Histology

Histology Notes

Histology

  • Histology is the study of tissues.
  • There are four fundamental tissue types: epithelial, connective, muscular, and nervous, each with distinct embryological origins.

Epithelial Tissues

  • Cells are closely packed with little extracellular matrix, avascular but innervated, resting on a basement membrane.
  • Functions:
    • Physical protection.
    • Exchange of substances between the environment and tissues (glandular epithelia).
    • Production of specialized secretions and sensory reception.
  • Types: lining, glandular (endocrine and exocrine), and sensory epithelia.
  • Embryological derivation:
    • Ectodermal: epidermis, nasal cavities, cornea, lens, inner ear, oral cavity, gums, hard palate, distal anal canal, parotid gland, adenohypophysis.
    • Endodermal: respiratory system, digestive system, part of the urinary and female genital systems, tympanic cavity.
    • Mesodermal: serous membranes, vessels (endothelium), ovaries, part of the urinary and genital system.

Lining Epithelia

  • Cells delimit a cavity with underlying connective tissue. They possess:
    • Apical (free) surface.
    • Basal surface on the basement membrane.
    • Lateral surface specializations for epithelial integrity.
Lateral Surfaces
  • Junctional complexes:
    • Occluding junctions: impermeable barrier, prevent molecule passage.
      • Maintain cell polarization.
      • Formed by transmembrane proteins (JAM) and extrinsic proteins (zonula occludens proteins).
      • Structural modifications occur in diseases like Crohn's.
    • Anchoring junctions: distribute forces.
      • Adherens Junctions (zonula adhaerens):
        • CADHERINS bind to actin filaments via CATENINS.
        • E-cadherin is calcium-dependent.
      • Desmosomes (macula adhaerens):
        • CADHERINS bind to PLAKOGLOBINS, which bind to intermediate keratin filaments.
    • Communicating Junctions (Gap Junctions or Nexus):
      • CONNEXONS form channels regulated by calcium.
      • Connexons are made of 6 connexins (homomeric or heteromeric).
      • Homotypic or heterotypic recognition.
Cell Membrane and ECM Junctions (Basement Membrane Domain)
  • Adhesion to connective tissue is mediated by junctional complexes.
    • Hemidesmosomes: half of a desmosome.
    • Focal Adhesions: half of an adherens junction.
    • PODOZOMES: point contacts.
  • Basement membrane layers:
    • Basal lamina (epithelial origin).
    • Reticular lamina (connective origin).
  • Transmembrane proteins and glycocalyx connect epithelial cells to connective tissue.
  • Integrins anchor cells to plakoglobins and intermediate filaments (hemidesmosomes) or actin filaments (focal adhesions).
  • Focal adhesions are important for cell migration.
Specializations of the Apical Domain
  • Microvilli: immobile, actin filaments inside, brush border in intestinal epithelia (increase surface area).
  • Stereocilia (stereovilli): immobile, actin, longer than microvilli, found in the epididymis (sperm maturation).
  • Cilia: mobile, 9+2 microtubule structure with dynein, present in respiratory epithelium (mucus movement).
    • Each cilium has a free portion and a basal body.
Cell Polarization
  • Nucleus and mitochondria near the basal part (energy).
  • Protein synthesis organelles in the space between nucleus and apical part.

