The term ovulation refers to the release of viable oocytes from the ovary. This process needs to be distinguished from the aspiration process used in assisted reproductive technologies for oocyte retrieval. Ovulation is a focal point in the reproductive process of all mammalian species. The temporal pattern of ovulation or its periodicity, however, varies among different mammals. In some species, ovulation is confined to intervals of increased sexual receptivity (estrus) during which copulation takes place. The dog, for example, ovulates only approximately twice each year during periods of “heat.” In the rabbit, however, ovulation occurs in response to coitus. Cyclic ovulation is characteristic of other species. In humans, as well as other primates, ovulation occurs neither during a period of heightened sexual activity as described above, nor as a consequence of each act of coitus. Instead, ovulation normally occurs once per month. Oocyte release is preceded by estrogen-induced distinctive changes that prepare the reproductive tract for possible conception. After ovulation, the ovarian follicle collapses rapidly; ingrowth of capillaries and fibroblasts from the theca cell layer and luteinization of the granulosa cells lead to formation of a corpus luteum. This gland is the most significant source of gonadal steroids during the postovulatory phase of the cycle; the predominant secretion of the corpus luteum is progesterone. In the absence of pregnancy, ovulation defines the time of succeeding menstruation, inasmuch as the corpus luteum has a proscribed finite life span of 14 ± 2 days unless “rescued” by human chorionic gonadotropin (hCG) elaborated by trophoblastic tissue of a conceptus. The secretions of the corpus luteum during the postovulatory phase essentially prepare the uterus for possible nidation of a fertilized egg.

The release of oocytes capable of fertilization is preceded by three significant steps in the female human: (1) oogenesis, the establishment of a line of female germ cells and their migration during embryonic life to the genital ridge; (2) folliculogenesis, the development of follicular structures that surround the oocytes; and (3) oocyte maturation, the resumption of the meiotic process in the oocyte just before ovulation.

Oogenesis

In embryonic life, germ cells arise extragonadally in the yolk sac entoderm and are first recognizable at approximately 4 weeks' gestational age. The primordial germ cells migrate by ameboid-like movement up the stalk of the yolk sac and along the wall of the hindgut, ultimately reaching the paired genital ridges. Mitotic activity, during migration and after arrival at the genital ridges, leads to a rapid increase in the number of germ cells. The stromal cells that surround the oocytes provide the follicular investments. In the female human embryo, the primordial germ cells are transformed into oogonia from the end of the second month to the seventh month of gestation. The final mitotic division of the oogonium results in formation of an oocyte, which then begins the process of meiosis (see below), in which the chromosomal complement is halved. At about the time of birth, all oocytes will have undergone meiosis to the diplotene stage. The oocytes then enter a quiescent phase, in which meiosis is suspended. Oocytes at this stage characteristically contain a nucleus, termed the germinal vesicle. The oocytes remain at this stage of development for varying periods of time.

Folliculogenesis

Folliculogenesis occurs before puberty. The oocyte is initially surrounded by a single layer of stroma-derived flattened epithelial cells, then by cuboidal cells, which proliferate and form multiple layers. The encompassing cells eventually differentiate into an outer theca and an inner granulosa cell layer. As the follicle continues to grow, the theca layer further differentiates into a theca externa and theca interna. The theca and granulosa cells serve as a major source of steroid hormones. Only after puberty has begun do pituitary gonadotropins stimulate the follicle, ultimately causing an accumulation of fluid within the follicular cavity, or antrum.

Ovulation occurs cyclically, beginning at the time of puberty and extending to the time of oocyte depletion. It is interrupted normally only by pregnancy and subsequent lactation. Cyclicity (discussed below) is dependent on sequential stimulation of follicles by pituitary gonadotropins. Release of the oocyte is contingent on maturation of the follicular structure in which it is enclosed. Gonadotropins are not essential for the initial steps of follicular development, but they are required for formation of the antrum. As the follicle continues to expand, the antrum enlarges and the follicle protrudes from the ovarian surface. At this stage, the follicle is termed a graafian follicle. Attainment of this stage of folliculogenesis is a prerequisite for ovulation.

