Sexually Transmitted Diseases - I. I. Mavrov 2005

Embryonic development of the reproductive organs. Sexual dimorphism

According to their functional purpose (development and release of Germ Cells, Fertilization, Nutrition, and gestation within the maternal body), male and FEMALE REPRODUCTIVE Organs are traditionally divided into three main components: Gonads, reproductive ducts, and copulatory organs. Their primordia begin to develop during the Embryonic period, with further growth continuing throughout intrauterine life and extending into the postnatal period.

Through the Sex Hormones they secrete, the reproductive organs exert a systemic influence on the entire body.

Due to shared embryonic origins, male and female reproductive organs exhibit a high degree of Homology.

The appearance of reproductive organ primordia is preceded by The Development of the Urinary System, which plays a guiding role in the Differentiation of the Urogenital apparatus. During embryonic development, the pronephros appears first, only to disappear completely within a week; this is followed by the mesonephros (primitive Kidney), which also undergoes regression, though parts of its structures are incorporated into the Urogenital System; and finally, the permanent (definitive) Kidneys are formed. The mesonephros is directly involved with the reproductive primordia, reaching its peak development by the end of the 2nd month. The gonads begin to form in embryos during the 4th week of intrauterine development, when bilateral elevations—known as genital ridges—appear on the ventral surface of the mesonephros. Primordial germ cells (gonocytes) migrate into these ridges from the yolk sac and penetrate the coelomic epithelium covering the gonads. Concurrently with gonadogenesis, paramesonephric ducts develop, which are initially present in both sexes. By the 6th week of embryonic development, the gonads remain histologically undifferentiated. Two pairs of ducts are present: the mesonephric ducts, which later contribute to The formation of the Epididymis and the male duct system, and the paramesonephric ducts, which serve as the precursors to the uterine tubes and Uterus. Thus, the initial development of both gonads and their associated ducts proceeds in a bisexual fashion.

In male embryonic development, the endocrine function of the developing Testis is established with the secretion of two inhibitory hormones: one triggers the regression of the paramesonephric ducts, while the other—testosterone—drives the development and differentiation of the mesonephric ducts along the male pathway. The Formation of the testis and its duct system originates from two sources: the genital ridge and the mesonephros. By the end of the 5th week, sex cords begin to form; active proliferation of gonocytes leads them to grow into the mesenchyme as cords surrounded by epithelial cells. These sex cords elongate intensively and become highly convoluted. By the middle of the intrauterine period, a small lumen forms within each epithelial cord, transforming it into a primitive convoluted seminiferous tubule. The epithelial cells of these tubules differentiate into sustentocytes (Sertoli cells), while the gonocytes differentiate into spermatogonia. Blood Vessels, nerves, and differentiating endocrine glandulocytes (Leydig cells) occupy the Connective Tissue surrounding the seminiferous tubules. A connective tissue zone forms beneath the surface epithelium, eventually developing into the tunica albuginea of the testis. The tubules of the mesonephros differentiate into efferent ductules, which, together with the initial segment of the mesonephric duct, form the epididymis. In the 4th month of intrauterine development, the testis begins its migration from the posterior abdominal wall toward the lesser pelvis. Between the 6th and 8th months, the testis descends into the Scrotum, a process that completes shortly before birth. As it descends into the scrotum, the testis carries along layers of the anterior abdominal wall, which give rise to the testicular tunics: the subcutaneous tissue transforms into the tunica dartos; the fascia and aponeurosis of the external oblique abdominal Muscle become the external spermatic fascia and the cremasteric fascia; the muscle fibers of the internal oblique and transversus abdominis give rise to the cremaster muscle; the transversalis fascia forms the internal spermatic fascia; and the vaginal process of the Peritoneum develops into the tunica vaginalis testis. Occasionally, testicular descent finishes shortly after birth.

The scrotum develops from the genital tubercle at the end of the 2nd month of intrauterine life. The genital tubercle is formed by the fusion of two swellings located in the lower Regions of the inguinal canals and comprises all layers of the abdominal wall, including the peritoneum. Initially, the peritoneum forms a shallow depression (the vaginal process) at the site of the future deep inguinal ring. This vaginal process gradually elongates distally, terminating in a broadened pouch beneath the Skin in the region of the future scrotum.

