Follicular growth and atresia. What factors are responsible for recruitment of an ovulatory follicle? - the answer is largely uncertain. There is undoubtedly a genetic predisposition to rates of ovulation - some mammals are polytocous (eg., insectivores, rodents, rabbit, opossum, pig, dog, and cat), others are monotocous (eg., primates, bats, goat, sheep, cattle, horse, kangaroo, deer, giraffe, camel, hippopotamus, elephant, and whale). Moreover, particular breeds or strains within an otherwise monotocous species can be litter-bearing (eg., Finnish Landrace, Booroola Merino, or Romanov ewes). It is unlikely, however, that privileged follicles are genetically-telecasted to ovulate at a preset time. Hormones modulate follicular development. A bit of chance is surely involved in the process of follicular selection.
Some hypophysectomy experiments indicate that preantral follicular growth occurs independent of gonadotropic influences; these results should be interpreted with caution. The pars tuberalis (a source of gonadotropins) is not always removed during hypophysectomy (ie., it is difficult to excise without rupturing the third ventricle). Indeed, numbers of follicles entering the "growing pool" are reduced after hypophysectomy.
Follicular cells divide, differentiate, and secrete fluid (antral formation) under the influence of FSH (LH also has some capacity in this regard); estradiol appears to have an intermediary part in these processes. The oocyte also contributes to folliculogenesis by secreting morphogens, such as growth differentiation factor-9.
Removal of one ovary will lead to augmented follicular growth within the remaining ovary. An increase in maturing follicles and rate of ovulation (polytocous species) is preceded by an overall decrease in ovarian output of inhibin and elevation in secretion of FSH (unilateral Sertoli cell compensatory hypertrophy also occurs in hemicastrated males). Thus, treatment of females with follicle-stimulating hormones (FSH, PMSG, or hMG) or immunization against inhibin will result in ovulation of more than the normal quota of follicles (superovulation). Furthermore, providing dietary supplementation so animals are gaining weight before breeding ("flushing") increases gonadotropin levels and ovulatory rate. It is unclear whether gonadotropins rescue follicles from atresia or stimulate the growth of healthy follicles.
Once follicular growth begins, it continues in an uninterrupted mode until either atresia or ovulation. Estimates for complete development to ovulation range from three weeks (rat) to six months (domestic animals). The sustained maturation of follicles requires growth of a thecal vascular wreath (angiogenesis). Most follicles undergo atresia at an early antral stage.
Atresia is a form of physiological cell death. Causative associations of atresia have not been delineated - both hormonal and nonhormonal theories abound. Sometime during the process of atresia affected follicles lose their capacity to synthesize estradiol - they have a high androgen (or progesterone) to estrogen ratio. An atretogen secreted by small- and medium-sized follicles, follicle regulatory protein, inhibits aromatase activity. It has been suggested that atresia is triggered by hypoxia or complement, or could involve follicular expression of a "death gene."
Ovulation. The gonadotropin surge initiates a series of events within the preovulatory follicle leading to resumption of oocyte meiosis, degradation and rupture of the follicular wall, expulsion of the maturing oocyte, and luteinization. It is widely assumed that LH is "the" ovulatory/luteinizing gonadotropin. Nevertheless, in several mammalian species highly purified preparations of FSH are capable of causing ovulation. Gonadotropin-releasing hormone agonists can induce ovulation by a direct ovarian effect in rats.
The immediate discussion will focus on the mechanics of follicular rupture and release of the ovum. Follicular mechanisms of LH-induced ovulation in spontaneous and reflex ovulators are similar.
Ovulation is extraordinary, not only because it involves a minority of the follicular cohort, but entails self-inflicted pathological dissolution of tissue. Manifestations of the ovulatory process are similar to those symptoms associated with acute inflammation. During the preovulatory period the thecal layer becomes edematous and the antrum accrues additional fluid ("preovulatory swelling") as a result of acute hyperemia (vasodilation, increased capillary/venule permeability). Granulosa cells of preovulatory follicles dissociate (Figure 4-51) and the oocyte (with adhering corona radiata) is freed from the cumulus pedicle and underlying mural granulosa. The thecal connective tissue network is degraded during the immediate preovulatory period; breakdown is associated with vascular damage, adhesion of platelets to endothelium, leukocyte chemotaxis (Figure 4-52), stasis of blood flow, and ischemia. Many women experience abdominal ovulatory pain (mittelschmerz) at midcycle.
