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Genitourinary Tract Early Embryology of the Genitourinary Tract

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Genitourinary Tract Early Embryology of the Genitourinary Tract SECTION IV Genitourinary Tract Early Embryology of the Genitourinary Tract Dale S. Huff The embryology of the genital and urinary tracts are inextricably interrelated and will therefore be described together. The development of the genitourinary tract can be divided into three phases. First, the primitive genitourinary tract develops in the early part of the fifth postfertilization week. Second, in the middle of the fifth week even before the first phase is completed, the definitive urinary tract and the bipotential genital tract begin to be superimposed upon the primitive genitourinary tract. Before the second phase is completed, the third phase, sexual differentiation of the bipotential genitourinary tract, begins when the bipotential genital tract begins to differentiate into either a male or female definitive genital tract beginning in the sixth week, a process that is not com- pleted until the 12th week. The postfertilization gestational ages are assigned following the revised data of O’Rahilly and Muller [1, Appendix 2]. Primitive Genitourinary Tract General The primitive genitourinary tract is composed of the paired mesonephric bodies, the paired mesonephric ducts, and the midline cloaca. The cloaca first appears in the mid- dle of the fourth week when the caudal curl of the embryo and the caudal intestinal portal define the hindgut from which the cloaca develops. The urorectal septum will divide the cloaca into a ventral half, which will become the urogenital sinus, and a dorsal half, which will become the rectum. The urogenital sinus will become the bladder and urethra, and, in the female, the introitus. The mesonephric bodies are composed of mul- tiple mesonephric nephrons that are arranged in single file from rostral to caudal. Each mesonephric nephron connects to the mesonephric duct, also called the primary excre- tory duct, which connects to the ventral or urogenital part of the cloaca. The meso- nephric nephrons produce urine, which drains through the collecting ducts into the mesonephric duct and into the cloaca. A mesonephric body is also called the meso- nephros. The mesonephros is the functional part of the primitive genitourinary tract and its development is described next. 95 Mesonephros and Mesonephric Duct The mesonephros develops from the intermediate the embryo to the mid lumbar region at approximately mesoderm. Its development is simultaneous with the the level of L2 to L3, where its most caudal attachment rostral to caudal wave of differentiation of the somites to the ectoderm is located. From this point of attach- from the paraxial mesoderm [2] and the differentiation ment it curves ventromedially away from the ectoderm of the overlying ectodermal ring within the ectoderm. and attaches to the urogenital portion of the cloaca, In the middle of the fourth week, the first somite appears at the level of L4 to L5, before the caudal growth of the in the paraxial mesoderm of the occipital region. Every mesonephros is completed. The caudal end of the few hours another pair of somites differentiates cau- mesonephros reaches only to the level of L2 to L3, the dally to the previous pair. Simultaneously, the adjacent site of the curve of the mesonephric duct and its intermediate mesoderm differentiates into the nephro- attachment to the ectoderm. The mesonephric genic cord. By the end of the fourth week, the occipital nephrons are not metameric and there are more and upper cervical somites appear, and the nephro- nephrons than somites. As new nephrons are added genic cord adjacent to these somites differentiates into caudally, the rostral nephrons and mesonephric duct primitive nephric structures called nephrotomes, which degenerate. A total of over 80 nephrons develop but, rapidly disappear. The nephrotomes are somewhat due to the simultaneous degeneration of the rostral analogous to the pronephros of lower vertebrates, but nephrons, when the last nephron differentiates and a true pronephros does not exist in humans [3]. attaches to the curve of the mesonephric duct at the By the beginning of the fifth week, the middle and level L2 to L3 only approximately 32 nephrons remain. lower cervical somites appear. The mesonephric duct The mesonephric duct is attached to the mesonephros arises from the dorsolateral border of the nephrogenic by a common investing mesenchyme and the attach- cord adjacent to somite C4 at the level of the upper ments of the connecting tubules of the mesonephric limb bud. The first mesonephric nephron immediately nephrons to the mesonephric duct. The attachment of differentiates within the nephrogenic cord adjacent to the caudal end of the mesonephros to the curve of the C6. Mesonephric nephrons are analogous to meta- mesonephric duct and the attachment of the curve of nephric nephrons and each is attached to the meso- the mesonephric duct to the ectoderm are the begin- nephric duct by its connecting tubule [4]. The nings of the development of the inguinal canal and the mesonephric duct separates