Conception and implantation

Oogenesis

Primordial germ cells originally appear in the yolk sac and can be identified by the fourth week of fetal development ( Fig. 2.1 ). These cells migrate through the dorsal mesentery of the developing gut and finally reach the genital ridge between 44 and 48 days post-conception. Migration occurs into a genital tubercle consisting of mesenchymal cells that appear over the ventral part of the mesonephros. The germ cells form sex cords and become the cortex of the ovary.

The sex cords subsequently break up into separate clumps of cells and, by 16 weeks, these clumped cells become primary follicles, which incorporate central germ cells.

These cells undergo rapid mitotic activity, and by 20 weeks of intrauterine life, there are about 7 million cells, known as oogonia . After this time, no further cell division occurs and no further ova are produced. By birth, the oogonia have already begun the first meiotic division and have become primary oocytes. The number of primary oocytes falls progressively and by birth is down to about 1 million and to about 0.4 million by puberty.

Meiosis

The process of meiosis results in 23 chromosomes being found in each of the gametes, half the number of chromosomes found in normal cells. With the fertilization of the egg by a sperm, the chromosome count is returned to the normal count of 46 chromosomes. Fusion of the sperm and the egg occurs when the first of two meiotic divisions of the oocyte has already been completed; with the second meiotic division occurring subsequently and being completed prior to the 23 chromosomes of the male gamete joining those of the female gamete within the nucleus of the cell, the zygote is formed, which will become the embryo.

In meiosis, two cell divisions occur in succession, each of which consists of prophase, metaphase, anaphase and telophase. The first of the two cell divisions is a reduction division, and the second is a modified mitosis in which the prophase is usually lacking ( Fig. 2.2 ). At the end of the first meiotic prophase, the double chromosomes undergo synapsis, producing a group of four homologous chromatids called a tetrad . The two centrioles move to opposite poles. A spindle forms in the middle, and the membrane of the nucleus disappears. During this prophase period of meiosis I, the double chromosomes, which are closely associated in pairs along their entire length, undergo synapsis, crossing over and undergoing chromatid exchange, with these processes accounting for the differences seen between two same-sex siblings despite the fact that the female gametes came from the same mother.

The primary oocytes remain in suspended prophase until sexual maturity is reached, or even much later, with meiosis I not recommencing until the dominant follicle is triggered by luteinizing hormone (LH) to commence ovulation. In anaphase, the daughter chromatids separate and move towards opposite poles. Meiosis II commences around the time the sperm are attached to the surface of the oocyte and is completed prior to the final phase of fertilization.

Thus, the nuclear events in oogenesis are virtually the same as in spermatogenesis, but the cytoplasmic division in oogenesis is unequal, resulting in only one secondary oocyte. This small cell consists almost entirely of a nucleus and is known as the first polar body. As the ovum enters the Fallopian tube, the second meiotic division occurs and a secondary oocyte forms, with the development of a small second polar body. In the male the original cell containing 46 chromosomes ultimately results in four separate spermatozoa, each being of the same size but containing only 23 chromosomes (see Spermatogenesis, later).

Follicular development in the ovary

The gross structure and the blood supply and nerve supply of the ovary have been described in Chapter 1 . However, the microscopic anatomy of the ovary is important in understanding the mechanism of follicular development and ovulation.

The surface of the ovary is covered by a single layer of cuboidal epithelium. The cortex of the ovary contains a large number of oogonia surrounded by follicular cells that become granulosa cells. The remainder of the ovary consists of a mesenchymal core. Most of the ova in the cortex never reach an advanced stage of maturation and become atretic early in follicular development. At any given time, follicles can be seen in various stages of maturation and degeneration ( Fig. 2.3 ). About 800 primary follicles are ‘lost’ during each month of life from soon after puberty until menopause, with only one or two of these follicles resulting in release of a mature ovum each menstrual cycle in the absence of ovarian hyperstimulation therapy. This progressive loss occurs irrespective of whether the patient is pregnant, on the oral contraceptive pill, having regular cycles or amenorrhoeic, with menopause occurring at the same time irrespective of the number of pregnancies or cycle characteristics. The vast majority of the follicles lost have undergone minimal or no actual maturation.

