Renewal of the epithelium in the descending colon of the mouse. IV. Cell population kinetics of vacuolated-columnar and mucous cells.

Proliferation and migration of cells in the vacuolated-columnar and mucous cell lines were studied in the descending colon of adult female mice given a single injection or a continuous infusion of 3H-thymidine and killed at various intervals from one hour to 12 days. This investigation was carried out using one mum-thick Epon sections which were radioautographed after staining with the periodic acid-Schiff technique and iron-hematoxylin. In the normalized crypts with ten equal segments, labeled vacuolated cells at one hour after injection of 3H-thymidine were encountered in the lower four segments and in decreasing numbers in segments 5 through 7. From the percent labeled cells in segments of the crypt, the birth rate and fluxes of cells were computed. Moreover, it was found that a cell in the vacuolated-columnar cell line would undergo three mitotic cycles on the average from its birth at the cryptal base to its extrusion from the surface; of these three cycles, the last one which took place from segment 3 to segment 7 appeared to be a changeover from dividing cells to non-dividing cells, in accordance with the "slow cut-off" model of Cairnie et al. ('65b). Mucous cells located in segments 1 through 6 of the crypt were capable of incorporating 3H-thymidine and thus capable of undergoing mitosis. However, the rate of turnover of mucous cells based on proliferative rate was found to be much lower than the rate of turnover of mucous cells based on the transit time in the non-dividing segments of the crypt. Since there was a concomitant overproduction of cells in the vacuolated cells and newly formed mucous cells in the lower portion of the crypt, it was concluded that some vacuolated cells would give rise to mucous cells. This putative transformation occurred in the lower four segments of the crypt. Mucous cells which were formed by transformation would migrate upward along the cryptal wall and accumulate more mucus in the theca; in doing so, they would undergo two divisions, on the average, before they became non-dividing mucous cells. In ascending the cryptal walls, both vacuolated-columnar cells and mucous cells appeared to migrate at a similar speed; they moved much slower at the base of the crypt and accelerated toward the upper portion of the crypt, but they migrated at a constant speed in the non-dividing segments of the crypt.     

Presence of cells combining features of two different cell types in the colonic crypts and pyloric glands of the mouse.

A small number of epithelial cells which combine features of two cell types were observed in the descending colon and pyloric stomach of the mouse. In the descending colon, where the base of the crypts is mainly composed of poorly differentiated "vacuolated" cells, a few of these cells contain, besides the characteristic "vacuoles," mucous globules identical to those in mucous cells or, less frequently, dense granules such as are found in entero-endocrine cells. Because there is evidence that the poorly differentiated vacuolated cells give rise to the other cells of the epithelium, those which also contain mucous globules or dense granules are likely to be differentiating into mucous cells or entero-endocrine cells respectively. In the pyloric stomach, where the glands are mainly composed of mucous cells, some of which are poorly differentiated, a few of the latter exhibit, besides the mucous globules, entero-endocrine type granules or features of caveolated cells. It is likely that the poorly differentiated mucous cells give rise to the other gland cells; and, therefore, those mucous-containing cells which also display dense granules or caveolated cell features are taken to be differentiating into entero-endocrine or caveolated cells respectively. Most of the cells containing two kinds of secretory materials are believed to be stem cells which initially contain a few vacuoles (colon) or mucous globules (pylorus) but are differentiating into a cell containing a different type of secretion. Rare observations of two kinds of secretory materials in a mature cell suggest that the transitional period may be prolonged, perhaps indefinitely.     

Morphology of the intestine of prefeeding and feeding adult lampreys,Petromyzon marinus (L): The mucosa of the posterior intestine and hindgut

The epithelium of the posterior intestine and hindgut of recently metamorphosed adult lampreys (Petromyzon marinus L.) prior to and during spontaneous feeding was examined using light and electron microscopy. These two regions differ slightly in their general morphology but possess the same mucosal cell types. Included are caveolated absorptive and mucous cells, which are not present in more cephalic regions of the intestine, and ciliated and enteroendocrine cells. During feeding, the caveolated cells undergo dramatic transformation in their structure, namely, through the acquisition of numerous heterophagic vacuoles. Due to their morphology and to the fact that there are low amounts of lipid, it is suspected that caveolated cells are primarily involved in the absorption of protein components from the ingested host blood and body fluids. Iron in caveolated cells may result from the degradation of ingested heme or reflect the excretion of bile products at this location in the intestine. Mucous cells are likely responsible for lubrication of the luminal surface and may be important as a stem cell for the mucosal epithelium.     

