The Growth of the Pituitary Body in the Female Rabbit

The investigations described in this paper were undertaken with a view to determining whether there was a post-pubertal sexual dimorphism in the weight of the pituitary body in the rabbit similar to that described in the rat by Hatai (1914). An endeavour was also made to detect changes in the size of the pituitary body in the female rabbit correlated with the various phases of the oestrus cycle, and with pregnancy, should these changes occur. The statement of several earlier workers—Compte (1898), Erdheim and Stumme (1909), Launois and Mulon (1903), Mayer (1910), Herring (1920)—that the size of the pituitary changes during pregnancy suggested that investigation might provide valuable results.

The rabbits used for this investigation were all supplied by dealers and were of mixed breeds. They were killed by a blow breaking the neck, or in a few cases by chloroform, and the live body weight obtained. The alimentary canal was then dissected out and weighed, its weight being subtracted from the whole body weight to get the clean body weight. The error involved by thus neglecting the tissues of the alimentary tract is small, about 4 per cent. (Robb, 1928–29), and approximately constant. In the case of pregnant rabbits the weight of the uterus and contents was also subtracted from the live body weight. The brain was lifted from the skull leaving the pituitary body, complete except for the hypophysial stalk, lying in the sella turcica. This was removed, dissected free of connective tissue, and weighed on a torsion balance having an accuracy of ± 0·5 mg.

The difference of k as determined by Robb for male rabbits and that now recorded for female rabbits suggested that there might be a sexual dimorphism in the size of the pituitary body in the two sexes, such as that recorded by Hatai (1914) in the rat. The data of 31 male rabbits are given in Table II (Appendix). These rabbits were all over 580 gm. cleaned body weight and thus do not afford sufficient data to determine the general nature of the growth curve. If it is assumed that this is a straight line relationship, such as that found in the female, and by Robb in the male, as is legitimate, they do provide sufficient data to determine the value k = 0·57. It can be seen, therefore, that there is no reason to suppose that the relation of the pituitary body weight to body weight is different in the adult males and females. Such a difference, if present, must be small.

The work of Fee and Parkes (1929) suggests that there is active secretion from the pituitary body within an hour after copulation. Animals were therefore killed (i) immediately after copulation, and (ii) at varying periods after copulation, to ascertain whether there was a detectable fall in the weight of the pituitary body correlated with this heightened secretory activity. In Table III (Appendix) are given the data for these animals, together with weights of three adult animals killed without having copulated. In all cases the ovaries were examined for ripe follicles or new corpora lutea to ensure that the animals were in oestrus or that copulation had been effective. In the three animals in oestrus the pituitary weight averages 0 ·00017 Per cent, of the cleaned body weight. The percentage weight immediately after copulation is slightly higher than this figure. The percentage weights in the animals killed at three hours and more after copulation vary inconsequently, but the average is again 0-00017 per cent It is evident that any change in size of the pituitary body connected with copulation, if it occurs, is too small to be detectable by the methods used.

Compte (1898), Launois and Mulon (1909), Erdheim and Stumme (1909) in man, and Mayer (1910) in the cow, have reported an increase in size of the pituitary body during pregnancy, and Erdheim and Stumme maintain that this increased size persists after pregnancy. Herring (1920), however, found in rats a decrease of 24 per cent, in the weight of the pituitary body after pregnancy. In Table IV (Appendix) are given data for five rabbits killed when 10–17 days pregnant, and while, except in the case of HR 15, the relation of the pituitary weight to body weight seems to be higher than in the non-pregnant animal, the variation from the fitted curve is not greater than in the other adult animals. The number of animals used is not sufficient to draw any other conclusions regarding a change in size during pregnancy, on account of the very large individual variation observed.

The relation between pituitary weight and body weight in the albino rat, as shown by Hatai, is similar in both males and females up to an age of about 40–50 days, when the animals weighed on the average about 45 gm. each. After this age the weight of the pituitary becomes increasingly heavier in the females than in the males. There is thus a different growth rate of the pituitary after puberty in males to that in the females. This interesting conclusion concerning the rat suggested that a similar post-pubertal sexual dimorphism in the weight of the pituitary might exist in other animals. This suggestion is not borne out by the data recorded for the rabbit, in which no significant difference in the size of the pituitary in males and females after puberty is detectable. No significant changes in the size of the pituitary, correlated either with pregnancy or oestrus, were detected. Such changes, if they occur, are so small that they are effectively masked by the large individual variation in size.

The expenses of this research were defrayed in part by a grant from King’s College, London : part of the material was given by Professor F. W. Rogers Brambell. I wish to express my thanks for this assistance. I am indebted also to Professor Brambell for his constant advice and help while the work was in progress.