Tissue refers to a group of similar cells that carries out a function. Histology is the science of tissues.
examples of tissues
Blood Tissue
Four categories of tissues
• Muscular tissue – designed to contract • Nervous tissue – conduct, sense, store information • Epithelial tissue – forms coverings (skin) & linings (blood vessels) • Connective tissues – support, transport, protect
Cellular junctions Cells may be separated by a matrix or connected together by cellular junctions.
1. Tight Junction • Cell membranes of adjacent cells are fused together - no space between cells • Tight junctions prevent the passage of unwanted & harmful substances • Examples: Blood Brain Barrier & Digestive Tract Cell 1
Cell 2
Interlocking membranes form tight junction between two cells
Cellular Junctions 2. Desmosome • Desmosomes form “spot welds” between cells • Provides structural support • Examples in the epidermis of the skin
Cells connected by desmosomes
Cellular Junctions 3. Gap Junction • Cells connected by protein ion channels • Ion channels allow for cell-to-cell diffusion of ions • Example: intercalated discs of cardiac muscle
Intercalated discs of cardiac muscle are one type of gap junction.
Types of Tissues: Epithelial Tissue Epithelial Tissue: • Lines cavities, such as the digestive & respiratory tracts • Forms coverings, such as the skin • Functions: protection, secretion, absorption, excretion
Epithelium: structure • Epithelium is anchored to a layer of connective tissue, called the basement membrane. • A Basal surface faces the basement membrane • The free (apical) surface is exposed to the open space (lumen)
Open space = lumen
Basement membrane
Characteristics of Epithelial Tissue • Tightly packed cells • No blood supply, instead they receive nutrients by diffusion • Readily divide, so they are continually replaced = rapid healing
Classifications of Epithelial Tissues Layers 1. Simple = Single layer of epithelial cells 2. Stratified = two or more layers of cells Shape of cells 1. Squamous = scale-like
Nucleus
2. Cuboidal = cube-like shape 3. Columnar = tall cells
Nucleus
Types of Epithelial Tissues Simple Squamous Epithelium • single layer of flattened cells • Common at sites of diffusion and filtration • lines air sacs of lungs (alveoli) • lines walls of capillaries
Squamous cells
Surface View
Alveoli of lung
Types of Epithelial Tissues Simple Cuboidal Epithelium • Single layer of cube-shaped cells • Lines ducts of certain glands & tubules within kidneys • Secretes chemicals in glands •Functions in reabsorption & secretion in kidneys
lumen
Cross-section of tubules in kidneys
Types of Epithelial Tissues Simple Columnar Epithelium • Single layer of elongated cells • Nuclei appear as a single row • Functions in absorption, secretion, or transport • Often contain goblet cells • Goblet cells secrete mucus onto apical surface • Some are ciliated, and others have microvilli
Simple Columnar Epithelium Some simple columnar epithelium are covered with cilia on their free surface. Cilia beat constantly in a rhythmic motion to propel substances across the cell surface.
• Cilia propels the egg through uterine tube
Simple Columnar Epithelium Some simple columnar epithelium are covered with microvili on their free surface. Microvilli are extensions of the cell membrane that increases the surface area of a cell.
• example: microvili on intestinal epithelium increases the amount of contact each cell makes with food. • Microvilli greatly enhance absorption of nutrients
Pseudostratified Columnar Epithelium
Psuedostratified columnar epithelium: • Appear striated, but are actually simple columnar • Each cell touches basement membrane • Cells appear to have two or more rows of nuclei • Usually ciliated • Contain goblet cells that secrete mucus
Pseudostratified Columnar Epithelium • Lines respiratory tract • Mucus from goblet cells traps debris & • Cilia moves the mucus (and trapped debris) away from the lungs
Stratified Squamous Epithelium • Several layers of cells = thick tissue • Superficial cells are squamous • Deeper cells are cuboidal or columnar • May be keratinized or nonkeratinized
Keratinized Stratified Squamous Epithelium • found in the epidermis (outer layer) of skin • Older, outer epidermal cells of skin are keratinized • Keratin = insoluble protein • Outer layer layer hardens & dies • Forms protective coating
Stratified squamous epithelium of epidermis
Non-keratinized Stratified Squamous Epithelium Non-keratinized (moist) stratified squamous epithelium lines esophagus, oral cavity, vagina, and anal canal.
Apical surface
Stratified squamous epithelium of vagina
Transitional Epithelium • Transitional epithelium is specialized to form an expandable lining • Found in the inner layer of urinary bladder & ureters • The Lining is several layers thick when bladder is contracted (empty). • Only few cell layers thick when urinary bladder is distended (full), and the cells appear squamous. Contracted bladder
Distended bladder
Glandular Epithelium • Glandular epithelium is specialized to produce & secrete substances • Glandular epithelium form glands
Exocrine Glands • Secretes substances through ducts onto open surface • Includes goblet cells, sweat glands, mammary glands
Duct of a sweat gland
Glandular Epithelium Endocrine Glands • Secretes substances (hormones) directly into blood or tissue fluid • Includes thyroid gland, pituitary gland, adrenal glands, ect.
Follicles of thyroid gland are composed of simple cuboidal epithelium
End of Section 1, Chapter 5