Oocyte Maturation

The oocyte itself grows during the preovulatory phase of follicular development. Normally, a close relationship exists between follicle development and oocyte maturation. Oocyte development, however, is suspended in the dictyate stage of the first meiotic division from embryonic life until just to ovulation. An oocyte remains in the germinal vesicle stage until after it has been stimulated by a preovulatory surge of luteinizing hormone (LH). Only then does meiosis resume.

Meiosis involves two divisions. In the first meiotic division, the homologous chromosomes are separate entities; homologous pairs separate from each other. In the second division, each chromatid pair is joined by a centromere, and each individual chromatid from the pair splits from its counterpart. The oocyte ovulates during metaphase II; reduction division is completed after the egg is penetrated by a spermatozoon.

Thus, for ovulation to occur, oocytes must be present in the ovary, the follicle must develop to the graafian stage, and the preovulatory oocyte must resume maturation. The follicle that proceeds to rupture in any given cycle is selected from a cohort of developing follicles and has been recruited early in that cycle or in the previous cycle. By the fifth day, the follicle designated to ovulate assumes a state of dominance, such that if it were to be destroyed or removed, none of the other members of the cohort would ovulate.

The majority of follicles do not achieve ovulation, but instead participate in a regressive phenomenon termed follicular atresia. It is characterized histologically by dissolution of the granulosa cell layer, fatty degeneration of cells, presence of debris in the antrum, collapse of the follicle, shrinkage of the oocyte, hyalinization, invasion of the follicle by theca, and fibrosis of the entire structure. Follicular atresia is evident throughout the ovulatory cycle. It also occurs in the neonatal ovary, through the prepubertal years, during gestation and lactation, and while ovulatory suppressants (oral contraceptives) are being used. The process of follicular atresia is similar between most laboratory animals and primates. The main difference is that multiple ovulations routinely occur in most subprimate species.

During each human ovulatory cycle, many ovarian follicles approach maturity, but as a rule, only one matures completely. The other developing follicles undergo the process of atresia. The accessory follicles that develop along with the dominant follicle in each cycle may possibly provide some hormonal support for the reproductive tract during the critical time interval between fertilization and implantation. Many smaller primordial follicles become atretic before they even reach maturity. The newborn human ovary contains approximately 2 million oocytes, but by the age of puberty, the follicle count drops to approximately 400,000.¹ The precise mechanisms responsible for follicular development and atresia are as yet unclear, but two distinct atretic processes most likely occur. One, affecting the primordial follicle, may be independent of hormonal influences; the other, affecting the mature follicle, may depend on endocrine factors. Studies by Pincus and Enzmann² in the rabbit ovary demonstrated that 10% of the less mature oocytes and 60% of the larger follicles are atretic.

Follicular atresia cannot be explained adequately without an understanding of the hormonal influences that affect the follicles. For example, estrogen, which enhances follicular maturation, may also encourage the premature occurrence of atresia. If an ovarian hormone is responsible locally for follicular atresia, it is not clear how the monthly occurrence of follicular atresia in both ovaries can be reconciled with unilateral ovulation. Many questions are unanswered as to (1) the process whereby a cohort of follicles is selected; and (2) the selection process that culminates with a single dominant follicle. For example, do mechanical factors affect circulation, oxygenation, and metabolism in follicular cells? What enzymes cooperate in the degenerative process? What is the nature of the selective mechanism that allows one or more oocytes to undergo ovulation while the majority regress?

Follicular atresia is a natural phenomenon that provides a built-in limit on the potential number of offspring of any species. Atresia not only regulates the fate of the follicles in each cycle, but also governs female reproductive life span by influencing the time of human menopause. A firmer grasp of the physiologic intricacies of follicular atresia may help explain the causes of multiple pregnancy and permit reduction of the multiple ovulations frequently associated with exogenous gonadotropin stimulation. Understanding the atretic process may also clarify the mechanism of action of certain presently used hormonal contraceptives, while suggesting alternate and newer approaches to limitation of fertility.

