HAPTER 42 EVELOPMENT OF THE REPRODUCTIVE ORGANS Bernadette S de Bakk - PDF document

HAPTER 4.2 -EVELOPMENT OF THE REPRODUCTIVE ORGANS Bernadette S de Bakker As part of the following chapter:DeRuiter,MC, Kleinrensink, G & de Bakker, BS (2018, in press). Anatomy of the female pelvis, pelvic organs & reproductive system.In: EAP Steegers e.a. (red). Textbook of obstetrics and gynaecology. A life course approach.Houten: Bohn Stafleu van Loghum.ÒDe bezigheid, die voor de vrouw te achten is,

ligt niet op studieterrein; zij behoeft lichamelijke, niet al te zware bezigheid.ÓDr. H. Treub, Leerboek der Gynaecologie, 1917 TLAS OF UMAN MBRYOLOGY 186 The indifferent stageAt embryonic day 19, the human embryo is a 0.6 mm wide threelayered germ disc. The three intraembryonic germ layers, the endoderm, intraembryonic mesoderm and ectoderm have been formed by the process of gastrulation in the socalled prim

itive streak. Already at 21 days, a subdivision in the mesodermal layer can be recognized (Fig. 1). From medial to lateral the following mesodermal components are formed; the axial mesoderm (or notochordal plate), the paraxial mesoderm from which the somites develop, the intermediate mesoderm that constitutes the urogenital system and the lateral plate mesoderm from which the body wall and circulatory system wi

ll be formed. We now focus on the intermediate mesoderm, because at embryonic day 26, when the embryo measures less than 3 mm, the first embryonal precursors of the urogenital system can be appreciated; the mesonephros and its mesonephric duct (Wolffian duct) (Fig. 2). The intermediate mesoderm constitutes the urogenital ridge, from which the embryonic kidney, the mesonephros, and the definitive kidney, the met

anephros develop. Also, the paramesonephric duct (MŸllerian duct) and the gonads are derivatives of the urogenital ridge. Because the urogenital ridge constitutes both the definitive kidneys and the internal genitalia, anatomical variants of the internal genitalia are often accompanied by the absence or dislocation of one or two kidneys, or presence of a horseshoe kidney.The most ventral of the three germ

layers, the endoderm, also contributes to the urogenital system. From de endodermal layer, the foregut, midgut and hindgut develop. It is the hindgut that contributes to the urogenital system since it forms the cloaca in very early staged embryos. The cloaca collects wastes from both the kidneys and the gut. At its ventral part, the cloaca passes into the allantois, a tubular structure that runs in the umbilica

l cord. The allantois serves as wastecollecting unit in animals that develop in eggs, because they are unable to wash out their waste products through the placenta. When the lumen obliterates in the fetal period, the allantois is called urachus. It can be recognized in adults as the median umbilical ligament (XanderÕs ligament) on the ventral body wall. Although the function of the allantois in humans has not

been fully elucidated yet, it does serve as the axis along which the umbilical arteries develop. Embryological remnants of the allantois or urachus sometimes form cysts in the trajectory of the median umbilical ligament, from the apex of the bladder to the umbilicus. When the lumen of the allantois failed to obliterate completely, urine can drain from the bladder to the umbilicus through a urachal fistuFrom day

31, the ureteric bud forms from the dorsolateral wall of the cloaca. It thereby initiates the surrounding mesoderm of the urogenital ridge to form the mesonephros. The ureteric bud will form the ureters, the calyces and the renal pelvis. Around day 42, when the embryo is approximately one cm in length, the cloacal membrane disappears, which means that the embryo is now able to dispose wastes from the cloaca in

the amniotic fluid. When the cloaca becomes separated from the primordial rectum and anal canal by the urorectal septum, the ventral part of the cloaca is called the urogenital sinus (Fig. 3). This separation completes around 50 days of development, when the crown rump length is approximately 1.5 cm. The urogenital sinus will form bladder and urethra in males. In females, the urethra still needs to become sepa

rated from the vagina in the fetal period, by means of the urogenital septum (Fig. 3). Besides the mesonephros, the mesonephric duct and the metanephros, the gonads also derive from the urogenital ridge. Since the urogenital ridges in an early stage of development bulged ventrally into the coelomic cavity, the gonads are lined with Development of the reproductive organs 187 4.2 5.1 coelomic epithelium. On th

e ventral surface of each of the two ridges the coelomic epithelium thickens. Together with gonadal mesenchyme from the urogenital ridge it constitutes the gonadal ridge from day 30. This gonadal ridge covers almost the entire ventral length of the urogenital ridge. It starts as a long flat ridge, but later in embryonic development it becomes relatively shorter and thicker. When the gonadal ridge is no longer a

ridge but merely an oval shaped organ, it is dubbed a gonad. Depending on the genetic makeup of the embryo, the gonads will become ovaries in females or testes in males. An ovotestis is a gonad which contains both testicular and ovarian tissue (Ortenberg et al. 2002). This condition was formerly known as true hermaphroditism and it can occur bilateral, when both gonads are affected, or unilateral, when the oth

er gonad is a normal ovary or testis. The testicular tissue in an ovotestis should be removed or carefully monitored due to its increased risk of gonadal cancer. Though this condition has not been thoroughly studied, mosaicism with a Y bearing cell line in the gonad is a plausible explanation for this rare variant (Andrade et al. 2017; Ortenberg et al. 2002)Fig. 1. Mesodermal differentiation in a human embryo o

f 22 days of development (5 pregnancy weeks). Left a dorsal view on a 3D reconstruction of the embryo. In green the developing neural tube which is already closed and covered with skin in the middle region. The dashed line indicates the level where the transverse histological section was positioned. Above right the histological section, and below it a schematic representation of the same section, where the meso

dermal differentiation is presented. The urogenital system develops from the intermediate mesoderm. The 3D reconstruction and histological section depict human embryo specimen No. 6330 of the Carnegie Collection of the Human Developmental Anatomy Center at the National Museum of Health and Medicine in Silver Spring, MD, USA, as published in the 3D Atlas of Human Embryology by de Bakker et al., 2016, with permis