112 Source o£ Equine Gonadotrophin 



Discussion 



With the additional facts which we now have at hand, let us again consider the 

 question of the source of equine gonadotrophin. The pituitary, chorion, and 

 endometrium can be considered to be the most likely sources. 



Pituitary. If the pituitary secretes the hormone, one must consider the possi- 

 bility that gonadotrophin is selectively taken up by endometrium and more 

 particularly by the endometrial cups, as the hormonal concentration of the 

 cups is much greater than that of the blood. Then one must assume that the 

 cups in turn secrete the hormone into the space between the maternal and 

 fetal tissues in a still more concentrated form. In other words, the cups would 

 be serving as excretory organs. Such a sequence of events seems to us extremely 

 unlikely. If the pituitary were the origin of the hormone then one must as- 

 sume an unusual activity for this gland. For instance, in mare 3 there is a 

 total of 2.4 million I.U. of hormone accounted for, whereas the pituitary con- 

 tains only 400 I.U. (table 2). This amount of hormone has accumulated in the 

 tissues over approximately a 25-day period, as the hormone first appears in 

 the bloom stream at about the 42d day of pregnancy. Thus it would be neces- 

 sary for the pituitary to secrete its total hormonal content 240 times daily to 

 account for the total hormone. It is true, of course, that some complications 

 arise in attempting to compare hormonal concentrations in the pituitary 

 with those in other tissues, as we have previously pointed out. Part of these 

 differences in action may be explained by the finding that the pituitary, unlike 

 mare serum, contains antagonistic substances and further that the pituitary 

 gonadotrophin is excreted in the urine (Evans and co-workers^. The differ- 

 ences in biological action of gonadotrophin obtained from the pituitary and 

 from serum or placental tissue might be used for a further argument against 

 the view of pituitary origin of serum gonadotrophin. 



Chorion. We have reviewed above the arguments used by Catchpole and 

 Lyons^ for the chorionic source of the hormone. The fact that the chorionic 

 epithelium overlying the cups gives evidence of special secretory activity 

 could be used in further support of this view. A study of hormonal concentra- 

 tions in the tissues, however, does not, in our opinion, support this theory. 

 The endometrial cups contain from 50 to goo times more hormone per gram 

 than the chorion in the 4 mares of the present study (table i) . The endo- 

 metrium in the cup region is separated from the chorion by the mass of edo- 

 metrial-cup secretion (pi. i, fig. 1). If the hormone is produced by the chorion, 

 one can account for the high activity of the endometrial-cup tissue in two ways: 

 gonadotrophin can diffuse slowly into the tissue from the mass of material 

 which we have referred to as endometrial-cup secretion; or it can pass to the 

 fetal blood, thence to the maternal blood at areas where there is close apposi- 

 tion of maternal and fetal membranes, and then be selectively absorbed from 

 the maternal blood by the endometrial cups. The fact that the hormone is a 

 protein and thus diffuses slowly renders the first possibility less tenable. The 



