Human Histology

• EPITHELIUM


  • Covers body surfaces, lines body cavities & constitutes glands
  • Creates a selective barrier between the external environment and the underlying Connective Tissue (CT) (e.g., Skin)
  • Facilitates or inhibits the passage of specific substances between external environment & underlying CT
• EPITHELIAL TISSUES

  • Cellularity: Composed almost entirely of cells (all cells tightly bound by cell junctions), forming a continuous sheet
  • Avascular: Containing no blood vessels
  • Innervated: Supports nerve fibers
  • Regeneration: Epithelial cells are frequently damaged or lost to abrasion, so they are replaced quickly
  • Polarity: Apical (free) and basal surfaces (attached to underlying connective tissue)
• EPITHELIAL TISSUEs: 8 MAJOR FUNCTIONS

  • Protection: Epithelial cells protect underlying tissue from mechanical injury, harmful chemicals and pathogens and dehydration
  • Sensation: Changes in the environment are detected by specialized epithelial cells found in the skin, eyes, ears, and nose, and on the tongue
  • Secretion: In glands, epithelial tissue is specialized to secrete specific chemical substances such as enzymes, hormones, and lubricating fluids
  • Excretion: Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine. Sweat is also excreted from the body by epithelial cells in the sweat glands
  • Diffusion: Simple epithelium promotes the diffusion of gases, liquids and nutrients. Because they form such a thin lining, they are ideal for the diffusion of gases (e.g., walls of capillaries and lungs)
  • Absorption: Certain epithelial cells lining the small intestine absorb nutrients from the digestion of food
  • Contractility: Some very specialized epithelial cells, found in the lining of the heart and respiratory tract, contain the contractile proteins myosin and actin, similar to muscle
  • Transportation: Some epithelial cells have cilia, short hair-like projections that propel fluid or particulate matter over tissue surfaces
• EPITHELIAL TISSUES: CLASSIFICATION

  1. Each epithelium tissue is given two names:

  2. The first name indicates the number of cell layers present. Simple- one layer (used in filtration), Stratified- more than one layer (wear and tear areas), Pseudostratified- one layer that looks like many (areas that need to stretch)

  3. The second describes the shape of its cells. Squamous- cells are wider than tall, Cuboidal- cells are as wide as tall, Columnar- cells are taller than wide, Transitional- cells have no specific shape

  4. Ciliated- If the apical edge of the epithelium has cilia
• Organ Epithelial functions

  • Skin - protective function
  • Glands - secretory function
  • GIT - absorptive function
  • Tongue & Nose - sensory function
  • Muscles - contractile function
  • Kidney - excretory function & maintains electrolyte balance
  • Testes - germ cell production
• EMBRYONIC ORIGIN of EPITHELIAL TISSUE


  Germ Layer Give rise to: Ectoderm Epidermis glands on skin, some cranial bones, pituitary and adrenal medulla, the nervous system, the mouth between cheek and gums, the anus. Mesoderm Connective tissues proper, bone, cartilage, blood, endothelium of blood vessels, muscle synovial membranes, serous membrane lining body cavities, kidneys lining of gonads. Endoderm Lining of airways and digestive system except the mouth and distal part of digestive system (rectum and anal canal); glands (digestive glands, endocrine glands, adrenal cortex)
• Embryonic origin layers

  • Ectodermis
  • Endodermis
  • Mesodermis
• Ectodermal Derivatives


  Surface ectoderm: epidermis & its derivatives (hair, nails, sweat glands, sebaceous glands & mammary duct glands, anterior pituitary gland, enamel of teeth, internal ear, corneal epithelium & eye lens
• Neuroectoderm


  Neural crest: cranial & sensory ganglia, adrenal medulla, melanocytes, pharyngeal arch cartilages, head mesenchyme, schwann cells, odontoblasts, neural tube: CNS, retina, pineal body, posterior pituitary gland
• Mesodermal Derivatives

  • Epithelium of the kidney & gonads, mesothelium, endothelium, adrenal cortex, seminiferous & genital duct epithelium, muscles of the trunk & skeleton, dermis of skin, connective tissue, spleen
• Endodermal Derivatives

