Development of the reproductive system

Definitions & Key takeaways

The early stage of both male and female reproductive system development are indifferent because it's impossible to distinguish between the male and female gonads at that time. It is until week seven that the primitive structures start to differentiate into female or male sex organs based on the present sex chromosomes. After the ovaries or testicles develop, further differentiation of the reproductive tract occurs, giving rise to corresponding male or female sex organs. Ovaries and the testis are initially developed in the abdomen and descend later during pregnancy to reach their final location in the pelvic cavity and the scrotum.

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Reproductive system development is the series of events that an embryo goes through to sexual differentiate into a male or female with regard to the gonads, genital ducts, and external genitalia.
The process starts at conception - when the gametes, the sperm and oocyte, fuse into a single cell that has either XX sex chromosomes in a female or XY sex chromosomes in a male - establishing the genetic sex of the embryo.
Through the first 5 weeks of development however, sexual development is basically identical for both sexes. At that point, the embryo is made up of three primitive germ layers: the ectoderm, the mesoderm and the endoderm.
The mesoderm also has three parts: the paraxial mesoderm, flanking the embryo’s future vertebral column, the intermediate mesoderm which is just lateral to it, and the lateral plate mesoderm which is the most lateral of all.
The intermediate mesoderm on both sides of the embryo condenses into two cylindrical structures called the urogenital ridges.
Each urogenital ridge runs parallel to the embryo’s future vertebral column, and organizes into a cylinder of mesoderm called the nephrogenic cord.
Most of the nephrogenic cord goes on to form urinary structures, but a strip of it in the middle gives rise to the gonads in males and females.
This portion that gives rise to the gonads is called the genital or sometimes gonadal ridge. The genital ridge has a mesoderm core and is covered with epithelium.
Gonad development, interestingly enough, starts in a tissue outside the embryo called the yolk sac - which is lined with endoderm cells, and connects to the embryo through the vitelline duct.
Early in development, some endoderm cells from the wall of the yolk sac differentiate into primordial germ cells, and they begin to migrate - physically move - along the vitelline duct, to the primitive digestive tract, and finally to the dorsal mesentery - a sheet of tissue that anchors the digestive tract to the posterior body wall.
From there, the primordial germ cells march along the dorsal mesentery until they reach the genital ridge - arriving there around week 6.
The primordial germ settle in the epithelium. and they send out chemical signals to the cells in the genital ridge, which makes them self-organize into an undifferentiated gonad - which can still develop into either testes or ovaries.
The epithelial layer of the gonad then forms primitive sex cords, which are epithelial projections that penetrate inwards, into the mesoderm layer of the gonad.
Around week 7, sex chromosomes start expressing genes that determine gonadal differentiation. In a male, genes in the sex-determining region of the Y chromosome - or SRY, for short - code for a protein called testis-determining factor, which initiates the development of testis.
The primitive sex cords mature into medullary cords, that grow longer and carry the primitive germ cells deeper into the mesoderm.
The surface epithelial layer of each gonad thins out to become the tunica albuginea. Later on the medullary cords develop into three ductal structures inside the testes: the seminiferous tubules, straight tubules, and the the rete testis,.
Of the three, the primordial germ cells settle down in the seminiferous tubules, and that’s where they mature into spermatogonia and lay dormant for a number of years.
During puberty, the spermatogonia awaken and start dividing over and over again to give rise to sperm, the male gametes.
During week 8, some cells in the wall of the seminiferous tubules differentiate into Sertoli cells. Sertoli cells surround the primordial germ cells, and secrete anti-mullerian hormone.
In addition, cells between the seminiferous tubules differentiate into Leydig cells, and they secrete testosterone. Anti-mullerian hormone and testosterone masculinize the rest of the male reproductive tract.
Now, in a female, since there’s no Y chromosome to secrete testis-determining factor, the undifferentiated gonads develop into ovaries.
In this case, the primitive sex cords also extend towards the center of the gonad, but they degenerate soon after. The surface epithelium proliferates once more, and forms a second set of projections called cortical cords.
The cortical cords rearrange to form nests of follicular cells that surround the primordial germ cells. A primordial germ cell and its ring of follicular cells make up a primordial ovarian follicle, and inside it, the primordial germ cell differentiates into an immature oocyte during fetal life.
The immature oocytes are halted in the first prophase of meiosis 1 until puberty, at which point they turn into secondary oocytes, the female gametes.
The rest of the reproductive tract acquires female characteristics in the absence of testosterone. The differentiation of the gonads leads to the phenotypic differentiation of the genital ducts and the external genitalia.
The genital ducts are initially undifferentiated, tubular structures that run down the embryo’s back inside the two nephrogenic cords on either side of the embryo.