Classification of Covering Epithelium

  • Based on morphology and function, using hematoxylin and eosin staining.
  • Criteria:
    • Number of layers:
      • Simple (monostratified): single layer on the basement membrane.
      • Pseudostratified: appears multilayered but is single-layered.
      • Compound (stratified): two or more layers, basal layer in contact with the basement membrane.
    • Shape of cells lining the cavity:
      • Squamous (flat): width is the greatest dimension.
      • Cuboidal (isoprismatic): all dimensions are equal.
      • Columnar (batiprismatic): height is the greatest dimension.
  • Combined classifications:
    • Simple squamous, cuboidal, columnar.
    • Compound squamous, cuboidal, columnar.
    • Pseudostratified.
    • Transitional (dimensions depend on functional state).
  • Compound epithelia are mostly squamous; cuboidal and columnar are rare.
  • The surface layer determines whether stratified epithelia are squamous, cuboidal, or columnar.
  • Basal layer cells are actively proliferating and appear cuboidal.
Logic Behind Localization of Epithelia
  • Epithelia obtain nutrients from underlying connective tissue via the basement membrane.
    • Simple squamous: thin, fragile, facilitates exchange (e.g., alveoli).
    • Stratified epithelium: robust, protective, not for absorption/secretion.
Types of Epithelia and Locations
  • Simple squamous: pulmonary alveoli, loop of Henle, endothelium (vessels), mesothelium (pleura, pericardium, peritoneum; reduces friction).
  • Simple cuboidal: endocrine and exocrine glands, straight tubules, rete testis, seminal vesicles, germinal epithelium of ovaries, terminal bronchioles, thyroid, kidney.
  • Simple columnar:
    • Non-ciliated: excretory ducts of glands, gastric and intestinal mucosa.
    • Ciliated: uterine tubes, part of the uterus, bronchioles.
  • Optical sections may show different structures depending on the sectioning plane.
  • Microvilli are always present in the small intestine, lost in the large intestine.
  • Pseudostratified columnar epithelium:
    • Nuclei at different heights, all cells on basement membrane.
    • Non-ciliated: epididymis, vas deferens (stereocilia).
    • Ciliated: respiratory passages, larynx, trachea, bronchi.
  • Stratified cuboidal and columnar: rare, protective, secretion/absorption function. Found in sweat gland ducts or transition points between epithelia.
  • Stratified squamous: robust, resistant to mechanical stress. Keratinized (epidermis) or non-keratinized (mucous membranes - mouth, pharynx, esophagus, vagina, rectum).
Epidermis
  • Stratified squamous keratinized epithelium, thickness varies from 50 μm to 1.5 mm. Nourished by the underlying dermis.
  • Keratinocytes exhibit various characteristics. Basement membrane folds form epidermal ridges and dermal papillae.
  • Layers (from basal to corneal):
    • Basal Layer (Stratum basale): hemidesmosomes, renewal and proliferation, keratin filaments (5 and 14).
      • Mitosis occurs only within this layer (except in the case of tumours).
    • Spinous Layer (Stratum spinosum): larger cells, increased keratin, partial proliferation, desmosomes only.
      • Odland bodies (lipid lamellae) and involucrin are present.
  • Granular Layer (Stratum granulosum): significant increase in keratin synthesis (2 and 9), keratohyalin granules accumulate (filaggrin, loricrin).
    • Lucid Layer (Stratum lucidum): may be present/absent, high mechanical stress areas like palms/soles.
    • Defined as thick skin, with a thickness between 400-1400 µm.
    • Without a lucid layer is thin skin, thickness ranging from 75-150 µm.
    • Cornified Layer (Stratum corneum): devoid of nuclei, barrier formation, desquamation occurs due to loss of contact between lamellae, proteins break bonds. Corneal cytomerization.
  • Functions of cornified layer:
    • Defense mechanism.
      • Thermal regulation.
      • Resistance to electric current (lipids).
      • Barrier to chemical agents.
      • UV radiation filter.
Cells Present in the Epidermis
  • Keratinocytes
  • Melanocytes: located in the basal layer, produce melanin (eumelanin-brown or pheomelanin-reddish) in premelanosomes, melanin-stimulating hormone (MSH) regulated, cytocrine secretion.
    • Average density of 500-1000 melanocytes/mm2.
  • Langerhans Cells: in the spinous layer, macrophage system, high phagocytic activity, lack desmosomes, E-cadherin adhesion, Birbeck granules.
    • Merkel Cells: in the basal layer, sensory function (proprioceptors), desmosome connections, mechanical stimuli release calcium ions/neurotransmitters.
Transitional Epithelium
  • Three layers:
    • Basal Layer (stem cells).
    • Intermediate Layer (club-shaped/pyramidal cells).
    • Superficial Layer (umbrella/dome-shaped cells).
  • Characteristic of the urinary tract (urothelium).
Division of Epithelia Based on Systems
  • Epidermis: Stratified squamous keratinized
  • Epidermal-associated Glands: Simple cuboidal/columnar or compound columnar.
  • Urinary System: Simple in the kidney; transitional from renal pelvis to bladder and urethra, transitions to columnar -> simple columnar.
  • Respiratory System: Pseudostratified ciliated columnar in nose, nasopharynx, larynx, trachea; ciliated simple columnar in bronchi; simple cuboidal; simple squamous alveoli in the Lungs.
  • Digestive System: Partially keratinized stratified squamous in the mouth; non-keratinized stratified squamous in oropharynx and esophagus; simple columnar in cardia, stomach, small intestine, large intestine; distal anal opening becomes keratinized.
  • Epithelial tissue never travels alone:
    • Mucous Membrane: Epithelium + connective tissue lining cavity communicating with exterior.
    • Serosal Membrane: Epithelium + connective tissue lining a cavity communicating with the interior.
    • Mucosa or Lamina Propria: Only connective tissue lining a cavity.