Oocytes in Prophase of Meiosis from Squash Preparations of Human Fetal Ovaries

From primordial germ cell to primordial follicle: mammalian female germ cell development

In mammals, the final number of oocytes available for reproduction of the next generation is defined at birth. Establishment of this oocyte pool is essential for fertility. Mammalian primordial germ cells form and migrate to the gonad during embryonic development. After arriving at the gonad, the germ cells are called oogonia and develop in clusters of cells called germ line cysts or oocyte nests. Subsequently, the oogonia enter meiosis and become oocytes. The oocyte nests break apart into individual cells and become packaged into primordial follicles. During this time, only a subset of oocytes ultimately survive and the remaining immature eggs die by programmed cell death. This phase of oocyte differentiation is poorly understood but molecules and mechanisms that regulate oocyte development are beginning to be identified. This review focuses on these early stages of female germ cell development.

FISHing for acrocentric associations between chromosomes 14 and 21 in human oogenesis

OBJECTIVE

The purpose of this study was to search for cytologic evidence of robertsonian translocation formation that involves chromosomes 14q and 21q in human oogenesis with the use of dual color fluorescent in situ hybridization with whole chromosome paints.

STUDY DESIGN

The oocytes from a chromosomally normal fetus at 23.5 weeks of gestation underwent cohybridization with chromosome specific DNA libraries from chromosomes 14 and 21. The nuclei were scored for the proportion of meiosis I prophase substages and for hybridization efficiency and were evaluated for the presence of hybridization signals that were suggestive of heterologous associations between chromosomes 14q and 21q in zygotene, pachytene, and diplotene.

RESULTS

A total of 1769 meiotic nuclei were analyzed. Of 272 informative nuclei at zygotene, pachytene, and diplotene, 1 nucleus at pachytene demonstrated hybridization signals for chromosomes 14 and 21 that could be consistent with a robertsonian translocation.

CONCLUSION

A heterologous association between chromosomes 14q and 21q that possibly represent robertsonian translocation formation was observed cytologically with the use of fluorescent in situ hybridization.

Ontogeny: the evolution of an oocyte

The physiology of oocyte development, maturation, and selection forms the basis for understanding ovarian follicle dynamics, reproductive function and ovulation induction. The number of ovarian follicles with which women are invested from birth and the rate of decline determine their reproductive lifespan. The processes of atresia and apoptosis constantly deplete this finite store. The initiation of follicle growth involves complex chemical signaling and ensures that a cohort of follicles is prepared for recruitment monthly. The effects of gonadotrophins, autocrine signals, and the degree of vascularization further determine dominance and the selection of an oocyte for ovulation.

Development of the first meiotic prophase stages in human fetal oocytes observed by light and electron microscopy

The development of the first meiotic prophase stages was studied in two series of human female embryos and fetuses aborted for social reasons. The first series (64 embryos or fetuses aborted at 6-24 weeks of gestation) was used mainly to perfect the methods applied to obtain chromosome preparations and synaptonemal complex spreads. The second series (37 embryos or fetuses aborted at 9-24 weeks of gestation) was used to establish the timing and to characterize the different stages of prophase I. Leptotene-zygotene figures were observed in some embryos at 10 weeks of gestation. Typical zygotene figures were seen at 11-22 weeks. Pachytenes were first observed at 12-13 weeks, and the proportion of these figures was usually lower than 40%. Diplotenes were seen in fetuses with a gestational age of 14 weeks or more. The duration of the process in the human female is thus about 3-4 weeks, a similar period to that described for the male.

Initiation of oogenesis in the human fetal ovary: ultrastructural and squash preparation study

Ovaries from 27 human fetuses at less than or equal to 12 weeks' gestational age were examined with electron microscopy and squash preparation studies. The findings demonstrated that meiosis begins in the fetal ovary between 11 and 12 weeks of age. This is several weeks after gonadal sex differentiation, which can be recognized in the testis at 6 weeks. Ovaries at earlier ages included primitive germ cells and oogonia. With advancing fetal age, mitotic activity and germ cell degeneration became increasingly evident. Preleptotene cells were found as early as 9 to 10 weeks, suggesting that meiotic capacity is present soon after gonadal sex differentiation but further progression into meiosis is delayed until some specific inductive influence is exerted.

The prophase stages in human foetal oocytes studied by light and electron microscopy

The progression of the prophase stages of meiosis in human foetal ovaries is reported from a series of aborted foetuses spanning the first two trimesters of gestation. A surface-spreading technique allowed cells to be examined at both light and electron microscope levels. The pachytene stage was specifically examined for evidence of synaptic or other abnormalities. Two observations of interest are the relatively high incidence with which errors of pairing occur and second the state of thickening observed when such unpaired axes are stained with silver.

Meiotic configurations in female trisomy 21 foetuses

Analysis of the meiotic configurations formed by the three No 21 chromosomes in oocytes from two trisomy 21 foetuses was undertaken using a spreading technique. Light microscope analysis of the first gave limited resolving power, such that over half the oocytes could not be classified as to presence or absence of trivalent or bivalent plus univalent. In the second, investigated at the electron microscope level, all 65 cells analysed were informative and precise detail of meiotic pairing in trivalents could be obtained. Two principal forms of trivalent occurred, one in which pairing was initiated at opposite ends of the three No 21's, each initiation point involving only two of the three homologous lateral elements; the other in which pairing was initiated by all three elements at the same end, a triple synaptonemal complex being formed. Only in one oocyte out of the 65 analysed at EM level, however, did triple pairing occur along the entire length of the No 21 trivalent. All others showed splitting into bivalent and univalent at some point along the structure. Unpaired regions within trivalents and all univalents were consistently seen to be thickened and dark staining with silver over the whole period from pachytene to diplotene. This contrasted with the desynapsing lateral elements of previously paired synaptonemal complexes which appeared thin by comparison at diplotene. The significance of the thickening remains, as yet, obscure.

The pathological assessment of ovarian neoplasms V: The germ cell tumours

All neoplasms of the ovary encountered in a 25-yr study period of King George V Memorial Hospital were classified according to the World Health Organisation (WHO) Histological Classification of Ovarian Tumours. Of just fewer than 1700 tumours, 467 fell into the category designated as germ cell tumours: 4 dysgerminomas, one endodermal sinus tumour, one primary choriocarcinoma, 6 immature and 454 mature teratomas and one mixed malignant germ cell tumour. Two additional cases of gonadoblastoma with associated dysgerminoma were encountered and included for discussion. A detailed histological assessment is presented and the major clinical correlates are documented.

Why are ovarian teratomas benign whilst teratomas of the testis are malignant?

A mechanism is suggested to account for the fact that most ovarian teratomas are benign (dermoid cysts) whereas testicular teratomas are almost always malignant. The mechanism advanced assumes that malignancy is cause by a recessive mutation, and then shows that the known facts of germ-cell development will produce the homozygous (malignant) state in the male but the heterozygous counterpart in the female. It is proposed that the benign nature of ovarian teratomas is a reflection of this heterozygous genotype.

Parthenogenic origin of benign ovarian teratomas

To determine the origin of benign cystic teratomas of the ovary, chromosome-banding studies were done on normal tissues and teratomas from five patients. The normal tissues were heterozygous (+/-) for 17 chromosome polymorphisms at or near the centromere, whereas the teratomas were uniformly homozygous (+/+ or -/-). These findings and those employing electrophoretic variants indicate that ovarian teratomas are parthenogenic tumors that arise from a single germ cell after the first meiotic division.