As already pointed out in the preceding chapter, the gonads, or primary reproductive organs, not only furnish either sperm cells or eggs, but they are also glands of internal secretion.
Whenever castration occurs before puberty, the so-called secondary sexual characters fail to develop. It has been shown that this result is due not to that part of the testis or ovary which produces the sperm cells or eggs, but to certain interstitial glandular cells of endocrine function, called “puberty glands” (Steinach).
There are three kinds of cells in the testis, namely, spermatogenic, Sertoli cells, and interstitial cells. The first two make up the walls of the seminiferous tubules, while the interstitial cells are grouped around the blood vessels in the spaces between the tubules. They form before the germ cells do and respond differently to various stains, thus indicating their specific character. Moreover, they do not succumb to the lethal effects of X-rays as quickly as do sperm cells.
When an animal is deprived of the hormones produced by the interstitial tissues of the testis, there is a tendency for distinctly male characteristics, such as the horns of the stag, the plumage colors of male birds, the distribution of hair that marks the male mammal, and particularly male behavior in courtship, to revert to a neutral condition. Interstitial glandular tissue, however, is not found in the testes of all vertebrates, and it is equally true that many vertebrates are without secondary sexual characters.
Small groups of interstitial cells were discovered in the ovary by Pfluger as early as 1863, but subsequent study thus far only goes to prove that they lack the morphological individuality of the interstitial testicular cells and may not belong to the same endocrine category. In the ovaries of certain mammals, including man, at least two hormones are produced, estrogen, formed by developing Graafian follicles, and progesterone, elaborated in the corpus luteum which is formed in the follicle, after the liberation of the egg, by proliferation of the cells of the zona granulosa.
Estrogen is associated with the development of secondary sexual characters and, after puberty, regulates oestrous and, in Primates, the menstrual cycle. Progesterone initiates the changes preparatory to implantation of the ovum and also may inhibit oestrous and menstruation, which does not occur when the corpus luteum persists as in pregnancy and some pathological conditions. Apparently degeneration of the corpus luteum, gradually cutting off the supply of progesterone near the end of gestation, is associated with the onset of milk production, or lactation, and the sensitizing of the uterine muscles to pituitrin, which leads to parturition. Pearl and Boring have also demonstrated the presence of lutein cells in the ovary of the hen, although the mammalian functions just enumerated do not occur in birds.
It will be noted that whatever endocrine tissues are present in either the ovaries or the testes serve in the long run the same general purpose of maintenance of the species as do the primary sex cells produced in the gonads, since secondary sexual characters contribute indirectly to the general function of mating and reproduction.