Sex Differentiation and Development, Puberty and Menopause

Introduction

Sex differentiation begins with chromosomes and genes that direct gonadal formation, internal ducts, and external genitalia. The SRY gene, androgen receptors, and hormonal signals guide male development, while absence of Y-linked factors leads to ovarian development and female differentiation.

• Reproduction ensures the continuity and survival of a species across generations.
• Nature achieves this through the formation of two biologically distinct sexes within the same species, called sexual dimorphism.
• Full sexual differentiation becomes evident after the onset of puberty.
• Puberty leads to maturation of the reproductive organs and development of secondary sexual characteristics.
• Sexually mature adults are capable of producing offspring.
• Understanding reproductive physiology requires study of sex differentiation and gonadal development. It also includes gonadal functions, puberty, male and female reproductive systems, pregnancy, childbirth, and lactation.

Sex Differentiation

Sex Determination

Normal Chromosomal Pattern

• Sex determination in humans depends on the chromosomal pattern present at fertilization.
• A normal human cell contains 46 chromosomes arranged in 23 pairs.
• Among these, 22 pairs are autosomes.
• One pair consists of sex chromosomes.

In Males

Males have 44 + XY chromosomes.

In Females

Females have 44 + XX chromosomes.

Sex Chromosomes

• Sex chromosomes determine genetic sex in humans. These chromosomes are the X chromosome and the Y chromosome.
• The X chromosome is larger and contains more genes than the Y chromosome.
• Females usually possess two X chromosomes (XX).
• Males usually possess one X and one Y chromosome (XY).
• During reproduction, every ovum contributes one X chromosome.
• Sperms contribute either an X chromosome or a Y chromosome. Therefore, the sperm determines the chromosomal sex of the offspring.
• Fertilization by an X-bearing sperm forms an XX zygote, which usually develops as a genetic female.
• Fertilization by a Y-bearing sperm forms an XY zygote, which usually develops as a genetic male.
• The normal karyotype of a genetic female is 44 autosomes + XX.
• The normal karyotype of a genetic male is 44 autosomes + XY.
• Because approximately half of sperms carry X and half carry Y, male and female conceptions are expected to occur in nearly equal numbers.
• In many populations, male births are slightly more frequent than female births. This small difference is influenced by multiple biological and environmental factors. It should not be explained only by faster movement of Y-bearing sperm, because scientific evidence for that theory is limited and inconsistent.
• Final sex ratio at birth can also be affected by fertilization success, embryo survival, and population factors. Thus, sex determination is a genetic event established at fertilization.

Sex Determination

Karyotyping

• Sex determination can be assessed by examining chromosomal composition.
• Karyotyping is a laboratory method in which chromosomes are visualized and arranged by size and structure. It helps identify the presence of XX or XY sex chromosomes. This method is useful for confirming chromosomal sex and detecting abnormalities.

Demonstration of Sex Chromatin

• Another simpler method is demonstration of sex chromatin or Barr body.
• Barr bodies can be identified in leukocytes or cells from the oral mucosa.
• In individuals with two X chromosomes, one X chromosome becomes inactive and condenses into a Barr body. Therefore, Barr body presence usually suggests an XX pattern.

Sex Differentiation

• Sex differentiation is the developmental process by which gonads, genital ducts, and external genitalia acquire male or female characteristics.
• During the first five weeks of gestation, male and female embryos have similar primitive gonads. This phase is called the indifferent gonad stage.
• Distinct sexual development usually begins around the sixth week of intrauterine life.
• Maturation continues after birth and progresses further during puberty.
• Sex differentiation converts the undifferentiated gonad into a functional testis or ovary.
• After fertilization, two principal cell lineages develop within the primitive gonad.
• One lineage forms Sertoli cells in the testis and granulosa cells in the ovary. These supporting cells nourish developing germ cells and assist their maturation.
• Sertoli cells help sperm development, while granulosa cells support ovarian follicles and produce estrogen precursors.
• The second lineage forms Leydig cells in the testis and theca cells in the ovary.
• Leydig cells secrete testosterone, which is essential for male reproductive development and spermatogenesis.
• Theca cells produce androgens that are converted to estrogens by granulosa cells.
• Proper coordination of genes, hormones, and cellular differentiation is necessary for normal sexual development.

