Histology — High-Yield Summary

Epithelial Tissue

• Classified by layers: simple (one layer) vs. stratified (multiple layers), and by surface cell shape: squamous, cuboidal, or columnar.
• Simple squamous: lines vessels (endothelium) and body cavities (mesothelium) — built for diffusion/filtration.
• Pseudostratified ciliated columnar: looks layered but every cell touches the basement membrane; lines the respiratory tract.
• Transitional (urothelium): lines the urinary tract; surface cells flatten as the bladder distends.
• Stratified squamous: keratinized in skin (barrier), non-keratinized in the esophagus/oral cavity/vagina (abrasion resistance).

Connective Tissue

• Fibroblasts produce the extracellular matrix (collagen, elastin, ground substance) — the most common resident cell.
• Mast cells store histamine/heparin in metachromatic granules and drive allergic/inflammatory responses.
• Macrophages phagocytose debris and pathogens and present antigen.
• Cartilage: hyaline (articular surfaces, trachea), elastic (ear, epiglottis), fibrocartilage (discs, menisci — avascular, no perichondrium).
• Bone: osteoblasts secrete osteoid → become osteocytes once mineralized matrix surrounds them; osteoclasts (multinucleated) resorb bone.

Blood Cells

• Erythrocytes: anucleate, biconcave, ~7.5 µm — carry oxygen via hemoglobin.
• Platelets: anucleate cytoplasmic fragments of megakaryocytes, involved in clotting.
• Granulocytes: neutrophils (multilobed nucleus, most abundant, acute infection), eosinophils (bilobed, eosinophilic granules, parasites/allergy), basophils (rare, coarse basophilic granules, allergic response).
• Agranulocytes: lymphocytes (large nucleus, thin cytoplasmic rim, adaptive immunity), monocytes (kidney-shaped nucleus, become tissue macrophages).

Muscle Tissue

• Skeletal muscle: striated, long multinucleated fibers, peripheral nuclei, voluntary control.
• Cardiac muscle: striated, branching fibers, one or two central nuclei, intercalated discs couple adjacent cells.
• Smooth muscle: non-striated, spindle-shaped cells with a single central nucleus, involuntary control (viscera, vessels).

Nervous & Lymphatic Tissue

• Neurons: large cell body (soma) with a prominent nucleolus, dendrites, and a single axon; Nissl bodies (rough ER) in the cytoplasm.
• Lymph node: cortex (B-cell follicles with germinal centers) surrounds the medulla (plasma cells, macrophages); paracortex holds T cells.
• Spleen: white pulp (lymphoid tissue around a central arteriole — immune surveillance) vs. red pulp (sinusoids — filters and recycles old erythrocytes).

Glands & Cell Junctions

• Secretion modes: merocrine (exocytosis, cell intact — eccrine sweat glands), apocrine (pinches off apical cytoplasm — mammary glands), holocrine (whole cell ruptures — sebaceous glands).
• Exocrine vs. endocrine: exocrine glands keep a duct to an epithelial surface; endocrine glands lose the duct and secrete hormones straight into surrounding capillaries.
• Cell junctions: tight junctions seal the paracellular space; desmosomes anchor intermediate filaments between cells (mechanical strength); gap junctions allow direct ion/small-molecule passage (electrical coupling); hemidesmosomes anchor cells to the basement membrane.
• Kupffer cells: liver-resident macrophages lining the sinusoids, part of the reticuloendothelial system.