This longitudinal section
through the upper portion of the human esophagus shows the four layers that
make up this muscular tube. (1) Closest to the lumen is the mucosa
(Mu), consisting of three components: a thick, nonkeratinizing
stratified squamous epithelium (Ep), resting on a lamina
propria (LP) of loose connective tissue, and bounded by the muscularis
mucosae (MM), a layer of smooth muscle, which is discontinuous in this
section. (2) Next is the submucosa (SM), a thick
layer of loose connective tissue. (3) Next comes the muscularis
externa (ME), consisting of two layers of muscle separated by a thin
layer of connective tissue (CTL). The muscle
fibers of the inner (circular) layer (IL) are
oriented with their long axes perpendicular to the long axis of the tube,
whereas those in the outer (longitudinal) layer
(OL) are oriented parallel to the long axis of
the tube. (4) The outermost layer is loose connective tissue. For
most of the esophagus this layer is called the adventitia
(A) and it is continuous with the surrounding connective tissue of
the mediastinum. In the terminal portion of the esophagus, after it passes
through the diaphragm and enters the peritoneal cavity, the outer layer becomes
covered with a layer of mesothelial cells and is called the serosa.
