Human Tissues: Epithelial Tissue

• Tissues are a group of similar cells working together to perform related functions, sharing a similar shape
• The four major tissue types in humans: epithelial, connective, muscle, and nervous

Epithelial Tissues

• Found throughout the body as a sheet of cells, cover body surfaces, lines organs and body cavities
• Epithelial tissue functions to create glands
• The free, or apical, surface is exposed to the outside
• Anchored to connective tissue via the basement membrane
• Most epithelial tissue lacks blood vessels, nourished by diffusion from connective tissues
• Quickly divide and reproduce
• Provide protection by being tightly packed
• Secretion, absorption, and excretion are other functions
• Classified by thickness, either simple or stratified
  • Simple: one cell layer
  • Stratified: more than one layer of cells
• Classified by shape: squamous, cuboidal, or columnar
  • Squamous: wider than tall, thin and flat
  • Cuboidal: as tall as wide, cube-shaped
  • Columnar: taller than wide, tall and elongated

Simple Squamous Epithelium

• A single layer of thin, flat cells with disc-shaped nuclei
• The function is to perform diffusion and filtration
• Located in the air sacs of the lungs, capillary walls, and linings of blood and lymph vessels
• Easily damaged

Simple Cuboidal Epithelium

• A single layer of cube-shaped cells with large, spherical nuclei
• Functions in secretion and absorption
• Covers ovaries and lines kidney tubules and glands

Simple Columnar Epithelium

• A single layer of elongated cells with round nuclei
• Cells may have cilia
• Functions to provide protection, absorption, and secretion; ciliated cells move particles
• The nonciliated type lines the digestive tract and uterus
• Columnar cells used for absorption have microvilli
• Scattered goblet cells secrete mucus

Pseudostratified Columnar Epithelium

• Has a stratified look but is really a single layer
• Nuclei are found at various levels
• Cilia and goblet cells are often present
• Functions in mucus secretion and cilia-aided movement
• Located in the respiratory system

Stratified Squamous Epithelium

• Possesses multiple layers of squamous cells
• Cells divide in deeper layers and top layers are flattened
• May be keratinized, meaning the surface cells have the protein keratin and are dead, creating a covering
• Protects underlying layers
• The keratinized type forms the outer layer of the skin known as the epidermis, while nonkeratinized type forms moist linings of the mouth and esophagus

Stratified Cuboidal Epithelium

• Has two or three layers of cuboidal cells
• Provides more protection than a single layer
• Found in the large ducts of mammary glands, sweat glands, and salivary glands

Transitional Epithelium

• Has several layers of cuboidal cells
• May resemble stratified squamous or stratified cuboidal depending on the tension
• Functions to readily stretch organs when distended
• Lines ureters, the urinary bladder, and the urethra

Glandular Epithelium

• Consists of cells specialzed to produce and secrete substances into ducts or body fluids, forms glands
• Glands are classified by where they secrete their products from
  • Exocrine glands: secrete products onto external body surfaces (skin) or into body cavities
  • Endocrine glands: secrete products into tissue or blood fluid; secretes hormones
• Exocrine glands possess 3 types
  • Merocrine glands: release fluid via exocytosis, examples include salivary and sweat glands
  • Apocrine glands: lose small portions of the cell bodies during secretion, examples include mammary glands
  • Holocrine glands: release entire cells that disintegrate, such as sebaceous glands
• Most exocrine glands are merocrine and secrete either serous fluid or mucus
  • Serous fluid is usually watery and lubricates
  • Mucus is secreted for protection and lines digestive, respiratory, and reproductive systems

Connective Tissues

• The most abundant and diverse tissue
• Binds structures together
• Provides support and protection, fills space plus stores fat, produces blood cells, fights infection, and helps repair tissue
• Cells separated by extracellular matrix, nonliving material between
  • The extracellular matrix is made of ground, adhesion proteins, fluid and fibers
  • Extracellular matrix ranges from fluid to solid
• Many possess a very vascular blood supply, with some having little to no vascularity
• Can reproduce but slower than epithelial cells
• Cells can be fixed, remaining in the tissue for a long time, or wandering, remaining temporarily in the tissue
• Fibroblasts produce fibers by secreting proteins into the extracellular matrix (fixed)
• Macrophages carry on phagocytosis and function as defensive cells (wandering)
• Mast cells release heparin, which prevents blood clotting, and histamine which promotes inflammation (fixed)
• Fibroblasts make 3 kinds of connective tissue fibers

