Histology: Epithelial Tissue
More Than Skin Deep
Epithelial membranes cover the internal and external surfaces of the body. Epithelia arise from all of the 3 primary germ layers: ectoderm (outer layer; e.g. skin & surface of sense organs), mesoderm (middle layer; e.g. lining of body cavities), and endoderm (inner layer; e.g. internal linings of gastrointestinal & respiratory tracts).
General characteristics of epithelial cells and tissues:
- Polar: Epithelial cells have structurally- and functionally-distinct apical and basal surfaces. The apical surface faces the external environment or lumen (apical → apex + al = “related to the peak/tip”) while the basal surface faces the basement membrane (basal → base + al = “related to the bottom/foundation”)
- Avascular: Epithelial tissue does not contain blood vessels, with few exceptions (e.g. stria vascularis of inner ear)
- Closely-connected continuous sheets: Epithelial cells typically fit closely together, forming continuous sheets of tissue. The lateral surfaces of these cells interact through junctional complexes (adhering junctions, tight junctions, and desmosomes) and gap junctions
- Supported by connective tissue: Epithelia rely on support from underlying connective tissue, facilitated by a layer of extracellular matrix called a basement membrane. It is formed by the basal lamina (lamina lucida + lamina densa) and the reticular lamina. The basement membrane has numerous functions, including roles in mechanical support and angiogenesis
Epithelial Membrane Classification
Number of Cell Layers
Simple [1 layer]
Stratified [2+ layers]
Pseudostratified [1 layer; staggered]
Shape of Cells*
Squamous [flat, “scale-like”]
Cuboidal [square; height ≈ width]
Columnar [tall, column-shaped]
*stratified epithelia are classified by the most superficial (i.e. apical) layer
Created with BioRender.com
The different specialized structures of epithelial tissues give us clues about their functions. Let’s take a closer look!
Epithelia provide a layer of protection forall underlying tissues from toxins, pathogens, trauma, etc.
e.g. stratified squamous keratinized epithelium of the skin
Absorption and/or Secretion
Depending on the location, some epithelia are involved in absorption (often facilitated by microvilli) or secretion
e.g. simple cuboidal epithelium of the choroid plexus
Some epithelia have motile cilia on their apical surface that move in coordinated waves to move particles (e.g. mucus)
e.g. ciliated pseudostratified columnar epithelium of the trachea
While the majority of epithelia are avascular, they are innervated; some are more extensively innervated than others
e.g. stratified squamous non-keratinized epithelium of the cornea
Comprised of organized collections of secretory epithelial cells, glands (also called glandular epithelia) are broadly divided into two categories: endocrine (without ducts) & exocrine (with ducts). In general, epithelial glands develop as a down-growth of epithelium into the connective tissue below. Some glands will separate from the surface epithelium whence it originated (endocrine), while others will maintain a connection via ducts (exocrine).
Without further ado, let’s dive right in to the *secretive* world of glands!
Since they are ductless, endocrine glands release their secretions—called hormones—directly into the bloodstream for distribution to target tissues with specialized receptors. The endocrine system plays a very important regulatory role throughout the body.
Examples include the pituitary gland, the ovaries & testes, and the pancreas*.
♫ One of these things is not like the other ♫
Arising from the epithelial lining of the intestine, the pancreas is comprised of two types of secretory unit:
Exocrine glands release their secretions into a lumen (from latin “lumen” = “light / an opening”) through an epithelial-lined tube called a duct. Like epithelial membranes, there are several criteria for the classification of exocrine glands
Structure of the Duct
Simple [duct doesn’t branch]
Compound [duct branches]
Shape of the Secretory Units
Tubular [long & thin / tube-like]
Acinar / Alveolar [spherical / flask-like]
Tubuloalveolar [mix of both types]
Mechanism of Secretion
Merocrine [exocytosis; secretory vesicles]
Apocrine [piece of apical cell pinches off]
Holocrine [breakdown of entire secretory cell]
Created with BioRender.com
While most exocrine glands are multicellular, goblet cells are the only example of unicellular exocrine glands in mammals!
These specialized epithelial cells secrete mucus are typically found in simple and pseudostratified columnar membranes. Goblet cells appear pale in H&E-stained sections due to the high concentration of mucin-containing secretory vesicles.
Work in Progress!
Dr. Matt & Dr. Mike’s Medical YouTube [YouTube]
Among NUMEROUS other topics, medical educators Dr. Mike Todorovic & Dr. Matt Barton have several fantastic videos about epithelial tissue, including form, function, and classification! I highly recommend the “Epithelia” playlist as a starting point.
Histology @ Yale – Epithelia Lab [Course Website]
An online course supplement developed by Dr. Peter Takizawa at Yale University. Complete with some background reading & interactive quizzes. Test yourself, then click “Answer”!
*Note: the links under “keywords” appear no longer to be supported*
Jerad Gardner, MD [YouTube]
What about pathology? Check out dermatopathologist & sarcoma pathologist Dr. Jerad Gardner’s YouTube channel! Not sure where to begin? I recommend starting with the “Dermpath for Beginners” playlist.