The gallbladder is a hollow organ that sits in a shallow depression below the right lobe of the liver, that is grey-blue in life. In adults, the gallbladder measures approximately 7 to 10 centimetres (2.8 to 3.9 inches) in length and 4 centimetres (1.6 in) in diameter when fully distended. The gallbladder has a capacity of about 50 millilitres (1.8 imperial fluid ounces).
The gallbladder is shaped like a pear, with its tip opening into the cystic duct. The gallbladder is divided into three sections: the fundus, body, and neck. The fundus is the rounded base, angled so that it faces the abdominal wall. The body lies in a depression in the surface of the lower liver. The neck tapers and is continuous with the cystic duct, part of the biliary tree. The gallbladder fossa, against which the fundus and body of the gallbladder lie, is found beneath the junction of hepatic segments IVB and V. The cystic duct unites with the common hepatic duct to become the common bile duct. At the junction of the neck of the gallbladder and the cystic duct, there is an out-pouching of the gallbladder wall forming a mucosal fold known as "Hartmann's pouch".
The gallbladder wall is composed of a number of layers. The gallbladder wall's innermost surface is lined by a single layer of columnar cells with a brush border of microvilli, very similar to intestinal absorptive cells. Underneath the epithelium is an underlying lamina propria, a muscular layer, an outer perimuscular layer and serosa. Unlike elsewhere in the intestinal tract, the gallbladder does not have a muscularis mucosae, and the muscular fibres are not arranged in distinct layers.
The mucosa, the inner portion of the gallbladder wall, consists of a lining of a single layer of columnar cells, with cells possessing small hair-like attachments called microvilli. This sits on a thin layer of connective tissue, the lamina propria. The mucosa is curved and collected into tiny outpouchings called rugae.
A muscular layer sits beneath the mucosa. This is formed by smooth muscle, with fibres that lie in longitudinal, oblique and transverse directions, and are not arranged in separate layers. The muscle fibres here contract to expel bile from the gallbladder. A distinctive feature of the gallbladder is the presence of Rokitansky–Aschoff sinuses, deep outpouchings of the mucosa that can extend through the muscular layer, and which indicate adenomyomatosis. The muscular layer is surrounded by a layer of connective and fat tissue.
The outer layer of the fundus of gallbladder, and the surfaces not in contact with the liver, are covered by a thick serosa, which is exposed to the peritoneum. The serosa contains blood vessels and lymphatics. The surfaces in contact with the liver are covered in connective tissue.
The gallbladder varies in size, shape, and position between different people. Rarely, two or even three gallbladders may coexist, either as separate bladders draining into the cystic duct, or sharing a common branch that drains into the cystic duct. Additionally, the gallbladder may fail to form at all. Gallbladders with two lobes separated by a septum may also exist. These abnormalities are not likely to affect function and are generally asymptomatic.
The location of the gallbladder in relation to the liver may also vary, with documented variants including gallbladders found within, above, on the left side of, behind, and detached or suspended from the liver. Such variants are very rare: from 1886 to 1998, only 110 cases of left-lying liver, or less than one per year, were reported in scientific literature.
An anatomical variation can occur, known as a Phrygian cap, which is an innocuous fold in the fundus, named after its resemblance to the Phrygian cap.
The gallbladder develops from an endodermal outpouching of the embryonic gut tube. Early in development, the human embryo has three germ layers and abuts an embryonic yolk sac. During the second week of embryogenesis, as the embryo grows, it begins to surround and envelop portions of this sac. The enveloped portions form the basis for the adult gastrointestinal tract. Sections of this foregut begin to differentiate into the organs of the gastrointestinal tract, such as the esophagus, stomach, and intestines.
During the fourth week of embryological development, the stomach rotates. The stomach, originally lying in the midline of the embryo, rotates so that its body is on the left. This rotation also affects the part of the gastrointestinal tube immediately below the stomach, which will go on to become the duodenum. By the end of the fourth week, the developing duodenum begins to spout a small outpouching on its right side, the hepatic diverticulum, which will go on to become the biliary tree. Just below this is a second outpouching, known as the cystic diverticulum, that will eventually develop into the gallbladder.