Greater omentum

Introduction

The greater omentum is a large, apron-like fold of peritoneum that drapes over the anterior surface of the abdominal organs. It serves multiple functions including fat storage, immune defense, and protection of the abdominal viscera. Its unique structure and mobility make it clinically significant in both normal physiology and surgical interventions.

Anatomy of the Greater Omentum

Origin and Attachments

The greater omentum originates from the greater curvature of the stomach and the proximal part of the duodenum. From this origin, it descends anteriorly, draping over the small intestine, and then folds back to attach to the transverse colon. These attachments create a double-layered peritoneal structure that allows for both mobility and coverage of the abdominal organs.

Structure and Layers

The greater omentum consists of four peritoneal layers that form two anterior and two posterior layers. Between these layers is a variable amount of adipose tissue, which acts as an energy reserve and provides cushioning for the underlying organs. Embedded within the omentum are blood vessels, lymphatics, and connective tissue that support its physiological functions.

Dimensions and Variations

The size of the greater omentum varies among individuals but can extend from the stomach to the pelvic cavity in adults. Its thickness and fat content are influenced by age, nutritional status, and body composition. Anatomical variations may include differences in length, width, and the extent of coverage over the intestines.

Relations and Surrounding Structures

The greater omentum has important anatomical relationships with several abdominal organs and structures, which influence both its physiological role and clinical relevance.

• Stomach and Transverse Colon: The greater omentum hangs from the greater curvature of the stomach and folds back to attach to the transverse colon, effectively covering the anterior surface of the small intestine.
• Small Intestine: It drapes over the jejunum and ileum, providing a protective layer against mechanical injury and infection.
• Diaphragm and Abdominal Wall: The superior portion lies in proximity to the diaphragm, while the lateral edges are adjacent to the anterior abdominal wall.
• Spleen, Liver, and Pancreas: The omentum is closely related to the spleen on the left side and the liver on the right, and its posterior layers are near the pancreas, facilitating immune surveillance in these regions.

Histology

The greater omentum is composed of specialized tissue layers that reflect its multifunctional role in the abdomen.

• Cellular Composition: The tissue contains adipocytes for fat storage, fibroblasts for structural support, and various immune cells including macrophages and lymphocytes for defense against infection.
• Blood Vessels and Lymphatics: The omentum contains a network of arteries, veins, and lymphatic vessels that supply both the omental tissue and the underlying abdominal organs.
• Connective Tissue Framework: Collagen and elastic fibers provide tensile strength and flexibility, allowing the omentum to move over the intestines while maintaining structural integrity.

Physiology and Function

The greater omentum plays several vital roles in maintaining abdominal homeostasis, contributing to metabolic, immune, and protective functions.

• Fat Storage and Energy Reserve: The omental adipose tissue serves as a significant energy reservoir, storing triglycerides that can be mobilized during periods of increased metabolic demand.
• Immune Surveillance: Rich in macrophages, lymphocytes, and milky spots, the omentum detects and responds to intra-abdominal infections and inflammatory processes.
• Wound Healing and Tissue Repair: The omentum can adhere to sites of injury or inflammation, promoting angiogenesis and tissue regeneration.
• Protection and Insulation: Its fatty layers cushion and insulate abdominal organs, reducing friction and mechanical injury during organ movement.

Clinical Significance

The greater omentum is important in various pathological and surgical contexts due to its mobility, vascularity, and immune functions.

Pathologies

• Omental torsion, which can lead to acute abdominal pain and ischemia
• Omental infarction, a rare cause of localized abdominal inflammation
• Omental cysts and tumors, including lipomas and mesotheliomas

Role in Surgery

• Used as an omental flap in reconstructive surgeries to promote healing and provide coverage for defects
• Provides protection to anastomoses and surgical sites due to its vascular and immunological properties

Imaging and Diagnosis

• CT scans and MRI are used to visualize omental anatomy and detect pathologies
• Ultrasound can identify fluid collections, masses, or thickening within the omentum

Embryology and Development

The greater omentum develops from the dorsal mesogastrium during embryogenesis, undergoing significant growth and morphological changes to form its adult structure.

• Origin from Dorsal Mesogastrium: Early in development, the dorsal mesogastrium hangs from the posterior abdominal wall and serves as the precursor to the greater omentum.
• Growth and Fusion of Peritoneal Layers: As the stomach rotates, the dorsal mesogastrium elongates and folds downward, eventually fusing its layers to create the double-layered structure of the mature greater omentum.
• Functional Maturation: During development, adipose tissue accumulates and vascular and lymphatic networks form, preparing the omentum for its roles in immunity, fat storage, and organ protection.

Comparative Anatomy

The greater omentum shows variations in structure and function among different species, reflecting adaptations to specific physiological and ecological needs.

• Variations in Mammals: In humans, it is a prominent apron covering the intestines, while in other mammals, its size and coverage may be reduced or modified according to body size and digestive anatomy.
• Evolutionary Significance: The mobility, fat storage, and immune functions of the omentum have evolved to provide protective and metabolic advantages, contributing to survival in diverse environmental conditions.

References

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