1. Figure 53-1 Mitosis and meiosis
Figure 53-1 Mitosis and meiosis. A, In mitosis, the two daughter cells are genetically identical to the mother cell. B, In male meiosis, the four daughter cells are haploid. Cell division I produces both recombination (i.e., crossing over of genetic material between homologous chromosomes) and the reduction to the haploid number of chromosomes. Cell division II separates the chromatids of each chromosome, just as in mitosis. C, Female meiosis is similar to male meiosis. A major difference is that instead of producing four mature gametes, it produces only one mature gamete and two polar bodies.
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2. Figure 53-2 Normal human karyotype
Figure 53-2 Normal human karyotype. The normal human has 22 pairs of autosomal chromosomes (autosomes) as well as a pair of sex chromosomes. Females have two X chromosomes, whereas males have one X and one Y chromosome.
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3. Figure 53-3 The location of the testis-determining region of the Y chromosome and an example of translocation. A, The Y chromosome is much smaller than the X chromosome. Giemsa staining of the chromosome results in alternating light and dark bands, some of which are shown here. The short or p arm of the Y chromosome is located above the centromere, whereas the long or q arm is located below it. The numbers to the left of the chromosome indicate the position of bands. The TDF is the SRY gene. B, Crossing-over events between normal X and Y chromosomes of the father can generate an X chromatid that contains a substantial portion of the TDF region and a Y chromatid that lacks its TDF. The figure shows both an equal and an unequal recombination event. If a sperm cell bearing an X chromosome with a translocated TDF fertilizes an ovum, the result is a male with a 46,XX karyotype, because one of the X chromosomes contains the TDF. Conversely, if the sperm cell carries a Y chromosome lacking its TDF, the result can be a 46,XY individual that appears to be female.
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4. Figure 53-4 The early gonad and germ cell migration
Figure 53-4 The early gonad and germ cell migration. A, The primordial germ cells originate in the endodermal endothelium of the yolk sac. B, The primordial germ cells migrate along the mesentery of the hindgut and reach the region of the urogenital ridge called the gonadal ridge. C, The indifferent gonad consists of an outer cortex and an inner medulla. D, The testis develops from the medulla of the indifferent gonad; the cortex regresses. E, The ovary develops from the cortex of the indifferent gonad; the medulla regresses.
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5. Figure 53-5 Transformation of the genital ducts
Figure 53-5 Transformation of the genital ducts. A, At the time the gonad is still indifferent, it is closely associated with the mesonephros, as well as the excretory duct (mesonephric or wolffian duct) that leads from the mesonephros to the urogenital sinus. Parallel to the wolffian ducts are the paramesonephric or müllerian ducts, which merge caudally to form the uterovaginal primordium. B, In males, the mesonephros develops into the epididymis. The wolffian duct develops into the vas deferens, seminal vesicles, and ejaculatory duct. The müllerian ducts degenerate. C, In females, the mesonephros and the wolffian (mesonephric) ducts degenerate. The paramesonephric or müllerian ducts develop into the fallopian tubes, the uterus, the cervix, and the upper one third of the vagina.
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6. Figure 53-6 Jost experiments
Figure 53-6 Jost experiments. A, Very early in development, both the wolffian (mesonephric) and the müllerian (paramesonephric) ducts are present in parallel. B, The wolffian duct develops into the vas deferens, the seminal vesicles, and the ejaculatory duct. The müllerian ducts degenerate. C, The paramesonephric or müllerian ducts develop into the fallopian tubes, the uterus, the cervix, and the upper one third of the vagina. The wolffian (mesonephric) ducts degenerate. D, Bilateral removal of the testes deprives the embryo of both AMH (also known as MIS) and testosterone, which are both testicular products. As a result of the absence of AMH, the müllerian ducts follow the female pattern of development. In the absence of testosterone, the wolffian ducts degenerate. Thus, the genetically male fetus develops female internal and external genitalia. E, After bilateral removal of the ovaries, müllerian development continues along normal female lines. Thus, the ovary is not required for female duct development. F, Unilateral removal of the testis results in female duct development on the same (ipsilateral) side as the castration. Duct development follows the male pattern on the side with the remaining testis. Virilization of the external genitalia proceeds normally. G, In the absence of both testes, administering testosterone preserves development of the wolffian ducts. However, because of the absence of AMH-which is a product of the testis-no müllerian regression occurs. H, In the presence of both ovaries, the testosterone promotes development of the wolffian ducts. Because there are no testes-and therefore no AMH-the müllerian ducts develop normally.
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7. Figure 53-7 Differentiation of the urogenital sinus
Figure 53-7 Differentiation of the urogenital sinus. A, The urorectal septum begins to separate the rectum (dorsal) from the urogenital sinus (ventral). The urogenital sinus is divided into a vesicle (i.e., urinary bladder) part, a pelvic part, and a phallic part. The common space into which the rectum and urogenital sinus empty-the cloaca-is closed by the cloacal membrane. B, At this stage, the rectum and the urogenital sinus are fully separated. The urogenital membrane separates the urogenital sinus from the outside of the embryo. C, The male has a common opening for the reproductive and urinary tracts. The prostatic utricle, which is the male homologue of the vagina, empties into the prostatic urethra. D, A solid core of tissue called the vaginal plate grows caudally from the posterior wall of the urogenital sinus. The lumen of the vagina forms as the center of this plate resorbs. Thus, the female has separate openings for the urinary and reproductive systems.
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8. Figure 53-8 Development of the external genitalia
Figure 53-8 Development of the external genitalia. A, Genital folds and genital swellings surround the cloacal membrane. B, Early in the fourth week of development-in both sexes-the genital tubercle begins to enlarge to form the phallus. C, In males, the genital tubercle becomes the glans penis. The urogenital folds fuse to form the shaft of the penis. The labioscrotal swellings become the scrotum. D, In females, the genital tubercle becomes the clitoris. The urogenital folds remain separate as the labia minora. The labioscrotal swellings become the labia majora where they remain unfused. Ventrally, the labioscrotal swellings fuse to form the mons pubis. Dorsally they fuse to form the posterior labial commissure.
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9. Figure 53-9 Testicular descent.
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