PERITONEUM

 PERITONEUM

Peritoneum is a large serous membrane lining the abdominal cavity.  Histologically it is composed of an outer layer of fibrous tissue, which give strength to the membrane and inner layer of mesothelial cells which secrete a serous fluid which lubricates the surface, thus allowing the movements of viscera.

The peritoneum is in the form of a closed sac which is invaginated by a number of viscera. As a result, the peritoneum is divided into:

• An outer or parietal layer

• An inner or visceral layer

• Folds of peritoneum by which the viscera are suspended by viscera are suspended.

The peritoneum which is a simple cavity, before being investigated by viscera becomes highly complicated.

PARIETAL PERITONEUM 

• It lines the inner surface of the abdominal and pelvic walls and the lower surface of the diaphragm. It is loosely attached to the walls by extraperitoneal connective tissue. And it can be easily stripped.

• Embryologically, it is derived from the somatopleuric layer of the lateral plate mesoderm.

VISCERAL PERITONEUM

• It lines the outer surface of the visceral, to which it is firmly adherent and cannot be stripped. Infact, it forms a part and parcel of the viscera.

• Embryologically, it is derived from the splanchnopleuric layer of the lateral plate mesoderm.

FOLDS OF PERITONEUM

• Many organs within the abdomen are suspended by folds of peritoneum. Such organs are mobile. The degree and direction of mobility are governed by the size and direction of the peritoneum fold. Other organs are fixed and immobile. 

Retroperitoneal organ - Those organs covered by peritoneum on one side . Such organs are said to be retroperitoneal.

Some organs are suspended by peritoneal folds in early embryonic life, but later become retroperitoneal.

Primarily retroperitoneal organs developed and remain outside of the parietal peritoneum. The oesophagus, rectum and kidneys are all primarily retroperitoneal.

Secondarily retroperitoneal organs were initially intraperitoneal, suspended by mesentery. Through the course of embryogenesis, they became retroperitoneal as their mesentery fused with the posterior abdominal wall. Thus, in adults, only their anterior surface is covered with peritoneum. 

Examples of secondarily retroperitoneal organs include the ascending and descending colon.

• Apart from allowing mobility, the peritoneal folds provide pathways for passage of vessels, nerves and lymphatics.

• Peritoneum folds are given various named:

• In general, the name of the fold is made up of the prefix 'mes' or 'meso' followed by the name of the organ, e.g. the fold suspending the small intestine or entron is called the mesentery, and a fold suspending part of the colon is called mesocolon.

• Large peritoneal folds attached to the stomach are called omenta, singular of which is omentum which means cover.

• In many situations, double layered folds of peritoneum connect organs to the abdominal wall or to each other . Such folds are called ligaments. These may be named after the structures they connect. For example, the gastrosplenic ligaments connect the stomach to the spleen. Other folds are named according to their shape, e.g. the triangular ligament of the liver.

The Peritoneum Membrane Development

The mesoderm is the origin of the peritoneum membrane development. From the trilaminar embryo of the mesoderm, the peritoneal membrane is formed. The first stage of development- It is when the intraembryonic coelom separates and splits the lateral plate of the mesoderm into two layers the mesoderm starts to differentiate. 

The two layers of the differentiated mesoderm then start developing into the two layers in the serous cavities. The layers that are known as the outer layer are called the parietal peritoneum layer and the inner visceral layer. The two layers are the components of the developed peritoneum. 

As an embryo develops, in the initial stages within the abdominal wall the various abdominal organs grow into the abdominal cavity from structures. And during this developmental process, the organs in the abdomen become enveloped in a layer of peritoneum membrane. The growing organs very organically "take their blood vessels with them" with them as there is no growth without blood supply from the abdominal wall. And mesentery the double layer within the inner visceral layer of the peritoneum is formed as it covers the blood vessels.

PERITONEAL CAVITY

 The viscera which invagated the peritoneal cavity completely fills it so that the cavity is reduced to a potential space separating adjacent layers is reduced to a potential space separating adjacent layers of peritoneum. Between these layers, there is a thin film of serous fluid secreted by the mesothelial cells. The fluid performs a lubricating function and allows free movement of one peritoneal surface over another.

Under abnormal circumstances, there may be a collection of fluid called ascites, or of blood called hemoperitoneum. Or of air called pneumoperitoneum within the peritoneal cavity.

The peritoneal cavity is divided broadly into two parts. The main, larger part is known as the greater sac, and the smaller part, situated behind the stomach, the lesser omentum and the liver, is known as the omental bursa or lesser sac. The two sacs communicate with each other epiploic foramen or foramen of Winsliw or opening into the lesser sac.

RELATION OF PERITONEUM TO VARIOUS VISCERA 

Relation to peritoneum	Organs
Intraperitoneal	Stomach, jejunum, ileum, caecum, appendix, transverse colon, and sigmoid colon
Partially covered	Ascending colon, descending colon, and rectum
Retroperitoneal	Duodenum, pancreas, kidney, ureter, and suprarenal
Subperitoneal	Urinary bladder, prodyrate, seminal vesicle, cervical utri, and vagina

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