At first sight, it would seem that nothing could be simpler than the subdivision into two sexes - female and male. But the development of sexual dimorphism is a complex process composed of many steps. It starts at conception and continues for many years.
The genetic sex of a person is determined by the sex chromosome carried by the fertilising sperm - if that sperm carries an X-chromosome, the new individual will be female (44 autosomes + XX sex chromosomes), but if it carries a Y-chromosome, the individual will be genetically male (44 + XY).
But the establishment of genetic sex is only the starting point of a process which continues after birth, through puberty and the reproductive years.
After conception, there follows a period of 6 weeks during which female and male embryos develop along almost identical pathways, and cannot be told apart on morphological grounds. This is called the indifferent period. (Of course, a chromosomal analysis at this time could distinguish the sexes.)
Then an event of great significance occurs - some special cells migrate into the rudimentary gonads*, having started their journey from the wall of the yolk sac. These are the primordial germ cells, the cells which will later give rise to the gametes after passing through meiotic divisions and morphological changes. (It is intriguing to speculate on why and how these important cells are set aside for their future role at such an early stage in development.)
* 'gonad' is the general name for the organ which produces or will produce, gametes, and can be applied to both sexes.
The rudimentary gonads form as swellings on the medial aspects of the mesonephric ridges on the posterior wall of the abdomen. Their initial development is the same in both sexes. Then the primordial germ cells arrive and begin to exert their influence on the further development of the gonads. In the genetic females, ovarian development takes place, with most of the changes occuring in the outer (cortical) regions of the gonad. In the male, it is the inner medulla which shows greater development with formation of the seminiferous tubules and rete testis. It seems that the Y-chromosome plays a key role in stimulating testicular differentiation.
During the indifferent period, two duct systems emerge. Later, only one duct system will be retained, and the other regresses. Which system is retained will depend on the genetic sex of the embryo.
The two systems are:
the mesonephric ducts - these we have considered already in relation to the mesonephros and developing urinary system
the paramesonephric ducts - a pair of ducts which develop in parallel with the mesonephric ducts. They are formed by infolding of the coelomic epithelium that covers the mesonephric ridge.
The paramesonephric duct opens into the intra-embryonic coelom at its cranial end, and at its caudal end passes medially until it meets the paramesonephric duct of the other side. Here, deep within the urorectal septum, the two ducts fuse, and contact a specialised region at the back of the urogenital sinus.
Caudal end of the embryo - hindgut and allantois (yellow),
mesonephric duct (yellow), paramesonephric ducts (light purple).
Note the mesonephric ridges and gonadal ridges
alongside the dorsal mesentery and hindgut.
fates of the duct systems
in the female - the paramesonephric ducts are retained.
They develop into the uterine tubes (with their delicately fimbriated ends opening into the future peritoneal cavity) and, where they meet caudally in the midline, the uterus. The paramesonephric ducts raise a transverse fold of peritoneum in the pelvic region as they course medially - this will form the broad ligament.
The vagina is formed by thickenings in the wall of the urogenital sinus - the sinovaginal bulbs - adjacent to the developing uterus.
The mesonephric ducts disappear almost completely.
in the male - the mesonephric ducts are retained.
The tubular structures within the testes become linked with the mesonephric ducts by a handful of mesonephric tubules alongside. All the other mesonephric tubules disappear, but these few which remain form the efferent ductules of the testis. The mesonephric duct itself differentiates into the epididymis and the duct deferens.
Recall that initially the metanephric urinary system drains via the distal portion of the mesonephric duct into the allantois, but as development proceeds that portion of the mesonephric duct is incorporated into the wall of the developing urinary bladder, until a stage is reached where the ureter (derived from the ureteric bud) opens into the bladder separately from the ductus deferens (derived from the mesonephric duct). The relative positions of these ducts then reverses - the ductus deferens shifts caudally until it opens (via the ejaculatory duct) into the urethra rather than the bladder. For details of this process, consult your textbook. Note how the trigone of the bladder is formed during this process of incorporation of the mesonephric ducts.
In the female, these are the vulval cleft bounded by the labia majora and labia minora, the clitoris, and the vaginal and urethral openings within the cleft.
In the male, these are the penis, with the urethra opening at its tip, and the scrotum containing the testes.
During the indifferent period the external genitalia are the same in both sexes. They develop from the mesodermal swellings (covered of course with ectoderm) which surrounds the coacal membrane. The swellings are:
a pair of cloacal folds
the genital tubercle
a pair of genital swellings.
When the urorectal septum develops, it subdivides the cloacal membrane into the anal portion and a urogenital portion. Thus, the original coacal folds become modified to form the anal and urogenital folds respectively. (The intervening region, where the urorectal septum fuses with the cloacal membrane, will later differentiate into the important perineal body.)