The Prostate Gland begins to form in the 3rd month of intrauterine development. By this time, in male embryos, the paramesonephric ducts have regressed, with the exception of their distal ends, which fuse and remain within the developing prostate as the prostatic utricle. By the end of the 3rd month, cords of urethral epithelium sprout into the connective tissue surrounding the Urethra, subsequently developing lumens. By the end of the 4th month, approximately 50 such outgrowths are formed, corresponding to the number of prostatic glands found in adults. By the 7th month, the lobular Structure OF THE prostate becomes apparent due to the development of fibromuscular septa.

The Penis develops from the indifferent genital tubercle covered by ectodermal epithelium. Genital folds flank the urethral plate on the underside of the tubercle. Following the closure of the urogenital sinus (at the 7th week of embryonic development), the urogenital opening forms at the posterior end of the urethral plate. Rapid proliferation of mesenchyme from the genital tubercle first gives rise to the corpora cavernosa, and by the 10th week of intrauterine development, the glans penis is formed. By this time, the deepening urethral plate transforms into the urethral groove. The genital folds grow toward each other and fuse by the 12th week to form the spongy urethra. The corpus spongiosum of the penis develops from the mesenchyme of the genital folds surrounding the urethra. Meanwhile, the external urethral orifice gradually shifts toward the glans penis. A skin fold, the prepuce (foreskin), expands around the margins of the glans. Initially, the inner layer of the prepuce is fused with the epithelium of the glans, but by the end of intrauterine development, a cleft forms between the glans skin and the inner preputial layer. By birth, the penis is fully formed.

During female development, the mesonephros and its duct regress, whereas the paramesonephric ducts undergo intensive growth; their paired sections give rise to the uterine tubes, while their unfused sections form the uterus and the upper part of the Vagina.

The Ovary originates in the embryo as a genital ridge (around the 31st–32nd day of development) on the ventral aspect of the mesonephros. Its composition includes: coelomic epithelium, which later gives rise to the surface epithelium and follicular Cells of the ovary; mesenchyme, which forms the connective tissue, interstitial, and smooth muscle elements of the ovary; and primordial germ cells (oogonia), the precursors to future ova. During the 3rd–4th weeks of development, oogonia proliferate actively and then migrate to the region of the genital ridges. In a 6-week embryo, the ovary acquires morphological signs of sexual differentiation: oogonia are distributed in nests among mesenchymal and epithelial cells throughout the thickness of the developing cortical layer, multiplying actively (their number reaching several millions). Oogonia (oocytes) entering the prophase of meiotic division I become surrounded by follicular cells, thereby forming primordial follicles. The entry of oogonia into prophase occurs asynchronously, resulting in specific developmental ratios where actively multiplying oogonia and oocytes coexist at various stages of embryonic ovarian development. Interstitial hormone-producing cells appear by the 28th week of fetal development.

The connective tissue framework of the ovary develops from the mesenchyme. By the 7th week of embryonic development, the ovary separates from the mesonephros, and the hilum of the ovary begins to form, allowing blood vessels, Lymphatic vessels, and later nerve fibers to penetrate the organ. The cranial portion of the mesonephros and its associated tubules form the rete ovarii. By the 26th–27th weeks, connective tissue cords fully invade the cortical layer, and the tunica albuginea of the ovary begins to form.

In newborns, the Ovaries are elongated and flattened with a smooth surface, weighing between 0.2 and 0.4 g. The number of germ cells ranges from 100,000 to 400,000, housed within primordial follicles.

The uterus develops from the paramesonephric (Müllerian) ducts. Duct formation begins in the embryo during the 2nd month of development. These duct primordia appear via the invagination of coelomic epithelium into the mesenchyme of the cranial region of the mesonephros at the level of the 1st thoracic vertebra. Growing as solid epithelial cords alongside the mesonephric ducts, they extend in a caudal direction toward the pelvis. In 6-week embryos, the coelomic epithelium of the paramesonephric ducts becomes elongated and pseudostratified. As growth continues, the paramesonephric ducts from opposite sides approach and come into tight contact without fusing immediately. By the 8th week, the intervening septa break down, and the ducts fuse. Fusion occurs first in the middle of the genital cord (the region of the future uterine cervix) and subsequently at its extremities, corresponding to the future Cytology/practical/108.html">Fundus of the uterus and the vagina. Definitive fusion of the paramesonephric ducts is observed by the 9th week; their caudal ends reach the urogenital sinus, push against its dorsal wall, and rupture into it between the 12th and 16th weeks. The cranial portions of the paramesonephric ducts remain separate and form the future uterine tubes. Prior to fusion, a lumen develops within the paramesonephric canals, converting them into tubular structures (the uterovaginal canal). By the 14th week, the upper (future uterine body), middle (cervix), and lower (vagina) segments can be distinguished within the canal. Between the 20th and 24th weeks of fetal life, final partitioning into the uterus, uterine tubes, and vagina takes place. The boundary between the uterine body and cervix is marked by a flexure in the canal; the cervix appears to "invaginate" into the upper segment of the vagina, forming the vaginal portion of the cervix.

