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OBJECTIVES

OBJECTIVES

After studying this chapter, you should be able to:

  • Name the key hormones secreted by Leydig cells and Sertoli cells of the testes and by Graafian follicles and corpora lutea of the ovaries.

  • Outline the role of chromosomes, hormones, and related factors in sex determination and development.

  • Summarize the hormonal changes that occur at puberty in males and females.

  • Outline the hormonal changes and their physiologic effects during perimenopause and menopause.

  • Describe the physiologic changes that occur in the female reproductive organs during the menstrual cycle.

  • Know the general structures of 17β-estradiol and progesterone, and describe their biosynthesis, transport, metabolism, and actions.

  • Describe the roles of the pituitary and the hypothalamus in the regulation of ovarian function, and the role of feedback loops in this process.

  • Describe the hormonal changes that accompany pregnancy and parturition.

  • Outline the processes involved in lactation.

SEX DIFFERENTIATION & DEVELOPMENT

CHROMOSOMAL SEX

The Sex Chromosomes

Sex is determined genetically by two chromosomes, called the sex chromosomes, to distinguish them from the somatic chromosomes (autosomes). Sex chromosomes are called X and Y. The Y chromosome is necessary and sufficient for the production of testes, and the testis-determining gene product is called SRY (for sex-determining region of the Y chromosome). SRY is a DNA-binding regulatory protein. SRY is a transcription factor that initiates transcription of genes necessary for testicular differentiation, including the gene for müllerian inhibiting substance (MIS; see further). Diploid male cells contain an X and a Y chromosome (XY pattern), whereas female cells contain two X chromosomes (XX pattern). As a consequence of meiosis during gametogenesis, each normal ovum contains a single X chromosome, but half of the normal sperm contain an X chromosome and half contain a Y chromosome. When a sperm containing a Y chromosome fertilizes an ovum, an XY pattern results and the zygote develops into a genetic male. When fertilization occurs with an X-containing sperm, an XX pattern and a genetic female results.

PUBERTY

Puberty is the period when the endocrine and gametogenic functions of the gonads have developed to the point where reproduction is possible. In girls, the first event is thelarche, the development of breasts, followed by pubarche, the development of axillary and pubic hair, and then by menarche, the first menstrual period. Initial menstrual periods are generally anovulatory, and regular ovulation appears about a year later. In the United States in recent years, puberty generally occurs between the ages of 8 and 13. The pulsatile secretion of GnRH brings on puberty.

THE FEMALE REPRODUCTIVE SYSTEM

THE MENSTRUAL CYCLE

The reproductive system of women (Figure 22–1), unlike that of men, shows regular cyclic changes that teleologically may be regarded as periodic preparations for fertilization and pregnancy. The cycle is a menstrual cycle, and ...

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