C2

• Epithelium consists of a sheet of aggregated cells of similar type, forming the external and internal surface of the body
• Epithelial cells proliferate into the underlying tissue to form glands and hair follicles
• Ectoderm is the origin of the epithelium of external body surfaces, while most of the epithelium of the digestive and respiratory systems originates from endoderm
• Mesoderm gives rise to the lining of the vascular system, closed body cavities, and parts of the urogenital system
• Epithelium is separated from the underlying connective tissue by a thin membrane, which may be a basement membrane composed of a basal lamina and a reticular lamina, or the basal lamina alone
• The basal lamina, common to all epithelia, contains type IV collagen, proteoglycans, laminin, and fibronectin
• Epithelial cells receive nutritional support by diffusion of tissue fluids from the underlying connective tissue since blood and lymph vessels do not penetrate the basement membrane
• Functions of epithelial cells include protection, sensory reception, absorption, secretion through glands, excretion of waste products, transport through ciliary action, and formation of barriers for selective permeability
• Classification of epithelium is based on the shape of epithelial cells and the number of layers present
• Simple epithelium is a single layer of cells resting on the basement membrane, found covering or lining wet surfaces
• Stratified epithelium is made up of two or more layers of cells with only the basal cell layer resting on the basement membrane
• Simple squamous epithelium is a single layer of thin, flat, scale-like cells, found lining internal surfaces like closed body cavities, the heart, and blood vessels
• Simple cuboidal epithelium is a single layer of cells whose width and height are approximately equal, found lining ducts of glands and covering certain eye structures
• Simple columnar epithelium consists of tall, narrow cells with oval nuclei located near the base, lining organs that perform secretory and absorptive functions
• Pseudostratified columnar epithelium appears to have several layers due to irregularly shaped and sized cells, found in the respiratory and reproductive systems
• Stratified squamous epithelium consists of several layers of cells, with only the superficial cells having a squamous shape
• Stratified squamous epithelium can be nonkeratinized or keratinized, each serving different functions and found in different body parts
• Stratified squamous epithelium consists of several layers:

  • Stratum basale: the deepest layer next to the basal lamina, rich in polyribosomes for tonofilament synthesis
  • Stratum spinosum: composed of polyhedral cells adhered by desmosomes, with spiny processes containing tonofibrils
  • Stratum germinativum gives rise to cells moving into upper layers
  • Stratum granulosum: layer of granulated cells, absent in non-keratinized epithelium
  • Stratum lucidum: layer of flattened keratinized cells, occurs only in non-hairy skin regions
  • Stratum corneum: consists of dead, keratinized cells resistant to environmental irritants
  • Stratum dysjunctum: group of cells in the outermost layer that become loose and separate
• Stratified cuboidal epithelium:

  • Consists of two or more layers with a surface layer of typical cuboidal cells
  • Often lines excretory ducts of glands
• Stratified columnar epithelium:

  • Composed of several layers
  • Superficial layer of tall, prismatic cells, deeper layers of smaller polyhedral cells
  • Found in the distal urethra, transitional epithelium, parotid and mandibular ducts, lacrimal sac and duct
• Transitional epithelium:

  • Pseudostratified type in the urinary system
  • Lines hollow organs capable of distention like the urinary bladder
  • Cell shape varies based on organ distention
• Special epithelial tissues:

  1. Ciliated epithelium: present in respiratory and reproductive systems

  2. Neuroepithelium: modified for sensory reception and transmission

  3. Pigmented epithelium: contains intracytoplasmic pigments like melanin

  4. Myoepithelium: associated with exocrine glands, contains myofibrils for secretion

  5. Other special epithelia: endothelium, mesothelium, epithelial-reticular cells, endocrine gland cells
• Surface modifications of epithelium:

  • Lateral surface modifications:
  • Glycocalyx: carbohydrate-rich surface coat
  • Junctional complexes: tight junctions, intermediate junctions, desmosomes, gap junctions
  • Basal border modification:
  • Basal lamina: structureless membrane supporting all epithelia
• Basal Border Modification:

  • Basal lamina: structureless membrane where all epithelia rest, with cell coats above and a network of reticular fibers below
  • Caveolae: bulb-like invaginations of the cell membrane
  • Hemidesmosomes: identical to one-half adenomere along the basal cell membrane
• Apical Border Modifications:

  • Microvilli: slender, cylindrical cellular processes projecting from apical surfaces, believed to increase surface area for absorption
  • Caveolae and pyknotic vesicles similar to those in the basal border
  • Cilia: small whip-like structures capable of vibratory beating or lashing movement
  • Kinocilia: long, slender, tapered cytoplasmic processes for movement of materials along epithelium
  • Stereocilia: extremely long, branched microvilli modified for facilitation of secretory activity
• Location of Different Types of Epithelium:

  • Simple squamous epithelium found in primordial follicles, endothelium, mesothelium, subarachnoid space, and more
  • Simple cuboidal epithelium in choroid plexus and ducts of glands like the kidney and pancreas
  • Simple columnar epithelium in the cervix, small intestine, efferent ducts, and more
  • Pseudostratified columnar epithelium in ductus epididymis, vesicular glands, and more
  • Stratified columnar epithelium in the distal portion of the urethra, parotid duct, and more
  • Stratified cuboidal epithelium in excretory ducts of glands and the external orifice of the urethra in sows
  • Transitional epithelium in the urinary bladder, ureter, bulbo-urethral gland, and conjunctiva of pig and ruminants
  • Stratified squamous epithelium in various locations including the esophagus, tongue, and vaginal mucosa
• Glands:

  • Unicellular glands consist of a single secretory cell like the goblet cell
  • Multicellular glands have more than one secretory cell forming adenomeres
  • Endocrine glands release hormones directly into intercellular fluid
  • Exocrine glands have ducts to transport secretory products
  • Glands can be simple or compound with different types of duct systems
• Types of Secretory Products:

  • Serous glands secrete clear watery products with zymogen granules
  • Mucous glands produce thick, viscous secretions for protection
  • Seromucous glands are a combination of serous and mucous cells
• Modes of Secretion:

  • Merocrine mode releases product as secretory granules through exocytosis
  • Apocrine mode discharges the apical part of the cytoplasm with the secretion
  • Holocrine mode secretes the whole cell with its secretion
  • Cytocrine mode transfers secretory material from one cell to another