The effect of estrogen on epithelial cell proliferation and differentiation in the crypts of the descending colon of the mouse: a radioautographic study

The regulatory role of estrogen on cell population kinetics in the descending colon was studied in intact female and ovariectomized mice. In the colonic crypts from intact mice, the crypt size (the number of epithelial cells per crypt column) and the proliferative activity of epithelial cells fluctuated slightly during the estrous cycle. Peak cellularity per crypt column was exhibted during estrus and early diestrus, whereas peaks in labeling index were seen during estrus and late metestrus. While the population size of mucous cells showed a minimal variation, the number of proliferative vacuolated cells per crypt column varied inversely with that of differentiated columnar cells during estrous cycle. The vacuolated cells were increased in number in the preovulatory phase and the columnar cells in the postovulatory phase. Three weeks after bilateral ovariectomy, the colonic crypt appeared to reach a new steady state, which was characterized by a small crypt size, a decrease in the number of differentiated cells, an increase in the relative number of proliferative cells and a relative increase in the proliferative activity of the crypt as compared to intact mice. When ovariectomized mice were treated with estrogen, the number of 3H-thymidine-labeled cells in the crypt was decreased as compared to untreated ovariectomized mice, the decrease being greater after a single injection than after multiple injections of estrogen, and the vacuolated-columnar cell line being affected more than mucous cell line. Meanwhile, the crypt size as well as the population size of differentiated cells in the crypt failed to return to normal after estrogen treatments. Thus, estrogen did not promote differentiation of epithelial cells in the crypt.     

A METHOD FOR QUANTITATION OF PROLIFERATIVE INTESTINAL MUCOSAL CELLS ON A WEIGHT BASIS: SOME VALUES FOR C57BL/6

A method for determining the number of intestinal mucosal crypts, S cells, and total proliferative cells, on a weight basis has been presented. The number of crypts was obtained (following injection of tritiated thymidine) by dividing the disintegrations per minute (dpm) per mg intestine by the dpm per crypt. Multiplication of the number of crypts per mg by the number of labeled cells per crypt (determined radioautographically) resulted in the number of S cells per mg intestine. Division of the number of S cells per mg by the fraction of proliferative cells in S (obtained by cell cycle analysis) resulted in the number of proliferative cells per mg intestine. Values for duodenum, jejunum, and ileum of male C57BL/6 mice are given.     

Light and electron microscopic cytochemistry of glycoconjugates in the rectosigmoid colonic epithelium of the mouse and rat

The several cell types in mouse and rat rectosigmoid colon have been examined with light and electron microscopic methods for localizing and characterizing complex carbohydrates. Mucous cells, also termed vacuolated cells, and goblet cells comprised most of the deep crypt epithelium in both species, and absorptive columnar cells and goblet cells mainly populated the more superficial epithelium of the upper crypts and main lumen. Occasional tuft cells and enteroendocrine cells were also encountered. Transitional cells structurally intermediate between mucous cells and absorptive cells contained granules characteristic of mucous cells and vesicles like those of columnar absorptive cells. These intermediate cells supported the concept of replacement of mucous by absorptive cells through transformation of mucous into absorptive cells. The intermediate cells also contained numerous lysosomes often in apparent fusion with mucous granules, indicating crinophagic disposal of mucous granules as a mechanism in the cell transformation. Glycoconjugate in absorptive cell vesicles resembled that coating the apical plasmalemma and appeared to represent the source of the glycocalyx of the brush border. Complex carbohydrate in these vesicles differed cytochemically from that of the mucous cell granules, which release their content into the crypt lumen. The absorptive cell vesicles, therefore, constitute an organelle distinct from the mucous cell granules rather than an atrophic form of the latter in a more mature cell. Goblet cells differed in failing to transform morphologically with age but changed in the cytochemical characteristic of their secretion during migration up the crypts. Terminal N-acetylglucosamine residues diminished, while terminal sialic acid-galactose dimers increased during the upward migration, indicating activation of glycosyl transferase synthesis in relation to goblet cell maturation. Glycoconjugate in secretion of mucous cell granules differed markedly from that in goblet cell granules, and content of both organelles differed from that of absorptive cell vesicles. However, secretion in mucous cell granules appeared generally similar for mice and rats with minor exceptions, and secretion in goblets of mice generally resembled that in goblets of rats. Cells interpreted tentatively as Kulchitsky cells stained for high content of fucose with the Ulex europeus I lectin. Globoid leukocytes infiltrating the epithelium of the rat but not the mouse rectosigmoid colon resembled globoid leukocytes in rat tracheal epithelium and, like the latter, appeared to derive from mast cells.     