The first stage of follicular development is characterized by enlargement of the ovum with the aggregation of stromal cells to form the thecal cells. When a dominant follicle is selected at about day 6 of the cycle, the innermost layers of granulosa cells adhere to the ovum and form the corona radiata . A fluid-filled space develops in the granulosa cells, and a clear layer of gelatinous material collects around the ovum, forming the zona pellucida . The ovum becomes eccentrically placed, and the Graafian follicle assumes its classic mature form. The mesenchymal cells around the follicle become differentiated into two layers, forming the theca interna and the theca externa .

As the follicle enlarges, it bulges towards the surface of the ovary and the area under the germinal epithelium thins out. Finally, the ovum, with its surrounding investment of granulosa cells, escapes through this area at the time of ovulation.

The cavity of the follicle often fills with blood but, at the same time, the granulosa cells and the theca interna cells undergo the changes of luteinization to become filled with yellow carotenoid material. The corpus luteum in its mature form shows intense vascularization and pronounced vacuolization of the theca and granulosa cells with evidence of hormonal activity. This development reaches its peak approximately 7 days after ovulation, and thereafter the corpus luteum regresses unless implantation occurs, when human chorionic gonadotropin (hCG) production by the implanting embryo prolongs corpus luteum function until the placenta takes over this role at about 10 weeks of gestation. The corpus luteum degeneration is characterized by increasing vacuolization of the granulosa cells and the appearance of increased quantities of fibrous tissue in the centre of the corpus luteum. This finally develops into a white scar known as the corpus albicans ( Fig. 2.4 ).

Hormonal events associated with ovulation

The maturation of oocytes, ovulation and the endometrial and tubal changes of the menstrual cycle are all regulated by a series of interactive hormonal changes ( Fig. 2.5 ).

The process is initiated by the release of the gonadotrophin-releasing hormone (GnRH), a major neurosecretion produced in the median eminence of the hypothalamus. This hormone is a decapeptide and is released from axon terminals into the pituitary portal capillaries. It results in the release of both follicle-stimulating hormone (FSH) and LH from the pituitary.

GnRH is released in episodic fluctuations, with an increase in the number of surges being associated with the higher levels of plasma LH commencing just before mid-cycle and continued ongoing GnRH action required to initiate the huge oestrogen-induced LH surge. Kisspeptin levels in the serum and urine have been studied during the menstrual cycle and found to be useful in the prediction of ovulation. During the first 5 days of the cycle, it is low. There is a surge around the eleventh day when the dominant follicle is about 1.2cm. The second surge, which is smaller, is around the fourteenth day. Serum kisspeptin levels correlate well with 17-β oestradiol (E ₂ ) levels. It appears that kisspeptin surge may be a good marker of the dominant follicle development prior to ovulation.

The three major hormones involved in reproduction are produced by the anterior lobe of the pituitary gland or adenohypophysis and include FSH, LH and prolactin. Blood levels of FSH are slightly higher during menses and subsequently decline due to the negative feedback effect of the oestrogen production by the dominant follicle. LH levels appear to remain at a relatively constant level in the first half of the cycle; however, there is a marked surge of LH 35–42 hours before ovulation and a smaller coincidental FSH peak (see Fig. 2.5 ). The LH surge is, in fact, made up of two proximate surges, and a peak in plasma oestradiol precedes the LH surge. Plasma LH and FSH levels are slightly lower in the second half of the cycle than in the pre-ovulatory phase, but continued LH release by the pituitary is necessary for normal corpus luteum function. Pituitary gonadotrophins influence the activity of the hypothalamus by a short-loop feedback system between the gonadotrophins themselves and the effect of the ovarian hormones produced due to FSH and LH action on the ovaries.

