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 paraaxial 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 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.
mDuring 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.
Antimullerian 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.