Although the process of ovulation is associated with an abrupt increase in plasma levels of LH, those local events within the ovary itself that lead to follicular rupture are not clearly understood. The precise mechanism (or mechanisms) of ovulation has been related to a number of factors:

1. Increase in intrafollicular pressure
   
2. Proteolytic enzyme activity on the follicular wall
   
3. Morphologic changes in the stigma that favor follicular rupture
   
4. Perifollicular ovarian smooth muscle contractions
   
5. Changes in the ovarian intercellular collagen bundles, such as increased distensibility and plasticity
   
6. Vascular alterations in the perifollicular vessels.
   

At present, no single hypothesis satisfactorily explains the entire mechanism of graafian follicle rupture. It is likely that many factors complement one another in the mechanics necessary to achieve ovulation. The recognition of smooth muscle and autonomic nerves within the ovarian stroma and the demonstration of contractions in the ovaries of several species (e.g., cat, rat, guinea pig, monkey, human)³ suggest the strong possibility that these elements are involved, directly or indirectly, in the ovulatory process. α-Adrenergic agents, such as norepinephrine, exert a stimulatory effect on ovarian contractions, whereas β-adrenergic agents are inhibitory. A heightened sensitivity to β-adrenergic agents has been observed after ovulation in the rabbit. The demonstration of an inhibitory effect of phenoxybenzamine⁴ on follicular rupture and the enhancement of ovarian contractility seen at about the time of ovulation in the rabbit⁵ may lend further support to the concept that ovarian smooth muscle is influential in bringing about ovum expulsion.

The precise nature of the role of ovarian nerves and neurohumoral agents in the ovulatory process is still unclear as well. Ovulation has been demonstrated in the unilaterally denervated ovary of gonadotropin-treated rabbits.⁶ Ovulation also has been achieved in perfused rabbit ovaries removed and placed in a perfusion chamber 8 hours after administration of hCG to the intact animal.⁷ Clearly, ovulation is preceded by a surge of pituitary LH or an ovulation-inducing hormone. This abrupt increase is quickly followed by acceleration of follicular growth accompanied by a number of other local phenomena within the ovary. In the rabbit, for example, the preovulatory phase is characterized by enhanced ovarian smooth muscle contractility and an increase in the prostaglandin content of the ovarian follicles. Such changes in prostaglandin content of follicles before ovulation may well reflect the importance of ovarian prostaglandins in the ovulatory process. Indeed, ovulation can be inhibited in the gonadotropin-treated rabbit and monkey by the administration of prostaglandin synthetase inhibitors.^{8, 9}

A number of observations have indicated that increased intrafollicular pressure cannot serve as the sole factor responsible for rupture of the mature follicle. These observations include micromeasurements of intrafollicular pressure made at different times in the reproductive cycle in laboratory animals, as well as the realization that cystic follicles may fail to rupture despite marked accumulation of intrafollicular fluid. Although vascular changes in the stigma region of the follicle occur in the periovulatory interval, the question of whether avascularity is essential for ovulation has never been clarified. Just before ovulation, the apical region becomes avascular, and in the periapical vessels, leakage of intravascular contents into intercapillary spaces reflects increased permeability. Such changes, as shown by vascular microcorrosion casts, are deemed necessary for the occurrence of ovulation. On the basis of morphologic and histochemical studies, Cajander and Bjersing¹⁰ have demonstrated evidence of increased lysosomal activity at ovulation time in surface epithelium covering the follicle. Release of proteolytic enzymes is described through the underlying layers of the follicle, and collagen disintegration and signs of cellular degeneration can be seen in the tunica albuginea. Cajander and Bjersing postulated that large amounts of sex steroids present in the follicle during the early preovulatory phase can stimulate growth of lysosomes, whose membranes may be labilized by the high levels of intrafollicular prostaglandins in the late preovulatory phase. A cascade of enzymatic steps resulting in collagenolysis participates in connective tissue remodeling associated with follicle wall rupture.¹¹ Other substances that appear to influence the process of ovulation at a local ovarian level include various cytokines, oxygen free-radicals, nitric oxide, and angiotensin II.

In the female human, ovulation begins at puberty and continues its cyclic occurrence for approximately three decades. Under normal circumstances, after termination or conclusion of pregnancy, the cyclic release of ova with associated physiologic effects is resumed after a relatively brief, but variable, interval. It would seem that this repetitious cycle of ovulation has an inherent drive to persist, a remarkable phenomenon considering the numerous delicate mechanisms responsible for this cyclic event. Female reproductive maturity is attained once complete follicular maturation and capacity for ovulation have become established. Both of these functions depend on stimulation of ovarian tissue by pituitary gonadotropins.

