Penis Didelphys

Figure 14.30 Sagittal sections showing formation of the uterus and vagina at various stages of development. A. Nine weeks. B. End of third month. C. Newborn.

The lumen of the vagina remains separated from that of the urogenital sinus by a thin tissue plate, the hymen (Figs. 14.29C and 14.30C), which consists of the epithelial lining of the sinus and a thin layer of vaginal cells. It usually develops a small opening during perinatal life.

The female may retain some remnants of the cranial and caudal excretory tubules in the mesovarium, where they form the epoophoron and paroophoron, respectively (Fig. 14.24B). The mesonephric duct disappears except for a small cranial portion found in the epoophoron and occasionally a small caudal portion that may be found in the wall of the uterus or vagina Later in life it may form Gartner's cyst (Fig. 14.24B).

CLINICAL CORRELATES Uterine and Vaginal Defects

Duplications of the uterus result from lack of fusion of the paramesonephric ducts in a local area or throughout their normal line of fusion. In its extreme form the uterus is entirely double (uterus didelphys) (Fig. 14.31 A); in the least severe form, it is only slightly indented in the middle (uterus arcuatus) (Fig. 14.31 B). One of the relatively common anomalies is the uterus bicornis, in which the uterus has two horns entering a common vagina (Fig. 14.31 C). This condition is normal in many mammals below the primates.

In patients with complete or partial atresia of one of the paramesonephric ducts, the rudimentary part lies as an appendage to the well-developed side. Since its lumen usually does not communicate with the vagina, complications are common (uterus bicornis unicollis with one rudimentary horn) (Fig. 14.31 D). If the atresia involves both sides, an atresia of the cervix may result (Fig. 14.31 £). If the sinovaginal bulbs fail to fuse or do not develop at all, a double vagina or atresia of the vagina, respectively, results (Fig. 14.31, A and F). In the latter case, a small vaginal pouch originating from the paramesonephric ducts usually surrounds the opening of the cervix.

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Figure 14.31 Main abnormalities of the uterus and vagina, caused by persistence of the uterine septum or obliteration of the lumen of the uterine canal.

EXTERNAL GENITALIA Indifferent Stage

In the third week of development, mesenchyme cells originating in the region of the primitive streak migrate around the cloacal membrane to form a pair of slightly elevated cloacal folds (Fig. 14.32A). Cranial to the cloacal membrane the folds unite to form the genital tubercle. Caudally the folds are subdivided into urethral folds anteriorly and anal folds posteriorly (Fig. 14.32B).

In the meantime, another pair of elevations, the genital swellings, becomes visible on each side of the urethral folds. These swellings later form the scrotal swellings in the male (Fig. 14.33 A) and the labia majora in the female (see Fig. 14.36B). At the end of the sixth week, however, it is impossible to distinguish between the two sexes (Fig. 14.34C).

Double Vagina

Figure 14.32 A and B. Indifferent stages of the external genitalia. A. Approximately 4 weeks. B. Approximately 6 weeks. C. Scanning electron micrograph of the external genitalia of a human embryo at approximately the seventh week. AF, anal fold; arrowhead, anal opening; GS, genital swelling; GT, genital tubercle; T, tail; UF, urethral fold.

Figure 14.32 A and B. Indifferent stages of the external genitalia. A. Approximately 4 weeks. B. Approximately 6 weeks. C. Scanning electron micrograph of the external genitalia of a human embryo at approximately the seventh week. AF, anal fold; arrowhead, anal opening; GS, genital swelling; GT, genital tubercle; T, tail; UF, urethral fold.

Development External Genital

Figure 14.33 A. Development of external genitalia in the male at 1 0 weeks. Note the deep urethral groove flanked by the urethral folds. B. Transverse sections through the phallus during formation of the penile urethra. The urogenital groove is bridged by the urethral folds. C. Development of the glandular portion of the penile urethra. D. Newborn.

Figure 14.33 A. Development of external genitalia in the male at 1 0 weeks. Note the deep urethral groove flanked by the urethral folds. B. Transverse sections through the phallus during formation of the penile urethra. The urogenital groove is bridged by the urethral folds. C. Development of the glandular portion of the penile urethra. D. Newborn.

