Carcinoid tumor GIT pathology

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Carcinoid Tumors of the GIT - Pathology

Definition and Nomenclature

Carcinoid tumors (now formally termed neuroendocrine tumors, NETs by the WHO) are neoplasms arising from neuroendocrine cells distributed throughout many organs, particularly the gut and endocrine pancreas. The term "carcinoid" (meaning "carcinoma-like") was coined because these tumors are slower-growing than carcinomas. The current WHO classification uses "low- or intermediate-grade neuroendocrine tumor," reserving "neuroendocrine carcinoma" for high-grade tumors resembling small cell carcinoma.

Robbins & Kumar Basic Pathology, p. 559

Epidemiology and Distribution

Most common site: GIT (>40% in the small intestine)
Second most common: Tracheobronchial tree / lungs
Peak incidence: Sixth decade, but can occur at any age
They constitute approximately 50% of all neuroendocrine tumors of the gut and about 5% of all colonic tumors

Cell of Origin

These tumors arise from enterochromaffin (Kultschitzky) cells - the APUD (Amine Precursor Uptake and Decarboxylation) cells of the gut mucosa. These cells are distributed throughout the GIT from stomach to rectum and normally produce vasoactive amines and peptides.

Morphology

Gross Pathology

Intramural or submucosal masses forming small polypoid lesions
Yellow or tan in color on cut section
Elicit an intense desmoplastic reaction that may cause kinking of the bowel and obstruction
Rarely exceed 3-4 cm at presentation

Histological Pathology

FIG. 13.17 Gastrointestinal carcinoid tumor (neuroendocrine tumor). (A) Carcinoid tumors often form a submucosal nodule composed of tumor cells embedded in dense fibrous tissue. (B) High magnification shows the bland cytology that typifies neuroendocrine tumors. The chromatin texture, with fine and coarse clumps, frequently assumes a "salt-and-pepper" pattern.

Composed of islands, trabeculae, strands, glands, or sheets of uniform cells
Cells have scant, pink granular cytoplasm and round-to-oval stippled ("salt-and-pepper") nuclei - a hallmark feature
Low mitotic activity in low-grade tumors
Dense-core neurosecretory granules visible on electron microscopy
On electron microscopy: characteristic dense-core granules
Robbins & Kumar Basic Pathology, p. 559-560

Classification by Location (Most Important Prognostic Factor)

The most important prognostic factor for GIT neuroendocrine tumors is anatomical location, divided into foregut, midgut, and hindgut:

1. Foregut Tumors (Stomach, Duodenum proximal to Treitz, Esophagus)

Rarely metastasize
Generally cured by resection
Gastric carcinoids may arise in the setting of autoimmune gastritis with hypergastrinemia (hormonal drive causing neuroendocrine cell hyperplasia)
Rare gastrin-producing foregut tumors (gastrinomas) cause Zollinger-Ellison syndrome - gastroduodenal ulcers, refractory GERD, diarrhea

2. Midgut Tumors (Jejunum and Ileum)

Most common location overall
Often multiple
Tend to be aggressive with greater metastatic potential
Depth of invasion, size, necrosis, and mitoses correlate with poor prognosis
Even lesions <1 cm in the midgut have been shown to metastasize - radical resection indicated

3. Hindgut Tumors (Appendix and Colorectum)

Appendix:
- Most common overall location for carcinoid (incidental finding in appendectomy specimens ~1%)
- Typically submucosal rubbery masses discovered incidentally
- Almost uniformly benign course
- Lesions <1 cm (95% of cases): appendectomy with negative margins is adequate
- Lesions ≥2 cm: right hemicolectomy recommended
- Carcinoid syndrome is rare (only ~2.9% if hepatic metastases develop)
Rectum:
- Tend to produce polypeptide hormones
- Usually discovered when small - metastasis is uncommon
- Present with abdominal pain and weight loss
Robbins & Kumar Basic Pathology, p. 560; Schwartz's Principles of Surgery, p. 1366

Immunohistochemistry and Markers

Carcinoid tumors stain positively for:

Chromogranin A (serum marker - also useful for postoperative surveillance)
Synaptophysin
Neuron-specific enolase (NSE)
Serotonin
Other peptides: bombesin, calcitonin, substance P

Carcinoid Syndrome

Carcinoid syndrome occurs in <10% of patients (due to liver's first-pass metabolism of vasoactive substances).

