Development of the genitourinary organs

Development of the urinary bladder. The formation of the urinary bladder in a 7-week embryo is associated with the transformation of the cloaca, allantois (urinary sac) and the caudal sections of the ducts of the primary kidney. The cloaca is divided by a frontal septum into an anterior section - the urogenital sinus, part of which goes to form the wall of the urinary bladder, and a posterior section - the future rectum. The allantois, the duct of the primary kidney and the paramesonephric duct are connected to the urogenital sinus. From the lower part of the allantois and the mouths of the ducts of the primary kidney in the 2nd month of embryonic development, the bottom and triangle of the urinary bladder are formed. From the middle part of the allantois, the body of the urinary bladder is formed, and from its upper part - the urinary tract, which then turns into a fibrous cord - the median umbilical ligament.

[ 1 ], [ 2 ]

Development of the genitals

In the human embryo, indifferent internal and external genital organs are initially formed, and then the internal and external male or female genital organs are formed in their final form.

The rudiments of the indifferent sex glands in the human embryo appear in the wall of the body cavity in the 4th week of embryonic development from the rudiments of the epithelium located anterior and medial to the rudiments of the right and left primary kidneys, extending from the 4th cervical to the 5th lumbar segments of the body. In the 5th week, a groove is formed from the cells lining the body cavity. Then the groove deepens, its edges converge and it turns into the paramesonephric duct, opening into the urogenital sinus. On the ventromedial surface of the primary kidney, the future sex gland begins to form. At this point, on each side of the root of the mesentery, a ridge-shaped elevation is formed - the urogenital fold. Subsequently, each of these folds is divided by a longitudinal groove into a medial part - the genital fold, where the gonad is then formed, and a lateral part, which is the primary kidney, as well as the duct of the primary kidney and the paramesonephric duct.

In the 7th week, the developing sex glands (gonads) begin to differentiate into either testes or ovaries. When the testes are formed, the ducts of the primary kidneys turn into the excretory ducts of the male sex glands, and the paramesonephric ducts are almost completely reduced. If the ovaries are formed, then the fallopian tubes, uterus, and part of the vagina develop from the paramesonephric ducts, and the ducts of the primary kidneys turn into rudimentary formations. The external genitalia are laid in the embryo in the 7th week of embryonic development in an indifferent form: in the form of a tubercle, genital folds, and ridges. From these rudiments, the external male or female genitalia then develop.

[ 3 ], [ 4 ], [ 5 ], [ 6 ], [ 7 ]

Development of internal male genital organs

In the 7th month of intrauterine development, the protein coat is formed from the connective tissue surrounding the developing male gonad. By this time, the gonad becomes more rounded, and strands are formed in it, differentiating into seminiferous tubules.

During the development of the male reproductive gland, the efferent ducts of the testicle are formed from the tubules of the primary kidney, and the duct of the epididymis is formed from the cranial part of the duct of the primary kidney. Several cranially located tubules of the primary kidney are transformed into the appendix of the epididymis, and the caudally located tubules are transformed into the appendix of the appendix of the testicle. The vas deferens is formed from the remaining part of the duct of the primary kidney (caudal to the epididymis), around which a muscular membrane is formed. The distal part of the duct deferens expands and turns into the ampulla of the duct deferens, and the seminal vesicle develops from the lateral protrusion of the duct. The ejaculatory duct is formed from the terminal narrowed part of the duct of the primary kidney, which opens into the male urethra - the male urethra.

The cranial end of the paramesonephric duct is transformed into a testicular appendage, and the prostatic utricle arises from the fused caudal ends of these ducts. The rest of these ducts are reduced in male embryos.

The testicle with its appendage and rudimentary formations do not remain in the place where they were laid, but in the process of development they shift in the caudal direction - the process of descent of the testicles (descensus testis) occurs. In this process, the main role is played by the guiding ligament of the testicle. By the 3rd month of the intrauterine period, the testicle is in the iliac fossa, by the 6th month it approaches the internal ring of the inguinal canal. In the 7-8th month, the testicle passes through the inguinal canal together with the vas deferens, vessels and nerves, which are part of the spermatic cord formed during the descent of the testicle.

