Reproduction 26 and Development Sex Determination Sex Chromosomes Determine Genetic Sex Sexual Diff erentiation Occurs Early in Development Basic Patterns of Reproduction Gametogenesis Begins in Utero The Brain Directs Reproduction Environmental Factors Infl uence Reproduction Male Reproduction Testes Produce Sperm and Hormones Spermatogenesis Requires Gonadotropins and Testosterone Male Accessory Glands Contribute Secretions to Semen Androgens Infl uence Secondary Sex Characteristics Female Reproduction Females Have an Internal Uterus The Ovary Produces Eggs and Hormones A Menstrual Cycle Lasts about One Month Hormonal Control of the Menstrual Cycle Is Complex Hormones Infl uence Female Secondary Sex Characteristics Procreation The Human Sexual Response Has Four Phases The Male Sex Act Includes Erection and Ejaculation Birth, and copulation, Sexual Dysfunction Aff ects Males and Females and death. That’s all Contraceptives Are Designed to Prevent Pregnancy the facts when you Infertility Is the Inability to Conceive come to brass tacks. Pregnancy and Parturition —T.S. Eliot, Sweeney Agonistes Fertilization Requires Capacitation The Developing Embryo Implants in the Endometrium The Placenta Secretes Hormones During Pregnancy Background Basics Pregnancy Ends with Labor and Delivery Positive and negative The Mammary Glands Secrete Milk During Lactation feedback Prolactin Has Other Physiological Roles Flagella Steroids Growth and Aging Agonist/antagonist Puberty Marks the Beginning of the Reproductive Years Up-and down-regulation Menopause and Andropause Are a Consequence of Aging Prostaglandins Hypothalamic-pituitary axis Prolactin Oxytocin Cross-section Spinal refl ex of intestinal villi Hot fl ashes (outlined in red). 900 Reproduction and Development magine growing up as a girl, then at the age of 12 or so, Sex hormones play a significant role in the behavior of fi nding that your voice is deepening and your genitals are other mammals, acting on adults as well as infl uencing the brain developing into those of a man. Th is scenario actually hap- of the developing embryo. Th eir role in humans is more con- I pens to a small number of men who have a condition known troversial. Human fetuses are exposed to sex hormones while in as pseudohermaphroditism {pseudes, false + hermaphrodites, the the uterus, but it is unclear how much infl uence these hormones dual-sex off spring of Hermes and Aphrodite}. Th ese men have have on behavior later in life. Does the preference of little girls the internal sex organs of a male but inherit a gene that causes for dolls and of little boys for toy guns have a biological basis or a defi ciency in one of the male hormones. Consequently, they a cultural basis? We have no answer yet, but growing evidence are born with external genitalia that appear feminine, and they suggests that at least part of our brain structure is infl uenced by are raised as girls. At puberty { pubertas, adulthood}, the period sex hormones before we ever leave the womb. when a person makes the transition from being nonreproduc- In this chapter we address the biology of human reproduc- tive to being reproductive, pseudohermaphrodites begin to se- tion and development. We begin our discussion with gametes that crete more male hormones. As a result, they develop some, but fuse to form the fertilized egg, or zygote. As the zygote begins to not all, of the characteristics of men. Not surprisingly, a con- divide (2-cell stage, 4-cell stage, etc.), it becomes fi rst an embryo fl ict arises: should these individuals change gender or remain (weeks 0–8 of development), then a fetus (8 weeks until birth). female? Most choose to change and continue life as men. Reproduction is one area of physiology in which we hu- mans like to think of ourselves as signifi cantly advanced over Sex Determination other animals. We mate for pleasure as well as procreation, and The male and female sex organs consist of three sets of struc- women are always sexually receptive (that is, not only during tures: the gonads, the internal genitalia, and the external genita- fertile periods). But just how diff erent are we? lia. Gonads { gonos, seed} are the organs that produce gametes Like many other terrestrial animals, humans have internal { gamein, to marry}, the eggs and sperm that unite to form a new fertilization that allows motile fl agellated sperm to remain in an individual. Th e male gonads are the testes (singular testis ), which aqueous environment. To facilitate the process, we have mating produce sperm ( spermatozoa ). Th e female gonads are the ova- and courtship rituals, as do other animals. Development is also ries, which produce eggs, or ova (singular ovum ). Th e undiff eren- internal, within the uterus, which protects the growing embryo 26 tiated gonadal cells destined to produce eggs and sperm are called from dehydration and cushions it in