Changes

At the anus, the '''columnar intestinal epithelium''' is replaced by the '''stratified squamous keratinised epithelium''' of the skin. The '''anal canal''' joins the bowel to the exterior and is the last 2-3cm of the alimentary tract. This is a short passage derived from the '''proctodeum''' (formed by invagination of the surface ectoderm). Sebaceous and large apocrine sweat glands both occur in this region in association with the anal sphincters. Before joining the anal canal, the rectum becomes dilated to form the ''rectal ampulla''. At the '''rectoanal''' junction, the lumen is constricted by longitudinal folds in the mucosa, these are normally pressed together to occlude the lumen. As the muscosa changes from columnar to cutaneous, three zones are created:

At the anus, the '''columnar intestinal epithelium''' is replaced by the '''stratified squamous keratinised epithelium''' of the skin. The '''anal canal''' joins the bowel to the exterior and is the last 2-3cm of the alimentary tract. This is a short passage derived from the '''proctodeum''' (formed by invagination of the surface ectoderm). Sebaceous and large apocrine sweat glands both occur in this region in association with the anal sphincters. Before joining the anal canal, the rectum becomes dilated to form the ''rectal ampulla''. At the '''rectoanal''' junction, the lumen is constricted by longitudinal folds in the mucosa, these are normally pressed together to occlude the lumen. As the muscosa changes from columnar to cutaneous, three zones are created:

1.'''Columnar'''

'''1.Columnar'''

The columnar zone has many longitudinal folds. It can be divided from the rectum by the ''anorectal line''. This is a line where the mucosa is stratified squamous epithelium containing lots of lymphoid tissue.  

The columnar zone has many longitudinal folds. It can be divided from the rectum by the ''anorectal line''. This is a line where the mucosa is stratified squamous epithelium containing lots of lymphoid tissue.  

2.'''Intermediate'''

'''2.Intermediate'''

The intermediate zone can be divided from the cutaneous zone by the ''anocutaneous line''.

The intermediate zone can be divided from the cutaneous zone by the ''anocutaneous line''.

3.'''Cutaneous'''

'''3.Cutaneous'''

The cutaneous zone is the skin. It is stratified squamous keratinised epithelium that surrounds the anus. The excretory ducts of the '''anal sacs''' open into this region.

The cutaneous zone is the skin. It is stratified squamous keratinised epithelium that surrounds the anus. The excretory ducts of the '''anal sacs''' open into this region.

[[Category:Large Intestine - Anatomy & Physiology]]

[[Category:Large Intestine - Anatomy & Physiology]]

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[[Category:To Do - Review]][[Category:To Do - AP Review]]