (white fibers) are composed of collagen, the body's most abundant protein, and often generate great tensile strength in bundles.
• Reticular fibers exist in networks, are delicate, support capillaries and nerve fibers and are a collagen form called reticulin.
• Elastic fibers (yellow fibers) consist of elastin, a rubbery protein with memory.

Connective Tissue Types

• Loose or ordinary connective tissue exists between tissues and organs.
• Adipose tissue exists under the skin and provides padding.
• Reticular tissue exists as the inner framework of the spleen, lymph nodes, and marrow.
• Dense irregular tissue exists in deep fascia, dermis, and scar tissue.
• Dense regular collagenous tissue exists in tendons and ligaments.
• Dense regular elastic tissue exists in walls of certain arteries.
• Compact bone forms the outer shell of bones.
• Cancellous bone exists inside bones.
• Hyaline cartilage is located in the nasal septum, articular surfaces, larynx, and rings in the trachea and bronchi.
• Fibrocartilage is located in vertebral disks and the symphysis pubis.
• Elastic cartilage is located in the external ear and auditory tube.
• Blood is a type of connective tissue.

Muscle Tissue

• There are three types of muscle tissue: skeletal, smooth, and cardiac.
• Skeletal muscle attaches to bones.
• Smooth muscle is located in the walls of the viscera including the stomach, intestines, and blood vessels.
• Cardiac muscle is unique to the heart and is called striated involuntary muscle.

Skeletal Muscle Characteristics

• Presents many cross striations
• Presents multinucleated cells
• Has a long, narrow, thread-like shape
• Each cell is approximately 3.75cm long
• Is well innervated by nervous tissue

Smooth Muscle Characteristics

• Presents narrow and long fibers, but shorter than striated muscle
• Presents one nucleus per fiber
• Lacks striations (smooth appearance)
• Includes cylinders that taper at both ends

Cardiac Muscle Characteristics

• Presents cross striations
• Presents unique intercalated disks joining untapered fiber ends
• Shows interpolated branching evident
• Unique to the heart

Nervous Tissue

• Nervous tissue has excitability and conductivity characteristics.
• Nervous tissue consists of neurons and neuroglia.
• Neurons contain a cell body (soma) and at least two processes: an axon carrying information away from the soma, and one or more dendrites carrying nerve signals toward the soma.
• Neuroglia connect, protect, and support neurons.

Tissue Repair

• Tissues can repair themselves.
• Phagocytic cells remove dead or injured cells, and regeneration restores any remaining gaps.
• Epithelial and connective tissues regenerate most easily.
• A fibrous mass remaining after injury forms a scar.
• A keloid is an unusually thick scar that may develop in the skin’s lower layer.