Organs

Lungs

• Surface Lining: The lungs are completely enveloped by a serous membrane called the visceral pleura.
• Histopathology: Under a microscope, the visceral pleura is distinctly composed of a single layer of flat mesothelial cells sitting atop a thin, supportive fibrovascular connective tissue layer containing blood vessels and lymphatics. This serous fluid-secreting layer gives the lungs their smooth, lustrous external appearance to prevent friction during breathing.

Liver

• Surface Lining: The liver is covered by a dense connective tissue capsule (Glisson’s capsule), which is itself covered by a layer of visceral peritoneum (mesothelium) over most areas.
• Histopathology Distinction: While it does feature a mesothelial coat, a histopathological diagnosis of an unidentified liver specimen relies heavily on its unique parenchymal architecture—namely, classic hexagonal hepatic lobules, cords of hepatocytes, central veins, and portal triads, rather than a prominent outer fibrovascular mesothelium.

Heart

• Surface Lining: The outer layer of the heart is the epicardium (visceral pericardium), which also consists of an outer mesothelium.
• Histopathology Distinction: While it matches the mesothelial description, the subepicardial fibrovascular layer of the heart uniquely contains a highly characteristic, thick cushion of adipose tissue (fat) alongside large coronary vessels and nerves, all tightly bound to deep layers of thick, branching cardiac muscle (myocardium).

Layers:

The pericardium (the sac enclosing the heart) has layers like an onion:

1. Fibrous Pericardium: The tough, outer connective tissue layer.
2. Serous Pericardium: The inner double-layered membrane, which splits into:
   • Parietal layer: Lines the inside of the fibrous sac.
   • Visceral layer (Epicardium): Adheres directly to the outside of the heart muscle itself.

Oral Cavity

• Surface Lining: The oral cavity is lined by a mucous membrane (oral mucosa), not a serous membrane.
• Histopathology Distinction: The oral cavity never contains mesothelium. Instead, because it is an environment exposed to mechanical friction from food, it is lined by a thick stratified squamous epithelium (which can be keratinized on the hard palate/gingiva or non-keratinized on the cheeks and floor of the mouth). It lacks a thin, lustrous mesothelial layer entirely.

Stomach

Parts

Cells

Fundus of stomach

• Muscle Layers: 3 Layers of Smooth Muscle
• Histological Breakdown: The stomach requires intense churning and mechanical breakdown of food. To do this, its muscularis externa is modified to include three distinct smooth muscle layers (ordered from innermost to outermost):
  1. Inner Oblique Layer: Unique to the stomach; provides the extra directional force needed for churning.
  2. Middle Circular Layer: Thickens at the pylorus to form the pyloric sphincter.
  3. Outer Longitudinal Layer: Runs along the length of the organ.

Intestine (Duodenum, Jejunum, Ileum)

• Muscle Layers: 2 Layers of Smooth Muscle
• Histological Breakdown: The entire small intestine follows the classic GI tunic plan. Its muscularis externa consists strictly of two smooth muscle layers:
  1. Inner Circular Layer: Compresses the lumen to mix chyme.
  2. Outer Longitudinal Layer: Shortens the tube to propel food forward via peristalsis.
• How to distinguish them individually under a microscope:
  1. Duodenum: Characterized by prominent Brunner’s glands in the submucosa layer, which secrete alkaline mucus to neutralize stomach acid.
  2. Jejunum: Features the most well-developed, tall Plicae circulares (circular folds) and long villi, but lacks specific identifying glands in the submucosa.
  3. Ileum: Characterized by prominent aggregates of lymphoid follicles known as Peyer’s patches in the submucosa (and sometimes extending into the mucosa).

Esophagus

• Muscle Layers: 2 Layers, but with a highly unique gradient of muscle types.
• Histological Breakdown: The muscularis externa of the esophagus also maintains a standard inner circular and outer longitudinal layout, but its tissue composition transitions radically from top to bottom depending on voluntary vs. involuntary control:
  • Upper 1/3: Composed exclusively of Skeletal muscle (for voluntary swallowing).
  • Middle 1/3: A mixed combination of both Skeletal and Smooth muscle.
  • Lower 1/3: Composed exclusively of Smooth muscle as it nears the stomach.

Spleen

The spleen is structurally divided into two distinct functional regions based on its appearance on fresh tissue cuts: the White Pulp and the Red Pulp. These two zones are separated by a narrow interface called the Marginal Zone.