The development of the testis of the Merino ram, with special reference to the orgin of the adult stem cells

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Abstract
In the ram, as in other mammals, the sex cords are made up of two types of cell: indifferent cells (derivatives of the coelomic epithelium) and primordial germ cells. In the cords, each type pursues a separate and independent line of development to become respectively the Sertoli cells and the stem cells (type A spermatogonia) of the adult testis. The principal changes taking place in the primordial germ cells (gonocytes) are a reduction in the size and number of the Feulgen-positive particles in the nuclei, the appearance and subsequent fusion of the nucleoli, and, finally, an increase in the size of the nuclei. While these changes are taking place, the cytoplasm increases in volume and inclusions become more numerous. Cells which have undergone all these transformations have been called prospermatogonia. The cells of the germ line are at first more centrally placed in the sex cords than the indifferent cells. Just before spermatogenesis begins, they migrate to the basement membrane of the seminiferous tubules. All germ cells in tubules in which spermatogenesis has been initiated are seen as prospermatogonia. These cells become flattened against the basement membrane, and their nuclei become more oval in shape. They thus become identical with the stem cells of the adult. Little change is evident in the nuclei of the indifferent cells until puberty. Feulgen-positive material is found in the form of coarse granules at earlier stages of development. At puberty, these granules become dispersed to give a much more homogeneous nucleus. Concurrently, nuclei increase in size, and single or double true nucleoli can be identified. During development, increases in cytoplasmic volume take place. Although cell boundaries between indifferent cells cannot be seen in fixed material, phase contrast observations of fresh material have demonstrated that some forms exist as mononucleate units. It could not be determined whether the same was true in the case of Sertoli cells. No striking change in the relative numbers of glandular interstitial cells could be observed at different stages of development.