healthy horses : eleven foals ( 7 males and 4 females under one year old ) 
 and an yearling female thoroughbred were used for evaluation of the development stages of 
 the gastric mucosae of healthy horses . details of the program for endoscopic examination are 
 summarized in table 1table 1.endoscopic examination of the healthy horsesm : male , f : female . the number of horses is shown in parentheses , eleven foals ( 7 
 males and 4 females under one year old ) and an yearling female thoroughbred were used 
 for evaluation of the development stage of the gastric mucosa of healthy horses .. m : male , f : female . the number of horses is shown in parentheses , eleven foals ( 7 
 males and 4 females under one year old ) and an yearling female thoroughbred were used 
 for evaluation of the development stage of the gastric mucosa of healthy horses . affected foals : in the clinical field , it has been empirically 
 demonstrated that foals suffering from gastric ulcer frequently exhibit depression , 
 dysorexia or diarrhea [ 1 , 3 , 11 ] . fifty - six foals having 
 these symptoms were used for examination of gastric ulcer by endoscope , as shown in table 2table 2.endoscopic examination of the affected horsesage ( days)number of examinations at each age ( days)malefemaletotal4305383160981761901091991120718121198224total332356 . we devised a grading system for the gastric ulcers of the foals according to 
 andrews criteria   and calculated the distribution 
 of the lesions . the grading system was as follows : a score of 1 indicates that the mucosa is 
 intact , but associated with areas of hyperemia with erosion ( fig . 1.a grading system for the gastric ulcers of foals ( modified andrews criteria , 1999 ) . 
 ( a ) score 1 : the mucosa is intact , but associated with areas of hyperemia with erosion 
 ( arrows ) . ( b ) score 2 : small , single or multifocal superficial ulcers with hyperemia 
 ( arrows ) . ( c ) score 3 : large , single multifocal or extensive superficial ulcers with 
 hyperemia ( arrows ) . ( d ) score 4 : extensive lesions with areas of apparent deep 
 ulceration ( arrow ) . ) . a score of 2 indicates a small , single or multifocal superficial ulcer with 
 hyperemia ( fig . a score of 3 indicates a 
 large , single , multifocal or extensive superficial ulcer with hyperemia ( fig . finally , a score of 4 indicates extensive 
 lesions with areas of apparent deep ulceration ( fig . 1d ) . a grading system for the gastric ulcers of foals ( modified andrews criteria , 1999 ) . 
 ( a ) score 1 : the mucosa is intact , but associated with areas of hyperemia with erosion 
 ( arrows ) ( b ) score 2 : small , single or multifocal superficial ulcers with hyperemia 
 ( arrows ) . ( c ) score 3 : large , single multifocal or extensive superficial ulcers with 
 hyperemia ( arrows ) . ( d ) score 4 : extensive lesions with areas of apparent deep 
 ulceration ( arrow ) . endoscopic examination : the endoscope ( vq-8303a , olympus , tokyo , japan ) 
 used in the present study has an effective length of 3,000 mm , a tip diameter of 10.6 mm , a 
 suction unit and an image recording apparatus . the endoscopic procedure was performed 
 according to previous reports [ 10 , 13 ] . for the purpose of sedation and pain relief , a 
 combined solution of xylazine ( 0.5 mg / kg , iv , nippon zenyaku kogyo co. , ltd . , koriyama , 
 japan ) and butorphanol ( 0.01 to 0.02 mg / kg , iv , bristol - myers squibb co. , tokyo , japan ) was 
 used before endoscopic examination . the endoscope was inserted via the nasal cavity of foals 
 and reached the stomach cavity expanded by air from a biopsy channel . endoscopic examination 
 was performed on 4 regions of the squamous mucosa in the lesser curvature ( lc - s ) , squamous 