The physiologic mechanisms regulating the initiation of puberty are obscure. Experimental and clinical data substantiate that the individual components of the reproductive system are capable of mature function well before the customary age of adolescence. The hypothalamic-pituitary-ovarian complex begins its function in utero. During midchildhood, hypothalamic secretion of gonadotropin-releasing hormone (GnRH) is restrained. There is also an enhanced inhibitory sensitivity to gonadotropic suppressing effects of sex steroids during childhood. As adolescence approaches, the central restraint diminishes, and the amplitude and frequency of GnRH pulses increase initially during periods of sleep.¹² Puberty does not represent sudden activation of a previously dormant system, but rather the arousal of a system active from the time of in utero existence. The sensitivity of hypothalamic receptors, which handle the inhibitory influences of the gonadal sex steroids, appears to decline as maturity progresses. In the normal prepubertal person and in immature children with gonadal dysgenesis, serum gonadotropin levels can be suppressed by smaller doses of estrogens than are required in mature adults. The required dose of estrogen for suppression increases with advancing age.^{12, 13} As a result of this mechanism, larger amounts of pituitary gonadotropins are secreted until the resultant increase in ovarian function establishes an equilibrium between pituitary and gonadal activity. Increasing sensitivity of the ovary to gonadotropins may be enhanced by insulin-like growth factor-1 from granulosa cells, which amplifies the effects of LH and follicle-stimulating hormone (FSH).¹⁴ The process of readjustment continues until the concentration of ovarian estrogens required for hypothalamic suppression exceeds the sensitivity of target tissues to these steroids. At this point, the physical characteristics of puberty become apparent. Ultimately, by a progressive process of equilibration among hypothalamic, pituitary, and ovarian functions, the relationship leading to cyclic folliculogenesis and ovulation is established in adulthood.¹²

Yen and co-workers^{15, 16} have shown a progressive rise in serum FSH concentrations with advancing age in girls between the ages of 8 and 14. By the age of 14, serum FSH values have doubled. LH is also detectable in serum at age 8, increasing progressively as sexual maturity is approached. By age 14, mean serum LH levels have approached adult levels. An increase in pituitary LH secretion during sleep in pubescent girls also indicates activation of the hypothalamic-pituitary system as a critical step in the initiation of puberty. This phenomenon may occur independent of gonadal steroids. The interactions between pituitary gonadotropins and ovarian steroids cannot be viewed as a self-contained system. To emphasize this point, Hopper and Yen¹⁷ have demonstrated a progressive increase in dehydroepiandrosterone as well as dehydroepiandrosterone sulfate with advancing age during puberty, suggesting that adrenal androgens may initiate central nervous system activation in puberty.

The ovarian follicle is a significant source of the steroid hormones: estrogens, progestogens, and androgens. It is important to consider individual components of the ovary as having a specific steroid secretory capacity. Both the theca and granulosa cells in the human ovary contain enzymes necessary to synthesize estrogens, progestogens, and androgens. As the follicle develops during the preovulatory phase of the cycle, granulosa cells have the capacity to secrete both estradiol and androstenedione, an androgen precursor. Estradiol, however, is the principal steroid in this phase. Estradiol produced by the granulosa cells provides an estrogenic environment that appears necessary both for proper oocyte development and for further proliferation of granulosa cells. Throughout most of the follicular phase of development, the granulosa cells serve as the major ovarian source of estrogens and, to a lesser extent, androstenedione. As ovulation is approached, the granulosa cell is transformed into a progesterone- and estradiol-secreting cell. Subsequently, after corpus luteum formation, progesterone and androstenedione represent the major products of the luteinized granulosa cell. In the atretic follicles, the granulosa cell continues to produce steroids; however, these are predominantly androgens. The thecal tissue synthesizes primarily androstenedione with only small amounts of estradiol, but in the intact follicle, thecal androgens may be transported to the granulosa cell to serve as precursors for estradiol synthesis. In atretic follicles, thecal tissue synthesizes significant quantities of androstenedione. Stromal tissue produces estradiol in the presence of high levels of FSH, but when FSH and LH levels both are high, stromal tissue produces more progesterone, androstenedione, and testosterone. During menopause, under the influence of high circulatory levels of gonadotropins, ovarian stromal androstenedione output is very high.¹⁸

The process of ovulation integrates four compartments that, when functioning normally, eventuate in the cyclic development of ovarian follicles and the release of at least one mature ovum during each cycle. Each of the four compartments is unique with regard to its anatomic characteristics as well as in the nature of its secreted hormone (or hormones). The compartments are as follows: (1) the ovaries, (2) the anterior lobe of the pituitary gland (hypophysis), (3) the hypothalamus, and (4) the environment (Fig. 1).