Dynamic histology of the antral epithelium in the mouse stomach: II. Ultrastructure and renewal of isthmal cells

The isthmus of typical mucous units of the pyloric antrum was investigated in 3- to 4-month-old CD1 mice using light and electron microscopy as well as 3H-thymidine radioautography. On the average, the isthmus measured 25 microns in length and was composed of 36 isthmal cells and two enteroendocrine cells. Isthmal cells generally displayed features found in embryonic cells, such as many free ribosomes, scant organelles, and a large reticulated nucleolus, and were, therefore, at an immature stage of development. Isthmal cells could be devoid of secretory granules ("granule-free cells," 2%) or contain a few small, spherical, PA-Schiff-positive, mucous granules in their apex. The granules in some of the cells had a variegated appearance and a diameter averaging 235 nm ("mottled granule cells," 39%); in other cells, the granules had a large diameter, 278 nm, with a pale background and a dense core ("core granule cells," 28%); while in still others they were homogeneously dark and measured 264 nm ("dense granule cells," 12%). Finally, some cells included a mixture of core and dense granules ("mixed granule cells," 14%). One hour after a single injection of 3H-thymidine, 37% of the isthmal cells were labeled. Each of the five isthmal cell types could acquire label and, therefore, divide. After one or more days of continuous 3H-thymidine infusion, all isthmal cells were labeled. Their turnover time was estimated to be 16.1 hr (t1/2 = 11.2 hr). The isthmus is thus composed of several cell types which are turning over rapidly. While all are relatively immature, the various types are thought to represent different developmental stages in the life history of an isthmal cell. A model devised on this basis proposes that the granule-free cells are stem cells, from which mottled granule cells are derived. These in turn evolve into either the dense granule cells of the upper isthmus or the core granule cells of the lower isthmus, or into the mixed granule cells (which are believed to develop eventually into dense granule cells or core granule cells). Maintenance of a steady state requires that the rapid production of isthmal cells be associated with rapid emigration; the dense granule cells presumably going to the pit and the core granule cells to the gland. The turnover of isthmal cells is accordingly described as following a "bidirectional pattern" of renewal.     

Dynamic histology of the antral epithelium in the mouse stomach: III. Ultrastructure and renewal of pit cells

The pit (foveola) of typical mucous units of the pyloric antrum was investigated in 3- to 4-month-old CD1 mice, using light and electron microscopy, sometimes combined with 3H-thymidine radioautography. Reconstruction of units from serial sections revealed that, on the average, the pit measured 151 microns in length and was lined by 184 mucus-containing pit cells. Of these, 164 were located along the wall of the pit, whereas 20 surrounded its opening on the free surface. For ultrastructural examination the pit was divided into equal thirds. The proximal third, located next to the isthmus and referred to as pit base, was composed of cells showing electron-dense mucous granules greater in number but similar in density and diameter to those of isthmal dense granule cells. Nucleoli were rather large, irregular, and reticulated; these and other features were indicative of partial differentiation. The appearance of the cells gradually changed with the distance from the isthmus. In the middle third or mid pit, cells had small, fairly rounded nucleoli, while mucous granules were more numerous than in the pit base but similar in appearance and size; these cells were considered to be mature. In the distal third or pit top-surface, granules became elongated and nucleoli shrank, and lysosomes and vacuoles greatly increased in number, indicating that cells were at a terminal stage. Indeed, some of the cells were extruded into the stomach lumen while others were phagocytosed by adjacent cells. Following a single injection of 3H-thymidine, labeling was found only in a small cohort of cells in the pit base. At the end of 1 day of continuous infusion, the cohort of labeled cells had reached the mid pit; by 2 days, the pit top; and by 3 days, the free surface, where cells were eventually lost. The renewal time of pit cells was assessed at 2.98 days (t1/2 = 1.8 days), giving a turnover rate of 33.5% per day. It is estimated that the divisions of pit base cells provide two-thirds of the cells needed daily for pit-cell renewal, while the other third is supplied by an influx of dense granule cells from the isthmus. These cells enter the pit and continuously migrate toward the gastric lumen, while differentiating in the pit base, maturing in the mid pit, and reaching a terminal stage at the pit top-surface. The progressive and orderly migration of pit cells is described as a "pipeline pattern" of renewal. It is completed in about 3 days when terminal cells are lost at the pit top-surface.     