Soon after this stage is reached in the 6th week, the urogenital and anal membranes begin to perforate. From now on the development of the external genitalia will diverge along different pathways, depending on the sex of the embryo and factors such as circulating hormones. The main difference between the two sexes is seen in the degree of fusion of the swellings around the urogenital region - fusion is more pronounced in the male embryo.
In the female embryo, the genital swellings emlarge to form the labia majora, and the urethral folds become the labia minora. Both pairs of folds remain unfused. The genital tubercle differentiates - with nearby tissues - into the clitoris. The short urethra opens directly into the vulval cleft, and the vagina - after canalisation - also communicates with this cleft, although its entrance is partially guarded by the membranuous hymen.
In the male embryo, the genital swellings enlarge and fuse caudally to form the scrotum. Later, the testes will descend into the scrotum. The urethral folds approach each other in the midline and fuse, forming the shaft of the penis, and enclosing the penile part of the urethra. The genital tubercle forms the glans penis, which at first is a solid structure. A lumen then develops from the tip of the glans and extends back until it joins the penile urethra. Thus, the urethra in the male has a composite origin, receiving a contribution from the urogenital sinus, being added to by closure of the urethral folds, and being completed by canalisation of the glans penis.
External genitalia - (a) during indifferent period,
(b) contact of urorectal septum with cloacal membrane,
(c) labia minora and majora of female perineum,
(d) formation of scrotum and shaft of penis,
(e) canalisation of glans penis to complete urethra.
descent of the gonads
The gonads develop on the posterior wall of the abdomen, but then in both sexes 'descend' to a more caudal position. The male gonads descend to a more caudal position than their female counterparts - the testes leave the abdominal cavity and enter the scrotum, where the slightly lower temperature (after birth) will be more conducive to sperm-formation at a later time. The ovaries descend into the pelvic region alongside the uterus.
The mechanisms involved in this descent include the actions of a structure called gubernaculum, and also differential growth processes, especially elongation of the abdominal region of the embryo/fetus.
The gubernaculum is a fibromuscular strand extending from the caudal part of the gonad down into the genital swelling of the same side. It has been proposed that this structure is actively contractile, but not everyone agrees with this view.
In the female embryo, the gubernaculum lies close alongside the developing uterus, and actually becomes fused with it. This subdivides the gubernaculum into two portions: one extending from the ovary to the uterus (later becomes the ligament of the ovary), and the other from the uterus to the genital swelling (later the round ligament of the uterus). The result is that the ovary normally descends only as far as the pelvic region, until it lies alongside the uterus on the posterior aspect of the broad ligament. (Very, very rarely there have been reports of an ovary located within a labium majus.)
In the male embryo, the gubernaculum does not become subdivided. The testis moves down the posterior abdominal wall and then through the anterior abdominal wall into the scrotum. As it passes through the abdominal wall it lies in relation to a tube of peritoneum called the processus vaginalis which extends down into the scrotum.
This migration of the testis is usually completed by the time of birth. The structures linking the testis with other abdominal structures - eg: the ductus deferens and the testicular vessels and nerves - lie for part of their course within the inguinal canal. The distal part of the processus vaginalis normally becomes separated from the peritoneal cavity and forms the sac-like tunica vaginalis that surrounds most of the testis.
Recall that as the gonads descend, the kidneys ascend, so that their relative positions are reversed.
So far, an outline of the morphological changes that occur during the development of the reproductive system has been given for both sexes. But other important changes take place at a finer level of detail while these morphogenetic processes are occuring.
For example, in the developing ovary the primordial germ cells divide repeatedly and produce a population of primary oocytes surrounded by follicle cells - the primordial follicles. About 6 million of these follicles are formed by the 5th month of development, but then a process of attrition begins as some follicles begin to degenerate. By the time of birth, the number of viable follicles and oocytes has dropped to about 1/6th of the original number, but these survivors have commenced the first meiotic division. Thus in the female, gamete formation and meiosis begin before birth. The process of reduction of oocytes numbers continues after birth, and is called atresia.
The female gametes then pass into a phase of suspended development which continues at least until puberty - and for some follicles will last for many years after that. During the reproductive years which follows puberty, groups of follicles recommence their development. With each ovarian (menstrual) cycle, a handful of follicles begin to mature, and the oocytes within continue their first meiotic divisions. But usually, only one follicle will actually ovulate in a given cycle - the others in a small group of maturing follicles become atretic.
Even at the time of ovulation, the oocyte is not really a true ovum - it has still to complete its meiotic preparations. Meiosis is only completed if fertilisation takes place - if not fertilised, the oocyte will die after about 24 hours, still halfway through its second meiotic division.