Requires: Hepatic metastatic disease (or primary bronchial/retroperitoneal tumor draining outside the portal system).

Mediators

The APUD cells produce: serotonin, histamine, kallikrein, bradykinin, prostaglandins

Clinical Features

Feature	Details
Flushing	Most common (~80%); 5-10 min episodes of facial/truncal erythema
Diarrhea	Explosive, watery; due to serotonin release
Bronchospasm	Wheezing
Carcinoid heart disease	Present in 60-70% of advanced cases; right-sided valvular fibrosis (tricuspid + pulmonary); leads to right heart failure
Abdominal cramps	Colicky pain

Diagnosis

24-hour urine 5-HIAA (5-hydroxyindoleacetic acid) - primary stable metabolite of serotonin
Plasma Chromogranin A
Cross-sectional imaging + somatostatin receptor scintigraphy
Maingot's Abdominal Operations, pp. 1985-1987

Malignant Potential and Prognosis

Location	Size	Metastatic Risk
Appendix <1 cm	Very low (~2%)	Benign course
Appendix 1-2 cm	Intermediate	Controversial
Appendix ≥2 cm	Higher	Right hemicolectomy
Midgut (any size)	Significant	Radical resection
Rectum (small)	Low	Local excision
Foregut	Low	Resection curative

5- and 10-year survival for patients with residual abdominal tumor and hepatic metastases approaches 60%, reflecting the relatively slow growth of these tumors.

Treatment Summary

Surgery: First-line for localized disease; debulking/hepatic resection for metastatic disease
Hepatic artery embolization / radiofrequency ablation: For widespread hepatic metastases
Somatostatin analogues (Octreotide): Palliates carcinoid syndrome in ~90%; inhibits vasoactive peptide release
Chemotherapy: Doxorubicin, 5-FU, dacarbazine, interferon-α - response rates ~20%
Monitoring: serial 5-HIAA and chromogranin A levels
Maingot's Abdominal Operations, pp. 1998-2001; Bailey & Love's Short Practice of Surgery, p. 1411

High-Grade Variant

High-grade neuroendocrine carcinomas (poorly differentiated) resemble small cell carcinoma histologically. In the GIT, these are most common in the jejunum and carry a much worse prognosis.

Sources:

Robbins & Kumar Basic Pathology, pp. 559-560 (primary pathology reference)
Maingot's Abdominal Operations, pp. 1983-2001 (carcinoid syndrome)
Schwartz's Principles of Surgery, p. 1366 (appendiceal carcinoids)
Bailey & Love's Short Practice of Surgery 28th Ed., p. 1411

Pathogens is morphology

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Pathogens and Their Morphology

A complete guide to the morphological characteristics of the major pathogen groups: bacteria, viruses, fungi, and parasites.

I. BACTERIA

A. General Characteristics

Bacteria are prokaryotes - they lack a membrane-bound nucleus and membrane-enclosed organelles. They are generally 0.5-5 µm in size. Their genome is a single, circular, double-stranded DNA chromosome located in the nucleoid region (not a true nucleus).

Key structures:

Cell membrane (phospholipid bilayer)
Cell wall (peptidoglycan - the basis for Gram staining)
Ribosomes (70S = 50S + 30S subunits)
No mitochondria - energy generated at cell membrane

B. Basic Bacterial Shapes

Shape	Term	Examples
Spherical	Coccus (pl. cocci)	Staphylococcus, Streptococcus, Neisseria
Rod-shaped	Bacillus (pl. bacilli)	E. coli, Bacillus, Clostridium, Salmonella
Comma-shaped	Vibrio	Vibrio cholerae
Spiral, rigid	Spirillum	Helicobacter pylori
Spiral, flexible	Spirochete	Treponema pallidum, Borrelia, Leptospira
Curved rod	Campylobacter	Campylobacter jejuni
Club-shaped	Pleomorphic	Corynebacterium diphtheriae

C. Arrangement of Cocci

Arrangement	Description	Example
Diplococcus	Pairs	Neisseria gonorrhoeae, S. pneumoniae
Streptococcus	Chains	Streptococcus pyogenes
Tetrad	Groups of 4	Micrococcus
Sarcina	Cuboidal packet of 8	Sarcina
Staphylococcus	Irregular clusters (grapelike)	Staphylococcus aureus