 mucosa in the greater curvature ( gc - s ) , glandular mucosa in the greater curvature ( gc - g ) and 
 pylorus ( p ) . statistical analysis of the mean scores was performed with the t - test . healthy horses : at the 6th day after birth , the margo plicatus was clear , 
 and serrated ridges were not noticeable . the squamous mucosa ( lc - s and gc - s ) wall was very 
 thin , and part of the spleen attached to the outer wall of the stomach was observed through 
 the thin gastric mucosa . the gc - g exhibited 
 a thin membrane - like appearance with a clear color . the margo plicatus became clear at 14 
 days old , and the formation of serrated ridges was not observed at that time . the squamous 
 mucosa wall thickened somewhat , had a translucent luster and appeared to be wet . the 
 squamous mucosa wall thickened further , and the clarity of the stomach wall was lost . the 
 gc - g was wet and had a surface structure containing irregularities that were finely 
 granular . at 28 days old , a serrated ridge was evident at the boundary of the squamous 
 mucosa and glandular part . the gc - g also thickened , and the granular structure of the mucosal surface 
 showed a more pronounced pink color . throughout the period from 6 to 28 days old , epithelial 
 desquamation although epithelial desquamation of the lc - s was noted until 60 days old , it was still 
 observed in the gc - s at 90 days old ( fig . old , the squamous mucosa wall had an increased thickness , and the serrated 
 ridges had grown , appearing similar to those observed in the squamous mucosa of a yearling 
 horse . after 30 days , the gm thickened and showed a reddish color . the thickened squamous mucosa wall of the yearling 
 lacked epithelial desquamation , and the surface of the mucous membrane showed dryness . the 
 mucosa of the gastric fundus and pylorus exhibited a shiny and smoothly granular structure . 
 during the period of endoscopic examination , there were no abnormalities in the gastric 
 mucosa in any of the foals . affected foals : the number of foals having a gastric ulcer during the 198 
 days after birth is summarized for each 30-day period in fig . the incidence ( 30.4% ) of gastric ulcer 
 sharply increases between 31 and 60 days old . the highest incidence ( 33.9% ) of gastric 
 ulcer is between 61 and 90 days old . the incidence ( 14.3% ) of gastric ulcer sharply 
 declines at 91 days old or older .. the minimum age of the foals having a gastric ulcer was 4 days old . the incidence of 
 gastric ulcer sharply increased by 30.4% after 31 days old . the highest incidence ( 33.9% ) of 
 gastric ulcer after 91 days old , the incidence of 
 gastric ulcer sharply declined ( 14.3% ) . the incidence ( 30.4% ) of gastric ulcer 
 sharply increases between 31 and 60 days old . the highest incidence ( 33.9% ) of gastric 
 ulcer is between 61 and 90 days old . the incidence ( 14.3% ) of gastric ulcer sharply 
 declines at 91 days old or older . the age ( days old ) at the time of endoscopic examination and distribution of gastric ulcers 
 in 56 foals are shown in fig . 3.fig . gastric 
 ulcers occurred more frequently in the squamous mucosa ( lc - s and gc - s ) ( 74.8% ) than in 
 the glandular mucosa ( gc - g and p ) ( 25.2% ) during the period of examination . gastric ulcer occurred more frequently in the squamous mucosa ( lc - s and gc - s ) ( 74.8% ) 
 than the glandular mucosa ( gc - g and p ) ( 25.2% ) during the period of examination . in the 
 group of foal at up to 30 days old , gastric ulcer was frequently observed in the gc - s . in 
 the group of foal at 31 to 60 days old , the ulcers increased in the lc - s in a manner similar 
 to that in the gc - s . in the group of foals at 61 to 90 days old , the incidence of gastric 
