Lecture 23 The Reproductive System

Lecture 23The Reproductive System

Evolution of Reproduction Among the Vertebrates Vertebrate sexual reproduction evolved in the ocean before vertebrates colonized land Most marine bony fish use external fertilization Male and female gametes are released into the water where fertilization occurs Most other vertebrates use internal fertilization Male gametes are introduced into the female reproductive tract • There are three strategies for internal fertilization • Oviparity • Fertilized eggs are deposited outside mother’s body to complete their development • Ovoviviparity • Fertilized eggs are retained within the mother to complete their development • Young obtain nourishment from egg yolk • Viviparity • Fertilized eggs are retained within the mother to complete their development • Young obtain nourishment from mother’s blood

Mammalian Breeding Patterns • Some mammals are seasonal breeders • Others have reproductive cycles • Periodic release of a mature ovum (ovulation) • Most female mammals have estrous cycles • Females sexually receptive to males only around time of ovulation (estrus) • Apes and humans have menstrual cycles • Females bleed when shedding inner lining of the uterus • Can copulate at any time in their cycle • Cats and rabbits are induced ovulators • Ovulation only after copulation due to LH secretion

Three Types of Mammalian Development • Monotremes are oviparous • Lay eggs • Young hatchlings obtain milk by licking mammary glands (they lack nipples) • Marsupials are viviparous • Give birth to incompletely developed fetuses • Complete development in mother’s pouch • Obtain food from nipples in mammary glands • Placentals are viviparous • Retain young in uterus for long periods of development • Fetuses are nourished by the placenta

How Sex is Determined in Mammals • In mammals, sex is determined early in embryonic development • Embryonic gonads are indifferent • Y chromosome converts them to testes • Responsible gene is SRY • Sex-determining region of the Y chromosome

Male Reproductive Organs • Thetestisproducessperm and testosterone • Enclosed in a hanging sac called the scrotum • Sperm need cooler temperature to develop • Spermatogenesis occurs in the seminiferous tubules • Sperm are then transferred to theepididymisfor storage and maturation • From there to the vas deferens & its ampulla • At ejaculation they pass into the urethra which empties through the penis • Accessory sex glands: • Empty their secretions into the ducts during ejaculation • Include the seminal vesicles, prostate gland, and bulbourethral glands

The Testis and Formation of Sperm Play Spermatogenesis

Thepeniscontains long cylinders of spongy tissue These get filled with blood causing an erection Physical stimulation is required for ejaculation 2-5 milliliters of semen are ejected This volume contains several hundred million sperm Plus secretions from the prostate and other glands The Male Penis

Mechanism and Effects of Testosterone Activity • Testosterone is synthesized from cholesterol • It must be transformed to exert its effects on some target cells • Prostate – it is converted into dihydrotestosterone (DHT) before it can bind within the nucleus • Neurons – it is converted into estrogen to bring about stimulatory effects • Testosterone targets all accessory organs and its deficiency causes these organs to atrophy • Male hormones make their appearance at puberty and induce changes in nonreproductive organs, including • Appearance of pubic, axillary, and facial hair • Enhanced growth of the chest and deepening of the voice • Skin thickens and becomes oily • Bones grow and increase in density • Skeletal muscles increase in size and mass • Testosterone is the basis of libido in both males and females

But It Doesn’t Always Work Right • Just because the body makes testosterone does not mean all the other cells will always respond properly • There are cases where the formation of the Mullerian ducts is inhibited • This leads to the default development – female external genitalia

Female Reproductive Organs • Ovaries are the primary female reproductive organs • Make female gametes (ova) • Secrete female sex hormones (estrogen and progesterone) • Accessory ducts include uterine tubes, uterus, and vagina • Internal genitalia – ovaries and the internal ducts • External genitalia – external sex organs • Labia major (homologous to male scrotum) • Labia minor (homologous to ventral penis) • Clitoris (homologous to the penis) • Erectile tissue hooded by the prepuce • The exposed portion is called the glans

Marsupials, such as opossums, have two unconnected horns, two cervices and two vaginas Male marsupials have a forked penis! A Comparison of Mammalian Uteruses

Events of Oogenesis • Production of female sex cells by meiosis • In the fetal period, oogonia (2n ovarian stem cells) multiply by mitosis and store nutrients • Primordial follicles appear as oogonia are transformed into primary oocytes • Primary oocytes begin meiosis but stall in prophase I • At puberty, one activated primary oocyte produces two haploid cells • The first polar body • The secondary oocyte • The secondary oocyte arrests in metaphase IIand is ovulated • If penetrated by sperm the second oocyte completes meiosis II, yielding: • One large ovum (the functional gamete) • A tiny second polar body