Differentiation of the Genetic Sex, Gonadal Sex and Phenotypic Sex

Genetic Sex

• Sex development involves three related levels: genetic sex, gonadal sex, and phenotypic sex.
• Genetic sex is determined at fertilization by the combination of sex chromosomes.
• A typical male karyotype is 44 autosomes + XY.
• A typical female karyotype is 44 autosomes + XX.
• In genetic males, the Y chromosome plays a major role in testicular development.
• The Y chromosome contains the SRY gene on its short arm.
• SRY activates pathways that convert the indifferent gonad into a testis. It produces testis-determining signals that initiate male gonadal differentiation.
• Without a functional Y chromosome or SRY gene, normal testicular formation usually does not occur.
• Testes then produce hormones required for further male development.
• Sertoli cells secrete anti-Müllerian hormone, which suppresses female internal duct development.
• Leydig cells secrete testosterone, which promotes development of male internal genital ducts.
• Conversion of testosterone to dihydrotestosterone is necessary for external male genitalia and prostate formation.
• Complete masculinization also requires functional androgen receptors in target tissues. These receptors are encoded by genes on the X chromosome.
• Therefore, both X-linked and Y-linked genes contribute to normal male development.
• In genetic females, the absence of a Y chromosome means SRY is not present. As a result, the indifferent gonad usually develops into an ovary.
• Ovarian development does not require the same testis-determining signals.
• Female internal genital structures develop when anti-Müllerian hormone is absent.
• External genitalia follow the female developmental pathway when significant androgen stimulation is absent.
• Phenotypic sex refers to the observable internal and external sexual characteristics produced by these genetic and hormonal processes.

Gonadal Sex

Male Gonadal Sex

• Gonadal sex refers to differentiation of the primitive gonad into a testis or ovary.
• In a normal genetic male, testicular differentiation begins around 6–7 weeks of gestation.
• Primitive seminiferous cords develop and later form seminiferous tubules.
• Sertoli cells surround and support the germ cells.
• Leydig cells appear by about 8–9 weeks of gestation.
• Leydig cells secrete testosterone, which promotes development of male internal and external reproductive structures.
• Sertoli cells produce anti-Müllerian hormone, which causes regression of Müllerian ducts.
• The SRY gene initiates testis formation through genetic regulation. It does not directly block estrogen production, but shifts gonadal development toward the male pathway.

Female Gonadal Sex

• In a normal genetic female, ovarian differentiation usually begins after 9 weeks of gestation.
• In germ cells, one X chromosome is later inactivated in somatic cells, while germ cells reactivate X chromosomes during early development.
• Germ cells multiply to form oogonia.
• Oogonia enter meiosis and become primary oocytes. These cells are surrounded by granulosa cells, forming primordial follicles.
• Ovarian stroma develops and gives rise to theca/interstitial cells.
• Primary oocytes remain arrested in prophase I (diplotene stage) until ovulation or follicular loss.
• Ovarian hormones, especially estrogens, support female reproductive development later in fetal life and puberty.

Phenotypic (Genital) Sex

• Phenotypic sex refers to the development of internal genital ducts, external genitalia, and related secondary structures. This differentiation depends mainly on fetal hormonal influences.
• In the presence of testicular hormones, male genital structures develop.
• In the absence of significant androgen action, the female developmental pathway usually proceeds.

In Males

• In males, the Wolffian (mesonephric) ducts are maintained during early fetal life.
• Under the influence of testosterone from Leydig cells, these ducts differentiate into:
  • Epididymis
  • Vas deferens
  • Seminal vesicles
  • Ejaculatory ducts

• Leydig cells become active around 8–10 weeks of gestation.
• Testosterone is essential for growth of male internal reproductive ducts.
• Conversion of testosterone to dihydrotestosterone promotes formation of external male genitalia.
• In males, Sertoli cells secrete anti-Müllerian hormone. This hormone causes regression of the Müllerian ducts through programmed cell death.
• As a result, female internal reproductive organs do not develop in a typical male fetus.

In Females

• In females, ovaries do not produce significant fetal testosterone. Therefore, the Wolffian ducts regress.
• Because anti-Müllerian hormone is absent, the Müllerian ducts persist and differentiate into:
  • Fallopian tubes
  • Uterus
  • Cervix
  • Upper part of the vagina

• Much of this internal genital differentiation is completed by about 18–20 weeks of gestation.
• Brain sexual differentiation is also influenced by genetic and hormonal factors during development.

Müllerian Inhibiting Substance (MIS)

• Müllerian Inhibiting Substance is also called anti-Müllerian hormone. It is a glycoprotein hormone produced mainly by Sertoli cells of the fetal and prepubertal testis. It belongs to the transforming growth factor beta superfamily.
• In male fetuses, secretion begins around 8–10 weeks of gestation. It causes regression of the Müllerian ducts, preventing development of female internal reproductive organs.
• Blood levels are high in infancy and early childhood.
• Concentrations gradually decline during puberty and remain lower in adult males.
• In females, granulosa cells of growing ovarian follicles produce anti-Müllerian hormone after puberty.
• In women, it reflects ovarian reserve and follicular activity.
• Adult female levels are usually lower than male childhood levels.

Functions of MIS

• Anti-Müllerian hormone causes regression of the Müllerian ducts in male embryos, preventing formation of female internal reproductive organs. It supports normal male reproductive development.
• During later fetal life, it may assist testicular descent.
• In both sexes, it may influence germ cell maturation.

Abnormalities of Sex Differentiation

• Abnormalities of sex differentiation are broadly classified as chromosomal or developmental disorders.

