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
(Lu) is the mucosa (Mu) characterized
by a thick, nonkeratinizing stratified squamous epithelium
(Ep) resting on a lamina propria of connective tissue. (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. The muscle fibers of the inner layer are
oriented with their long axes perpendicular to the long axis of the tube,
whereas those in the outer layer are oriented
parallel to the long axis of the tube. (4) The outermost layer
is an adventitia (Ad) of loose connective tissue
that is continuous with the surrounding connective tissue of the mediastinum.