Oestrogen production increases in the first half of the cycle, then falls to about 60% of its follicular phase peak following ovulation, and a second peak occurs in the luteal phase. Progesterone levels are low prior to ovulation but then become elevated throughout most of the luteal phase. These features are shown in Fig. 2.5 .

Prolactin is secreted by lactotrophs in the anterior lobe of the pituitary gland. Prolactin levels rise slightly at mid-cycle but are still within the normal range and remain at similar levels during the luteal phase and tend to follow the changes in plasma oestradiol-17 β levels. Prolactin tends to control its own secretion predominantly through a short-loop feedback system on the hypothalamus, which produces the prolactin-inhibiting factor, dopamine. Oestrogen appears to stimulate prolactin release in addition to the release of various neurotransmitters, such as serotonin, noradrenaline (norepinephrine), morphine and enkephalins, by a central action on the brain. Antagonists to dopamine such as phenothiazine, reserpine and methyltyrosine also stimulate the release of prolactin, whereas dopamine agonists such as bromocriptine and cabergoline have the opposite effect.

The endometrial cycle

The normal endometrium responds in a cyclical manner to the fluctuations in ovarian steroids. The endometrium consists of three zones, and it is the two outer zones that are shed during menstruation ( Fig. 2.6 ).

The basal zone ( zona basalis ) is the thin layer of the compact stroma that interdigitates with the myometrium and shows little response to hormonal change. It is not shed at the time of menstruation. The next adjacent zone ( zona spongiosa ) contains the endometrial glands, which are lined by columnar epithelial cells surrounded by loose stroma. The surface of the endometrium is covered by a compact layer of epithelial cells ( zona compacta ) that surrounds the ostia of the endometrial glands. The endometrial cycle is divided into four phases:

• 1.

  Menstrual phase. This occupies the first 4 days of the cycle and results in shedding of the outer two layers of the endometrium. The onset of menstruation is preceded by segmental vasoconstriction of the spiral arterioles. This leads to necrosis and shedding of the functional layers of the endometrium. The vascular changes are associated with a fall in both oestrogen and progesterone levels, but the mechanism by which these vascular changes are mediated is still not understood. What is clear clinically is that the menstruation due to the shedding of the outer layers of the endometrium occurs whether oestrogen or progesterone, or both, fall, with the loss generally being less if both the oestrogen and progesterone levels fall (as at the end of an ovulatory cycle), and heavier when only the oestrogen level falls (as in an anovulatory cycle).

• 2.

  Phase of repair. This phase extends from day 4 to day 7 and is associated with the formation of a new capillary bed arising from the arterial coils and with the regeneration of the epithelial surface.

• 3.

  Follicular or proliferative phase. This is the period of maximal growth of the endometrium and is associated with elongation and expansion of the glands and with stromal development. This phase extends from day 7 until the day of ovulation (generally day 14 of the cycle).

• 4.

  Luteal or secretory phase. This follows ovulation and continues until 14 days later when menstruation starts again. During this phase, the endometrial glands become convoluted and ‘saw-toothed’ in appearance. The epithelial cells exhibit basal vacuolation, and by the mid-luteal phase (about day 20 of a 28-day cycle), there is visible secretion in these cells. The secretion subsequently becomes inspissated and, as menstruation approaches, there is oedema of the stroma and a pseudodecidual reaction. Within 2 days of menstruation, there is infiltration of the stroma by leukocytes.

It is now clear that luteinization of the follicle can occur in the absence of the release of the oocyte, which may remain entrapped in the follicle. This condition is described as entrapped ovulation or luteinized unruptured follicle (LUF) syndrome and is associated with normal progesterone production and an apparently normal ovulatory cycle. Histological examination of the endometrium generally enables precise dating of the menstrual cycle and is particularly important in providing presumptive evidence of ovulation.

Production of sperm