Development of the reproductive system

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.