Structural characteristics of the mucosa of the small intestine in gnotobiote rats]

Histological and electron microscopic investigations of mucosal strucutre of the small intestine in gnotobiotic and conventional rats have demonstrated that mucosal index (lenear dimentions of villi/crypt dimentions ratio) is equal to 3--4 and 1.4--2.5, respectively. In gnotobiotic rats the number of mitotic figures in crypts is less, migratory-desquamative parameters of enterocytes are 5--6 days, cranio-caudal gradient of linear dimentions of villi, crypts and the number of goblet-shaped cells is not expressed. The goblet-shaped cells and cells with acidophilic granules have secretion of moderate intensity. The number of cells infiltrating epithelium of villi and crypts is greater when microbes are present in the gut lumen. A considerable difference in the structure of the connective tissue basis of the gnotobiotic and conventional rats mucosa are also connected with presence or absence of microflora in the gut lumen.     

Status of the mucous membrane of the jejunum of the rat after desympathization

Desympathization of the rat jejunum has been performed by means of periarterial sympathectomy of the cranial mesenteric artery. In total preparations of the jejunum wall, by means of water-aldehyde method, catecholamines have been revealed for controlling desympathization effectiveness. Signs of reinnervation appear in 180 days after the operation. In paraffin sections, stained by general histological methods, morphometric analysis of the most important parameters of the mucous membrane is performed (height of the villi, depth of the crypts, villus/crypt index, height of the epithelium, amount of goblet cells and interepithelial lymphocytes) Height of the villi and villus/crypt index increase in 1, 3 and 14 days, while in 7 days after the operation depth of the crypts increases, and the index mentioned decreases. Height of epithelium in the villi decreases in 14, 30 and 90 days. In 3-30 days after the operation amount of interepithelial lymphocytes increases.     

THE EFFECT OF TRANSPOSITION TO JEJUNUM ON EPITHELIAL CELL KINETICS IN AN ILEAL SEGMENT

Epithelial cell kinetics were studied in an ileal segment after transposition to proximal jejunum. The number of cells per villus column in the transposed ileum increased after 4-7 days to reach values normal for jejunum after 14-30 days. This increase was accompanied by a simultaneous increase in the number of cells per crypt column up to 130% of values in jejunum and ileum in situ. The percentage of labelled crypt cells, after labelling with ³H-thymidine, and the relative size of the proliferative cell compartment in the crypt in the transposed ileum did not differ from values in the ileum in situ at any time interval after surgery. The total proliferative activity per crypt, which was determined by scintillation counting of isolated crypts after ³H-thymidine labelling, increased two-fold from 7 days after surgery. Cell migration studies showed that the increase in the number of villus cells was probably not caused by a change in the life span of the epithelial cells. It seems that the increase in the number of villus cells in ileal epithelium after transposition to proximal jejunum is brought about by an enlargement of the crypt, while the relative size of the proliferative cell compartment in the crypt remains unchanged.     

Prolactin and ergocryptine effects on mucus glycoproteins of the rat ileum

Summary The present work reports observations from a first study on the effect of prolactin on mucous cells of the mammalian ileum. Sprague-Dawley rats were treated with prolactin or with the prolactin-inhibitor ergocryptine. Light microscopic histochemical study revealed that ergocryptine increased the number both of Alcian Blue-positive mucous cells and of the total number of mucous cells in the ileal crypts. Prolactin treatment apparently decreased the number of Alcian Blue pH 1.0-positive (sulphated glycoprotein-containing) mucous cells on the villi but was without effect on crypt cells. The implications of these observations are discussed.     