D. Gram Staining - The Fundamental Classification

The Gram stain divides bacteria based on cell wall composition:

Feature	Gram-Positive	Gram-Negative
Color	Purple/violet	Pink/red
Cell wall	Thick peptidoglycan (20-80 nm)	Thin peptidoglycan (2-7 nm) + outer membrane
Outer membrane	Absent	Present (contains LPS/endotoxin)
Teichoic acids	Present	Absent
Periplasmic space	Narrow	Wide
Toxin type	Exotoxins (mainly)	Endotoxin (LPS) + exotoxins

Important Gram-positive pathogens:

Cocci: Staphylococcus aureus, Streptococcus pyogenes, S. pneumoniae, Enterococcus
Rods/Bacilli: Bacillus anthracis, Clostridium tetani, C. perfringens, C. difficile, Listeria, Corynebacterium diphtheriae

Important Gram-negative pathogens:

Cocci: Neisseria meningitidis, N. gonorrhoeae
Rods: E. coli, Klebsiella, Salmonella, Shigella, Pseudomonas, Haemophilus influenzae, Campylobacter, Helicobacter pylori, Vibrio cholerae, Yersinia pestis

E. Special Morphological Features

1. Spore-forming bacteria (endospores - highly resistant structures):

Bacillus anthracis - central spore
Clostridium tetani - terminal spore ("drumstick")
C. perfringens - subterminal spore
C. botulinum - subterminal spore

2. Capsule-forming bacteria (polysaccharide coat; anti-phagocytic):

Streptococcus pneumoniae (polysaccharide capsule - visualized by Quellung reaction)
Klebsiella pneumoniae (mucoid colonies)
Haemophilus influenzae type b
Neisseria meningitidis
Cryptococcus neoformans (fungal - detected by India ink)

3. Flagella (motility):

Monotrichous - single polar flagellum: Vibrio cholerae, Pseudomonas
Lophotrichous - tuft at one pole: Helicobacter pylori (multiple at one end)
Amphitrichous - both poles: Campylobacter
Peritrichous - all around: E. coli, Salmonella, Listeria
Non-motile (no flagella): Shigella, Klebsiella, Yersinia pestis

4. Fimbriae/Pili:

Hair-like protein appendages; shorter and more numerous than flagella
Function in adhesion to host cells
Sex pili (F-pili): mediate conjugation (DNA transfer)
E. coli, Neisseria gonorrhoeae (type IV pili)

F. Acid-Fast Bacteria (Mycobacteria)

Do not Gram stain well due to high mycolic acid content in cell wall. Identified by Ziehl-Neelsen (ZN) stain - appear red against blue background.

Organism	Disease
Mycobacterium tuberculosis	Tuberculosis
M. leprae	Leprosy
M. avium complex	Opportunistic infection

Morphology: Slender, beaded rods (~2-4 µm); slow-growing; obligate aerobes; waxy hydrophobic cell wall.

G. Atypical Bacteria (No Cell Wall or Different Structure)

Organism	Morphology	Key Feature
Mycoplasma	Pleomorphic (no fixed shape)	No cell wall; smallest free-living organism; resistant to beta-lactams
Chlamydia	Obligate intracellular; elementary body (infectious) + reticulate body (replicating)	No peptidoglycan; cannot make own ATP
Rickettsia	Small coccobacilli	Obligate intracellular; arthropod-borne
Spirochetes	Helical, flexible	0.1-0.5 µm wide, 5-250 µm long; motility via endoflagella (axial filaments)

Spirochete morphology details:

Treponema pallidum (syphilis): very thin, tightly coiled (~8-14 coils), 6-15 µm; cannot be cultured
Borrelia burgdorferi (Lyme disease): loosely coiled, 10-30 µm; visualized on Giemsa
Leptospira (leptospirosis): hooked ends, 6-20 µm; "shepherd's crook" shape

II. VIRUSES

A. General Structure

Viruses are non-cellular obligate intracellular parasites. They consist of:

Nucleic acid core - either DNA or RNA (never both), single- or double-stranded
Capsid - protein coat surrounding the nucleic acid; made of capsomeres
Envelope (some viruses) - lipid bilayer derived from host cell membrane; contains viral glycoproteins

