Cells & Tissue

Connective tissue and immune cells

Mast Cells

• MCQ Buzzwords: Perivascular location, granular cytoplasm, ovoid dark nucleus, metachromasia with Giemsa or Toluidine blue.
• Key Secretions: Histamine (vasodilation, increased vascular permeability), Heparin (anticoagulant), Tryptase, Eosinophil chemotactic factors, and Leukotrienes
• Clinical Context: Mediates Type I Hypersensitivity reactions (anaphylaxis, asthma, urticaria) via IgE cross-linking.

Plasma Cells

• MCQ Buzzwords: "Clock-face" or "Cartwheel" chromatin pattern in an eccentric nucleus, abundant basophilic cytoplasm, and a prominent perinuclear halo (which represents a highly developed, unstained Golgi apparatus).
• Key Secretions: Antibodies / Immunoglobulins (IgG, IgA, IgM, IgE, IgD).
• Clinical Context: Terminally differentiated B-lymphocytes. Malignant proliferation leads to Multiple Myeloma (look for Bence-Jones proteins in urine).

Macrophages (Histiocytes)

• MCQ Buzzwords: Large size, indented/kidney-shaped or horseshoe-shaped nucleus, abundant cytoplasm filled with vacuolar lysosomes or phagocytosed debris.
• Key Secretions: $IL-1, IL-6, TNF-\alpha (acute phase response triggers), IL-12 (induces Th1 differentiation), and Nitric Oxide (NO).
• Clinical Context: Antigen-presenting cells (APCs) derived from blood monocytes. In specific granulomatous diseases, they fuse to form multinucleated Langhans giant cells (horseshoe pattern of nuclei) or Foreign body giant cells (disorganized nuclei pattern).

Eosinophils

• MCQ Buzzwords: Bilobed nucleus (looks like spectacles/sunglasses) and packed with intense, bright pink/red acidophilic/eosinophilic granules.
• Key Secretions: Major Basic Protein (MBP) (highly toxic to helminths/worms), Eosinophil Cationic Protein (ECP), and Histaminase (to degrade histamine and limit mast cell reactions).
• Clinical Context: Key defenders against parasitic infections; highly elevated in allergic conditions (asthma, drug reactions).

Neutrophils

• MCQ Buzzwords: Multilobed nucleus (usually 3 to 5 lobes connected by thin strands), pale pink cytoplasm containing fine primary (azurophilic) and secondary (specific) granules. Often referred to as Polymorphonuclear leukocytes (PMNs).
• Key Secretions: Myeloperoxidase (MPO), Collagenase, Lysozyme, and Lactoferrin.

•Clinical Context: The first responders to acute bacterial infection and tissue necrosis. They form the primary cellular component of pus.

Intercellular junctions function

Gap junctions

• Structure: Composed of transmembrane protein complexes called connexons (each formed by six connexin proteins) that align between adjacent cell membranes to create physical, open channels.
• Function: They allow the direct, rapid exchange of small cytoplasmic molecules, ions Ca, and metabolic signals between adjacent cells without entering the extracellular space.
• Key Locations: Crucial in cardiac muscle (at intercalated discs) and single-unit smooth muscle to facilitate electrical coupling, allowing the tissue to contract synchronously as a single functional unit (syncytium).

Tight junctions (Zonula occludens)

• Structure: Interlocking strands of transmembrane proteins (primarily claudins and occludins) that fuse the outer plasma membranes of adjacent cells together near their apical surface.
• Function: They form a tight, impermeable seal that prevents the paracellular leakage of water, ions, and molecules between cells.
• Key Locations: Found heavily in intestinal epithelial linings to force nutrients through transcellular pathways, and forming the basis of the blood-brain barrier.

Anchoring junctions

• Structure & Classification: This is a broad category of junctions that physically fasten cells together or to the underlying extracellular matrix. They connect to the cellular cytoskeleton rather than leaving open pathways. The group includes:
  • Zonula adherens (Adherens junctions): Connect actin filaments of one cell to another via cadherin proteins.
  • Macula adherens (Desmosomes): Connect intermediate filaments (keratin) between cells like spot-welds to resist intense mechanical shearing forces (prominent in the skin and cardiac muscle).
• Function: They provide structural stability and mechanical strength, keeping tissues from tearing apart under stress. They do not allow the exchange of cytoplasmic material.

Focal junctions (Focal adhesions)

• Structure: A specialized subtype of anchoring junction that utilizes transmembrane integrin proteins to link the internal actin cytoskeleton of a cell down to proteins within the extracellular matrix (like fibronectin or laminin).
• Function: They anchor cells to the basement membrane and play a dynamic role in cell migration, signaling, and sensing mechanical changes in the extracellular environment.

Ito Cells

Ito cells, also classically known as hepatic stellate cells (HSCs), perisinusoidal cells, or fat-storing cells, are a highly specialized cell type found in the microscopic architecture of the liver.

Under a microscope, Ito cells reside within a critical microenvironment called the Space of Disse (the perisinusoidal space). This is the narrow anatomical gap located between the fenestrated endothelial cells of the liver sinusoids and the cords of functional liver cells (hepatocytes).

1. The Quiescent State (Healthy Physiology)

In a normal, healthy liver, Ito cells remain in a resting, or quiescent, state. Their primary responsibilities are:

• Vitamin A Storage: They are the principal storage site for Vitamin A (retinoids) in the entire human body, keeping it packed away safely within characteristic lipid droplets in their cytoplasm.
• Microenvironment Maintenance: They produce quiet amounts of extracellular matrix (ECM) to keep the delicate architecture of the Space of Disse stable.

2. The Activated State (Chronic Liver Injury & Pathology)

When the liver suffers from chronic inflammation or injury (such as from chronic alcohol abuse, chronic Hepatitis B or C, or Non-Alcoholic Fatty Liver Disease/NASH), adjacent damaged hepatocytes and Kupffer cells (liver macrophages) release inflammatory cytokines like TGF-β (Transforming Growth Factor-beta).

White vs Brown Adipose

RBC