  • Respiratory system epithelium (trachea, bronchi & lungs), alimentary system epithelium (GIT), liver, GB, pancreas, urinary bladder & urethra, thyroid, parathyroid & thymus gland epithelium, auditory tube & tonsils
• Classification of epithelial tissue based on number of cell layers: Simple epithelium tissue is one layered, stratified epithelial tissue is more than one layered
• Classification of epithelial tissue based on shape of cell: Squamous Epithelium has flattened cells, Cuboidal Epithelium has cubic cells, Columnar Epithelium has column cells
• In stratified epithelium, only the shape of the cells that form the surface layer is used in classifying the epithelium
• Special classification of epithelium: Pseudostratified appears layered though not all of the cells rest on the basement membrane, Transitional Epithelium lines the lower urinary tract, Endothelium lines the vascular system, Mesothelium lines close cavities of the body
• Surface modification of epithelial cells includes apical, lateral, and basal domains
• Apical/Free Domain is directed towards the external surface or the lumen of an enclosed cavity or tube. It includes structures like microvilli and stereocilia
• Microvilli


  • Cytoplasmic processes that extend from the cell surface
  • Its number correlates with its absorptive capacity (transport fluid & metabolites)
  • Found in intestines (striated border), kidney tubule (brush border)
• Stereocilia (stereovilli)


  • Microvilli of unusual length & are immotile
  • Found in epididymis of male reproductive system, hair cells of the ear
• Cilia (Kinocilia)


  • Motile cytoplasmic structures capable of moving fluid & particles along epithelial surfaces
  • Found in trachea (sweep trapped mucus), fallopian tubes (help transport ova & fluid to the uterus)
• Primary Ciliary Dyskinesia (PCD) (Immotile Cilia Syndrome) is an autosomal recessive hereditary disorder (1 in 20,000 births)
• PCD in Kartagener’s Syndrome results in dysfunctional cilia, respiratory distress (bronchitis, sinusitis), otitis media (middle ear infection), persistent cough & asthma
• PCD can also be present in Polycystic Kidney Disease and Hydrocephalus (fluid within the brain)
• Flagella

  • Much longer than cilia
  • Found in spermatozoa of humans having 1 flagellum/tail, propels the cell along the female genital tract
• LATERAL Domain (LD) communicates with adjacent cells & is characterized by specialized attachment areas or cell-to-cell adhesions through junctions
• Cell Adhesion Molecules (CAM) are unique CHONs in the lateral domain
• Epithelial cells must remain firmly attached to the basement membrane and to one another to form a complete cover or lining
• Four common intercellular connections are tight junctions, adhering junctions, gap junctions, and anchoring junctions (desmosomes)
• Occluding junctions (tight junctions)

  • Located at the most apical point, also known as "zonula occludens"
• Clinical Significance of Occluding junctions includes Cl. Perfringens (food poisoning bacteria), H. pylori (stomach bacteria in gastric ulcer & cancer), Rotavirus (infant enteritis)
• Tight junctions anchor cells to each other, prevent substances from passing between cells, found in intestinal lining
• Anchoring junctions


  • Provide mechanical stability to epithelial cells by linking the cytoskeleton of one cell to the cytoskeleton of an adjacent cell
  • Responsible for cell-to-cell recognition, morphogenesis & differentiation, also known as "zonula adherens"
• Adhering junctions are often deep to tight junctions, form all the way around the cell, support apical surface, allow passage between cells below the apical surface
• Communicating junctions (Gap Junctions)


  • Found in epithelia, intestinal, smooth, cardiac & vascular muscles & nerves, allow cells to exchange ions, fluid & regulate transport of molecules & metabolites
• Gap junctions tunnel between two plasma membranes, found in muscles, enable sharing of ions and proteins, enable fast communication between cells
• Desmosome


  • Also known as "Macula Adherens" or "Anchoring Junction", situated just below the zonula adherens, numerous in the stratified squamous keratinizing epithelium of the skin
• Anchoring junctions attach cells to each other, bind epithelium together, bind muscle cells, resist shear force, allow bending and twisting
• BASAL Domain (BD) rests on the basal lamina anchoring the cell to
• Epithelial Tissues

  • Intercellular Connections
  • Basal Domain (BD)
  • Basement Membrane
  • Renewal and Repair
• Anchoring junctions

  1. Attach cells to each other

  2. Bind epithelium together

  3. Bind muscle cells

  4. Resist shear force

  5. Allow bending and twisting