Chapter 4 Histology Notes: Glands, Epithelial Junctions, and Connective Tissue

Glands and Secretion

  • Epithelial cells that make and secrete a product form glands.
  • Gland products are aqueous (water-based fluids) that usually contain proteins.
  • Secretion is the process by which a gland obtains substances from the blood, chemically transforms them into a product, and discharges it from the cell.
  • Protein product synthesis pathway: rough endoplasmic reticulum (RER) → secretory granules → Golgi apparatus → exocytosis to release.
  • Glands classification depends on where they release their product: endocrine vs exocrine.

Endocrine vs Exocrine Glands

  • Endocrine glands: ductless; secrete hormones directly into surrounding tissue fluid.
  • Exocrine glands: use ducts to carry products to epithelial surfaces.
  • Hormones (endocrine) will be discussed in a later chapter.

Unicellular vs Multicellular Glands

  • Most glands are multicellular (composed of many cells).
  • Goblet cell as an example of a unicellular exocrine gland.

Goblet Cells and Mucin

  • Goblet cells are unicellular exocrine glands that typically secrete mucin.
  • Mucin + water forms mucus, a gelatinous substance that protects and lubricates many internal body surfaces.
  • Goblet cells are typically found in pseudostratified columnar epithelia or columnar epithelia.
  • Structure visible in TEM: rough endoplasmic reticulum and Golgi apparatus involved in production and shipping of proteins.

Exocrine Gland Variety and Secretory Units

  • Exocrine glands come in many varieties: mucus-secreting glands, sweat and oil glands, salivary glands; liver and pancreas are included as exocrine glands (in tissues with exocrine function).
  • Goblet cells produce mucin (a protein).

Multicellular Exocrine Glands: Structure and Classification

  • Basic parts: epithelium-lined duct and a secretory unit that produces the product.
  • Duct structure:
    • Simple glands: unbranched ducts.
    • Compound glands: branched ducts.
  • Secretory unit structure:
    • Tubular: secretory cells form tubes.
    • Alveolar (acinar): secretory cells form spherical sacs; alveolus = small hollow cavity.
    • Tubuloalveolar: combination of tubular and alveolar units.
  • Terminology:
    • Alveolar and acinar are commonly used synonymously.
    • Acinus derives from Latin for grape/berry; acinar refers to grape-like sacs.

Visualizing Gland Structures

  • Diagrams distinguish tubular vs alveolar (alveolar means sacs), simple vs compound ducts.
  • Exam tip: you may be tested on general structures; be aware that exact body locations are not always required unless specified.

Epithelial Surface Features and Cell Junctions

  • Epithelial tissues are composed of tightly joined cells with apical and basal regions; basal surface rests on a boundary with connective tissue (basement membrane).
  • Apical surface modifications include specialized features for function.
  • Major lateral cell junctions: tight junctions, desmosomes, gap junctions; plus adhesive belt junctions contribute to the junctional complex.

Tight Junctions (Zonula Occludens)

  • Form a belt-like junction around the periphery of each cell at the apical region.
  • Function: create a watertight seal that closes off extracellular space, preventing water and many molecules from passing between adjacent epithelial cells.
  • Example: in the intestinal lining, tight junctions prevent digestive enzymes, ions, and microorganisms in the lumen from seeping into the bloodstream.
  • Not completely impermeable; some may be leaky to certain ions.

Adhesive Belt Junctions (Zonula Adherens)

  • Located near tight junctions as part of the junctional complex.
  • Comprised of transmembrane linker proteins that bind to actin microfilaments of the cytoskeleton.

Desmosomes (Anchoring Junctions)

  • Also called anchoring junctions; function like spot welds or rivets to resist shear stress between cells.
  • Structure: cytoplasmic plaques of proteins; linker cadherins extend into the extracellular space and interdigitate like teeth of a zipper.
  • Provide strong cell-to-cell adhesion; common in epithelial tissues and cardiac muscle.
  • You do not need to know every protein (e.g., specific cadherins or keratin filaments) in detail, but know what a desmosome is and where it is found.

Gap Junctions

  • Made of connexin proteins forming channels between adjacent cells.
  • Allow small molecules and ions to pass directly from cytoplasm of one cell to the cytoplasm of a neighboring cell.
  • Important for tissues requiring synchronized activity (e.g., heart muscle).

Basal Lamina and Basement Membrane

  • Basal surface of epithelium secretes basal lamina, a thin region of protein fibers.
  • Basal lamina is part of the basement membrane that anchors epithelia to connective tissue.
  • Functions:
    • Selective filter for molecules diffusing from capillaries into epithelium.
    • Scaffolding along which regenerating epithelial cells migrate.
    • Together with reticular fibers from connective tissue, forms the basement membrane.

Apical Surface Features: Microvilli and Cilia

  • Microvilli (plural): finger-like membrane extensions with a core of actin filaments; extend surface area for absorption and transport; contain membrane channels and carriers.
  • Cilia (plural): highly motile extensions with a core of microtubules held together by cross-linking proteins and motor proteins; capable of movement.
  • Locations discussed: female reproductive tract, respiratory tract.
  • Note: Cilia structure and details extend into cell biology; not all details are required for the first exam.

Connective Tissue Overview

  • Four main classes of connective tissue: connective tissue proper, cartilage, bone, and blood (blood is not a typical solid connective tissue; it has a fluid extracellular matrix).
  • Functions (illustrative): form the basis of the skeleton (hyaline cartilage and bone), store and carry nutrients, protect and surround structures like vessels and nerves, defend against infection.
  • ECM components: ground substance (proteoglycans and glycosaminoglycans) and protein fibers (collagen, elastic, reticular).
  • Cells that produce ECM: fibroblasts (main producers in most connective tissues); chondroblasts (cartilage); osteoblasts (bone).
  • Blood plasma matrix is produced by the liver, not by blood cells themselves.
  • Embryonic origin: mesenchyme.

