Kinetics of tissue proliferation in colorectal mucosa during post-natal growth.

The main developmental event in the colorectal mucosa during post-natal growth is a dramatic increase in the number of crypts of Lieberkühn, resulting from a longitudinal fission of pre-existing crypts. In the present study, the kinetic aspects of this process have been analysed, using extensive gland and cell counts involving the entire colon and rectum of 24 male BD IX rats distributed into four age groups. The number of crypts was found to rise from an average 4652 to 423,800 between birth and adulthood; the corresponding ratios of bifurcating glands were 13.55 and 0.67%, respectively. Crypt production attained its maximum 18 days after birth with an hourly increment of 519 units. The time spent by replicating glands in the bifurcating stage ('fission time') averaged 6.9--10.5 hr. The mean number of epithelial cells per crypt rose from 249 in 4-day old rats to 635 in adults. The estimated total number of epithelial cells in the colon and rectum was one million in newborns and 248 million in adults. The increment in cell number peaked 3 weeks after birth with a value of 310,000/hr. During the first few days after birth, all cells produced in the epithelium were retained. Cell loss thereafter rapidly progressed, reaching 70% of the cell production in 3-week old animals.