Penile Didelphys

Figure 14.34 A. Genitalia of a male fetus at 14 weeks, showing fusion of the scrotal swellings (S). Arrow, epithelial tag. B and C. Dorsal and ventral views, respectively, of the genitalia of a female fetus at 11 weeks. The genital tubercle at this stage is longer than in the male (A), and the genital swellings (GS) remain unfused.

Figure 14.34 A. Genitalia of a male fetus at 14 weeks, showing fusion of the scrotal swellings (S). Arrow, epithelial tag. B and C. Dorsal and ventral views, respectively, of the genitalia of a female fetus at 11 weeks. The genital tubercle at this stage is longer than in the male (A), and the genital swellings (GS) remain unfused.

External Genitalia in the Male

Development of the external genitalia in the male is under the influence of an-drogens secreted by the fetal testes and is characterized by rapid elongation of the genital tubercle, which is now called the phallus (Figs. 14.33 A and 14.34A). During this elongation, the phallus pulls the urethral folds forward so that they form the lateral walls of the urethral groove. This groove extends along the caudal aspect of the elongated phallus but does not reach the most distal part, the glans. The epithelial lining of the groove, which originates in the endoderm, forms the urethral plate (Fig. 14.33B).

At the end of the third month the two urethral folds close over the urethral plate, forming the penile urethra (Figs. 14.33B and 14.34A). This canal does not extend to the tip of the phallus. This most distal portion of the urethra is formed during the fourth month, when ectodermal cells from the tip of the glans penetrate inward and form a short epithelial cord. This cord later obtains a lumen, thus forming the external urethral meatus (Fig. 14.33C).

The genital swellings, known in the male as the scrotal swellings, arise in the inguinal region. With further development they move caudally, and each swelling then makes up half of the scrotum. The two are separated by the scrotal septum (Figs. 14.33D and 14.34A).

CLINICAL CORRELATES Defects in the Male Genitalia

In hypospadias fusion of the urethral folds is incomplete, and abnormal openings of the urethra occur along the inferior aspect of the penis, usually near the glans, along the shaft, or near the base of the penis (Fig.14.35). In rare cases the urethral meatus extends along the scrotal raphe. When fusion of the urethral folds fails entirely, a wide sagittal slit is found along the entire length of the penis and the scrotum. The two scrotal swellings then closely resemble the labia majora. The incidence of hypospadias is 3-5/1000 births, and this rate represents a doubling over the past 15 to 20 years. Reasons for the increase are not known, but one hypothesis suggests it could be a result of a rise in environmental estrogens (endocrine disruptors, see Chapter 7).

Epispadias is a rare abnormality (1/30,000 births) in which the urethral meatus is found on the dorsum of the penis. Instead of developing at the cranial margin of the cloacal membrane, the genital tubercle seems to form in the region of the urorectal septum. Hence a portion of the cloacal membrane is found cranial to the genital tubercle, and when this membrane ruptures, the outlet of the urogenital sinus comes to lie on the cranial aspect of the penis (Fig. 14.35C). Although epispadias may occur as an isolated defect, it is most often associated with exstrophy of the bladder.

In exstrophy of the bladder, of which epispadias is a constant feature, the bladder mucosa is exposed to the outside (Figs. 14.16A and 14.35C). Normally the abdominal wall in front of the bladder is formed by primitive streak mesoderm, which migrates around the cloacal membrane. When this migration does not occur, rupture of the cloacal membrane extends cranially, creating exstrophy of the bladder.

Micropenis occurs when there is insufficient androgen stimulation for growth of the external genitalia. Micropenis is usually caused by primary hypogonadism or hypothalamic or pituitary dysfunction. By definition, the penis is 2.5 standard deviations below the mean in length as measured along

Abnormal urethral orifices

Abnormal urethral orifices

Penis Didelphys

Hypospadias

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  • Antje
    What is a double Vagina?
    7 years ago

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