During the embryonic period, the uterine cavity is lined with low columnar epithelium, which becomes tall as the fetus develops (especially within the cervix). By the 18th week of embryonic development, uterine glands differentiate for the first time as tubular structures; by the 28th week, initial signs of secretory activity appear in the Glandular Epithelium of the endometrium and endocervix; muscular elements of the uterus develop from the surrounding mesenchyme between the 12th and 16th weeks; the myometrium is formed by the 28th week; and the Development of the muscular layers is completed by the 32nd week.

By the time a female infant is born, her uterus is approximately 3–4 cm long, with the body being twice as short as the cervix.

The uterine tubes form from the upper third of the paramesonephric ducts. During the 11th–12th weeks of intrauterine development, muscular and connective tissue layers appear around these canals. All Structural elements of the uterine tube wall are distinguishable by the 18th–22nd weeks of intrauterine development. By this stage, longitudinal folds of the mucosa are well-defined. By the 28th week, these folds enlarge, and arborizing structures become noticeable on the mucosa of the newborn's uterine tube, lined with a single layer of columnar epithelium. The first cilia appear on the epithelial cells of the fimbriae at the 16th week of intrauterine development; circular and longitudinal muscle layers form by the 26th–27th weeks; and maximal development of the mucosal epithelial layer is reached by the 30th–31st weeks. Blood vessels develop within the outer connective tissue layer.

Initially, the vagina appears as a solid epithelial vaginal plate that increases in length and matches the length of the uterus by the end of the 3rd month of intrauterine development. The larger upper portion of the vagina is formed by the uterovaginal canal resulting from the fusion of the paramesonephric ducts, while the smaller lower portion and the vestibule develop from the urogenital sinus. Lumens gradually appear at the upper and lower ends of the vaginal plate (spanning almost its entire length), transforming it into a hollow organ. In most cases, the vaginal lumen is not fully developed at birth and may remain filled with epithelium in certain areas throughout the first year of life. Longitudinal and transverse growth of the vagina continues until the middle of the embryonic period. However, the region bordering the wall of the urogenital sinus remains unchanged. As a result, a circular fold develops here; once the vaginal lumen opens, this fold forms an annular disc situated at the future boundary between the vagina and the vestibule (this is the primordium of the hymen). Separation of the vagina from the vaginal portion of the cervix occurs through rapid proliferation of the vaginal plate epithelium, which invades the surrounding mesenchyme to form an epithelial ridge at the site of the future boundary. The vaginal portion of the cervix forms in the center of this ridge, while the epithelial ridge itself transforms into the vaginal fornices.

Beginning in the 4th month of intrauterine development, external genitalia differentiation takes place: the genital tubercle develops into the Clitoris, the genital folds form the Labia minora, and the genital swellings give rise to the Labia Majora.

Thus, sexual differentiation is already evident at Early stages of embryonic development when the respective organs are established. Morphophysiological sex differences between males and females are referred to as Sexual Dimorphism. Sexual differentiation encompasses not only the Reproductive System but the entire Organism. For instance, a clear link exists between sex and hepatic function. Numerous metabolic processes in the Liver (Functional Characteristics of its cells) are responsive to sex hormones (androgens and estrogens) and exhibit pronounced sex-related differentiation—some traits predominate in males, others in females. Sexual dimorphism of the Brain has also been established, underlying two distinct types of somatic behavior. Both the neuroendocrine Functions of the brain that coordinate reproductive processes (hypothalamic regulation of gonadotropin secretion) and functions unrelated to reproduction (appetite, thirst, body growth) are sexually differentiated. Consequently, the brain and the liver act simultaneously as both targets and "organizers" of systemic sexual differentiation.