Oocytes, primary

Fig. 2. Part of an oocyte (primary follicle) of a dog. Numerous mitochondria in close contact with the endoplasmic reticulum (i). Follicle epithelium (F), basal membrane (B), collagen fibrils of the connective tissue (X). 1 8,500.

Xenobiotic induced disruption of female fertility follows essentially the same pattern as that of the male and can be caused by changes in pituitary-hypothalamic function, primary disruption of ovarian structure or hormone secretion, or changes in the rate of hormone deactivation. In addition, there may be changes in the synthesis of estrogen induced production of the yolk protein by the liver (vitellogenesis), which in turn can lead to failure to lay down sufficient yolk in the developing oocytes. Vitellogenesis provides a valuable biomarker for endocrine dysfunction in both sexes,but is more properly considered as part of the liver function. 

Pivotal pre-approval safety and efficacy was assessed in a randomized, open label, multicentre study of infertile females undergoing in vitro fertilization and embryo transfer. The primary efficacy parameter was the mean number of oocytes retrieved, which (at 13.6) was similar to the number retrieved when urinary-derived hCG was used. Serious potential side effects can include ovarian over/hyper-stimulation, sometimes with pulmonary or vascular complications. The product is marketed by Serono Inc. 

The first cell division of meiosis occurs in the primary oocyte but the process is arrested during prophase and remains so until puberty. Just before ovulation, meiosis, which has been arrested since before birth, resumes. The first division halves the number of chromosomes to produce the haploid secondary oocyte. The process is the same as that in spermatozoa (Chapter 20 see Figure 20.29 ) except that the two resulting haploid daughter cells are unequal in size. One is the large functional secondary oocyte whereas the other is much smaller and is known as the first polar body. The second meiotic division is arrested at metaphase. It is completed only at fertilisation. Once again, the division is unequal. One cell is large, the secondary oocyte. The other is small, a second polar body, which is discarded. 

A follicle is a sac that contains extracellular fluid, cells and the ovum. The name comes from the Latin for a little bag, folliculus. At birth an ovary contains several million follicles, each of which contains an immature ovum (the primary oocytes). These are known as... 

VCD is ovotoxic, causing follicle destruction in both rats and mice. After 30 days of intraperitoneal dosing with 80mg/kg the number of oocyte-containing primordial and primary follicles was reduced 80% in rats and 92% in mice. 

A loss of primary oocytes will irreversibly affect a woman s fecundity, but this is difficult to measure directly. Reproductive dysfunction can be studied by the evaluation of irregularities of menstrual cycles and onset of menarche and menopause. However, menstrual cyclicity and onset of menopause and menarche are affected by many parameters, such as age, genetics, nutritional status, stress, exercise, certain drugs and the use of contraceptives that alter endocrine feedback. The length of the menstrual cycle, particularly the follicular phase (before ovulation), can vary between individuals and may make it difficult to measure significant effects in groups of women (Burch et al., 1967 Treloar et al., 1967). 

In the case of oogenesis, the primary oocyte arrests after the first meiosis, before birth, and the next S phase is not until after fertilization. Therefore, the oocyte is more resistant to chemical mutagens. 

In the humans, the primordial germ cells are differentiated from about the sixth week of gestation and are consequently susceptible from then onward. In the female, production of the primary oocytes, which involves the first meiotic division, occurs in fetal life. These primary oocytes do not mature into ova until puberty, with the second meiotic division yielding one ovum from each primary oocyte (Fig. 6.48). 

All primary oocytes are already present about a week before birth. Many of these remain in an arrested diplotene stage of meiosis for many months before ovulation or loss by atresia. Stages of primary oocyte development are defined by the cytologic appearance of the oocyte and its follicle. 

Some or all events leading to sex-chromosome loss or nondisjunction are either not inducible in or not recoverable from some of the germ cell stage.378 However, very few chemicals have been tested, and none in all germ cell stages. Sex-chromosome loss has been shown to be induced by triethylenemelamine in spermatids and spermatozoa, 55 by isopropyl methanesulfonate in primary oocytes within 6 wk of being ovulated,138 and by hycanthone methanesulfonate in primary oocytes within 1 wk of being ovulated.38 ... 

The ovaries perform two major functions (i) the storage, maturation, and expulsion of healthy haploid germ cells (i.e., oocytes) for fertilization, and (ii) the synthesis and secretion of hormones to prepare the reproductive tissues for the establishment and maintenance of pregnancy, to properly regulate gonadotropin secretion from the hypothalamic-pituitary axis, to induce appropriate sexual behaviors, and to provide lactation. The two primary functional units in the ovaries are the maturing follicles and the corpora lutea. 

The frequency of the menstrual/estrous cycle varies among species. For example, monoestrous species (e.g., dogs, cats) exhibit 1-2 cycles per year, whereas polyes-trous species (e.g., rodents, primates) exhibit more frequent cycles of shorter duration. Primordial follicles, each consisting of an oocyte surrounded by a single layer of flattened somatic cells known as granulosa cells, are recruited to become primary follicles, a transition marked by increased oocyte size, formation of a glycoprotein matrix (zona pellucida) around the oocyte, and transformation to cuboidal-shaped... 

Q1 The primordial follicles in which ova develop, form in the embryo and the maximal number 1-2 million, is present at birth. Follicles are located in the cortex of the ovary and each contains a primary oocyte. Following sexual maturation at puberty about 200 000 follicles remain, some of these will develop into primary follicles. During reproductive life a number of the primary follicles develop further, leading to ovulation, the remainder decline (a process known as atresia). 

Cyclin B/CDKl is the primary regulator of the G2/M transition, and its activity is required for entry into mitosis. It was termed the maturation-promoting factor (MPF) because it was originally shown to be essential for Xenopus oocytes maturation... 

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