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Chp.7 Development and Sex Determination 7.1 The Human Reproductive System

Chp.7 Development and Sex Determination 7.1 The Human Reproductive System. The human reproductive system Gonads produce gametes (testes in males, ovaries in females) Ducts to transport gametes Genital structures for intercourse and fertilization. Single-celled Zygote.

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Chp.7 Development and Sex Determination 7.1 The Human Reproductive System

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  1. Chp.7 Development and Sex Determination7.1 The Human Reproductive System • The human reproductive system • Gonads produce gametes (testes in males, ovaries in females) • Ducts to transport gametes • Genital structures for intercourse and fertilization

  2. Single-celled Zygote • We all begin as a single cell, the zygote, produced by the fusion of a sperm and oocyte • Zygote • Fertilized egg that develops into a new individual

  3. Male and Female Gametes • Gametes • Unfertilized germ cells • Sperm • Male gamete • Oocyte • Female gamete

  4. Male and Female Gonads • Gonads • Organs where gametes are produced • Testes • Male gonads that produce spermatozoa and male sex hormones • Ovaries • Female gonads that produce oocytes and female sex hormones

  5. Elements of the Male Reproductive System Table 7-1, p. 152

  6. The Female Reproductive System Table 7-2, p. 153

  7. 7.2 A Survey of Human Development from Fertilization to Birth • Human development begins with fertilization and the formation of a zygote • Fertilization • Fusion of two gametes to produce a zygote • Zygote • Fertilized egg that develops into a new individual

  8. Fertilization

  9. The Blastocyst • Cell divisions in the zygote form an early embryonic stage called the blastocyst • Blastocyst • The developmental stage at which the embryo implants into the uterine wall • Inner cell mass • A cluster of cells in the blastocyst that gives rise to the embryonic body

  10. Implantation • Implantation • The embryo implants in the uterine wall, and membranes develop to support the embryo • Trophoblast • Outer layer of cells in the blastocyst that gives rise to the membranes surrounding the embryo

  11. Embryonic Membranes and Placenta • Chorion • Two-layered structure formed from trophoblast • Releases human chorionic gonadotropin (hCG) hormone which maintains uterine lining and stimulates endometrial cells to produce hormones • Grows and forms villi that exchange nutrients and wastes with maternal blood circulation, eventually forming the placenta

  12. Fertilization to Implantation Fig. 7-6a, p. 156

  13. Fertilization to Implantation Fig. 7-6b, p. 156

  14. Development is Divided into Three Trimesters • First trimester • First month: basic tissue layers form; most of the body is divided into paired segments • Second month: most major organ systems are formed • Third month: embryo becomes a fetus; sexual development is initiated

  15. Development is Divided into Three Trimesters • Second trimester • Increase in size and organ-system development • Bony parts of skeleton form • Heartbeat is heard with a stethoscope • Fetal movements begin • Third trimester • Rapid growth • Circulatory and respiratory systems mature • Birth is a hormonally induced process at the end of the 3rd trimester

  16. Human Development Fig. 7-7ab, p. 158

  17. Human Development Fig. 7-7cd, p. 159

  18. Keep In Mind • Most of the important organ forming events in human development occur in the first trimester • The remaining months are mainly a period of growth

  19. 7.3 Teratogens Are a Risk to the Developing Fetus • The embryo and fetus are sensitive to chemical and physical agents that can cause birth defects • Teratogen • Any physical or chemical agent that brings about an increase in congenital malformations • Radiation, viruses, medications, alcohol

  20. Teratogens and their Impact on Organ Formation Fig. 7-8, p. 160

  21. Alcohol is a Teratogen • Fetal alcohol syndrome (FAS) • A constellation of birth defects caused by maternal alcohol consumption during pregnancy • Alcohol is the most common teratogenic problem and leading cause of preventable birth defects • There is no “safe” amount of alcohol consumption during pregnancy

  22. Fetal Alcohol Syndrome • A preventable form of birth defect • US Surgeon General: “Drinking during pregnancy may cause mental retardation and other birth defects.”

  23. 7.4 How Is Sex Determined? • Mechanisms of sex determination vary from species to species • In humans, the presence of a Y chromosome is associated with male sexual development, and the absence of a Y chromosome is associated with female development • In other animals, reptiles for example, environmental interactions help determine sex.

