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Abstract The histomorphological studies of the rabbit ovary during pregnancy and pseudopregnancy revealed that, at 12 hrs post mating and 14 hrs post injection of an ovulatory dose of human chorionic gonadotropin (HCG) the ovary showed clear signs of the preovulatory changes. The highly folded fimbrae of the uterine tube became closely related to the surface of the ovary. Several preovulatory follicles (blister like) could be demonstrated and appeared bulging from the outer ovarian surface epithelium. The preovulatory changes included, thinning in the apical region of the preovulatory follicle and in the ovarian tissue covering it. Edema was demonstrated in the all layers of the apical follicular wall (membrane granulosa, theca interna and theca externa) and in the covering tunica albuginea. Massive leucocytic and RBCs infiltration of the all layers of preovulatory follicle and in tunica albuginea especially in the apical follicular region. The ovarian surface epithelium above the apical follicular region of the preovulatory follicles became formed of a single layer. Large and electron dense intracyoplasmic bodies could be demonstrated in the ovarian surface epithelial cells. The tunica albuginea became disintegrated and contained active fibroblast cells and contained dissociated and degenerated collagen fibers. Smooth muscle like cells could be demonstrated in the theca externa of the preovulatory follicles in addition to active fibroblasts. Two types of cells could be demonstrated in the theca interna, theca interna cells and elongated cells (Fibroblast like). The granulosa cells were widely dissociated and in the apical region of the preovulatory follicles, the membrane granulosa became thin and formed of few widely separated cells and may be completely lost especially in the stigma. Stigma was a thin non vascular area in the surface of the ovary covering the thin apical region of the preovulatory follicles. It was formed of a single layer of the ovarian surface epithelium and thin layer of sub-epithelial degenerated connective tissue. Dissociation of the Call-Exner’s bodies and cumulus expantion could be demonstrated in the preovulatory follicles of the rabbit ovary. Mitotic figures could be demonstrated clearly in the zona granulosa of the preovulatory follicles. With transmission electron microscope the granulosa cells of the preovulatory follicles still contained well developed rough endoplasmic reticulum, numerous polyribosomes, moderately dense bodies and mitochondria. Oocyte maturation was a prominent feature of the preovulatory follicles of the rabbit ovary. Oocyte maturation was included nuclear and cytoplasmic maturation. The oocyte and its surrounding cells (corona radiata and cumulus oopherus cells) disconnected from the membrana granulosa became loosen and free in the follicular fluid. The present study showed that, luteinization process in the rabbit started after ovulation. It was characterized by the transformation of the postovulatory follicle into a capsulated functional highly vascular corpus luteum by gradually replacement of the central blood clot with highly vascular luteal tissue. It could be divided into 3 stages; the first stage, was the stage of migration and proliferation which characterized by migration of theca interna cells, vascular endothelial cells and fibroblast cells from the vascular theca interna to the non-vascular mural granulosa to became intermingled with each other. It was also characterized by the proliferation of these all types of cells. It lasted till 3 days post mating or induction: At 40 hrs post induction, the rabbit ovary contained several slightly raised dark red spots representing the corpora haemorrhagica. The corpus haemorrhagicum appeared as large post ovulatory follicle filled partly or completely with blood, contained no oocyte and showed the following changes: the cells of the mural granulosa were infolding inward, mitotic figures demonstrated in them, the basement membrane became completely lost and theca interna cells became intermingled with granulosa cells. Second stage (from 3 to 7 days of pregnancy and from ovulation to 3 days of pseudopregnancy) was the stage of differentiation and angiogenesis which characterized by the differentiation of the granulosa cells and theca interna cells to form large (granulosa) lutein cells and small (theca) lutein cells respectively and massive angiogenesis to form a highly complicated network of blood capillaries. This differentiation was accompanied by morphological, cytological and cytochemical changes in the both types of lutein cells as appearance of lipid droplets and smooth endoplasmic reticulum. PAS positive granules could be demonstrated in the cytoplasm of the large lutein cells. The developing corpus luteum in this stage became well organized and formed of vascularized compact cellular mass surrounded by highly vascularized connective tissue capsule. Some developing corpus luteum still contained central blood clot and in the other the central blood clot disappeared and replaced by the developing cells of the corpus luteum or connective tissue. The ovarian cortex was occupied by numerous corpora lutea in different stages of the development; not all the developing corpora lutea were in the same stage of the development but some in early stage of the development and the other in advanced stages of the development. Four different types of cells could be demonstrated in the developing corpus luteum, large (granulosa) lutein cells, small (theca) lutein cells, fibroblasts and vascular endothelial cells. The third stage (from 7 to 14 days of pregnancy