Compartment 1: The Ovaries

The ovary produces steroid hormones (estrogens, progestogens, and androgens). Estrogens are primarily produced by granulosa cells, progestogens by theca and granulosa cells, and androgens by theca cells and stromal cells. Although the premenarcheal ovary possesses the capacity for steroid production, controlling mechanisms at the level of the anterior lobe of the pituitary gland and hypothalamus (discussed above) do not provide the appropriate level of stimulation to effect significant ovarian steroidogenesis until just before menarche. The anterior lobe of the pituitary gland produces protein hormones (FSH, LH, prolactin) that regulate ovarian function. FSH and LH influence ovarian folliculogenesis and steroid hormone secretion via direct action on the ovary. Prolactin influences ovarian function indirectly through modulation of gonadotropin release; there is also compelling evidence that prolactin may have a direct, but incompletely understood, action on ovarian function.¹⁹

Compartment 2: The Hypophysis

Histochemical techniques and immunocytochemical methods have demonstrated that LH and FSH are largely individually produced by distinct gonadotropes, but to a lesser extent are secreted by the same cells.²⁰ The observation that both gonadotropins can arise from common cells may explain the midcycle rise of FSH release that accompanies the preovulatory LH surge. Prolactin-secreting cells are also identifiable histologically. These distinct cells, termed lactotropes, are more laterally situated in the hypophysis than the gonadotropes.²¹ The first few days of the ovulatory cycle are characterized by a predominance of FSH secretion. The midcycle surge of gonadotropins brought about by GnRH is dominated by LH; the concomitant rise in FSH is of lesser magnitude. After ovulation, serum levels of LH and FSH abruptly decline to their preovulatory range.

Compartment 3: The Hypothalamus

LH and FSH secretion as well as that of other hypophysiotropic hormones, specifically the peptide hormones elaborated by the hypothalamus, is regulated mainly by the central nervous system. Ovarian function is regulated by GnRH (a decapeptide), prolactin-inhibiting factor (either a dopamine-like substance or dopamine itself), and thyrotropin-releasing hormone (a tripeptide), which stimulates release of thyroid-stimulating hormone but also serves as a prolactin-stimulating agent. These hypothalamic peptides stimulate the synthesis and secretion of hormones by the anterior lobe of the pituitary gland. GnRH originally was thought to be exclusively related to LH secretion and was thus termed “luteinizing hormone-releasing factor.” It is now apparent that both FSH and LH secretion are dependent on GnRH. The change in nomenclature from LH-releasing factor to the term gonadotropin-releasing hormone reflects this peptide's dual control over both gonadotropins, and also emphasizes that this factor fulfills the criteria necessary to be identified as a hormone. No exclusive FSH-releasing substance has yet been identified.

The following are two important characteristics of hypothalamic control over gonadotropic secretions: (1) varying responsiveness of the pituitary gonadotrope to GnRH throughout the cycle; and (2) pulsatile release of gonadotropins in response to episodic elaboration of GnRH into the hypothalamic-hypophyseal portal vasculature. The pulses of GnRH vary in their frequency and amplitude. The sensitivity of gonadotropes to GnRH depends on circulating levels of estrogens and progestogens, which vary with time in the cycle.

Compartment 4: The Environment

The environmental compartment is not anatomically defined, unlike the other three compartments involved in ovarian functions. It consists of a variety of heterogeneous factors that ultimately influence hypothalamic function. These factors include, among others, diet, climate, medications, light-dark cyclicity, level of stress, and the functional status of other endocrine structures. This spectrum of environmental features—internal and external—is translated into input recognizable by the hypothalamus via biochemical conversion of these factors to neurochemical transmitters. Among these neurochemical transmitters are norepinephrine, dopamine, epinephrine, serotonin, and gamma-aminobutyric acid. Catecholamine and serotonin neuron innervation of the median eminence is particularly important in regulating secretions of the hypothalamic nuclei that control pituitary-ovarian function. In addition to monamines and amino acids, peptides appear to play a modulating role. The vasculature of the hypothalamus and pituitary is tridirectional,^{22, 23} with flow toward (1) the systemic circulation, (2) the anterior lobe of the pituitary, and (3) the brain. This vascular arrangement facilitates both short and ultrashort feedback relationships acting between the hypothalamus and hypophysis.