PANETH AND GOBLET CELL RENEWAL IN MOUSE DUODENAL CRYPTS

Proliferation of Paneth and goblet cells of mouse duodenal crypts was studied by high resolution light microscope radioautography. In one group of mice, blood levels of thymidine-³H were sustained for up to 12 hr by repeated injections of isotope to facilitate identification of proliferating cells. In these animals, many goblet cell nuclei incorporated thymidine-³H whereas only 1 of 6261 tabulated Paneth cells was labeled. Cells intermediate in structure between undifferentiated and goblet cells and between undifferentiated and Paneth cells were identified and their light and electron microscopic features are described. A significant number of these "intermediate" cells incorporated thymidine-³H into their nuclei. Another group of mice received a single injection of thymidine-³H. These animals were killed 4 hr to 29 days after isotope administration. Goblet cells and intermediate cells with labeled nuclei were identified 4 hr after thymidine-³H but could not be seen after 15 days. In contrast, Paneth cells with labeled nuclei were not observed until 24 hr after thymidine-³H but were still present at 29 days, long after labeled undifferentiated, goblet, and intermediate cells had disappeared. We conclude that differentiated Paneth cells in mouse duodenum do not normally proliferate, but, instead, arise by differentiation from undifferentiated crypt cells or from intermediate cells. Moreover, once formed, Paneth cells persist in crypts for a prolonged period. In contrast, intermediate cells and crypt goblet cells proliferate actively and are less stable cell populations than differentiated Paneth cells. The precise function of the intermediate cells is not known, but they may represent transition forms between undifferentiated cells and the more matrure secretory cells. Damage of crypt epithelial cells, thought to be due to radiation effects, was evident in both groups of mice.     

The response of the small intestine of the protein-deficient rat to infection with Nippostrongylus brasiliensis

The intestinal response of the protein-deficient Wistar rat was examined after primary infection with 1500 larvae of Nippostrongylus brasiliensis. Protein-deficient animals failed to expel N. brasiliensis after 15 days at a time when nutritionally normal animals had expelled more than 99% of the worm burden. Morphology of the small intestine of protein-deficient animals before infection showed small villi and crypt hypoplasia, followed after infection by sustained crypt hyperplasia and increased mitotic index of crypts. Protein deficiency was associated with fewer mucosal mast cells, goblet cells and intraepithelial lymphocytes. There was an impaired response of mucosal mast cells and goblet cells to infection. This could explain the deficiency of worm expulsion in these protein-deficient animals.     

The effect of continuous irradiation on cell proliferation and maturation in small intestinal epithelium.

Autoradiographic studies and scintillation counting of crypt material after pulse labelling with 3H-thymidine showed that during continuous irradiation with 290 rads/day a reduced proliferative activity is present in the crypts of rat small intestine after 1 day of irradiation and of normal activity during the remaining period (5 days) irradiation. After cessation of irradiation an increase in proliferative activity can be observed after 1 day of recovery. From the time (36-48 hr after starting of the irradiation) that the number of villus cells is reduced an expansion of the proliferation zone in the crypt was observed. Both effects last until 1 day of recovery after cessation of irradiation. The process of crypt cell maturation and of villus cell function has also been studied during and after continuous irradiation by micro-chemical enzyme analyses in isolated crypts and villi. It was found that the expansion of the proliferation zone in the crypt is accompanied by a decrease in activity of only those enzymes (i.e. non-specific esterases) which normally become active during crypt cell maturation. The activity of enzymes normally present mainly in the functional villus cells remained relatively unaffected by changes in crypt cell kinetics. A hypothesis of different regulation mechanisms of the proliferative activity in the intestinal crypt and a possible explanation of the different behaviour of various enzyme activities as a result of changes in crypt cell proliferation is discussed.     

Epithelial crypts: A complex and enigmatic olfactory organ in African and South American lungfish (Lepidosireniformes,Dipnoi)

African lungfish (Protopterus ) seem unique among osteognathostomes in possessing a potential vomeronasal organ homolog in form of accessory epithelial crypts within their nasal cavity. Many details regarding structural and functional properties of these crypts are still unexplored. In this study, we reinvestigate the issue and also present the first data on epithelial crypts in the South American lungfish Lepidosiren paradoxa . The nasal cavities of L. paradoxa and Protopterus annectens were studied using histology, scanning electron microscopy, and alcian blue and PAS staining. In both species, the epithelial crypts consist of a pseudostratified sensory epithelium and a monolayer of elongated glandular cells, in accordance with previously published data on Protopterus . In addition, we found a new second and anatomically distinct type of mucous cell within the duct leading into the crypt. These glandular duct cells are PAS positive, whereas the elongated glandular cells are stainable with alcian blue, suggesting distinct functions of their respective secretions. Furthermore, the two lungfish species show differently structured crypt sensory epithelia and external crypt morphology, with conspicuous bilaterally symmetrical stripes of ciliated cells in L. paradoxa . Taken together, our data suggest that stimulus transport into the crypts involves both ciliary movement and odorant binding mucus.     