Collagenous Fibers

• Thick, made of collagen
• Found in long, parallel bundles
• Strong and flexible, but not very elastic, located in bones ligaments and tendons
• Also known as white fibers

Elastic Fibers

• Made of springlike protein called elastin
• Not as strong, very elastic for stretching
• Referred to as yellow fibers
• Located in vocal cords

Reticular Fibers

• Thin collagenous fibers
• Highly branched, provide delicate support
• Found in the spleen
• Two major categories: connective tissue proper and specialized connective tissue

Loose Connective Tissue

• Connective tissue proper contains loose connective tissue and dense connective tissue
• Loose connective tissue has less collagenous fibers
• Dense connective tissue has abundant collagenous fibers
• Includes areolar, adipose, and reticular connective tissues

Areolar Tissue

• A type of loose connective tissue that forms thin membranes throughout the body
• Binds skin to underlying organs and fills spaces between muscles
• Lies under epithelium with many blood vessels, nourishes epithelial cells
• Widespread throughout the body

Adipose Tissue

• A type of loose connective tissue that develops when adipocyte cells store fat as droplets and enlarge
• Provides a protective cushion and insulation of heat, and stores energy
• Located beneath the skin, between muscles, around kidneys and behind eyeballs, and around certain organs and joints

Dense Connective Tissue

• Many closely packed collagenous fibers and a network of elastic fibers
• Has few cells and poor blood supply, slows healing process
• Includes tendons and ligaments
  • Tendons connect muscles to bones
  • Ligaments connect bones to bones
• Specialized connective tissue: cartilage, bone, and blood

Cartilage Specialized Connective Tissue

• Cartilage which is rigid, provides support and protection
• Cartilage cells are called chondrocytes and found in chambers called lacunae, singular is lacuna
• Has no direct blood supply, heals slowly
• The 3 types of cartilage: hyaline, elastic, and fibrocartilage

Hyaline Cartilage

• Made of fine, white collagenous fibers
• The most common cartilage
• Provides support and resists compressive stress
• Located on ends of bones, the soft part of the nose, and parts of respiratory passages

Elastic Cartilage

• Contains many elastic fibers in the matrix
• Maintains shape but allows for flexibility
• Located in the external ear and larynx

Fibrocartilage

• Very tough with many collagenous fibers
• Functions as a shock absorber for structures under pressure
• Forms intervertebral discs between vertebrae and cushions bones in the knees

Bone Specialized Connective Tissue

• The most rigid connective tissue
• Has different salts between cells that make them hard
• Supports body structures, protects important organs, and serves as attachment points for muscles
• Stores minerals and contains marrow, produces blood cells
• There are several layers
  • Central canals, or Haversian canals, containing a blood vessel
  • Bone cells, called osteocytes, evenly spaced around the canals
  • Bone matrix lamellae found between the cells
• Cells clustered around a central canal is called osteon or Harversian system

Blood Specialized Connective Tissue

• Transports materials between internal cells to other cells that perform exchange externally
• The fluid extracellular matrix is blood plasma
• Formed elements: red blood cells, white blood cells, and platelets

Muscle Tissues

• Able to contract allowing for movement of body parts
• Three types of muscle tissues are skeletal, smooth, and cardiac

Skeletal Muscle Tissue

• Located in muscles attached to bones
• Makes possible voluntary movement controlled by conscious effort
• Cells have Striations that are alternating light and dark sections
• Have multiple nuclei per cell
• Nerve cells stimulate cells to contract

Smooth Muscle Tissue

• No striations
• Possess one large nucleus per cell
• Movement is involuntary and not controlled by conscious thought
• Makes up walls of stomach, intestine, bladder, uterus, and blood vessels

Cardiac Muscle Tissue

• Only found in the heart
• Is striated and cells are branched
• Each cell is connected to other cells by an intercalated disc to pass signals through cells quickly

Nervous Tissues

• Located the brain, spinal cord, and peripheral nerves
• Cells are called neurons or nerve cells
• Neurons pass electrical impulses along processes called axons
• Also includes neuroglia
• Provides support and supplies nutrients to the neurons

Tissues are groups of similar cells performing related functions and sharing shapes. The four major tissue types in humans are epithelial, connective, muscle, and nervous tissue. Epithetlialt tissues can be simple or stratified, and squamous, cuboidal, or columnar.