Size: Generally 20-300 nm (smaller than bacteria)

B. Capsid Symmetry

Type	Description	Examples
Icosahedral	20 triangular faces; 12 vertices (spherical-appearing)	Adenovirus, Picornavirus, Herpesvirus (inner capsid), Hepatitis B
Helical	Capsomeres arranged in a helix around nucleic acid	Rabies, Influenza, RSV, Measles
Complex	Neither icosahedral nor helical	Poxviruses (brick-shaped), Bacteriophages

C. Enveloped vs. Non-Enveloped

Feature	Enveloped	Non-Enveloped (Naked)
Structure	Lipid bilayer + glycoproteins	Capsid only
Stability	Labile (destroyed by detergents, heat, drying)	More stable in environment
Transmission	Requires close contact; respiratory/sexual/blood	Can survive on surfaces; fecal-oral
Examples	Herpesvirus, HIV, Influenza, Hepatitis B/C, Rabies, CMV, EBV	Adenovirus, Poliovirus, Hepatitis A, Norovirus, Rotavirus, Parvovirus

D. DNA vs. RNA Viruses

DNA Viruses (most replicate in nucleus):

Family	Morphology	Examples
Herpesviridae	Enveloped icosahedral; 150-200 nm	HSV-1/2, VZV, CMV, EBV, HHV-6/8
Adenoviridae	Non-enveloped icosahedral; 70-90 nm	Adenovirus (pharyngitis, conjunctivitis)
Poxviridae	Complex, brick-shaped; 200-400 nm (largest DNA virus)	Smallpox, Vaccinia, Molluscum
Hepadnaviridae	Enveloped; partially dsDNA; 42 nm (Dane particle)	Hepatitis B virus
Papillomaviridae	Non-enveloped icosahedral; 55 nm	HPV (warts, cervical cancer)
Parvoviridae	Non-enveloped icosahedral; 18-26 nm (smallest DNA virus)	Parvovirus B19

RNA Viruses:

Family	Genome	Morphology	Examples
Picornaviridae	+ssRNA	Non-enveloped icosahedral; 28-30 nm	Poliovirus, Rhinovirus, HAV, Enterovirus
Flaviviridae	+ssRNA	Enveloped icosahedral; 40-60 nm	HCV, Dengue, Zika, Yellow fever, WNV
Togaviridae	+ssRNA	Enveloped icosahedral; 70 nm	Rubella, Chikungunya
Coronaviridae	+ssRNA	Enveloped helical; 80-220 nm; crown-like spikes	SARS-CoV-2, MERS-CoV
Orthomyxoviridae	-ssRNA (segmented, 8 segments)	Enveloped helical; 80-120 nm; HA + NA surface proteins	Influenza A, B
Paramyxoviridae	-ssRNA	Enveloped helical; 150-300 nm	Measles, Mumps, RSV, Parainfluenza
Rhabdoviridae	-ssRNA	Enveloped helical; bullet-shaped; 75x180 nm	Rabies virus
Retroviridae	+ssRNA (diploid); reverse transcriptase	Enveloped icosahedral; 80-120 nm	HIV-1/2, HTLV
Reoviridae	dsRNA (10-12 segments)	Non-enveloped; double-layered icosahedral; 70-80 nm	Rotavirus, Reovirus
Bunyaviridae	-ssRNA (3 segments)	Enveloped; 80-120 nm	Hantavirus, Rift Valley fever
Filoviridae	-ssRNA	Enveloped helical; long filamentous; up to 14,000 nm	Ebola, Marburg

HIV (detailed morphology):

Enveloped, ~120 nm spherical
Outer envelope: gp120 (binding) + gp41 (fusion) glycoproteins
Matrix: p17
Conical capsid: p24 (truncated cone shape - diagnostic on EM)
Genome: 2 copies of +ssRNA + reverse transcriptase (p66/p51), integrase, protease

III. FUNGI

A. General Morphology

Fungi are eukaryotes with a cell wall made of chitin (not peptidoglycan). They reproduce by spores.