Along the apical surface of the follicle an avascular stigma forms (macula pellucidum) - marking the time of impending rupture (Figure 4-53). Extrusion of fluid through the stigma occurs in a passive manner (Figure 4-54); there is no substantial increase in intrafollicular pressure as rupture approaches. The increase in antral volume is accommodated by an increase in distensibility of the follicular wall. A decline in the tensile strength of the follicular wall allows disruption to occur at a constant (ie., capillary) pressure. The point around the circumference of the follicle of least resistance is the apex (it is not supported by interstitium). There is some bleeding at ovulation from disrupted vessels adjacent to the stigma.
The timing of follicular rupture following the onset of the surge of LH varies between species, but is predictable within species. The preovulatory interval in animals of small body size (eg., rodents and rabbits) is about 12 hours. Larger-framed species (eg., farm animals and humans) ovulate approximately 24 to 36 hours following the preovulatory rise in LH.
Luteinizing hormone induces follicular synthesis of prostaglandins and progesterone, release of histamine from resident mast cells, and urokinase plasminogen activator secretion from the circumjacent ovarian surface epithelium. In the sheep both PGE2 and PGF2a increase initially. Prostaglandin E2 causes dispersion of granulosa cells. Progesterone, by activating PGE2-9-ketoreductase, preferentially stimulates synthesis of PGF2a - at the expense of PGE2 accretion. An unopposed role for PGF2a in the final stages of the ovulatory process is pivotal; it activates (via plasmin and collagenases) degradation of thecal collagen (the primary supportive element of the follicular wall) and causes vasoconstriction. Tumor necrosis factor, cleaved from thecal endothelium by plasmin, induces cellular death within the formative ovulatory stigma. A decrease in follicular stability and rupture is precipitated by collagenolysis, tissue ischemia, and cellular deletion. Contractions of smooth muscle of the theca externa, caused by PGF2a, aid in evacuating follicular fluid and in propelling the egg from the cavity of the ruptured follicle onto the surface of the ovary (Figure 4-55).
Ovulation is a rate-limiting event dictating fertility and efficiency of reproduction. Consequently, a sizable research effort has been mounted to understand this process - in hope of elucidating points of potential control.
Failure to ovulate can be the consequence of hypothalamic-pituitary or ovarian malfunctions. Follicular cyst formation in animals sometimes results in chronic circulatory elevations in estradiol and nymphomania. Polycystic ovarian disease in women (Stein-Leventhal syndrome) is characterized by excessive follicular androgen production, amenorrhea, obesity, and hirsutism (male-like growth of body hair).
Oogenesis. In contrast to males, most mammalian females are born with their lifelong complement of gametes (and their ovaries remain housed within the abdominal cavity). During fetal and early postnatal life immature germ cells (oogonia) undergo mitosis, enter prophase of meiosis I, and become arrested (dictyate phase). Oogonia in meiotic arrest grow in size (accumulate cytoplasm and begin to form a zona pellucida) to yield primary oocytes.
Resumption of meiosis I is initiated by the preovulatory surge of LH, beginning at puberty, and then at each ovulatory cycle. Breakdown of the germinal vesicle (nuclear membrane) and condensation of chromatin are taken as morphological signs of reentry of a primary oocyte into meiosis (Figure 4-56). Maturation of the oocyte takes place within those (preovulatory) follicles that are sensitive to LH. Biochemical signals that regulate maturation of the oocyte are communicated through very fine processes of the corona radiata that course through the zona pellucida; the elaborate network of gap junctions then interconnect corona cells with the cumulus oophorus and mural granulosa. Oocytes do not express receptors for gonadotropins.
Granulosa cells secrete a peptidergic factor, designated oocyte maturation inhibitor (OMI), that maintains the primary oocyte in an immature state within healthy follicles. Indeed, when oocytes are isolated from developing follicles and placed in culture they mature spontaneously. Other substances shown to inhibit maturation of the oocyte include hypoxanthine, adenosine, activators of protein kinase C, cGMP, and cAMP. In vivo the transient increase in cAMP and disruption of cell-to-cell channels (decrease transfer of OMI?) triggers resumption of meiosis I (prolonged exposure to cAMP impedes maturation). Prostaglandin E2, several growth factors, and GnRH stimulate oocyte maturation. Maturation-promoting factors (p34 kinase-cyclin) produced by the activated oocyte mediate resumption of meiosis I.
At the end of meiosis I (near the time of ovulation), the primary oocyte divides into a secondary oocyte and the first polar body (Figure 4-57). Meiosis is arrested again in metaphase II by an inhibitory protein called cytostatic factor; this compound is degraded by a calcium-sensitive protease (calpain) at fertilization. Parthenogenesis - nonsexual development of an ovum beyond the second stage of meiotic arrest, is rare.
Penetration of the oocyte by spermatozoa is the stimulus for completion of meiosis II in most mammals. In the dog and fox a primary oocyte is ovulated and sperm penetration occurs before meiosis I is completed. The secondary oocyte divides into the ootid (1N) and a second polar body (the first polar body sometimes divides). The majority of cytoplasm is retained by the oocyte (unequal cytokinesis) during reductional divisions - providing a source of stored food for the developing embryo.
