Human Reproduction and Development Chapter 42

Modes of Reproduction 1. Sexual reproduction Meiosis (gamete formation) followed by fertilization Offspring show genetic variation 2. Asexual reproduction Mitosis Single parent produces offspring Offspring are genetically identical 3. Hermaphrodites -produce eggs and sperm, some can self-fertilize Ex.-Tapeworms…why is this an advantage? Parasite, lives in a animal’s gut, doesn’t see another of the same species very often to mate

Cost of Sexual Reproduction Specialized cells and structures must be formed (to deliver/receive sperm) Reproductive timing; pheromones, Visual signals, special courtship, and parental behaviors can be costly Nurturing developing offspring, either in egg or body, requires resources from mother Reproductive timing: sperm and egg must mature at same time, requires hormonal regualtion, reponsiveness to environmental cues

Costs of Sexual Reproduction

Gamete Formation

The Reproductive System Gonads – primary sex organs Testes in males Ovaries in females Gonads produce gametes (sex cells) and secrete hormones Sperm – male gametes Ova (eggs) – female gametes

Male Reproductive System Accessory organs(Seminal vesicle, Prostate gland, Bulbourethral gland) External genitalia (penis, Scrotum: testes, epididymis)

Testes Spermatogenesis Seminiferous tubules Tightly coiled structures Function as sperm-forming factories Sperm travels from testes to epididymis Produce testosterone Figure 16.1

Epididymis Comma-shaped, tightly coiled tube Function: to mature and store sperm cells (at least 20 days) Expels sperm with the contraction of muscles in the epididymis walls to the vas deferens

Vas Deferens Carries sperm from epididymis to the ejaculatory duct Vasectomy – cutting of the vas deferens at the level of the testes to prevent transportation of sperm

Urethra Extends from the base of the urinary bladder to the tip of the penis Carries both urine and sperm Sperm enters from the ejaculatory duct

Semen Mixture of sperm and accessory gland secretions Advantages of accessory gland secretions Seminal Vesicles: Fructose provides energy for sperm cells Prostate Gland: raises pH of semen helps neutralize the acidic environment of vagina Bulbourethral gland: lubricating mucus clears urethra of urine Semen inhibits bacterial multiplication Elements of semen enhance sperm motility

External Genitalia Scrotum Penis Divided sac of skin outside the abdomen Maintains testes at 3°C lower than normal body temperature to protect sperm viability Penis Delivers sperm into the female reproductive tract Internally, three areas of spongy erectile tissue around the urethra

Sperm Formation: Seminiferous Tubules

Spermatogenesis Production of sperm cells Begins at puberty and continues thru-out life In the seminiferous tubules (inside testes) Spermatogonium (2n) divides by mitosis to form primary spermatocyte (2n) Meiosis produces haploid spermatids Spermatids mature to become sperm Spermatogenesis takes 64 to 72 days

Sperm Formation (in seminferous tubule in testes) head midpiece with mitochondria tail with core of microtubules

Anatomy of Mature Sperm The only human flagellated cell DNA is found in the head Little cytoplasm 48-72 hour lifespan (up to 5 days) Figure 16.5b

Testosterone Production Produced in Testes 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

Regulation of Male Androgens (Sex Hormones)

Female Reproductive System Ovaries Duct System Uterine tubes (fallopian tubes) Uterus Vagina Urethra is NOT part of reproductive system in females External genitalia

Female Reproductive Organs

Female Reproductive Organs

Ovaries Produces eggs Composed of ovarian follicles (sac-like structures) Ovulation – egg is mature and the follicle ruptures Occurs every 28 days Ruptured follicle is transformed into a corpus luteum Figure 16.7

Fallopian Tubes (Oviducts) Receive the ovulated oocyte Cilia move the oocyte towards the uterus (takes 3–4 days) Provide a site for fertilization Attaches to the uterus Does not physically attach to the ovary

Uterus Hollow organ Cervix – narrow outlet that protrudes into the vagina Functions of the uterus Receives a fertilized egg Retains the fertilized egg Nourishes the fertilized egg Endometrium Inner layer Allows for implantation of a fertilized egg Sloughs off if no pregnancy occurs (menses)

Vagina Extends from cervix to exterior of body Serves as the birth canal Receives sperm during sexual intercourse

Oogenesis Making eggs Total supply of eggs are present at birth Ovulation begins at puberty Many eggs “die off” in earlier stages of development, before ovulation occurs Reproductive ability ends at menopause Oocytes are matured in developing ovarian follicles

Oogenesis Girl is born with 2 million primary oocytes already in ovaries Each oocyte has entered meiosis I and stopped Meiosis resumes, with the first menstrual cycle Meiosis completed only after fertilization Unfertilized egg: lifespan 24 hours

Ovarian Cycle

Oogenesis Figure 16.10

Menstrual Cycle Function: produce an environment that is hospitable to the growing embryo Lining of uterus builds up cyclically Bleeding is a by-product resulting from no pregnancy The fertile period for a human female occurs on a cyclic basis

Menstrual Cycle Cyclic changes of the endometrium Regulated by cyclic production of estrogens and progesterone Menstrual cycle lasts about 28 days Day 1: first day of bleeding Ovulation around day 14 Stages of the menstrual cycle Menses: endometrium is sloughed Follicular phase: prior to ovulation Luteal phase: after ovulation

Hormonal Control of the Ovarian and Uterine Cycles Figure 16.12a, b

Hormonal Control of the Ovarian and Uterine Cycles Figure 16.12c, d

Menstrual Cycle Overview

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

Hormone Production by the Ovaries Progesterone Produced by the corpus luteum Production continues until LH diminishes in the blood Helps maintain pregnancy

Female Hormonal Control

Cycle Overview GnRH secretion affects LH and FSH secretion by pituitary LH and FSH affect follicle maturation Estrogen and progesterone from ovary affect uterus

Probability of Pregnancy 10 days…that’s about 2 weeks after the start of her period. Wilcox, A. J et al. BMJ 2000;321:1259-1262 What day on a regular cycle is there the highest probability of pregnancy? Copyright ©2000 BMJ Publishing Group Ltd.

Fertilization Sperm penetrates to egg cytoplasm Secondary oocyte undergoes meiosis II; forms mature egg Egg nucleus and sperm nucleus fuse to form diploid zygote

Birth Control Options