Cells in Connective Tissue

  • Fibroblasts: produce collagen and elastic fibers of ECM.
  • Chondroblasts: produce ECM in cartilage.
  • Osteoblasts: produce bone matrix.
  • In blood: there are no fibroblasts, chondroblasts, or osteoblasts; plasma matrix is liver-derived.
  • Other cell types in areolar tissue:
    • Adipocytes (fat cells) in adipose tissue.
    • Leukocytes (white blood cells): neutrophils, lymphocytes, eosinophils, monocytes/macrophages.
    • Mast cells: involved in inflammation and allergic responses.
    • Plasma cells: produce antibodies.

Fibers in Connective Tissue ECM

  • Collagen fibers: stain pink; strong in resisting shear and tension.
  • Reticular fibers: bundles of a special type of collagen; form networks; common in lymphoid organs (reticulum means network).
  • Elastic fibers: contain elastin; provide recoil after stretching.
  • Ground substance: gel-like matrix produced by fibroblasts; fills spaces between fibers and cells; contains proteoglycans and glycosaminoglycans; does not stain like fibers.
  • Ground substance also binds tissue fluid and cushions/protects; interstitial fluid can become edema if not drained by lymphatics.

Ground Substance and Extracellular Matrix (ECM)

  • ECM composed of ground substance and fibers; ground substance is not stained as densely as fibers in histology slides.
  • Ground substance stores tissue fluid and proteoglycans/GAGs; acts as a cushion and barrier, and aids diffusion.

Areolar Connective Tissue: A Model Tissue

  • Areolar connective tissue is a model tissue for studying connective tissue properties.
  • Structure: gel-like matrix with all three fiber types (collagen, elastic, reticular) and a variety of cells (fibroblasts, macrophages, mast cells, adipocytes, neutrophils, lymphocytes).
  • Location and function: underlies epithelial tissue, surrounds small nerves and blood vessels; flexible yet supportive; fights infection (macrophages, leukocytes).
  • Important for studying ground substance, fibers, and cell types.

Adipose Tissue

  • Adipose tissue is composed mainly of adipocytes (fat cells).
  • Adipocytes contain a large lipid droplet that pushes the nucleus to the side; low water content per cell.
  • Richly vascularized to supply nutrients for stored fat; storage of triglycerides.
  • Functions: reserve fuel, insulation, protection/support of internal organs.
  • Locations: subcutaneous tissue (hypodermis), around kidneys, behind eyeballs, within the abdomen, breasts.
  • How to identify: adipose tissue rich in adipocytes with prominent fat vacuoles; identify adipose tissue, adipocytes, and fat vacuoles.
  • Brown adipose tissue (BAT): more mitochondria, highly vascularized, produces heat directly; important in newborns for thermogenesis; present in small amounts in adults (near shoulder blades, neck, and anterior abdominal wall).

Reticular Connective Tissue

  • Found in lymphoid organs (lymph nodes, spleen) and as the framework (stroma) of bone marrow.
  • Structure: network of reticular fibers with supporting cells and lymphocytes scattered within the ground substance.

Areolar Tissue and Integumentary System Connections

  • Integumentary system (Chapter 5) focuses on skin (epidermis and dermis).
  • About 20% of the dermis is composed of areolar connective tissue near the epidermis.
  • Areolar tissue underlies epithelial tissue, surrounds small nerves and blood vessels, and participates in immune defense and fluid handling.

Practical and Exam Relevance

  • Areolar connective tissue and its components are commonly tested in lab practicals: identify ground substance, fibroblasts, and the three fiber types (collagen, elastic, reticular).
  • Be able to name tissue types and cells from histology images, including identifying adipose tissue with adipocytes and fat vacuoles, and recognizing brown adipose tissue by its higher vascularity/mitochondria.
  • Know the differences between loose (areolar, adipose, reticular) and dense connective tissues (dense irregular, dense regular, elastic).
  • Understand embryonic origin (mesenchyme) and the general pathway of ECM production by fibroblasts.
  • Recognize the role of basement membrane in attachment, diffusion, and regeneration, and the function of the basal lamina as a selective barrier.

Quick Reference: Key Terms to Know

  • Gland, secretion, endocrine, exocrine, goblet cell, mucin, mucus, secretory unit, duct, tubular, alveolar, acinar, tubuloalveolar.
  • Tight junction (zonula occludens), adhesive belt junction (zonula adherens), desmosome, gap junction.
  • Basal lamina, basement membrane, interstitial fluid, ground substance, proteoglycans, glycosaminoglycans (GAGs).
  • Fibers: collagen, elastic, reticular; cells: fibroblast, chondroblast, osteoblast, adipocyte, leukocyte, mast cell, plasma cell, macrophage.
  • Adipose tissue types: white adipose tissue, brown adipose tissue.
  • Reticular connective tissue and stromal framework (stroma).

Ethical/Practical Implications

  • Understanding tissue structure-function relationships informs diagnoses and understanding of disease (e.g., edema due to disrupted ground substance, infection risk in loose connective tissue, fibrosis in dense connective tissue).
  • Exam preparation emphasizes identification and basic function rather than exhaustive molecular detail, aligning with typical first-year histology objectives.
  • Emphasis on canonical model tissues (areolar) helps students build a transferable framework for recognizing tissues in various organs.