Handbook of Clinical Gender Medicine. Группа авторов
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Название: Handbook of Clinical Gender Medicine

Автор: Группа авторов

Издательство: Ingram

Жанр: Зарубежная психология

Серия:

isbn: 9783805599306

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СКАЧАТЬ step is development of the phenotypic sex, comprising internal and external genitalia. It includes gonadal development. Testis formation starts with Sertoli cell proliferation and differentiation under the influence of SRY, which must reach a threshold level in a defined temporal window; otherwise, the ovary-determining pathway begins. Sertoli cells surround primitive germ cells and induce mitotic arrest. The formation of testis cords from the medullary region of the primitive sex cord is the first visible sign of male differentiation. From the 6th to the 12th week, the organotypic patterns of the testis are established. Proliferation of Sertoli cells leads to the characteristic enlargement of the early testis. The number of Sertoli cells appears to be closely related to the number of germ cells, testicular size, and sperm count in later life. During the evaluation of a newborn with ambiguous genitalia, small gonads can indicate gonadal dysgenesis. Later, this finding may explain a delayed onset of puberty or male subfertility.

      Vascularization of the testis starts with formation of a large artery under the coelomic surface. The coelomic vessel is formed following SRY expression, with a role for androgen transport suggested. Later, anastomoses to the arteria ductus deferentis and arteria cremasterica are formed, and the blood-testis barrier is created by the Sertoli cells. The tunica albuginea comprises mesenchymal tissue forming a thick fibrous capsule. It is a specialized structure in both sexes. In the male, it forms the testicular septations and the mediastinum testis. It is involved in the compartmentalization of the testis, and laminin deposition can be demonstrated at early stages. The tunica contains contractile elements. It promotes the transport of spermatozoa in adult life, maintains the interstitial pressure, and controls testicular blood flow.

      The tunica albuginea of the ovary appears to have a regulatory role in the maintenance of the germ cells. The tunica of the testis is thicker than that of the ovary. In newborn boys, the tunica is white or bluish, in newborn girls it is yellow or brown, and follicular cysts can be seen.

      Local androgen action is required for the differentiation of the epididymis and the vas.

      The gubernaculum and the suspensory ligaments are relevant for the position of the gonad. Incomplete regression of the caudal suspensory ligament may contribute to the abdominal position of the gonads in some patients with XY disorders of sex development (DSD). In newborns with ambiguous genitalia, ultrasound of the inguinal region and the lower abdomen is the first step. If no gonad is found, laparoscopy should be considered.

      Reproductive Ducts

      Mesonephric and Paramesonephric Ducts

      Development of the External Genitalia

      The initial signs of masculinization are an increased distance between the anus and genital structures. Circulating androgens, and conversion to dihydrotestosterone (DHT), induce genital tubercle (GT) growth. Tubularization of the urethral plate leads to formation of the urethra and is important for the development of corpus spongiosum and other penile structures. The fossa navicularis forms independently (ectodermal ingrowth is debated). Differentiation of the external genitalia starts at 8 weeks, the coronal sulcus separates the glans and penile shaft by 12 weeks, and the glandular urethra forms by 16 weeks. The prepuce is formed following ventral closure of the glans, and separation of skin folds from the glans is not complete until birth. Involved genes, proteins, and pathways are SHH, FGFs, BMPs, and androgen/androgen receptor.

      The scrotum is formed by fusion of the labioscrotal folds under androgen action, leaving a visible raphe. Normal scrotal insertion of the gubernaculum testis is essential for the inguino-scrotal phase of the descensus. The posterior aspect of the testis is not normally covered by the processus vaginalis and is fixed to the scrotal skin. Between 63 and 77 days, feminization or differentiation from the masculinized form begins. The phallus does not lengthen but bends forward or caudally. Before 20 weeks, there is a slow phase of growth of the genital swellings that cover superior and lateral aspects of the clitoris. The anogenital distance does not change, but the phallic portion of the urogenital sinus remains open, and genital folds do not fuse. At 14-20 weeks, the vagina opens into the pelvic portion of the urogenital sinus and it becomes the vaginal vestibule. After fetal ovarian follicular growth (20-22 weeks), there is rapid ventral outgrowth of the perineum, the urethral and vaginal openings separate, and the urethra is brought to the surface. The clitoris becomes incorporated into the fused anterior ends of the genital folds (labia minora), which continue to grow posteriorly. Genital swellings lateral to the labia minora become the labia majora, anteriorly continuous as the mons pubis. The growth of the labia minora is greater than that of the labia majora; they are seen protruding out of the labia majora at 23-25 weeks of gestation. After 26 weeks, the labia majora have grown sufficiently to cover the labia minora [1, 2]. At midtrimester there is no difference in the distribution of androgen receptors between male and female fetuses in the external genitalia; estrogen receptors are present only in the genitalia of the female fetus. It is not known when estrogen receptors appear or what induces their appearance. Their lack may be what protects male fetuses from the effects of maternal estrogen.

      Development СКАЧАТЬ