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Abstract
SUMMARY
A total number of 43 rabbits ranged from 0 day up to 16 weeks of age in addition to pregnant and post partum rabbit were used in the present study. Results of the present work could be summarized as follows: The histomorphological studies of the postnatal development of the rabbit ovary revealed that at 0 day the ovary was relatively very small and resemble rice grain in shape. Female rabbit was born in a relatively immature stage with germ cells have not entered meiosis. The ovary at this early stage of the postnatal development was formed of a compact cellular mass contain mainly one population of the female germ cells (oogonia) and no ovarian follicles. The germ cells were arranged in groups to form cell nests. Most of the germ cells in the ovary at this stage of development were oogonia in premeiotic resting phase. Oogonia in metaphase, anaphase and telophase of mitotic division were also demonstrated at 0 day of postnatal development. The limit between the outer cortex and inner medulla was not clear. The outer surface epithelium was formed of a single layer of cuboidal or columnar cells and in some areas, it was formed of multiple layers cuboidal or columnar cells. The primitive tunica albuginea under the outer surface epithelium was difficult to demonstrate at this early stage of postnatal development. The rabbit ovary at 1 day still resemble 0 day.
With advancement of age the rabbit ovary at 1 week of age still showed slow ovarian growth. Occasionally ovarian surface epithelial cells appeared to extend into the cell nests and contact the germ cells. The primitive cortex and medulla of the developing ovary contained three types of cells that could be identified by the light microscope, germ cells, pregranulosa cells or follicular cells and mesenchymal cells or stroma cells. The rabbit ovary at 1 week of age showed two populations of germ cells; the first population were oogonia whereas the second population were oocytes in different stages of the first meiotic prophase; leptotene, zygotene, pachytene and diplotene stages. These two cell populations where arranged to form cell nests, the cellular components of the nests were separated from each other by stroma cells. Meiosis in the female rabbit ovary started postnatally. The ovary at this stage of postnatal development contained no ovarian follicles.
In the rabbit ovary of 2 weeks of age; the limit between the cortex and medulla still not clear. The peripheral ovarian cortex was occupied by nests of naked oocytes and somatic cells. Few oogonia still demonstrated among the cell populations of the cell nests. Degenerated or apoptotic germ cells were clearly demonstrated at this stage of development. The first evidence of the primordial follicles formation was demonstrated in the inner portion of the ovarian cortex of the rabbit ovary of two weeks of the age. So the folliculogenesis in the rabbit ovary occurred postnatally at two weeks of age.
With the advancement of the development the rabbit ovary at the 4th weeks of age was formed of an outer cortex and inner medulla. The tunica albugenia was well organized and formed of dense white fibrous connective tissue contained mainly collagenous fibers and few connective tissue cells. The ovarian medulla became more developed and formed of loose connective tissue contained blood and lymph vessels. Primordial follicles comprised the majority of the ovarian germ cells content and were mainly located in the peripheral cortical area under tunica albugenia, although primary and growing follicles could be also demonstrated in the deep cortical area near the well vascularized medulla. The limit between the outer cortex and the inner medulla of the rabbit ovary became well developed than those observed at the previous postnatal developmental stages. The cell nests were completely resolved and the oogonia and the naked acolytes could not be demonstrated at this age. Call-Exner bodies were first demonstrated in the growing follicles of the rabbit ovary at 4 weeks of age.
The small antral follicles firstly demonstrated in the rabbit ovary at the 6th weeks of age in the deep inner part of the cortex near the well vascularized medulla. Follicular atresia was prominent at this age of postnatal development.
Biometrical studies of the rabbit ovary revealed that at 8 weeks of age of postnatal development there was a pronounced change in the ovarian weight. The ovarian weight was suddenly increased to reach about 53.33 mg for the right ovary and about 36.67 mg for the left ovary. There was also a pronounced increase in the weight of the reproductive tract to reach about 3.16 gm. The ovarian cortex contained all stages of the follicular development; primordial and primary follicles at the outer peripheral part while growing, small antral and large antral follicles were located in the deep cortex near the medulla. The large antral follicles firstly demonstrated at this age of the postnatal development. So folliculogenesis in the rabbit has short duration (6 weeks). The interstitial gland of the rabbit ovary was firstly demonstrated at 8 weeks of age. These glands were well developed in the rabbit ovary and were represented by groups of a well vascularized polyhedral or rounded cells separated from each other by connective tissue contained blood vessels. Each cell contained lightly stained rounded vesicular nucleus and faintly stained vacuolated cytoplasm. Another characteristic feature of the rabbit ovary at 8 weeks of age was the appearance of the blood follicles; the blood follicle was abnormal large follicles contained blood in its follicular cavity and represented an early stage of follicular atresia.
At 16weeks of age the mean weight of the reproductive tract was greatly increased and correlated with the great increased in the mean weight of the rabbit ovary. The mean ovarian weight and dimensions was greatly increased than those recorded in the previous developmental stages. The rabbit ovary at this stage of postnatal development was elongated in shape and has vesicular appearance. The two ovary still located in the sub lumber region caudal to the kidneys nearly at the same level. The ovarian surface epithelium was well organized and formed of flattened cells which measured about 3.25 µm in height. The tunica albugenia also was well organized and was formed of dense collagenous fibers contained few connective tissue cells. A prominent and characteristic feature of the mature rabbit ovary was the presence of fully developed interstitial glands which occupied most of the ovarian cortex. These glands were well developed in the rabbit ovary and were represented by groups of a well vascularized polyhedral or rounded cells separated from each other by connective tissue contained blood vessels. Each cell contained lightly stained rounded vesicular nucleus and faintly stained vacuolated cytoplasm. The ovarian cortex at this age of postnatal development characterized by the presence of the ovarian follicles in all developmental stages.