 ulcer sharply increased by more in the gc - s than in the lc - s . gastric ulcers formed 
 predominantly in the gc - s between 4 and 90 days old and had a peak incidence between 61 and 
 90 days old . in the group of foals at 91 to 120 days old , the incidence of ulcers was 
 reduced , and they were found predominantly in the squamous mucosa ( lc - s and gc - s ) . at over 
 120 days , there were no differences observed in the incidence of ulcer in the 4 parts of the 
 gastric mucosa . gastric 
 ulcers occurred more frequently in the squamous mucosa ( lc - s and gc - s ) ( 74.8% ) than in 
 the glandular mucosa ( gc - g and p ) ( 25.2% ) during the period of examination . the age ( days old ) at the time of endoscopic examination , and the grading scores of gastric 
 ulcers in 56 foals are shown in fig . 4fig . 4.the average score of gastric ulcers in all parts of the gastric mucosa . in the group 
 of foals at up to 30 days old , the mean  sd grading score of ulcers for each mucosal 
 site was 0.53  0.24 ( lc - s , 0.13  0.13 ; gc - s , 0.75  0.16 ; gc - g , 0.88  0.40 ; p , 0.38 
  0.26 ) . in the group of foals at 31 to 60 days old , the mean  sd grading score for 
 each mucosal site was 0.53  0.21 ( lc - s , 0.24  0.14 ; gc - s , 0.71  0.22 ; gc - g , 0.69  
 0.27 ; p , 0.50  0.20 ) . in the group of foals at 61 to 120 days old , the mean  sd 
 grading score in the gc - s was between 1.26  0.25 and 1.13  0.55 . the mean  sd 
 grading score in the gc - g was between 2.00  0.38 and 1.50  0.60 . in the group of 
 foals at more than 120 days old , the mean  sd grading score for each mucosa was 1.19 
  0.68 . the grade of gastric ulcer shows a tendency to increase with age in days .. the average score for gastric ulcers in each of the parts showed a tendency to 
 increase over time . in the group of foals at up to 30 days old , the mean  sd grading score 
 for ulcers at each mucosal site was 0.53  0.24 ( lc - s , 0.13  0.13 ; gc - s , 0.75  0.16 ; gc - g , 
 0.88  0.40 ; p , 0.38  0.26 ) . in the group of foals at 31 to 60 days old , the mean  sd 
 grading score for each mucosal site was 0.53  0.21 ( lc - s , 0.24  0.14 ; gc - s , 0.71  0.22 ; 
 gc - g , 0.69  0.27 ; p , 0.50  0.20 ) . in the group of foals at 61 to 120 days old , the mean  
 sd grading score in the gc - s was between 1.26  0.25 and 1.13  0.55 . the mean  sd grading 
 score in the gc - g was between 2.00  0.38 and 1.50  0.60 . in the group of foals at more 
 than 120 days old , the mean  sd grading score in each mucosa was 1.19  0.68 . the average score of gastric ulcers in all parts of the gastric mucosa . in the group 
 of foals at up to 30 days old , the mean  sd grading score of ulcers for each mucosal 
 site was 0.53  0.24 ( lc - s , 0.13  0.13 ; gc - s , 0.75  0.16 ; gc - g , 0.88  0.40 ; p , 0.38 
  0.26 ) . in the group of foals at 31 to 60 days old , the mean  sd grading score for 
 each mucosal site was 0.53  0.21 ( lc - s , 0.24  0.14 ; gc - s , 0.71  0.22 ; gc - g , 0.69  
 0.27 ; p , 0.50  0.20 ) . in the group of foals at 61 to 120 days old , the mean  sd 
 grading score in the gc - s was between 1.26  0.25 and 1.13  0.55 . the mean  sd 
 grading score in the gc - g was between 2.00  0.38 and 1.50  0.60 . in the group of 
 foals at more than 120 days old , the mean  sd grading score for each mucosa was 1.19 
  0.68 . healthy horses : a few reports have described the development process of 
 the gastric mucosa of neonatal foals using an endoscope [ 9,10,11 , 13,14,15 ] . however , long and continuous 
 observations of the mucosal development of the stomach for 119 days after birth have not 
 been reported as far as we know . in this report , endoscopic observation revealed the 
 development process of the gastric mucosa of the foal , during the 4 months following birth . 
