Müllerian ducts

Epithelial-lined grooves develop from the cephalad end of the mesonephros late in the sixth week of gestation ( Fig. 15-1 ). The müllerian grooves that form in the nephrogenic blastema posterior to the wolffian (mesonephric) ducts become tubes as the margins close, forming the müllerian (paramesonephric) ducts .

Relation of the müllerian to the wolffian ducts

The unattached cranial portion of the müllerian duct is the primordium of the uterine tube. The ovary develops medial to the duct ( Fig. 15-2 A). The metanephros now is formed into a kidney, attached by the ureter to the urogenital sinus .

The kidney “ascends,” covered by the adrenal gland ( Fig. 15-2 B). The round ligament joins the müllerian duct to the inguinal canal.

The fused caudal part of the müllerian ducts forms the uterovaginal canal, later to become the uterus and vagina ( Fig. 15-2 C). The open end of the müllerian duct will form the infundibulum of the uterine tube and will descend with the ovaries enclosed in a peritoneal fold, the broad ligament.

The growing tip of the fused müllerian ducts lies medial to the wolffian duct. Both systems empty into the urogenital sinus on the müllerian tubercle (Müller’s hillock). The two ducts are intimately related; experimental arrest of the wolffian duct blocks further development of the müllerian duct.

Müllerian tubercle

The müllerian ducts fuse to form the utricular cord, which subsequently becomes canalized ( Fig. 15-3 A). The cord impinges on the dorsal wall of the urogenital sinus at the junction of the vesicourethral canal and the pelvic part of the urogenital sinus.

The lining of the urogenital sinus proliferates locally and joins with that of the utricular cord to form the sinoutricular cord, which is composed of a mixture of sinus endoderm and the epithelial linings of the wolffian and müllerian ducts ( Fig. 15-3 B). This combination forms the müllerian (sinus) tubercle (see Figure 14-3 ). The utricular cord will form the vagina, and the sinus will form the introitus.

The openings of the wolffian ducts become sealed, and the ducts subsequently regress.

As the urinary tract separates from the hindgut and the urogenital membrane becomes perforated to form the urogenital orifice, the urogenital sinus above the membrane shortens. This exposes the separate openings of the vagina and the urethra. The rectum and anus are displaced posteriorly to make room for the separate vaginal opening.

Near the urogenital sinus, between the bladder and the rectum, the wolffian ducts fuse to form the genital cord, a structure that divides the pelvis coronally, forming a fossa in the peritoneal cavity behind the bladder. In the female, this space persists as the uterovesical pouch; it becomes obliterated in the male.

Uterovaginal development

The uterus and cervix develop from the tubular portion of the fused müllerian ducts. The müllerian primordium itself will become the lining of the uterus, and the adjacent mesenchyme will form the endometrial stroma and myometrium. Should müllerian fusion be incomplete, uterus didelphys with septate vagina may result ( Figs. 15-4 and 15-5 ).

A number of uterine anomalies may result from failure of proper fusion of the müllerian ducts or from failure of some portion of the system to develop properly. This patient, being investigated for infertility, had a single vagina and a single cervix. Radiologic contrast material injected into the endocervical canal delineates a possible septal structure separating two uterine horns. It was unclear whether the findings represented a subseptate uterus (a partial partition of the endometrial cavity) or a uterus bicornis unicollis (a single cervix and lower uterine cavity with two horns).

(Image courtesy of James Liu, MD.)

Hysteroscopy in the patient shown in Figure 15-4 showed findings consistent with a subseptate uterus: a septum partially partitioned the endometrial cavity. The septum was resected endoscopically.

(Image courtesy of James Liu, MD.)

The cervix becomes twice the size of the uterine body by the time of birth, after which both structures regress by 66% and 33%, respectively.

The cranial unfused portions of the müllerian ducts develop as the wolffian ducts regress. The pelvis widens during maturation so that the ducts assume a more transverse position as they develop into uterine tubes. The müllerian ducts fuse, but the proximal portion remains patent as the uterine canal ( Fig. 15-6 A). The enlargement of the pelvis pulls the urogenital sinus away from the uterine portion, allowing space for development of the vagina .

The vagina passes through a solid stage by invasion of mesenchyme and by internal proliferation of epithelium that form a solid epithelial utricular cord.

Subsequently, spaces appear within the utricular cord by regression of the epithelial lining, so that by the 20th week, the vagina has a complete lumen ( Fig. 15-6 B).

Should recanalization be incomplete or the epithelium fail to invade the mesenchyme, vaginal atresia is the result. Atresia may also be secondary to failure of that portion of the müllerian duct below the cervix to reach the urogenital sinus, leaving only a shallow depression at the site of the normal vaginal orifice. Hematocolpos will appear with the onset of menstruation. The entire uterus may be absent if the distal ends of the müllerian ducts fail to form.

