Serosa and adventitia
Anatomical structures don't float around freely in the body. They are embedded in connective tissue that holds them on their place. If a structure is removed from the body a bit of this surrounding connective tissue comes with it. When seen under the microscope, this attached connective tissue is seen as a thin discontinuous layer of tissue remnants with a frayed border around the structure. This layer is named 'adventitia'.
The situation is different for structures that are covered by visceral peritoneum, such as large portions of the digestive tract. Under the microscope, these structures are seen to have a thin layer with a smooth border around the structure; the attached visceral peritoneum. In histology this layer is called 'serosa' after serous membrane. Serosa thus is the same as visceral peritoneum.
Hence:
- a structure with a serosa = a structure that is lined by visceral peritoneum
- a structure with an adventitia = a structure that is NOT lined by visceral peritoneum, (but instead is surrounded by connective tissue fixed to it).
Some parts of the digestive tract have an adventitia as outer layer instead of a serosa. In this case the gut tube is directly embedded and fixed in surrounding connective tissue. For instance, the thoracic oesophagus has an adventitia because it is not covered by peritoneum.
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