 immediately after birth , part of the spleen attached to the outside stomach wall and after 28 days of development , the stomach wall was thick enough to 
 prevent observation of the spleen through the wall . the appearance of the mucosal surface 
 was very similar to that of a yearling horse . during examination , formation of epithelial 
 desquamation on the mucosal surface was characteristically observed in the squamous mucosa 
 ( lc - s and gc - s ) . histologically , the stratum corneum of the squamous mucosa forms beginning 
 in the late fetal stage , and the stratum corneum grows slowly thicker after birth . the keratinized epithelium of the stratum corneum 
 detaches from the mucosal surface gradually , and this is called epithelial desquamation . epithelial desquamation is considered a physiological phenomenon and is characteristic of 
 the development process of the gastric mucosa [ 11 , 
 15 ] . in our study , endoscopic examination revealed 
 the physiological development process of the gastric mucosa during the 4 months following 
 birth . earlier studies reported that epithelial desquamation of the squamous mucosa 
 disappears at around 40 days old . however , 
 epithelial desquamation was observed in the lc - s in our foals until 60 days old and in the 
 gc - s until 90 days old . this finding suggests that there is a difference in the development 
 of the gastric mucosa by region and that desquamation can continue over a long term , a term 
 longer than studies have previously reported . affected foals : as reported in western countries , gastric ulcers of foals 
 in japan ( hidaka district ) occur between 30 and 90 days old , but the reason for this is 
 still unclear . a deficiency in passive immunity is 
 considered to be one of the internal factors that contribute to gastric ulcers that occur 
 frequently in this period [ 7 , 8 ] . many severe diseases , such as pneumonia ( e.g , rhodococcus 
 equi ) , enteritis ( e.g , 
 rotavirus ) [ 5 , 6 , 22 ] , infectious 
 arthritis and osteomyelitis occur frequently in foals with immunodeficiency . on the other hand , 
 changes in feed and rearing environment can be considered as external factors . in the hidaka 
 district , the weaning of foals usually begins at around 100 days old and ends at around 180 
 days . in the healthy foals , the mother s milk and 
 secretion of by the foal prompts the proliferation of epithelial cells and maturation of the 
 gastric mucosa in preparation for weaning . while 
 living with the mother , the growing foal may frequently eat the concentrated feed given to 
 the mare . the decreases in gastric juice ph , 
 intake of the mother s milk and saliva secretion caused by this surreptitious eating are 
 considered to disturb the development of the gastric mucosa before controlled weaning . our 
 study revealed that maturation of the gastric mucosa of the foal occurs up to at least 90 
 days old . before weaning , surreptitious ingestion of the mother s feed therefore , the 
 deficiency of passive immunity and failure of feeding management can be considered important 
 factors in the occurrence of gastric ulcers in foals before weaning . the increased gastric 
 ulcer score along with age in the affected foals suggests that these factors may contribute 
 to formation of severe lesions before weaning of the foals . on the other hand , ulcers in the weaning period after 120 days old showed a high score in 
 the present study . weaning of foals may be involved in weakening the immune system and 
 weight loss . the gastric ulcers were found to have occurred mostly in the squamous mucosa ( lc - s and 
 gc - s ) throughout the examination period . the results suggest that gastric ulcers may occur 
 given the specific background of the development process of the mucous membrane and 
 structure of the stomach . the mucosal surface of the gm is protected by mucus produced by 
 the surface mucus cells , and secretion of digestive juice is not active in the lactation 
 period . the membrane of the squamous mucosa ( lc - s and gc - s ) lacks surface mucus cells and is 
 protected solely by the action of saliva . the mother s milk in the lactation period 
 contributes to protect the mucosal membrane and to the development of the gastric mucosal 
 epithelium . therefore , anatomical and functional 
 differences of the stomach , which are horse specific , may cause the differences in sites of 
 lesions and scores of gastric ulcers caused by the various internal and external factors . in 
 other words , when the causes of gastric ulcer overlap , the squamous mucosa is the most 
 affected region , and the lesion may occur most frequently in the gc - s , in which development 
 of the mucosa continues until around 90 days old . the process of development of 
 the gastric mucosa of healthy foals during the 119 days from the 6th to the 125th day after 
 birth was revealed in this study . characteristic changes were seen in the squamous mucosa 
 ( lc - s and gc - s ) , and the maturation of the squamous mucosa ( lc - s ) was more rapid than that 
 in gc - s . the epithelial desquamation of the squamous mucosa ( lc - s and gc - s ) indicates the 
 process of development of the squamous mucosa membrane and is considered to be a normal 
 physiological phenomenon . gastric ulcers formed predominantly in the squamous mucosa 
 ( especially in the gc - s ) due to the immature mucosa of the foals before the weaning period , 
 and the peak incidence was between 61 and 90 days .