The first is the mesonephric, or Wolffian duct, which gives rise to the male reproductive duct system. The second is the paramesonephric, or Mullerian duct, which is lateral to the mesonephric duct and gives rise to the female reproductive duct system.
These ducts start in the thoracic and upper lumbar region, and continue down the embryo’s back, until they open into a part of the cloaca called the urogenital sinus, which gives rise to urinary tract structures and the external genitalia of both sexes.
In males, mullerian inhibiting factor makes the paramesonephric - or mullerian - ducts degenerate into two vestigial bodies called the appendix testis.
The mesonephric ducts, on the other hand, grow longer under the influence of testosterone, and form the reproductive ducts outside the testis The efferent ductules look a bit like the teeth of a comb, and they connect the intratesticular straight tubules to the epididymis.
The epididymis is where sperm go to mature after they’ve been produced in the testes. After the epididymis, there’s the vas deferens, through which sperm will be released during ejaculation.
An accessory gland called the seminal gland develops from each vas deferens, and it secretes a nourishing fluid for the travelling sperm.
At the end of each vas deferens, below the opening of the seminal glands, there’s an ejaculatory duct, which opens into the urogenital sinus which forms the male urethra.
In females, on the other hand, the ovaries don’t make testosterone, so the wolffian ducts degenerates; and the ovaries don’t make mullerian inhibiting factor either - so the paramesonephric ducts persist.
In their upper regions, the two paramesonephric ducts form the fallopian tubes. In their lower regions, the ducts fuse and form the uterovaginal primordium, which is a tubular structure from which the uterus, the cervix, and the upper third of the vagina develop.
The lower two thirds of the vagina develops from the urogenital sinus when the external sex organs differentiate. The external sex organs also go through an undifferentiated stage, during which the lowermost portion of the urogenital sinus opens up in the genital region.
Flanking the opening on either side, there are two sets of symmetrical folds: the urethral folds, and the labioscrotal swellings.
Above the opening, there’s a small bit of tissue called the genital tubercle, which first turns into a primordial phallus for both sexes.
The differentiation of the external genitalia into male external sex organs starts when testosterone levels begin to rise.
An enzyme called 5-alpha-reductase, which can be found in many tissues in our body, including the skin of the external genitals, converts testosterone into a dihydrotestosterone, it’s more potent form.
Starting with week 9, dihydrotestosterone masculinizes the external sex organs of male embryos. The urethral folds fuse on the midline and form the penile urethra.
The primordial phallus grows longer, and its tip forms the glans penis, while its body gives rise to the body of the penis, which encloses the penile urethra.
The labioscrotal swellings fuse as well, giving rise to the scrotum. For genetically female embryos, in the absence of testosterone, the urethral folds remain unfused and form the labia minora.
The labioscrotal swellings form the labia majora, and only fuse in their anterior portion to give rise to the mons pubis.
The primordial phallus shrinks and turns into the clitoris. All in all, phenotypical differentiation is complete around week 12 - so this is about as early as you could tell the sex of an unborn baby.
Finally, there’s one more step in the development of the reproductive system. The gonads have to descend to their adult positions, in both males and females!
Starting with males, the testes initially develop near the anterior abdominal wall, but they are anchored by the gubernaculum, a fibrous cord that attaches to the bottom of the scrotum by a fibrous cord called the gubernaculum.
The gubernaculum starts to shorten, pulling the testis down towards the scrotum through the inguinal canal. The inguinal canal forms when a projection of peritoneum called the processus vaginalis herniates through the abdominal body wall.
By week 12, the testes reach the inguinal canal, and move towards the scrotum. On their way, they pull the vas deferens and the testicular artery and vein with them through the inguinal canal.
These structures eventually form the spermatic cord. The journey is usually over by the last month of pregnancy, and the abdominal muscle layers that came along form the layers of the scrotum.
Now, in female embryos, there’s also a gubernaculum that attaches the inferior part of the ovary to the future labia majora, and it shortens to pull the ovaries down the same way it would if they were testis.
However, when the uterus forms, the middle of the gubernaculum attaches to it and separates in a superior portion known as the cranial genital ligament, which becomes the ovarian ligament, anchoring the ovaries slightly above and on either side of the uterus.
The inferior portion, called the caudal genital ligament becomes the round ligament, connecting the uterus to the labia majora.
All right, as a quick recap: The reproductive system goes through an undifferentiated phase of development, which is the same for both sexes, but in week 7, the primitive structures differentiate into male or female sex organs, according to the sex chromosomes that the baby has had since conception.
First, the gonads develop into ovaries or testicles, and the differentiation of the gonad acts on the rest of the reproductive tract, which gives rise to correspondent male or female sex organs.
Both the ovaries and the testis initially develop in the abdomen, but descend during pregnancy to reach the pelvic cavity or the scrotum.