By the 20th week, the site of the cervix may be identified. The junction of the vagina with the urogenital sinus is marked by the hymen. The portion of the urogenital sinus below the müllerian tubercle regresses and becomes the vestibule, and the müllerian tubercle comes to lie at the level of the hymen. The hymen is usually perforated but may seal off the vagina. The urethra and vagina are separated by the urovaginal septum to the level of the perineum.

As the müllerian ducts fuse, the broad ligaments are formed coronally from peritoneum, leaving one pouch behind, the uterorectal pouch (Douglas), and one in front, the uterovesical pouch. Within the broad ligament, mesenchyme proliferates to form connective tissue and smooth muscle that becomes the parametrium.

Fate of the wolffian duct

At 9 weeks, mesonephric tubules persist in conjunction with the wolffian duct , which extends to the urogenital sinus beside the uterus and vagina ( Fig. 15-7 A).

Vestigial structures remain from the wolffian ducts and tubules near the müllerian ducts. The epigenital portion becomes the epoöphoron and the paragenital part, the paroöphoron ( Fig. 15-7 B). The vesicular ovarian appendix or hydatid forms from detached cranial portions of the former wolffian duct.

With regression of the wolffian ducts, remnants persist in the cervical and vaginal walls as Gartner’s ducts in one-fifth of women. Because the ureter is a derivative of the wolffian duct, delayed separation of the ureteric bud would result in an ectopic ureteral opening anywhere along the course of Gartner’s duct, including the introitus (see Fig. 13-10 ).

The homologue of the male gubernaculum is the segment of mesenchyme in the inguinal fold that comes to lie in the ovarian ligament above and in the round ligament below, the latter having secondary attachments to the uterus. The ovarian ligament forms the proximal part of the gubernaculum and the round ligament, as the termination of the gubernaculum, will extend it into the inguinal canal. The homologue of the male processus vaginalis is the vaginal sac (or canal of Nuck) that extends into the labium majus, only to be obliterated before birth.

Female gonadal development

The ovary arises from the genital ridge during the seventh week as epithelial cells and mesenchyme differentiate. Initially, celomic epithelium forms the medullary cords, as well as the ovarian rete. Epithelial cords (cortical cords) form irregularly and remain attached to the surface of the genital ridge, with those lying deepest forming the rete. The primordial germ cells proliferate in the cortical epithelial cords but remain otherwise unchanged during the development of the ovary, in contrast to the activity of the germ cells in the testis that differentiate from gonocyte to spermatogonia. At about 16 weeks, under the influence of the feminizing genes on the short arms of the two X chromosomes, the cortical cords separate into cell clusters, the primordial follicles, that consist of an oogonium surrounded by a single layer of follicular cells. The oogonia become primary oocytes that, when covered with follicular cells, form a primary follicle.

Perinatally, the follicles grow within a multilayered granulosa that becomes organized into a cellular theca interna and a fibrous theca externa. During the first 3 to 6 months of life, the follicles degenerate but return at puberty.

The germinal epithelium becomes a single layer distinct from the tunical albuginea that forms the fibrous capsule covering the ovarian tissue. Separation from the mesonephros occurs as the ovary develops a mesentery, the mesovarium. The ovary has descended into the pelvis by the end of the third month, and it subsequently rotates laterally.

The derivation of reproductive structures in the male and female is outlined in Table 15-1 .

Development of the female urethral musculature

Early in development (around 3 weeks), the components of the musculature of the bladder neck and trigonal system and of the ventral urethral wall are similar in both male and female fetuses.

The smooth urethral musculature that lies in midurethra surrounds all but the dorsal part, where it inserts into the extension of the superficial trigone. More caudally, the muscle inserts into a dorsal condensation similar to that developed in the dorsum of the membranous urethra of the male. Although at first some of the smooth muscle surrounds the vaginal primordium, as the vagina grows, it loses this dorsal extension, leaving a small amount of the smooth urethral musculature attached to the lateral urethral wall or to the urethrovaginal septum.

The striated urethral musculature is continuous from the base of the bladder to the perineum. It has two parts, one distributed about the urethra and the other about the urogenital sinus. Both are laid down early in development and show little change thereafter. The exception is the extension of the lower border of the urogenital sphincter that lies below the pelvic floor, which becomes attached laterally to the ischiopubic ramus.

Even though mesenchyme originally encircled the urethra, the muscle bundles formed in the cranial part incompletely surround the urethra, being open dorsally where they appear to insert. The bundles of the caudal part cover the vagina on both sides and insert into the uterovaginal septum.