from the nephrogenic caudal gonadal ligaments. This coordinated rostral to cord and grows caudally between the dorsolateral caudal differentiation of somites, mesonephros, and border of the nephrogenic cord and the ectoderm by mesonephric duct is so rapid that within 1 to 2 days, elongation of its proliferating blind tip. Every few hours early in the middle of the fifth week, all of the thoracic a new somite differentiates caudal to its predecessor and lumbar somites have been added [6], the devel- [5] and new mesonephric nephrons differentiate cau- opment of the mesonephric bodies and mesonephric dally to their predecessors and attach to the indepen- duct is complete, the mesonephric duct has attached dently elongating mesonephric duct. The caudal to the cloaca, and the development of the inguinal growth of the mesonephric duct is faster than the ros- canal and caudal gonadal ligaments has begun [2] tral to caudal differentiation of mesonephric nephrons. (see Figs. IV-1 and IV-2). The continued development of As the duct elongates caudally, it is attached to the the inguinal region and the caudal gonadal ligaments ectoderm at multiple sites. It follows the caudal curl of is described below. 96 Color Atlas of Fetal and Neonatal Histology Otocyst Nasal disc Pharyngeal arches Optic cup Ectodermal ring (shaded) Anterior neuropore Upper limb Allantois Level of C4 Cloacal membrane Mesonephric duct Midlumbar level Mesonephric vesicles Lower limb Mammary crest FIGURE IV-1. Left lateral view of the embryo at the beginning of chyme, which invests the mesonephros and mesonephric duct, the fifth postfertilization week. The mesonephros extends from is not shown in this figure so that the individual mesonephric the midcervical to the midlumbar area and follows the caudal nephrons can be seen. The lower limb bud has appeared curl of the embryo. The caudal development of the meso- within the ectodermal ring at the midlumbar area in continuity nephros is complete and the caudal end of the mesonephros with the cloacal membrane. The precursors of the caudal is at the mid lumbar area. The mesonephric duct curves ven- attachments of the gonads and gonadal ducts to the inguinal tromedially around the distal end of the mesonephros at the canal at the midlumbar area are already established. Several midlumbar area where it is attached to the ectodermal ring at developmental fields within the ectodermal ring are depicted the future site of the internal inguinal ring and canal. This is (shaded area). the first curve of the mesonephric duct. The ­common mesen- AB Midlumbar level Tail gut Cloaca (most caudal area of Cloacal attachment between membrane Lower limb mesonephric duct and ectodermal ring Cloaca Mesonephric * Allantois duct * Caudal end of Mesonephric Hind gut mesonephric duct * * vesicles Mesonephric vesicles Most caudal area of attachment between mesonephric duct and ectodermal ring FIGURE IV-2. Closer views of the caudal end of the primitive of the mesonephric seen on end is attached the ectodermal genitourinary tract early in the fifth week. A, Left lateral view. ring at the asterisks at the future site of the internal inguinal ring The mesonephros follows the caudal curl of the embryo. At the and canal at the origin of the lower limb. The ureteric bud will midlumbar area, the first curve of the mesonephric duct is arise from the segment of the mesonephric duct between its attached to the overlying ectodermal ring (not shown) attachment to the ectodermal ring and its entrance into the between the asterisks at the future site of the internal inguinal cloaca. ring and canal. B, Dorsal view of the caudal curl. The first curve Early Embryology of the Genitourinary Tract 97 Ectodermal Ring The ectodermal ring is a placodal strip of thickened of the entire embryo of all vertebrates, both nonmamma- ectoderm that encircles the embryo in the boundary lian and mammalian [8]. Developmental fields arise within between the dorsal and ventral halves of the body. It the ectodermal ring and the underlying specialized first appears during the end of the fourth postfertilization mesoderm for the nose, eyes, ears, pharyngeal appara- week in the face and extends caudally on both sides of tus, upper limbs, mammary glands, lower limbs, inguinal the embryo like an inverted U to cover the lateral head, region, external genitalia, and anus (see Fig. IV-1). neck, thorax, abdomen, and the inguinal areas at the The mesonephric duct is in contact with the ectoder- same time as the somites, mesonephros, and meso- mal ring at various points from the midcervical to the nephric duct are differentiating [7]. The two limbs of the midlumbar regions. Its attachment to the ectodermal inverted U meet caudally in the ventral midline over the ring at the level of C4 through C8, the site of the upper cloacal membrane to complete the ring on the second limb bud, is so close that some believe the duct arises in day of the fifth week at the same time that the develop- whole or in part from the epithelium of that part of the ment of the somites, mesonephros, and mesonephric ectodermal ring rather than entirely from the nephro- duct reaches the midlumbar region.
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