Chapter 20 Reproduction 20-1

Sexual Differentiation Early embryonic gonads can become testes or ovaries Y chromosome induces formation of testes Testosterone (T) from testes induces formation of male sex organs In absence of T, female sex organs develop 20-4

Sex Determination Each zygote inherits 23 chromosomes from mother and 23 from father Producing 23 pairs of homologous chromosomes 22 of these are autosomal chromosomes 23rd pair are sex chromosomes XX in females; XY in males Y (contains 80 genes) is believed to be derived from X (contains 1098 genes) Thus, in males ~1000 genes are only represented by a single allele from the X chromosome Leading to a large # genetic diseases caused by X-linked genes 20-6

Genomic Imprinting The cells of a zygote contain 23 pairs of homologous chromosomes Thus each gene is represented either by the same or an alternative allele (form) from each homolog Usually both alleles are expressed However, in ~80 genes either the maternal or paternal allele is silenced Accomplished by chemically modifying DNA bases e.g., by methylation Called epigenetic changes because they don’t change the DNA sequence but they are heritable Called genomic imprinting 20-9

Formation of Testes and Ovaries 1st 40 days after conception, gonads of males and females are similar Cells that will give rise to sperm and eggs migrate from yolk sac to developing gonads Gonads have potential to become testes or ovaries until testis-determining factor (TDF) causes conversion to testes TDF is produced by SRY (sex-determining region of Y) gene 20-10

Development of Testes Leydig cells begin secreting T at 8 weeks of development and peak at 12-14th week This masculinizes embryonic structures T levels then decline to very low levels until puberty Testes descend into scrotum shortly before birth Because spermatogenesis requires 3oC lower temperature than body Spermatogenesis doesn't occur in cryptorchidism (undescended testes) 20-13

Sexual Differentiation: Internal Embryonic Development Figure 26-3a: Sexual development in the human embryo

Sexual Differentiation: External Genitalia Figure 26-3b: Sexual development in the human embryo

Pathway for Sexual Development: Review for Genes to Organs Figure 26-4: Role of the SRY gene in male development

Formation of Testes and Ovaries continued Seminiferous tubules (STs), which produce sperm in adults, appear within 43-50 days following conception Contain germinal cells which will produce sperm and nongerminal Sertoli cells (appear at day 42) At ~day 65, Leydig cells (produce T) appear in clusters around STs In absence of TDF, ovaries develop Ovarian follicles (functional units of ovary) don't appear until day 105 20-12

Development of Accessory Sex Organs Male accessory sex organs are derived from wolffian ducts Include epididymis, vas deferens, seminal vesicles, and ejaculatory duct Female accessory sex organs are derived from Müllerian ducts and include uterus and fallopian tubes 20-14

Disorders of Embryonic Sexual Development Hermaphroditism occurs when individuals have both ovary and testis tissue Pseudohermaphrodites have either testes or ovaries but have accessory organs and external genitalia that are incompletely developed or inappropriate Most common cause of female pseudohemaphroditism is congenital adrenal hyperplasia In males, one cause is testicular feminizing syndrome in which testes are normal but there are no receptors for T Individual develops very female appearance but is infertile 20-17

Gonads During Development and Childhood Embryonic testes produce lots of T during 1st trimester of pregnancy to masculinize fetus T production declines during 2nd trimester Ovaries don’t produce much sex steroid until puberty Both testes and ovaries remain inactive after birth until puberty 20-18

Adrenal Cortex: Steroid Hormone Production Figure 23-2: Synthesis pathways of steroid hormones

Testosterone Production The most important hormone of the testes Produced in interstitial cells Functions of testosterone Stimulates reproductive organ development Underlies sex drive Causes secondary sex characteristics Deepening of voice Increased hair growth Enlargement of skeletal muscles Thickening of bones

Hormone Production by the Ovaries Estrogens Produced by follicle cells Cause secondary sex characteristics Enlargement of accessory organs Development of breasts Appearance of pubic hair Increase in fat beneath the skin Widening and lightening of the pelvis Onset of menses

Regulation of Spermatogenesis Figure 26-11: Hormonal control of spermatogenesis

Ovary: Details of Histology & Physiology Figure 26-12d: ANATOMY SUMMARY: Female Reproduction

Menstrual Cycle: Egg Maturation, and Endometrial Growth Follicular phase Egg matures Ovulation Egg released Luteal phase Corpus luteum Endometrium Prep for blastocyst No Pregnancy Menses Figure 26-13: The menstrual cycle

Endocrine Control of Menstrual Cycle: Follicular Phase FSH stimulates follicular development Estrogen: + feedback, limits more follicles

Endocrine Control of Menstrual Cycle: Ovulation  Estrogen  LH "surge" & FSH spike  egg release Inhibin pushes FSH down ,  new follicle development

Endocrine Control of Menstrual Cycle: Luteal phase Granulosa cells form corpus luteum  progesterone  progesterone & estrogen maintain endometrium Inhibin continues to limit new follicular development

Endocrine Control of Menstrual Cycle: Late Luteal phase Pregnancy: maintain  progesterone, estrogen & inhibin No pregnancy:  progesterone, estrogen & inhibin Menses,  FSH & LH  new follicle development

Onset of Puberty FSH and LH secretion is high for 1st 6 months of life, but falls to very low levels until puberty At puberty hypothalamus increases GnRH secretion This stimulates increased LH and FSH Which stimulates sex steroid secretion Which drives changes in secondary sex characteristics and menarche (1st menstrual flow) Growth of pubic and axiliary hair is due to androgen secretion from adrenal cortex 20-21

Onset of Puberty continued At beginning of female puberty, high E stimulates growth High pubertal T in boys causes growth spurt that lags that of girls Age of puberty in girls depends on % body fat and physical activity Girls with low body fat and high activity levels enter puberty at older age 20-22

Testosterone and Age Secretion of T declines gradually and varyingly in men> 50 Causes are unknown Not due to low GnRH, LH, or FSH because their levels are elevated 20-30

Factors Affecting Menstrual Cycle Release of GnRH is regulated not only by hormonal feedback but also by input from higher brain centers Olfactory system can send activity to hypothalamus in response to pheromones Can cause the “dormitory effect” (Ch 11) in which cycles of roommates become synchronized 20-74

Factors Affecting Menstrual Cycle continued Limbic system (involved in emotions; Ch 8) input to the hypothalamus in times of stress can cause functional amenorrhea (cessation of menstruation) Also occurs in thin or athletic females with low body weight Appears to be related to reduced leptin secretion by small adipocytes 20-75

Menopause Is cessation of ovarian activity and menstruation at ~50 years Ovaries are depleted of follicles and thus produce no E LH and FSH are high because of no negative feedback Lack of E from ovaries is most responsible for hot flashes, osteoporosis, and increased risk of atherosclerosis 20-78