  24. Mechanisms of Sex Determination Fig. 7-10, p. 162

  25. Chromosomal Sex Determination Fig. 7-11, p. 163

  26. Human Sex Ratios • Sex ratio • The proportion of males to females, which changes throughout the life cycle • The ratio is close to 1:1 at fertilization • The ratio of females to males increases as a population ages

  27. Keep In Mind • Chromosomal sex is determined at fertilization • Sexual differentiation begins in the seventh week and is influenced by a combination of genetic and environmental factors

  28. Sexual Development begins in the Seventh week of Gestation Fig. 7-13a, p. 165

  29. 7.5 Defining Sex in Stages: Chromosomes, Gonads, and Hormones • Chromosomal sex is established at fertilization • Other aspects of sex depend on the interaction of genes and environmental factors, especially hormones

  30. Factors in Sexual Differentiation • The formation of male and female reproductive structures depends on: • Gene action • Interactions within the embryo • Interactions with other embryos in the uterus • Interactions with the maternal environment

  31. Three Levels of Sexual Determination • The chromosomal sex of an individual (XX or XY) can differ from the phenotypic sex • Sex of an individual is defined at three levels • Chromosomal sex • Gonadal sex • Phenotypic sex

  32. Exploring Genetics: Sex Testing in the Olympics—Biology and a Bad Idea • Beginning in the 1968 Olympic Games, sex-testing of all female athletes was required, involving identification of Barr bodies • Barr body testing is unreliable, leading to false positive or false negative results • Fails to consider androgen insensitivity • New tests instituted in 1992 evaluated the presence of a male-determining gene called SRY • At the 2000 Olympic Games, genetic screening of female athletes was abandoned due to numerous criticisms of the techniques

  33. Gonadal Sex Differentiation • For the fist 7 or 8 weeks, the embryo is neither male nor female • Two undifferentiated gonads • Both male and female reproductive duct systems develop • Genes cause gonads to develop as testes or ovaries, establishing gonadal sex • Alternative pathways can produce intermediates

  34. Human Embryo: Week 8 • Sexual differentiation begins Fig. 7-13, p. 166

  35. Y Chromosome and Testis Development • SRY gene • Sex-determining region of the Y chromosome • Located near the end of the short arm of the Y chromosome • Plays a major role in causing the undifferentiated gonad to develop into a testis

  36. Hormones of the Testes and Male Development • Testosterone • A steroid hormone produced by the testis • Male sex hormone • Müllerian inhibiting hormone (MIH) • Hormone produced by developing testis that causes breakdown of Müllerian (female) ducts in the embryo

  37. Female Development • Requires the absence of the Y chromosome and the presence of two X chromosomes • Embryonic gonad develops as an ovary • In the absence of testosterone, the Wolffian duct system degenerates • In the absence of MIH, the Müllerian duct system forms female reproductive system

  38. Male and Female Differentiation Fig. 7-12, p. 163

  39. Pathways of Sexual Differentiation Fig. 7-14, p. 167

  40. 7.6 Mutations Can Uncouple Chromosomal Sex from Phenotypic Sex • Early in development, the Y chromosome signals the indifferent gonad to begin development as a testis • Hormones secreted by the testis control later stages of male sexual differentiation, including the development of phenotypic sex

  41. Androgen Insensitivity • A mutation in the X-linked androgen receptor gene (AR) causes XY males to become phenotypic females • Testosterone is produced, but not testosterone receptors; cells develop as females • Androgen insensitivity (CAIS) • An X-linked genetic trait that causes XY individuals to develop into phenotypic females

  42. Mutations can cause Sex Phenotypes to Change at Puberty • Pseudohermaphroditism • An autosomal genetic condition that causes XY individuals to develop the phenotype of females • Caused by mutations in several different genes • Affected individuals have both male and female structures, but at different times of life • At puberty, females change into males

  43. 7.7 Equalizing the Expression of X Chromosomes in Males and Females • Human females have one X chromosome inactivated in all somatic cells to balance the expression of X-linked genes in males and females

  44. Dosage Compensation • Females have two X chromosomes, males have one; yet the amount of gene product is the same • Dosage compensation • A mechanism that regulates the expression of sex-linked gene products

  45. Barr Bodies and X Inactivation • Lyon hypothesis (proposed by Mary Lyon) • Dosage compensation in mammalian females • Random inactivation of one X chromosome in females equalizes the activity of X-linked genes in males and females • Barr body • A densely staining mass in the somatic nuclei of mammalian females • An inactivated X chromosome, tightly coiled

  46. X Chromosomes and Barr Bodies Fig. 7-16, p. 169

  47. Female Mammals are Actually Mosaics for X Chromosome Expression • In females, some cells express the mother’s X chromosome and some cells express the father’s X chromosome • Inactivated chromosome can come from either mother or father • Inactivation occurs early in development • Inactivation is permanent; all descendants of a particular cell have the same X inactivated

  48. Mosaic Expression in Female Mammals Fig. 7-17, p. 170

  49. Mosaic X Chromosome Expression in Human Females Fig. 7-18, p. 171

  50. X Inactivation Center • X Inactivation begins and is regulated from the X inactivation center (Xic) of the X chromosome • X inactivation center (Xic) • Region on the X chromosome where inactivation begins

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