Tumor Pathology: Growth Patterns, Structure, and Atypism

Posted on Mar 5, 2026 in Biomedical Sciences

I. Types of Tumor Growth

  • Expansile Growth: The tumor grows as a cohesive mass, pushing aside adjacent tissues. It forms a capsule, demarcating it from normal tissue (e.g., fibroadenoma).
  • Infiltrative (Invasive) Growth: Tumor cells invade and destroy surrounding tissues without clear boundaries. This is a hallmark of malignant neoplasms.
  • Exophytic Growth: Grows outward from an epithelial surface into a free space (e.g., lumen of the stomach, colon, or bronchus).
  • Endophytic (Ulcerative) Growth: The tumor grows primarily inward, infiltrating the wall of a hollow organ, often leading to ulceration of the overlying surface (e.g., ulcerative gastric carcinoma).
  • Multicentric Growth: Arises from multiple independent foci within the same organ (e.g., transitional cell carcinoma of the bladder).

II. General Structure of the Tumor

  • Parenchyma: The proliferating neoplastic cells. This determines the tumor’s name and biological behavior.
  • Stroma: The supportive, non-neoplastic connective tissue framework, consisting of blood vessels, lymphatics, extracellular matrix, and inflammatory cell infiltrate.

Macroscopic Features

  • Shape: Nodular, polypoid, fungating, ulcerative, diffuse, etc.
  • Size: Varies greatly.
  • Color: Often resembles the tissue of origin (e.g., lipomas are yellow).
  • Consistency: Soft (medullary), firm (scirrhous), or variable.
  • Demarcation: Well-circumscribed (often benign) or poorly defined/infiltrative (malignant).

Microscopic Features

  • Architectural Atypia: Loss of normal tissue organization (polarity, layered structure).
  • Cellular Atypia: See section III.
  • Relationship to Basement Membrane: In situ neoplasia (dysplasia/carcinoma in situ) shows atypia confined within an intact basement membrane. Invasion is the hallmark of malignancy, where tumor cells breach the basement membrane into the stroma.

III. The Concept of Atypism

Atypism (Anaplasia) is a set of morphological and functional deviations of tumor cells from normal, reflecting cellular and genetic instability.

Types of Atypism

  • Morphological (Cytological) Atypism: Cellular pleomorphism, nuclear atypia, hyperchromasia, increased nuclear/cytoplasmic ratio, irregular nuclear contour, coarse chromatin, and prominent (often multiple) nucleoli. Includes abnormal mitoses and cytoplasmic alterations.
  • Structural (Architectural) Atypism: Loss of normal tissue architecture and polarity, disordered arrangement of cells, and abnormal relationship between parenchyma and stroma. For glandular epithelium: irregular gland size, shape, and crowding; cribriform patterns.
  • Biochemical (Metabolic) Atypism: Altered metabolic pathways (e.g., aerobic glycolysis – Warburg effect) and synthesis of abnormal products (e.g., oncofetal antigens, ectopic hormones).
  • Immunological Atypism: Expression of tumor-associated antigens and possible loss of histocompatibility antigens.
  • Functional Atypism: Loss of specialized function of the original tissue or acquisition of new functions (e.g., paraneoplastic syndromes).

IV. Degree of Tumor Differentiation

  • Well-Differentiated: Tumor cells closely resemble normal mature cells of the tissue of origin. Typical of benign and some low-grade malignant tumors.
  • Moderately-Differentiated: Intermediate state with discernible features of origin but clear atypia.
  • Poorly-Differentiated: Tumor cells show minimal resemblance to the normal tissue, with marked atypia and high proliferative activity.
  • Undifferentiated (Anaplastic): Cells are so primitive that the tissue of origin cannot be determined morphologically. They display extreme pleomorphism and high aggressiveness.

Note: The degree of differentiation (grading: G1 to G4) is a key prognostic factor in malignancies: poorer differentiation leads to a higher grade and generally more aggressive behavior.

V. Histogenesis of Tumors

Histogenesis is the origin of a tumor from a specific cell type or tissue and serves as the basis for classification.

Major Histogenetic Groups

  • Epithelial Tumors: Originates from surface, glandular, or parenchymal epithelium. Benign: Adenoma, Papilloma. Malignant: Carcinoma (e.g., adenocarcinoma, squamous cell carcinoma).
  • Mesenchymal Tumors: Benign: Lipoma, leiomyoma, chondroma. Malignant: Sarcoma (e.g., liposarcoma, leiomyosarcoma, osteosarcoma).
  • Melanocyte Tumors: Benign: Nevus. Malignant: Melanoma.
  • Neural and Neuroectodermal Tumors: Glioma, meningioma.
  • Hematopoietic and Lymphoid Tumors: Leukemias, lymphomas.
  • Germ Cell Tumors: Teratoma, seminoma.
  • Embryonic Tumors (Blastomas): Nephroblastoma, neuroblastoma.