Glandular epithelium

Introduction to Glandular Epithelium

  • Focus on epithelial cells that comprise glands in the human body.
  • Glandular epithelium is the second major type of epithelium.

Types of Epithelium

  • Covering Epithelium:
    • Covers the surface of the body and internal organs.
  • Glandular Epithelium:
    • Composed of cells with different shapes than those in surface epithelium.
    • Contains basal laminae similar to covering epithelium.

Structure and Classification of Glands

  • Connective Tissue Surrounding Glands:

    • Glands are typically surrounded by connective tissue called stroma.
    • Cells that perform the main function of the gland (production of hormones, enzymes, lipids) are called parenchyma.
    • Secretory structures and ducts determine the gland's structure.

  • Secretory Granules/Vesicles:

    • Found in cytoplasm of parenchyma cells.
    • Often consist of hormones or enzymes, especially in endocrine glands.

  • Mucins/Mucinogens in Gastrointestinal Tract:

    • Secretions that are glycoproteins or combinations of proteins, lipids, and carbohydrates, such as those in mammary glands.

Modes of Secretion in Glands

  • Classification by Secretion Mode:

    1. Ducts (Exocrine): Products are secreted through ducts to target organs.
    2. Blood (Endocrine): Products, such as hormones, enter the bloodstream and reach various organs.

  • Classification by Cell Number:

    • Unicellular Glands: One cell (e.g., goblet cells).
    • Multicellular Glands: More than one cell.

  • Modes of Secretion Based on Cell Structure:

    • Holocrine: Entire cell is secreted; cell dies (e.g., sebaceous glands).
    • Merocrine: Typical exocytosis; cell remains intact (e.g., salivary glands).
    • Apocrine: A portion of the cell is pinched off but the cell remains alive (e.g., mammary glands).

Types of Secretions

  • Mucous Secretions:

    • Contain glucosylated proteins (mucins).

  • Serous Secretions:

    • Composed mainly of non-glucosylated proteins.

  • Mixed Glands:

    • Contain both mucous and serous components.

Example of Simple Gland: Goblet Cells

  • Goblet Cells:

    • Simple unicellular exocrine gland found in the intestinal epithelium.
    • Structure: Basal part and apical part with microvilli.
    • Apical part contains secretory vesicles filled with mucinogens that create a mucosal film for lubrication.

  • Histological Identification:

    • Identifiable by PAS staining to visualize glycoproteins.
    • Apical microvilli present at their surface.

Comparison of Goblet Cells and Absorptive Cells

  • Structural Differences:
    • Goblet cells: Secretion function.
    • Absorptive cells: Absorption function (columnar cells with microvilli).

Multicellular Gland Organization

  • Secretory Units:

    • Multicellular glands organized into secretory units that communicate and coordinate secretion.

  • Classification by Structural Organization:

    1. Simple Glands:
    • Duct leads directly to secretion site.
    1. Compound Glands:
    • Multiple ducts combine before reaching target organ.

Types of Simple Glands

  • Tubular Glands:
    • Simple tubular (e.g., intestinal glands).
    • Tubular branched (multiple tubes combined).
    • Coiled (spiral structures).
    • Acinar (rounded structures).

Complex Glands: Organization

  • Compound Glands:
    • Each secretory part ends in a colector duct that merges repeatedly into a final duct leading to a target organ.
    • Examples: Pancreas and salivary glands.

Terminology and Structures of Glands

  • Stroma and Capsule:
    • Capsule encases the gland.
    • Septa project into the gland, dividing it into lobes and lobules.
    • Blood vessels coursing through stroma.

Secretory Types in Glands

  • Duct Structure:
    • Ducts collect secretions from acini and channel them to target organs.

Endocrine Cells

  • Endocrine vs. Exocrine:
    • Endocrine glands release hormones directly into blood, no ducts are needed.
    • Organized into groups or individual cells (e.g., enteroendocrine cells).

Hormonal Signaling

  • First Messengers: Hormones that act as signals to initiate biological responses.
  • Second Messenger System: Activated receptors influence internal cellular pathways.
  • Hormonal Regulation: Specific hormones have specific targets depending upon receptor presence.

Major Endocrine Glands

  • Pituitary Gland: Master gland located in the skull, controlled by the hypothalamus.
  • Regulates several other glands like thyroid, adrenal, pancreas, and gonads.

Hormonal Feedback Loops

  • Hormone levels regulated by feedback mechanisms.
    • Pituitary hormones control target gland hormone levels, which in turn may inhibit pituitary release.

Structural Differences Among Endocrine Glands

  • Thyroid Gland: Follicular structure with colloid storage (thyroglobulin).
  • Adrenal Glands: Sections (zona glomerulosa, zona fasciculata, zona reticularis, and medulla) for different hormones.
  • Pancreas: Both exocrine and endocrine functions, with pancreatic acini for enzyme secretion and islets of Langerhans for hormone secretion.

Tumors of Epithelial Origin

  • Epithelial tumors: Can be benign (adenoma) or malignant (carcinoma).
  • Common Cancers: Include breast, lung, and colon cancer, often categorized as carcinoma or adenocarcinoma depending on origin.