The mesorectum: hypothesis on its evolution.
BACKGROUND
Two points are controversial in the anatomy of the mesorectum: (1) its origin; and (2) the existence of the lateral ligaments. We studied these structures in animals and in human fetuses.


METHODS
Dissections were performed on quadrupedal mammals (29 dogs and 32 pigs) and 28 primates (Macaca apes). Moreover, macroslices of Macaca ape and of 182 human fetuses were examined histologically.


RESULTS
In quadrupedal mammals, we found no traces of any adipose masses comparable to the human mesorectum nor were there ligaments of suspension. In the ape, the adipose tissue in the mesosigmoid forms an adipose cuff that completely surrounds the extraperitoneal rectum. Two dense connective bands were found between the lateral wall of the pelvis and the perirectal tissue. Both the mesorectum and the lateral ligaments were clearly identified in the sections of human fetus only at the end of the fifth month but not earlier.


CONCLUSIONS
On the basis of our analysis of 3 animal species, we conclude that the mesorectum and lateral ligaments are absent in quadrupedal mammals but are present in primates. Therefore, we hypothesize that these structures appeared with the attainment of the upright position, even though other hypotheses are possible.
Nano
M
M
Department of Clinical Pathophysiology, University of Turin, Turin, Italy. mario.nano@unito.it
Prunotto
M
M
Ferronato
M
M
Solej
M
M
Galloni
M
M
D000818:Animals; D017885:Collateral Ligaments / Q000196:embryology; D004210:Dissection; D004285:Dogs; D005205:Fascia / Q000196:embryology*; D005333:Fetus; D006801:Humans; D010388:Pelvis / Q000033:anatomy & histology; D012007:Rectum / Q000033:anatomy & histology* / Q000196:embryology; D040521:Subcutaneous Tissue / Q000196:embryology; D013552:Swine