Biometrical studies of the rabbit ovary during the postnatal development revealed that, there was a relatively slow ovarian growth from birth to 6 weeks of age. The mean ovarian weight was about 2.45 mg at birth and reached about 11.68 mg at 6 weeks of age. At 8 weeks of postnatal development there was a pronounced growth in the rabbit ovary. The ovarian weight was suddenly increased to reach about 45 mg. At 16 weeks of age the ovarian weight was increased approximately 2 folds and reached about 92.5 mg.
Quantification of the female germ cells in the rabbit ovary during the postnatal development revealed that; the germ cells population per unit area was higher at the early stages of the postnatal development and reached a maximal value (peak) at one week of age. Increase in the germ cells number at the early stages of the postnatal development reflects the increase in the germ cells mitosis. At two weeks of age the mean number of the female germ cells in the rabbit ovary was markedly decreased. This decrease of the germ cells population was also paralleled with reduced mitosis and high rate of the germ cells apoptosis. Germ cells population declined gradually with the advancement of age and reached a minimal value at 16 weeks of age. In the mature rabbit ovary about 2% of the peak number of the germ cells was conserved. So the rabbit ovary characterized by increased germ cells apoptosis.
The growth of the oocyte in the rabbit ovary during the postnatal period occurred in two distinct phases; in the first growth phase, the growth of the oocyte was parallel with the growth of the follicle. So the growth of the oocytes and follicles was correlated in appositive and liner manner. While in the second growth phase the size of the oocyte remains nearly constant despite continuous of the follicular growth.
Zona pellucida was demonstrated in the primary follicles around the oocyte larger than 30 µ in diameter. Zona pellucida was started to appear as PAS positive membrane located between the oocyte and the granulose cells of the primary follicles. It was more clear and thicker in the growing and antral follicles than in primary follicles. The mean thickness of the zona pellucida was about 1.40 µ in primary follicles, 4.57 µ in the growing follicles, 4.86 µ in the small antral follicles and 3.24 µ in the large antral follicles.
Call-Exner’s bodies were special structres among the granulosa cells of the growing and antral follicles. Call-Exner’s bodies were firstly demonstrated in the growing follicles at 4 weeks of age. Call-Exner’s bodies were more frequently demonstrated in the large antral follicles than in growing and small antral follicles. The mean number of Call-Exner’s bodies in the growing follicles was about 4.4 bodied while in the small antral follicles was about 7.7 bodied and in large antral follicles was about 8.6 bodied in the histological section. They were located in both mural and cumulus oophorus regions of the granulosa cell population. The Call-Exner bodies were also demonstrated at early stages of the follicular atresia.
Ovarian follicles can degenerate at any stage of follicular development. Atretic follicles were conspicuous in the rabbit ovary especially at 6- 8 weeks of age; the time of first appearance of the interstitial gland. It was often difficult to find healthy follicles among the numerous degenerated ones. Increase in the follicular atresia lead to an increase in the size of the interstitial gland.
Polyovular follicles (2-4 oocytes) at different stages of development and atresia were a characteristic feature of the rabbit ovary.
The two ovaries in the female rabbit were active in the folliculogenesis and ovulation at the same time. More than one follicle selected to reach the mature stage and either induced to ovulate or regressed.
It is well established that ovulation in the rabbit is non spontaneous or induced ovulation. Sexually mature female’s rabbit were mated with a buck of proven fertility. At the second week of pregnancy or mid gestation (14 days post coital) the mean ovarian weight and dimensions was greatly increased than those recorded at the non pregnant rabbit ovary. The average number of the corpora lutea per ovary was about 4 corpora lutea. The corpora lutea were well developed and occupied most of the ovarian cortex.
Each corpus luteum was surrounded by connective tissue capsule. The lutein cells varied in shape; they were large polyhedral or elongated in shape. Each lutein cell contained one or two spherical, vesicular and centrally located nuclei. The lutein cells of the rabbit corpus luteum at this stage have a large diameter reaching about 34.33 µm. The cytoplasm was slightly vacuolated (contained less lipid DROPlets). During this stage the corpora lutea reached the maximum size, the cell boundaries of the lutein cells were well clear, one or more surfaces of the lutein cells were bounded by capillaries, few or no PAS positive granules could be demonstrated in the cytoplasm of the lutein cells.
The corpus luteum of the pregnant rabbit remains morphologically active until nearly the end of the pregnancy. Pronounced sings of retrogression of the corpora lutea of the rabbit ovary were demonstrated during the postpartum period. The corpus luteum was markedly decreased in size and became less vascularized. Apoptosis of the lutein cells could be demonstrated in the regressing corpus luteum of the rabbit. The lutein cells were markedly decrease in the size, have ill clear cell boundaries and most of their nuclei were eccentric, pyknotic and deeply stained. The cytoplasm was more vacuolated. The density of the connective tissue between the lutein cells was markedly increased. The mean number of the lutein cells per unit area of the CL was greatly increased than those recorded at the middle and late stage of pregnancy.