Primordial follicles contain oogonia. Primary, secondary, and tertiary follicles contain a primary oocyte. A secondary oocyte can be found within the preovulatory follicle just before rupture or (unfertilized) within the oviduct. Therefore, the threat of an ovarian autoimmune response directed toward female meiotic cells is negligible (ie., secondary oocytes are not kept in reserve). An ootid is transformed into a zygote upon fusion of its haploid pronucleus with that of a spermatozoon (Figure 4-58).
Corpus luteum. Morphological and functional correlates of luteinization are exhibited before ovulation. Preovulatory follicles produce a yet to be characterized luteinization stimulator (small follicles presumably produce an inhibitor). Hyperplasia (mitosis) and hypertrophy of granulosa ("large" luteal) cells continues during the immediate postovulatory period. Steroidogenic cells of the theca interna differentiate into "small" luteal cells (Figure 4-59). The basement membrane surrounding the granulosa layer of the periovulatory follicle dissolves and the connective tissue matrix of the developing CL is remodeled. Luteal tissue becomes highly vascularized (Figure 4-60). Changes in follicular tissues following ovulation are similar of those alterations in tissue associated with wound healing and tumorigenesis.
Ovulation and luteinization can be mutually exclusive events (Table 4-8). For example, drugs that suppress activity of cyclooxygenase prevent ovulation without affecting luteinization of the follicular wall (Figure 4-61) or circulatory profiles of progesterone characteristic of an otherwise normal luteal phase; these structures accrue abnormal amounts of fluid. The "luteinized unruptured follicle syndrome" is due to a chronic follicular inflammatory response (ie., failure of PGF2a to terminate the preovulatory hyperemic reaction induced by proinflammatory agents, such as histamine). Luteinized ovarian cysts are not uncommon among high-producing dairy cows, swine (Figure 4-62), and women.
Not all species (eg., pigs and marsupials) require pituitary luteotropic support after ovulation - production of progesterone by the CL is autonomous during the cycle and not influenced by hypophysectomy. Most mammals require an intact pituitary gland to assure proper development and function of the CL. The primary luteotropin is LH. There continues to be uncertainty concerning the luteotropic role of prolactin in mammals other than rodents or the dog.
Large luteal cells produce progesterone independent of LH. Small luteal cells produce very little progesterone without LH. Receptors for prostaglandins are localized on large luteal cells. Sensitivity of the CL to PGF2a (ie., transformation of small cells into large cells) increases as the luteal phase progresses.
In contrast to solid tumors, CL predictably regress at the end of a cycle or pregnancy; this process involves PGF2a (the cat is an apparent exception). Thus, a common theme underlying the mechanics of ovulation and luteolysis is the central role of PGF2a. Inasmuch as the follicle survives the damaging actions of PGF2a, in its transformed state, it will be destroyed. Several theories for the mechanism of luteolysis have been postulated: vasoconstriction (ischemia); inhibition of luteotropic hormonal action; lipid peroxidation and decline in membrane fluidity (disrupting enzymatic functions, ion channels, or LDL uptake); stress-response; down-regulation of StAR; apoptosis (internucleosomal DNA cleavage); and autoimmune/inflammatory attack. Eosinophils and mononuclear cells have been found within regressive CL of some species. Eosinophils are typically attracted into sites of antigen-antibody reactions. Macrophages are involved in tissue resorption (structural regression). The preovulatory prolactin surge plays an undefined role in morphological regression of CL in rodents.
Source of luteolytic agent differs among mammals. In farm animals and rodents the uterus is obligatory for luteolysis; thus, the lifespan of the CL is extended by hysterectomy. Prostaglandin F2a reaches the CL either by a specialized local utero-ovarian vascular system (eg., ruminants and rodents) or the systemic circulation (eg., horse, rabbit) (Figures 4-63, 4-64, and 4-65); it is less efficiently metabolized within the periphery of the latter species. Pigs utilize a combined vascular pathway. The ovary is the site of production and luteolytic action of PGF2a in the opossum, dog, ferret, and primate. Curiously, there is anatomical evidence of a local utero-ovarian system in primates (the functional significance of this relationship is unknown and might simply represent an evolutionary vestige).
The exact nature of the signalling processes responsible for endometrial or intraovarian production of PGF2a remain to be clarified; ovarian steroid hormones and oxytocin are evidently involved. Administration of pharmacological amounts of progesterone, estradiol, or oxytocin causes premature luteolysis in several mammals (Figure 4-66). In contrast, estradiol is luteotropic in pigs and rabbits.