Morphological studies on the development of bovine ovarian follicle and oiicyte

Abstract

Recently some authors have described that in several kinds of animals the cumulus oophorus is loosened by the enlargement of its intercellular spaces and the oocyte is easily discharged from the mature follicle at the time of ovulation. We have been performing the study of morphological changes in normal bovine reproductive organs, and this report dealt with the morphological observation of the development of ovarian follicles and the examination whether the above mentioned fact is the case in the cow. The ovarian follicles of Japan native cows obtained from the slaughterhouse and Holsteins kept in our laboratory were used. Serial sections of the cumulus were made and stained with hematoxylin-eosin. Results and discussions are as follows:(1) The relative growth curve between the major axis of the oocyte and that of the follicle (Ma in chart 1) was almost the same with that obtained in white mouse by BRAMBELL. This curve was divided into two phases; at the end of the first phase the ooecyte gained about 3 times of the major axis, and then about 3.6 times at the end of the second phase compared with that in the primary follicle. The relative growth curve between the minor axis of the oocyte and the major axis of the follicle (Mi in chart 1) showed the descendance in the last stage of the second phase. It is, however, very difficult to decide the cause of the result; it might be because of fixation, a few cases, or the reduction of oocyte size by the first maturation division.(2) The antrum began to appear in follicle of 437 × 294 microns in the major and minor axes.(3) The zona pellucida began to appear on the surface of the oocyte with the average size of 82.4 × 63.2 microns in the beginning of the second phase. The thickness of the zona pellucida which was 1.0 × 3.51.5 × 3.5 microns did not markedly increase or decrease in spite of the growth of the oocyte.(4) The cell, so-called “stellate cell”, which has the delicate processes and connects each other was seen in the follicular epithelium when the follicle grew larger than the secondary follicle. These stellate cells were localized in the region of the cumulus and became indistinct in the other region following the formation of the cumulus oophorus.(5) Although the cumulus of the follicle of 15mm. in diameter situated near the rupture point of the follicle, in the follicle of 611 mm. in diameter it located on the opposite site. These facts may be suggesting the movement of the cumulus. In the mature follicle of 1217 mm. in diameter, however, the cumulus situated on various positions of the follicular wall.(6) In the follicle more than 1mm. in diameter several small processes of the theca interna were found more distinctly in the region of the cumulus, but we could not find the penetration of the capillaries into the membrana granulosa.(7) In the follicle with the numerous corpuscles which are deeply stained with hematoxylin, the membrana granulosa was very thin because of the disappearance of its cells and the small processes of the theca interna were not clear in the region other than the cumulus. The oocyte and the cells of the cumulus, however, did not show the degenerative finding.(8) Follicular epithelial cells adjacent to the theca interna varied their forms according to the growth of follicle. They were flattened or cubical in the primary follicles, columnar when the antrum appeared, and thereafter cubical again in the follicles more than 6mm. in diameter.(9) The corona radiata was found to have several cell cords which run radially and connect it to the surrounding and among these cords several spaces were always seen after the formation of the cumulus. The space itself, however, was found to originate from the ones already present in the secondary follicle.