Two basic morphological forms:

Form	Description	Examples
Yeast	Unicellular, round/oval; reproduce by budding	Candida, Cryptococcus
Mold (Hyphae)	Multicellular; grow as filamentous branching hyphae; network = mycelium	Aspergillus, Rhizopus, dermatophytes
Dimorphic	"MOLD in the cold, YEAST in the heat" - mold at 25°C, yeast at 37°C (body temperature)	Histoplasma, Blastomyces, Coccidioides, Paracoccidioides, Sporothrix

B. Key Fungal Pathogens and Morphology

Organism	Morphology	Key Feature
Candida albicans	Oval budding yeast (3-6 µm) + pseudohyphae + germ tubes at 37°C	Germ tube test (+); most common opportunistic fungus
Cryptococcus neoformans	Round yeast (4-20 µm) with large polysaccharide capsule	India ink - clear "halo" around organism; urease (+)
Aspergillus fumigatus	Septate hyphae with acute angle (45°) branching	V-shaped branching; conidiophore with conidia
Mucor/Rhizopus	Broad, non-septate (aseptate) hyphae with wide-angle (90°) branching	"Ribbon-like" hyphae; causes mucormycosis
Histoplasma capsulatum	Yeast inside macrophages (2-4 µm); mold phase has tuberculate macroconidia	Bird/bat droppings; Ohio/Mississippi river valleys
Blastomyces dermatitidis	Broad-based budding yeast (8-15 µm); thick double-refractile cell wall	"Figure-8" appearance
Coccidioides immitis	Spherules (20-60 µm) containing endospores in tissue	Arthroconidia in mold phase; desert Southwest
Pneumocystis jirovecii	Cysts (5-8 µm) with 8 intracystic bodies; trophic forms	Stained by GMS (silver) stain; CD4 <200
Sporothrix schenckii	Cigar-shaped yeast in tissue; mold has "rosette" conidia arrangement	"Rose gardener's disease"; lymphocutaneous
Dermatophytes	Septate hyphae + macroconidia/microconidia	Trichophyton, Microsporum, Epidermophyton; infect keratinized tissue only

C. Special Stains for Fungi

GMS (Gomori Methenamine Silver): Black fungi on green background
PAS (Periodic Acid-Schiff): Magenta/pink fungi
India ink: Cryptococcus capsule (negative stain)
Calcofluor white: Fluorescent staining of chitin

IV. PARASITES

A. Protozoa (Unicellular Eukaryotes)

1. Intestinal Protozoa

Organism	Morphology	Key Feature
Entamoeba histolytica	Trophozoite (15-60 µm): motile, pseudopods, ingested RBCs; Cyst (10-20 µm): 4 nuclei	"Flask-shaped" ulcers; amoebic dysentery
Giardia lamblia	Trophozoite: pear-shaped, 2 nuclei, 4 pairs of flagella ("falling leaf" motility); Cyst: oval, 4 nuclei	Ventral sucking disk; mountain stream water
Cryptosporidium	Oocysts (4-6 µm): very small, acid-fast; 4 sporozoites per oocyst	Acid-fast stain; immunocompromised
Balantidium coli	Largest protozoan (50-100 µm); ciliated trophozoite; bean-shaped macronucleus	Only ciliate infecting humans

2. Blood/Tissue Protozoa

Organism	Morphology	Key Feature
Plasmodium falciparum	Ring forms (appliqué/accole); multiple rings per RBC; banana-shaped gametocytes	RBC not enlarged; causes malaria
P. vivax/ovale	Ring forms + trophozoites; enlarged RBCs; Schüffner dots	Amoeboid trophozoite
P. malariae	Band-form trophozoite; "rosette" schizonts	Normal or small RBCs
Trypanosoma brucei	Spindle-shaped trypomastigote (~25 µm); undulating membrane; single flagellum	Sleeping sickness; tsetse fly
Trypanosoma cruzi	Trypomastigote with C- or U-shape; "sickle" form; amastigotes in tissue	Chagas disease; reduviid bug
Leishmania	Promastigote (flagellated, extracellular) in sandfly; amastigote (non-flagellated, intracellular) in macrophages	Sandfly transmission
Toxoplasma gondii	Tachyzoites: crescent/banana-shaped (3x6 µm); bradyzoites in tissue cysts	Definitive host = cat; congenital infection

3. Vaginal Protozoa

| Trichomonas vaginalis | Pear-shaped, 5 flagella (4 anterior + 1 in undulating membrane); no cyst stage | Jerky "tumbling" motility; STI |