a layer of fl uid. germ cells. Th e internal genitalia consist of accessory glands and Humans are sexually dimorphic { di- , two + morphos , ducts that connect the gonads with the outside environment. Th e form}, meaning that males and females are physically distinct. external genitalia include all external reproductive structures. This distinction is sometimes blurred by dress and hairstyle, Sexual development is programmed in the human genome. but these are cultural acquisitions. Although everyone agrees Each nucleated cell of the body except eggs and sperm contains that male and female humans are physically dimorphic, we are 46 chromosomes. Th is set of chromosomes is called the diploid still debating whether we are behaviorally and psychologically number because the chromosomes occur in pairs: 22 matched, dimorphic as well. or homologous, pairs of autosomes plus one pair of sex chromo- somes ( Fig. 26.1 a). Th e 22 pairs of autosomal chromosomes RUNNING PROBLEM direct development of the human body form and of variable characteristics such as hair color and blood type. Th e two sex Infertility chromosomes, designated as either X or Y, contain genes that Peggy and Larry have just about everything to make them direct development of internal and external sex organs. Th e X happy: successful careers, a loving marriage, a comfortable chromosome is larger than the Y chromosome and includes home. But one thing is missing: after fi ve years of marriage, many genes that are missing from the Y chromosome. they have been unable to have a child. Today, Peggy and Eggs and sperm are haploid cells with 23 chromosomes, Larry have their fi rst appointment with Dr. Coddington, an one from each matched pair and one sex chromosome. When infertility specialist. “Finding the cause of your infertility egg and sperm unite, the resulting zygote then contains a unique is going to require some painstaking detective work,” set of 46 chromosomes, with one chromosome of each matched Dr. Coddington explains. He will begin his workup of pair coming from the mother and the other from the father. Peggy and Larry by asking detailed questions about their reproductive histories. Based on the answers to these questions, he will then order tests to pinpoint the problem. Sex Chromosomes Determine Genetic Sex Th e sex chromosomes a person inherits determine the genetic sex of that individual. Genetic females are XX, and genetic males are XY ( Fig. 26.1 b). Females inherit one X chromosome from 901 Reproduction and Development whether development proceeds along male or female lines. Th e HUMAN CHROMOSOMES presence of a Y chromosome means the embryo will become (a) Humans have 23 pairs of chromosomes: 22 pairs of autosomes male, even if the zygote also has multiple X chromosomes. For and one pair of sex chromosomes. X and Y chromosomes (lower instance, an XXY zygote will become male. A zygote that in- right) mean that these chromosomes came from a male. The autosomes are arranged in homologous pairs in this figure. herits only a Y chromosome (YO) will die because the larger X chromosome contains essential genes that are missing from the Y chromosome. In the absence of a Y chromosome, an embryo will de- velop into a female. For this reason, a zygote that gets only one X chromosome (XO; Turner’s syndrome) will develop into a fe- male. Two X chromosomes are needed for normal female repro- ductive function, however. Once the ovaries develop in a female fetus, one X chromo- some in each cell of her body inactivates and condenses into a clump of nuclear chromatin known as a Barr body . (Barr bodies in females can be seen in stained cheek epithelium.) Th e selec- tion of the X chromosome that becomes inactive during devel- opment is random: some cells will have an active maternal X chromosome and others have an active paternal X chromosome. Because inactivation occurs early in development—before cell division is complete—all cells of a given tissue will usually have the same active X chromosome, either maternal or paternal. Concept Check Answers: End of Chapter 1. Name the male and female gonads and gametes. (b) X and Y chromosomes determine sex. Each egg produced by a female (XX) has an X chromosome. Sperm produced by a male (XY) have either an X chromosome or a Y chromosome. Sexual Diff erentiation Occurs Early in Development Female parent Male parent The sex of an early embryo is difficult to determine because XX XY reproductive structures do not begin to diff erentiate until the seventh week of development. Before diff erentiation, the embry- onic tissues are considered bipotential because they cannot be morphologically identifi ed as male or female. The bipotential gonad has an outer cortex and an inner XXEggs X Sperm Y medulla ( Fig. 26.2 a). Under the infl uence of the appropriate developmental signal (described below), the medulla will de- velop into a testis.