and from 3 to 7 pseudopregnancy) was the stage of hypertrophy and it was characterized by increased in the size of the large lutein cells as a result of increasing in the amount of cell organelles and inclusions which characterized the highly active steroid secreting cells as lipid droplets and smooth endoplasmic reticulum. The mean diameter of the large lutein cells was three times larger than granulosa cells of preovulatory follicle and small lutein cells. The maximum (peak) of luteal activity reached at 3-7 days post induction of ovulation and 14 days post matting where the corpus luteum became fully developed and reached its maximum size. Leucocytic infiltration could be demonstrated clearly in the post ovulatory follicle (corpus haemorrhagicum) and in early developing corpus luteum especially in the central blood clot. They included neutrophils, eosinophils, lymphocytes and macrophages. Blood platelets could also be demonstrated in the central blood clot in between the RBCs. The corpus luteum of rabbit was still functionally active nearly until the end of pregnancy and few days before the end pseudopregnancy where the corpus luteum started to regress or involute. Luteolysis or corpus luteum regression in rabbit occurred in two phases, the first of which was the functional regression (at the end of pregnancy) and was associated with marked decreased in progesterone production. The second phase (at 18 days of pseudopregnancy and 10 days postpartum), termed structural regression and occurred after initial decline in progesterone output. It characterized by the death of the large lutein cells and transformation of the small lutein cells into interstitial gland cells. The histological examination of the rabbit ovary at the end of the pregnancy revealed that, the cytoplasm of the large lutein cells was more vacuolated than the previous stages; lipid droplets were increased in the cytoplasm of the large lutein cells than the previous stages and some lipid droplets were coalesced with each other to form medium and large lipid droplets or vacuoles. The pericapillary spaces, the space between the endothelial basement membrane and the surface of lutein cells was enlarged and became more prominent than the previous stages. With transmission electron microscope the large lutein cells of the parturient rabbit ovary were characterized by disappearance of the whorled smooth endoplasmic reticulum, appearance of clears (lucent) interconnecting or coalescing autophagic vacuoles containing fibrillar structure or cytoplasmic remnants, appearance of many myelin figures and by an increase in the number of the moderately osmophilic (dense) lipid droplets of different shapes and sizes. Many pleomorphic mitochondria of different shapes (rounded, oval and rod shape) could be demonstrated between the lipid droplets and between the lipid droplets and the nucleus. The nucleus was large euochromatic with irregular outline and contained central rounded nucleolus. The cell membrane of the large lutein cells facing the blood capillaries and the lateral cell membranes between two adjacent large lutein cells had many cytoplasmic projections or folds. The small lutein cells were present singly between the large lutein cells and the blood capillaries. They were small in size with large oval or rounded central nucleus and their cytoplasm contained many slightly dense lipid droplets, many rounded and oval mitochondria and Golgi aparatus. At 18 days of pseudopregnancy and 10 days postpartum (10 days lactation): The ovary appeared vesicular and contained several vesicles representing the large antral follicles and many small white areas representing the regressed corpora lutea. At this stage the corpus luteum of the rabbit ovary showed clear signs of structural regression (luteolysis). Structural regression of corpora lutea of the pregnant and pseudopregnant rabbit ovary was characterized by autophagic and apoptotic programmed cell death of large lutein cells and differentiation and transformation of the small lutein cells into interstitial gland cells. Careful examination of the regressed corpora lutea of pregnancy and pseudopregnancy at different stages of the luteolytic process in different ovaries and in different serial sections revealed that, there were different patterns of structural luteolysis in the rabbit according to the presence or absence of the central blood clot. Leucocytic infiltration could be demonstrated in between luteolytic large lutein cells and in the newly formed blood vessels. Neutrophils, eosinophils, monocytes, lymphocytes and macrophages could be demonstrated. During luteiniztion, the corpus luteum of rabbit ovary gradually increased in size (diameter) and reached its maximum size at the mid of gestation period (14 days post mating) and at 7 days of pseudopregnancy then (during functional luteolysis) it slightly decreased in size around the end of pregnancy and pseudopregnancy followed by (during structural luteolysis) marked decreased in size at 10 days postpartum and 18 days of pseudopregnancy. This increased and decreased in the size of the corpus luteum was accompanied by an increased and decreased in the size of large lutein cells. Also during structural regression the mean number of large lutein cells was markedly and significantly decreased while the mean number of small lutein cells was significantly increased. The follicular dynamic in the rabbit ovary during pregnancy and pseudopregnancy showed waves of development (growth) and regression (atresia). All stages of the follicular development could be demonstrated in the rabbit ovary during pregnancy and pseudopregnancy especially the large antral follicle. |