B. Helminths (Worms)

1. Nematodes (Roundworms) - Cylindrical, unsegmented

Organism	Size/Morphology	Key Feature
Ascaris lumbricoides	Large (15-35 cm); 3 lips around mouth	Largest intestinal nematode
Enterobius vermicularis	Small (1 cm); pointed tail (pinworm)	Scotch tape test; perianal pruritus
Trichuris trichiura	Whip-shaped; narrow anterior, thick posterior (whipworm)	Barrel-shaped eggs
Hookworms (Ancylostoma/Necator)	Cutting plates (Ancylostoma) or cutting plates with semilunate plates (Necator)	Blood loss; hypochromic anemia
Strongyloides stercoralis	Rhabditiform (free-living) vs. filariform (infective) larvae	Autoinfection; filariform larvae in stool
Wuchereria bancrofti	Sheathed microfilariae; nocturnal periodicity	Lymphatic filariasis; elephantiasis
Loa loa	Sheathed microfilariae; diurnal periodicity	Calabar swellings; eye worm
Onchocerca volvulus	Unsheathed microfilariae in skin	River blindness; black fly

2. Cestodes (Tapeworms) - Flat, segmented

Organism	Morphology	Key Feature
Taenia saginata	Unarmed scolex (no hooks); up to 10 m long; >12 uterine branches per proglottid	Beef tapeworm
Taenia solium	Armed scolex (hooks + suckers); <7 m; <12 uterine branches per proglottid; cysticerci in tissue	Pork tapeworm; neurocysticercosis
Echinococcus granulosus	Small tapeworm (3-6 mm) in dog intestine; hydatid cysts in human liver/lung	Hydatid disease; brood capsules with scolices
Diphyllobothrium latum	Broadest tapeworm (up to 15 m); scolex has bothria (grooves, no suckers)	Fish tapeworm; B12 deficiency

3. Trematodes (Flukes) - Flat, non-segmented, leaf-shaped

Organism	Morphology	Key Feature
Schistosoma mansoni	Adult male has gynecophoral canal holding female; eggs with lateral spine	Intestinal schistosomiasis; Africa
S. haematobium	Eggs with terminal spine	Urinary schistosomiasis; bladder cancer
S. japonicum	Small eggs with rudimentary lateral spine	Most eggs per worm; cerebral disease
Clonorchis sinensis	Small (12-20 mm); Chinese liver fluke; flask-shaped eggs with operculum	Bile duct; cholangiocarcinoma
Fasciola hepatica	Large (30 mm); sheep liver fluke; large operculated eggs	Liver fluke; biliary obstruction
Paragonimus westermani	Lung fluke; reddish-brown, 8-16 mm; operculated eggs	Hemoptysis; raw crab

C. Ectoparasites

Organism	Morphology	Key Feature
Sarcoptes scabiei	Oval mite (0.3-0.4 mm); 4 pairs of legs; burrows in skin	Scabies; intense pruritus
Pediculus humanus	Louse with clinging claws; nits attached to hair	Head/body lice
Phthirus pubis	Crab louse - short, broad body with crab-like claws	Pubic lice

Summary: Quick Identification Table

Pathogen Type	Cell Type	Cell Wall	Size	Unique Feature
Gram + bacteria	Prokaryote	Thick peptidoglycan	0.5-5 µm	Purple on Gram stain
Gram - bacteria	Prokaryote	Thin PG + outer membrane (LPS)	0.5-5 µm	Pink on Gram stain; endotoxin
Mycobacteria	Prokaryote	Mycolic acid-rich	2-4 µm	Acid-fast (ZN stain)
Mycoplasma	Prokaryote	None	0.1-0.3 µm	Smallest free-living organism
Viruses	Non-cellular	None (has capsid ± envelope)	20-400 nm	Obligate intracellular
Fungi	Eukaryote	Chitin	3-30 µm	GMS/PAS staining
Protozoa	Eukaryote	None or minimal	1-100+ µm	Often have cyst + trophozoite stages
Helminths	Eukaryote (multicellular)	Cuticle	mm-meters	Eggs/larvae used for identification

Sources: Jawetz, Melnick & Adelberg's Medical Microbiology, 28th ed. (Fundamentals of Microbiology, pp. 6-140); Medical Microbiology, 9th ed.; Sherris & Ryan's Medical Microbiology, 8th ed.

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