Introduction

Mullerian inhibiting factor (MIF), also called the anti-Mullerian hormone (AMH), plays a significant role in sexual differentiation. It is produced by the Sertoli cells in male fetuses and signals the regression of the Mullerian ducts, fallopian tubes, and uterus. Androgens, which are secreted by Leydig cells, induce differentiation of Wolffian ducts into male internal and external genitalia. 

Development

Approximately around six weeks of gestation, the SRY gene is expressed on the Y chromosome, which initiates the formation of the testes. Mullerian inhibiting factor, testosterone, and insulin-like factor 3, which are all produced by the testis, control male sex differentiation.[1] Stimulated by follicle-stimulating hormone (FSH), Sertoli cells secrete the Mullerian inhibiting factor, which induces the regression of Mullerian or paramesonephric duct. The expression of Mullerian inhibiting factor prevents the development of female internal genitalia. Without Mullerian inhibiting factor, the Mullerian duct develops into the fallopian tubes, uterus, and upper vagina. In response to the luteinizing hormone (LH), Leydig cells produce testosterone, which is responsible for the virilization of male internal genitalia, as well as the development of male secondary sex characteristics during puberty. Testosterone stabilizes the Wolffian or mesonephric duct, which differentiates into the epididymis, seminal vesicle, and vas deferens.[2] Testosterone is converted into dihydrotestosterone by 5-alpha-reductase. Dihydrotestosterone induces the differentiation of external male genitalia such as the prostate and penis.[3] 

Cellular

SRY gene, found on the Y chromosome, promotes the expression of SOX9, which plays a crucial role in the development of the testes. Under the influence of SRY and SOX9, the genital ridges differentiate into the testes. Testes are composed of transverse cords that contain Sertoli, germ, and peritubular myoid cells. Sertoli cells induce differentiation of Leydig cells via hedgehog signaling. Leydig cells are present in the interstitium, in between the transverse cords. Sertoli cells reinforce their own differentiation via prostaglandin D2 and FGF9 signaling by maintaining SOX9 expression. SOX9 also inhibits FOXL2, which prevents the differentiation of supporting cells into ovarian granulosa cells.[4] 

Molecular

Mullerian inhibiting factor is composed of two identical 70kDa subunits linked by disulfide bonds, which form a dimeric glycoprotein.[5][1] Mullerian inhibiting factor signals via two types of serine/threonine kinase receptors. The type 2 receptor is specific for MIF due to its similarity to the receptors of the transforming growth factor-beta (TGF-B) family. AMH shares a homologous C-terminus with the other members of the TGF-B family. When bound to its receptor, intracellular Smad molecules are signaled to enter the nucleus to function as transcription factors. The molecular identification of the type 1 receptor is uncertain. The three types of type 1 receptors under research include alk2, alk3, and alk6. The type 1 receptor could potentially activate Smad molecules specific to bone morphogenetic proteins.[5]

Mechanism

Gonadotropins and androgens regulate the secretion of the Mullerian inhibiting factor. The gonadotropin-releasing hormone regulates LH and FSH secretion. FSH binds to its Sertoli cell receptor to stimulate the production of AMH.[6] From gestation to puberty, MIF is high, and testosterone is low in males.[2] LH binds to its Leydig cell receptor, causing the release of testosterone. Testosterone acts on its receptor on Sertoli cells and inhibits the production of MIF.[6] MIF gets stimulated by FSH but downregulated by testosterone. Therefore, in puberty, the increase in testosterone causes a decrease in MIF.[2] After gestation in females, ovarian granulosa cells of preantral and small antral follicles produce MIF. The level of MIF increases with an increase in the number of preantral and small antral follicles. The expression of MIF declines when the follicle reaches about eight millimeters in diameter.[7] 

Testing

The levels of the anti-Mullerian hormone are assessable via an enzyme-linked immunosorbent assay.[8] The Gen II AMH assay is the currently adopted standard. The Gen II AMH assay uses a more stable antibody to bind to AMH, making this assay more sensitive. There was undetectable cross-reactivity with inhibin A, activin A, FSH, and LH.[9] 

Pathophysiology

Persistent Mullerian duct syndrome is a type of male pseudohermaphroditism defined by the presence of female internal genitalia and male internal and external genitalia. This condition can also result from a mutation in the gene encoding MIF or a mutation in the MIF receptor. A mutation in the MIF gene will result in the lack of production of MIF. A mutation in the MIF receptor will result in the insensitivity of the Mullerian ducts to the hormone.[10][1] Lack of production or lack of response to MIF does not regress the Mullerian ducts and, therefore, these individuals have female internal genitalia. Individuals with persistent Mullerian duct syndrome present with a normal male phenotype; however, unilateral or bilateral cryptorchidism can be found on examination.[8] 

Clinical Significance

Anti-mullerian hormone serves as a marker for ovarian reserve. It is used to determine the age at menopause and ovarian insufficiency. The number of primordial follicles is fixed at birth in females, but the follicles remain dormant until puberty. The dormant follicles do not produce AMH, but once recruited for development, AMH gets secreted by preantral and small antral follicles.[11] AMH rises with the number of growing follicles in the ovaries and reaches a plateau by twenty-five years. As age increases, the number of follicles decreases and, therefore, leads to a decline in serum levels of AMH. After twenty-five years of age, it gradually declines until menopause, eventually falling below detectable levels. It has an advantage as a biochemical marker compared to the measurement of ovarian steroid hormones, as there are no fluctuations with the menstrual cycle. Serum levels for a fertile woman range from 1.0 ng/ml to 4.0 ng/ml; under 1.0 ng/ml is considered low and indicates a diminishing ovarian reserve.[12] 

Measuring ovarian reserve predicts pregnancy chances in women, which is essential in couples dealing with infertility. Additionally, It has been useful in monitoring the fertility potential of women with cancer by measuring the serum levels before and after treatment to assess chemotherapy's effect on the ovaries. Serum AMH, which is secreted by granulosa cells of ovaries, can be used to diagnose ovarian granulosa cell tumors, monitor efficacy of surgery, and assess for recurrence. [9] In women with polycystic ovarian syndrome, AMH is two to three-fold higher than women with normal ovulation. AMH correlates to the number of preantral and small antral follicles. Patients with PCOS have a higher number of small antral follicles, resulting in increased AMH production. Patients using oral contraceptives have a decreased number of small antral follicles and, therefore, a lower AMH level.[13]

Serum AMH is a marker of the function of Sertoli cells, while testosterone is a marker of the function of Leydig cells. In persistent Mullerian duct syndrome, serum testosterone is normal, and AMH is undetectable or normal. Normal levels suggest normal production and means that the mutation is present in the receptor of AMH. Low or undetectable levels suggest reduced production and that the mutation exists in the AMH gene. If both testosterone and AMH levels are undetectable, this is suggestive of a lack of testicular function such as in anorchia or Klinefelter syndrome. Both are also low in mixed disorders of sex development because fetal hypogonadism results in dysfunction of Leydig and Sertoli cells.[6] 

The anti-mullerian hormone also plays a role as a biochemical marker for testing testicular dysfunction in cryptorchid males. The levels of AMH tend to be significantly lower in bilateral undescended testis compared to unilateral.[14]