Chromosomal Abnormalities

Common chromosomal conditions include Turner syndrome, Klinefelter syndrome, androgen insensitivity syndrome, triple X syndrome, and ovotesticular disorder of sex development.

Turner’s Syndrome

• Turner syndrome is a form of gonadal dysgenesis caused by partial or complete absence of one X chromosome.
• The usual karyotype is 45,X.
• Common features include short stature, webbed neck, streak ovaries, and delayed puberty.
• Patients often present with primary amenorrhea and infertility. It usually results from meiotic nondisjunction or chromosomal loss.

Klinefelter’s Syndrome

• Klinefelter syndrome is a common sex chromosome disorder in phenotypic males.
• The usual karyotype is 47,XXY.
• Patients have male external genitalia, small firm testes, and impaired seminiferous tubule development.
• Puberty occurs, but testosterone may be low or relatively insufficient.
• Common findings include tall stature, sparse body hair, gynecomastia, primary hypogonadism, and infertility.
• Learning difficulties may occur, but intellectual disability is not universal.

Superfemales

• Triple X syndrome is a chromosomal condition with karyotype 47,XXX.
• Many individuals have normal appearance and remain undiagnosed.
• Some may have tall stature, mild learning difficulties, or menstrual irregularities.
• Fertility is often normal.

Testicular Feminization Syndrome

• Androgen insensitivity syndrome was previously called testicular feminization syndrome.
• Affected individuals usually have 46,XY karyotype and testes.
• Because target tissues do not respond adequately to androgens, external genitalia appear female or undervirilized.
• Breasts develop at puberty due to estrogen conversion.
• Pubic and axillary hair may be sparse.
• The uterus is absent because testes produce anti-Müllerian hormone.
• The vagina is often short or ends blindly.
• Patients commonly present with primary amenorrhea or inguinal masses.

True Hermaphroditism

• Ovotesticular disorder of sex development was previously termed true hermaphroditism.
• Both ovarian and testicular tissue are present in the same individual.
• External genitalia may be ambiguous, male, or female.
• Internal reproductive structures vary according to the gonadal tissue present.
• Karyotypes may include 46,XX, 46,XY, or mosaic patterns. These conditions require multidisciplinary evaluation, hormonal assessment, imaging, genetics, and individualized long-term care.

Other Chromosomal Abnormalities

Other chromosomal abnormalities may arise from structural or numerical errors during meiosis.

Transposition of Chromosome

• Translocation can move part of one chromosome to another chromosome.
• Transfer of the SRY gene to an X chromosome may produce an XX male phenotype.

Deletion of a part of Chromosome

Deletion of the Y chromosome region containing SRY may result in an XY female phenotype.

Nondisjunction of Chromosome

• Nondisjunction is failure of chromosomes to separate properly during cell division. This causes aneuploidy, such as trisomy 21, which leads to Down syndrome.
• Trisomy 21 is an autosomal, not sex chromosome, abnormality.

Developmental Abnormalities

• Developmental abnormalities of sex differentiation often result from hormonal disturbances, receptor defects, or enzyme deficiencies.
• Some structural abnormalities may also occur without hormonal causes.

Pseudohermaphroditisms

• Older terms such as pseudohermaphroditism are now replaced by disorders of sex development. These conditions involve mismatch among chromosomal sex, gonads, and external genital appearance.

Female Pseudohermaphroditisms

• In 46,XX disorders of sex development, the individual has ovaries and female chromosomes but varying degrees of genital virilization.
• Excess androgen exposure during fetal life, especially between 8 and 13 weeks of gestation, can masculinize external genitalia.
• A common cause is congenital adrenal hyperplasia, usually due to 21-hydroxylase deficiency.
• Maternal androgen-secreting tumors or androgenic medications may also cause virilization.
• Internal female organs such as uterus and fallopian tubes are usually present because testes and anti-Müllerian hormone are absent.

Male Pseudohermaphroditisms

• In 46,XY disorders of sex development, the individual has testes or impaired testicular tissue with undervirilized or female-appearing external genitalia.
• Causes include defective testicular development, reduced testosterone synthesis, or impaired androgen action.
• If testes fail to produce adequate anti-Müllerian hormone, Müllerian structures may persist.
• If testosterone production is low, Wolffian duct development is incomplete.
• 5-alpha reductase deficiency reduces conversion of testosterone to dihydrotestosterone, causing undervirilized external genitalia at birth.
• Virilization may increase at puberty because testosterone rises.
• Androgen insensitivity syndrome results from defective androgen receptors.
• In complete forms, testes produce anti-Müllerian hormone, so the uterus is absent.
• External genitalia are typically female, and the vagina may be short or blind-ending.
• Accurate diagnosis requires genetics, hormone testing, imaging, counseling, and individualized management.

Enzyme Deficiencies

• 17-alpha hydroxylase deficiency can cause 46,XY disorders of sex development with undervirilization.
• Some forms of congenital adrenal hyperplasia also alter steroid synthesis and sexual differentiation.