At birth, a female’s ovaries contains all the oocytes she will ever produce ~ 2 million oocytes are arrested in prophase I of the first meiotic division At puberty, the release of FSH causes the resumption of meiosis I in a few oocytes However, only one becomes dominant and is ovulated Mature egg cells are calledova (singular,ovum) This cycle is repeated about every 28 days The Ovary and Formation of an Ovum

Fertilization of the egg occurs high in the Fallopian tubes (also called uterine tubes or oviducts) The fertilized egg is now called a zygote It is transported to the uterus A muscular pear-shaped organ about the size of a fist It narrows to a muscular ring called the cervix Leads to the vagina Fertilization

The fertilized egg is pushed down the oviducts by the rhythmic contraction of its smooth muscles The journey takes 5-7 days The uterus is lined with a stratified epithelial membrane called the endometrium The zygote attaches to this layer and begins embryonic development! If the egg is not fertilized, the surface layer of the endometrium is shed during menstruation The underlying layer generates a new surface layer during the next cycle Fate of the Zygote

Hormones Coordinate the Reproductive Cycle • The female reproductive cycle is composed of two distinct phases separated by ovulation • Follicular phase • Egg reaches maturation • Ovulation • Ovary ruptures and releases the egg • Luteal phase • Body continues to prepare for pregnancy • A family of hormones coordinates these phases

Follicular Phase • Development of the egg within the ovary • The oocyte and its surrounding mass of tissue is called the follicle • FSH secretion triggers the maturation of several follicles and resumption of meiosis in their oocytes • But only one achieves full maturity • FSH also causes the ovary to secrete estrogen • Negative feedback by estrogen, causes the hypothalamus to stop the pituitary’s FSH output

Luteal Phase • The body is prepared for fertilization • Hypothalamus causes the anterior pituitary to begins secreting luteinizing hormone (LH) • LH inhibits further estrogen production • It also causes the wall of the follicles to burst • Oocyte is ovulated into oviducts • LH directs the repair of the ruptured follicle, which becomes the corpus luteum • The corpus luteum begins to secrete the hormone progesterone • Progesterone inhibits FSH • It also thickens the endometrium preparing for fertilization

Luteal Phase • If fertilization does not occur, progesterone production stops and the luteal phase ends • Thickened endometrial layer sloughs off • This causes the bleeding associated with menstruation • If fertilization does occur, the corpus luteum is maintained by human chorionic gonadotropin (hCG) • hCG is a hormone produced by the embryo • It is tested for in all pregnancy tests • Two other hormones are of importance • Prolactin • Stimulates milk production • Oxytocin • Initiates milk release • Induces labor

Structure of Lactating Mammary Glands • Modified sweat glands consisting of 15-25 lobes that radiate around and open at the nipple • Areola – pigmented skin surrounding the nipple • Suspensory ligaments attach the breast to underlying muscle fascia • Lobes contain glandular alveoli that produce milk in lactating women • Compound alveolar glands pass milk to lactiferous ducts, which open to the outside

Contraception • Several different methods are available • These differ in their effectiveness and acceptability to different couples • Abstinence • Simplest and most reliable way • Natural family planning, or the rhythm method • Prevention of egg maturation • Birth-control pills • Estrogen and progesterone • Shut down production of the pituitary hormones FSH and LH • Birth-control injections • Birth-control patches • Prevention of embryo implantation • Intrauterine devices (IUDs) • RU486(“morning after pill”) • Sperm blockage • Condoms • Diaphragms • Sperm destruction • Spermicidal jellies • Foams • Sterilization • Vasectomyin males • Tubal ligationin females

Sexually Transmitted Diseases • STDs are diseases that are spread from one person to another through sexual contact • They include: • Acquired Immune Deficiency Syndrome (AIDS) • Caused by the human immunodeficiency virus (HIV) • Gonorrhea • Caused by the bacterium Neisseria gonorrhoeae • Chlamydia • Caused by the bacterium Chlamydia trachomatis • Syphilis • Caused by the bacterium Treponema pallidum • Genital Herpes • Caused by the herpes simplex virus type 2 (HSV-2) • Most common STD in the US • Genital Human Papillomavirus(HPV) • Some types cause genital warts • HPV is a key factor in virtually all types of cervical cancer

Lecture 23 The Reproductive System