A variety of peptide hormones are produced by the neurons in an area of the median eminence that terminates on capillary loops. These neurohormones are released into the hypothalamic-hypophyseal portal system. Opioid peptides such as β-endorphins and enkephalins also play a major role in the regulation of hypothalamic-pituitary function. Opioid receptors have been identified in the hypothalamus as well as in other regions of the central nervous system. Administration of opiates and enkephalin analogs results in inhibition of LH secretion and an increase in prolactin release.²⁴ Opioids suppress LH secretion by inhibiting GnRH neurons in the arcuate nucleus; catecholamines also regulate the elaboration of GnRH. Dopamine inhibits LH secretion, whereas norepinephrine may stimulate LH release.²⁵

The pulsatile release of GnRH by the hypothalamus was first described by Knobil.²⁶ These classic studies, carried out in the Rhesus monkey, revealed that the secretion of LH and FSH is periodic, with pulses at approximately 90-minute intervals in response to the episodic release of GnRH. Continuous infusion of GnRH downregulates the pituitary gland and results in failure of gonadotropin secretion. The use of long-acting analogs of GnRH also inhibits gonadotropin production. Thus the intervals between physiologic episodes of either GnRH elaboration or exogenous GnRH, delivered with an infusion pump, allow the gonadotrope to recover its responsiveness to GnRH, thereby replenishing GnRH receptors. The ultimate elaboration of gonadotropins results from a multitude of factors. The major factor is an appropriate program of GnRH pulses with varying amplitude and frequency delivered via the hypothalamic-hypophyseal portal system. Modulating this mechanism are catecholamines, neuropeptides, opioids, and feedback signals originating at all four compartments.

The secretions elaborated by the four compartments (ovaries, hypophysis, hypothalamus, and environment) act in an integrated fashion to regulate the two major ovarian functions—ovulation and steroid production—via feedback mechanisms.

The basic system depends on pulsatile elaboration of GnRH by the arcuate nucleus which is located in the medial basal hypothalamus. The elaboration of GnRH represents a final common pathway regulated by norepinephrine (positive effect) and dopamine (negative effect), modulated by endogenous opiates (negative effect). Catecholestrogens, synthesized from estrogens in the brain, may also play a role in modulating the influence of catecholamines on GnRH elaboration.²⁷ Circulating estradiol inhibits FSH secretion directly by suppressing hypothalamic GnRH release. Increasing levels of estradiol simultaneously act at the hypothalamus to bring about the preovulatory surge of LH in response to GnRH. Progesterone, produced by the dominant follicle just before ovulation and by the corpus luteum after ovulation, acts in concert with estradiol to inhibit hypothalamic pulsations of GnRH, as well as to trigger the preovulatory surge of LH and FSH. The interrelationships between estradiol and progesterone and pituitary gonadotropin elaboration constitutes a long loop feedback system. An example of a short loop feedback mechanism is the negative effect exerted by gonadotropins on their own release via inhibition of hypothalamic elaboration of GnRH. An ultrashort loop feedback explains the theoretic ability of hypothalamic secretions to regulate their own release through local actions. This level of regulation of the hypothalamic-pituitary ovarian axis is mediated by catecholamines, norepinephrine, dopamine, serotonin, and endorphins, as well as by various other peptides that may act as neurotransmitters in the central nervous system, such as vasoactive intestinal peptide, cholecystokinin, neurotensin, and gamma-aminobutyric acid.

The elaboration of LH by the pituitary in response to GnRH is also episodic and exhibits alterations in amplitude and frequency. LH pulse mean amplitude is maximal in the early and midluteal phase and low in the follicular and late luteal phases. LH pulse mean frequency is maximum in the follicular phase, declines in the luteal phase, and is lowest in the late luteal phase.²⁸ These alterations in LH release generally correlate with FSH patterns, even though FSH is less responsive than LH to GnRH. The elaboration of LH and FSH by the anterior lobe of the pituitary gland ultimately results in ovarian steroid secretion, folliculogenesis, follicular rupture, and corpus luteum formation and maintenance.

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