Mesocolon

The mesocolon refers to the fold of peritoneum that suspends parts of the colon from the dorsal abdominal wall, conveying vessels, lymphatics, and nerves. It is subdivided anatomically according to the segment of the colon it supports.

a. Ascending Mesocolon (Mesocolon ascendens)

• Location: Attaches the ascending colon to the dorsal body wall.

• Species Variations:

  • In ruminants, this portion is particularly elaborate, due to the spiral configuration of the colon.

  • In dogs and cats, it is less extensive and supports the short ascending colon.

b. Transverse Mesocolon (Mesocolon transversum)

• Location: Connects the transverse colon to the dorsal abdominal wall, crossing the midline.

• Position: Usually lies cranial to the root of the mesentery and may be associated with the pancreas in some species.

• Function: Contains the middle colic artery and vein.

c. Descending Mesocolon (Mesocolon descendens)

• Location: Attaches the descending colon to the dorsal body wall on the left side.

• Length and Mobility: Long and relatively mobile, especially in carnivores like dogs and cats.

• Vascular Supply: Houses the left colic artery and associated lymphatics.

d. Sigmoid mesocolon (Mesocolon sigmoideum)

• While this structure is more clearly defined in humans, in domestic animals, the sigmoid colon may be less prominent or absent depending on the species, especially in carnivores like dogs and cats.

Species Differences

• Carnivores (e.g., dog, cat):

  • Relatively simple mesocolon structure.

  • The descending mesocolon is long and allows significant displacement during surgeries like colotomy.

• Ruminants (e.g., cattle, sheep, goats):

  • Complex due to the spiral colon.

  • The mesocolon of the spiral loop is shared with the centrifugal and centripetal coils, forming a flattened disc adhered to the mesojejunum.

• Pigs:

  • The ascending colon forms a conical spiral. The mesocolon follows this spiral and is intimately related to the mesojejunum.

• Horses:

  • The mesocolon reflects the great complexity of the large colon.

  • The mesocolon of the large colon is relatively short and limits mobility, predisposing to impaction and displacement (e.g., nephrosplenic entrapment).

General Structure and Function

The mesocolon is a serous membrane derived from the embryonic dorsal mesentery. It serves the following roles:

• Suspension and Support: Maintains the colon’s position within the abdominal cavity.

• Conduit for Neurovascular Structures: Contains mesenteric arteries, veins, lymphatics, and autonomic nerves supplying the colon.

• Mobility Regulation: Allows limited movement of the large intestine while preventing volvulus in most cases.

Embryological Origin

The mesocolon develops from the dorsal mesentery during embryogenesis. It is initially continuous with the mesentery of other gut segments but becomes regionally differentiated as the gut rotates and elongates.