Modeling Cell Dynamics in Colon and Intestinal Crypts: The Significance of Central Stem Cells in Tumorigenesis

Colon and intestinal crypts have been widely chosen to study cell dynamics because of their fairly simple structures. In the colon and intestinal crypts, stem cells (SCs) are located at very bottom of the crypt, fully differentiated cells (FDs) are located in the top of the crypt, and transit-amplifying cells (TAs) are in the middle of the crypt between FDs and SCs. Recently, it has been discovered that there are two types of stem cells in the intestinal crypts: central stem cells (CeSCs) and border stem cells. To investigate dynamics of mutants in colon and intestinal crypts, we develop a four-compartmental stochastic model, which includes two SC compartments, and TAs and FDs compartments. We calculate the probability of the progeny of marked or mutant cells located at each of these compartments taking over the entire crypt or being washed out from the crypt. We found that the progeny of CeSCs will take over the entire crypt with a probability close to one. Interestingly, the progeny of advantageous mutant TAs and FDs will be washed out faster than disadvantageous mutants. Saliently, the model predicts that the time that the progeny of wild-type central stem cells will take over the mouse intestinal crypt is around 60 days, which is in perfect agreement with an experimental observation.     

Development of the pattern of cell renewal in the crypt-villus unit of chimaeric mouse small intestine

We have previously shown that the epithelium of each adult intestinal crypt in chimaeric mice is derived from a single progenitor cell. Whether the crypts are monoclonal from the outset-that is, are formed by the proliferation of a single cell-or whether their formation is initiated by several cells was not known. Here we report that many crypts contain cells of both chimaeric genotypes in the neonatal period indicating a polyclonal origin at this stage of morphogenesis. The cellular organization of the early neonatal crypt is therefore different from that of the adult crypt, which includes a zone of 'anchored' stem cells above the crypt base. Within 2 weeks, however, the crypt progenitor cell and its descendants displace all other cells from the crypt and the crypt attains monoclonality. The distribution of enterocytes on chimaeric villi in the neonate shows a mottled pattern of mosaicism which is progressively replaced by coherent sheets of cells from the crypts, and within two weeks the orderly adult clonal pattern is established.     

Morphometric and Kinetic Studies On the Changes Induced In the Intestinal Mucosa of Rats By Intraperitoneal Administration of Quinacrine

The intraperitoneal administration of large doses of quinacrine in rats results in a state of enteromegaly affecting mainly the distal small bowel, caecum and proximal colon. This enteromegaly is associated with mucosal crypt hyperplasia, and hypertrophy and hyperplasia of the Muscularis propria. In order to investigate the changes in the intestinal mucosal crypts, morphometry and a metaphase-arrest experiment with vincristine were undertaken on a group of rats given 12 mg of quinacrine hydrochloride by intraperitoneal injection daily for 5 days 2 weeks previously, and comparisons drawn with a group of control animals. In the quinacrine-treated animals there was marked enteromegaly affecting the distal small bowel, caecum and proximal colon, and in these segments there was clear crypt hyperplasia. Proximal and distal to the dilated bowel hyperplasia was not seen. No consistent pattern of change in crypt-cell birth rate was evident. the mechanisms by which quinacrine may effect kinetic and morphometric changes in the intestinal crypts are considered.     

CHANGES IN INTESTINAL CELL KINETICS IN THE SMALL INTESTINE OF LACTATING MICE

The enlargement of the small intestine of mice during lactation is due, at least in part, to hyperplasia in the mucosal crypts and villi. The number of cells per crypt increases by 130% and the cell production rate by 63% after 15 days of lactation. These parameters were measured from crypt squashes and sections using both double-label and PLM techniques. Neither the numbers of crypts and villi in the small intestine nor the turnover time of post-mitotic cells on the villi changed. A number of factors appear to act in concert during lactation to trigger this increase in epithelial cell number in the small intestine. The experiments reported suggest a role for the increased quantity of food consumed by the lactating animal, for changing hormonal levels, and for the increased demands placed on the body by milk production.