The reproductive period for women usually lasts for 30-35 years, after which the ovarian cycles cease (menopause). Even if she ovulates every month throughout her reproductive years, only 300-400 oocytes will be released - of the 6 million or so potential eggs present in the ovaries before birth.
In the male, gametogenesis does not begin until puberty, and then production of spermatozoa becomes a continuous, production-line process that continues into old age. (Recall that a single ejaculate contains 300 million spermatozoa.)
Thus, gametogenesis follows quite different time-scales in the two sexes, and invloves different numbers of gametes, even though the same fundamental meiotic events are observed in oocytes and spermatocytes.
disordered development of the reproductive system
It was pointed out earlier that development of 'femaleness' or 'maleness' is a complex process, involving for example genetic factors, the appropriate development of the reproductive system and other body structures, and hormonal influences before and after birth. A better understanding of these interacting processes can be gained from a study of the various disorders that arise.
disorders of sexual development
It is surprisingly difficult to define 'femaleness' or 'maleness'. Neither law nor medicine seems to have a well-established definition. However, it seems that at least 4 criteria must be taken into account:
1 genetic sex
female - 44 autosomes + XX
male - 44 autosomes + XY
2 gonadal sex
a normal female has 2 intra-abdominal ovaries
a normal male has 2 extra-abdominal testes
3 phenotypic sex
the form of the external and internal genitalia and general body form –
the female has relatively wide hips compared with shoulder span; a vulval cleft with urethral and vaginal openings, labia majora & minora, and a small imperforate clitoris; at puberty the breasts develop, the pubic hair has a horizontal upper margin, the forehead hairline is straight and facial hair inconspicuous; subcutaneous fat is deposited generously over hips and thighs
the male has relatively narrow hips compared with shoulder span, and a penis conveying the urethra to its tip. At puberty, males develop conspicuous facial hair, pubic hair which extends to the umbilicus and a greater amount of body hair compared with the female. Later in life, the forehead hairline recedes and baldness may occur
4 psychological sex
a psychological libido usually directed towards members of the other sex
Thus, for a person to be a 'normal' woman or a 'normal' man, each of the four criteria listed above should correspond to one sex only. If one or more does not match, the condition is known as 'intersex'.
determination of sex
The genetic sex of an individual is determined at fertilisation. A sperm carrying an X chromosome will establish - with the X chromosome contributed by the egg - a genetic female, and a sperm carrying a Y chromosome will establish a genetic male. It is thought that there are female-inducing genes in the autosomes and perhaps the X chromosomes, while the Y chromosome probably carries male-inducing genes. The Y chromosome is dominant over the X chromosome and also the autosomes. In humans, two sex chromosomes are necessary for the development of a normal gonad.
The presence of a testis will induce male differentiation of the other organs. Absence of a testis, even if there is no ovary, will produce female differentiation of the other sex organs.
Before 7 weeks, the embryonic gonad is similar in both sexes. Nonetheless, it displays two main zones, the cortex and the medulla. The cortex plays the major role in ovarian differentiation, the medula in testicular differentiation. Genetic factors are responsible for the prevalence of either the cortex or the medulla. The presence of testes inhibits growth of the paramesonephric duct system. Absence of only one testis allows formation of a female-like duct system on that side only. This means that the suppresive effect of a testis is localised and may not be produced by systemic androgens as originally thought. At a slightly later stage, testicular or exogenous androgens induce fusion of the urethral folds and genital swellings to form the penile shaft and scrotum. Timing is critical, since the presence of androgens after the normal period of differentiation of the external genitalia results only in clitoral hyperthrophy, not fusion of the labial folds.
Variations in sexual development may stem from genetic factors or other sources.
a) chromosomal intersex
The sex chromosomes act primarily on the gonads. The Y chromosome is dominant and testis-producing, but if it is accompanied by more than one chromosome, development of a normal testis is prevented. Disorders of the sex chromosomes generally arise during the meiotic divisions of gametogenesis, but errors of mitosis may occur during embryonic development to give mosaicism - ie: an embryo with two or more cell populations, each with a different chromosomal complement. In addition to anomalies in the number of whole chromosomes, it is possible to have localised anomalies of a single sex chromosome sufficient to disrupt normal development. On the other hand, the presence of seemingly normal sex chromosomes does not guarantee development of a normal gonad; true hermaphrodites with both testicular and ovarian tissues have been found to have apparent normal chromosomal constitutions.
i) Turner's syndrome (44 + XO)
This syndrome is characterised by normal prepubertal female external genitalia, short stature (less than 5 feet), webbing of the neck; usually amenorrhea after puberty, with no development of secondary sex characteristics, or slight breast development and occasional menstruation. The ovaries and uterus are poorly developed, and there is a high level of urinary gonadotrophins. Treatment is oestrogen therapy to encourage development of secondary sex characteristics.
