Welcome to the wonderful world of histology. Histology is how we study the body on a cellular level. This week, we are getting more specific and looking at one organ, in particular, the liver
The liver is a highly diverse organ and has many functions. It produces energy and synthesises cholesterol and proteins. It also converts carbohydrates and proteins into fatty acids and triglycerides. It regulates blood glucose concentration and detoxifies metabolic waste, toxins and drugs. It also synthesises and secretes bile.
So it has a lot of functions, but what does all of this look like histologically?
The liver is covered in a collagenous connective tissue capsule which is then covered in simple squamous epithelial, mesothelial cells. The liver can be divided into hexagonal lobules. Each lobule has a branch of the hepatic vein, called the central vein, at its centre and a portal triad in each corner. The portal triad is branches of vessels that enter the porta hepatis, these are the hepatic portal vein, the hepatic artery and a component of the bile duct. There are three vessels present here, hence being called the portal triad.
Lymphatic vessels are also typically seen lying alongside the portal triad.
The main cells of the liver are the hepatocytes. These are polyhedral epithelium that appears as chord-like structures. They are actually plates of cells with gaps between them to allow blood to flow. These gaps in blood flow are called sinusoids. Blood flows from the centre of a lobule outwards. Microvilli line the sinusoids and help to propel hepatocytes outwards. Sinusoids are fenestrated meaning they are full of holes; this allows the liquid component of blood to flow freely and have access to the hepatocytes but the blood cells themselves are too big to pass through these fenestrations.
Within the sinusoids are macrophages called Kupffer cells. These remove worn-out red blood cells and help to filter the blood. The hepatic stellate cells are other structures found under the microscope in the liver. They are scattered in the spaces alongside the sinusoids and are modified fibroblasts that make connective tissue. They also act as a store for vitamin A.
The liver functions to produce bile. The bile is produced by all hepatocytes. Bile is a solution that contains bilirubin, a pigment responsible for the breakdown of haemoglobin in the spleen and when excreted, it gives faeces its brown colour. Bile is also responsible for the breakdown of fats and is carried via bile canaliculi towards the bile ducts.
The histology of the liver is very extensive due to it being such a large and important organ.