The prostate gland develops from the epithelium of the developing urethra in the form of cellular cords (up to 50), from which the lobules of the gland are subsequently formed. The bulbourethral glands develop from the epithelial outgrowths of the spongy part of the urethra. The ducts of the prostate gland and bulbourethral glands open with their mouths in those places where these glands were laid down during intrauterine development.

[ 8 ], [ 9 ]

Development of internal female reproductive organs

In the ovary of a female embryo, the zone of connective tissue under the layer of rudimentary epithelium is less pronounced than in the male gonad. Cellular strands are less noticeable, and sex cells are scattered in the mesenchymal stroma of the organ. Some of these cells grow more actively, they become larger, are surrounded by smaller cells, and the initial - primordial - ovarian follicles are formed. Later, the cortex and medulla of the ovary are formed. Blood vessels and nerves grow into the latter. As they develop, the ovaries also descend, but to a much shorter distance than the testicles. From the place of their origin, the ovaries shift together with the fallopian tubes to the pelvic area. The descent of the ovaries is accompanied by a change in the topography of the fallopian tubes, which change from a vertical to a horizontal position.

During the development of the ovary, the remaining tubules and duct of the primary kidney become rudimentary - appendages of the female reproductive gland. The cranially located tubules and the adjacent part of the duct turn into the ovarian appendage (epidermis), and the caudal ones - into the parovary. The remains of the duct of the primary kidney can be preserved in the form of a continuous or discontinuous cord lying on the side of the uterus and vagina - this is the longitudinal duct of the ovarian appendage (Gartner's duct; ductus epoophori longitudinalis).

The paramesonephric ducts give rise to the fallopian tubes, and the fused distal parts form the uterus and proximal vagina. The urogenital sinus forms the distal vagina and its vestibule.

Development of the external genitalia

In the 3rd month of intrauterine development, the genital tubercle arises from the mesenchyme in front of the cloacal membrane. At the base of the genital tubercle in the direction of the anus is the urogenital (urethral) groove, which is limited on both sides by genital folds. On both sides of the genital tubercle and genital folds, crescent-shaped elevations of the skin and subcutaneous tissue are formed - the genital ridges. These formations represent an indifferent rudiment of the external genitalia, from which the external male or female genitalia subsequently develop.

[ 10 ]

Development of the external male genitalia

In male embryos, the indifferent rudiments undergo complex changes. The genital tubercle begins to grow rapidly and lengthen, turning into the cavernous bodies of the penis. On their lower (caudal) surface, the genital folds become higher. They limit the urogenital (urethral) slit, which turns into a groove. Then, as a result of the fusion of the edges of the groove, the male urethra and the spongy body of the penis are formed. In the process of growth, the urogenital opening from its original position at the root of the penis moves, as it were, to its distal end.

The closure (fusion) of the urethral groove remains as a scar called the penile suture. Simultaneously with the formation of the male urethra, the foreskin is formed over the distal end of the penis. This is due to the development of a skin fold around the head of the penis.

The genital ridges become more convex, especially in the caudal sections, they converge and fuse along the midline. At the site of fusion of the genital ridges, a suture of the scrotum appears, which extends from the root of the penis to the anus through the entire perineum.

[ 11 ]

Development of the external female genitalia

In female embryos, the genital tubercle transforms into the clitoris. The genital folds grow and turn into the labia minora, which border the urogenital slit on the sides, which opens into the urogenital sinus. The distal part of the genital slit becomes wider and turns into the vestibule of the vagina, into which the female urethra and vagina open. By the end of intrauterine development, the vaginal opening becomes significantly wider than the opening of the urethra. The genital ridges transform into the labia majora, in which a significant amount of fatty tissue accumulates, then they cover the labia minora.

[ 12 ]