ii) Triple X female (44 + XXX)
Sometimes called 'super female' – however the condition may be associated with amenorrhea, underdeveloped breasts, infantile external genitalia, and learning difficulties.
iii) Klinefelter's syndrome (44 + XXY)
The external genitalia are male-like, but the testes are very small and generally no spermatozoa are produced. There may be gynaecomastia, sometimes a straight forehead hairline, and a horizontal upper margin to the pubic hair. Other congenital defects may be present. The histology of the testes are distinctive: hyalinisation of the semiferous tubules and massive clumps of interstitial Leydig cells. Urinary secretion of gonadothrophin is high. Occasionally associated with mental impairment. Treatment is not usually required. Incidence is about 3 per 1000 live male births.
b) gonadal intersex
i) True hermaphroditism
Ovarian and testicular tissues are present in the same person. The possibilities are:
testis + ovary
testis + ovotestis
ovotestis + ovotestis
The external genitalia may be male-like or female-like. There are no exclusive characteristic features to distinguish true hermaphrodites. The majority are reared as boys, but most develop breasts at puberty. The karyotype is usually 44 + XX, but XX/XY mosaics occur. The only treatment is hormone therapy and plastic surgery to enhance any phenotypic tendency.
ii) Gonadal dysgenesis
The subjects are tall, eunochoid, and female-like with normal female external genitalia. They have a uterus but no ovaries. There is an absence of breast development. The karyotype is usually 44 + XY. Treatment is oestrogen therapy to improve secondary sexual characteristics.
c) hormonal sex
The mechanisms involved include:
abnormal response of target organs to hormonal factors
abnormal endogenous secretion of hormones by the fetus
transplacental transmission of hormones from mother to fetus. (These hormones may either be administered to the mother during pregnancy, or they may originate from a pathological source in the mother.)
i) Masculinisation in the female
The subjects are genetically female. The degree and type of masculinisation depends on whether the causative factors exerted their effect in prenatal life, childhood, or in adult life. Masculinisation that begins before birth modifies development of the genital organs, and the degree of virilisation may be sufficient to give rise to mistaken identification of sex at birth. Prenatal masculinisation may be due to adrenal or ovarian pathology in the mother or the fetus. Congenital adrenal hyperplasia is due to an inborn error of metabolism - a defect in the synthesis of cortisol by the suprarenal cortex. Cortisol production is low and the pituitary secretes excessive amounts of ACTH, with the consequence that the suprarenal cortex becomes enlarged, but still unable to synthesise hydrocortisone, and hydroprogesterone is released into the blood stream. There is hypertrophy of the clitoris and persistence of the urogenital sinus. The virilisation progresses after birth causing precocious growth of pubic hair, deepening of the voice, no breast development, stunted growth, and infertility. Treatment is by cortisone. Masculinisation can also be caused before birth by exogenous hormones with androgen-like actions. For example, synthetic progestins are given to mothers to maintain pregnancies that are threatening to miscarry. Some of these have been found to produce masculinisation. Alternatively, androgens may be produced within the mother because of some pathological condition, and passed across to the fetus where they cause masculinisation. Clitoral enlargement is produced, and fusion of the labia majora may occur.
ii) Feminisation in males
Feminisation of a genetic male may occur during fetal life. The main type is testicular feminisation. It is due either to very early failure of the fetal testes to develop, or to defective response of tissues to fetal androgens. Patients with the testicular feminisation syndrome have female-type external genitalia and body form. At puberty they develop breasts but do not grow pubic or axillary hair and do not menstruate. The vagina is absent or poorly developed, there is no uterus, and the testes are undescended. Often, no treatment is required since the person is physically and psychologically female, even though the karyotype is 44 + XY.
There is another condition which may lead to mistaken identification of sex at birth: hypospadias. This is due to failure of output of androgens by the fetal testes at the time of closure of the urethral folds.
Here are some questions about the development of the reproductive system:
What is meant by the genetic sex of an individual?
Describe the migration made by the primordial germ cells from the yolk sac to the gonads.
Describe normal development of the uterus and vagina.
In the male embryo, how do the developing semiferous tubules become linked with the mesonephric duct?
Give an embryological explanation of the course taken by the ductus deferens in relation to the ureter just behind the bladder.
Are there any vestiges (remnants) of the duct system that regresses in each sex?
How is the prostate gland formed?
Compare the development of the ovary with that of the testis.
How is the broad ligament formed in the female embryo?
What is meant by undescended testis (cryptorchidism)? What might be the consequences of this condition?
Why do the gonads receive their blood supply from high on the posterior abdominal wall?
What might happen if the tunica vaginalis remains in communication with the peritoneal cavity?
What changes occur in females and males at puberty?
Describe development of the penis. What is hypospadias?
What is meant by atresia?