Lecture 23
The 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
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There are three strategies for internal fertilization
1.
2.
3.
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
 The testis produces sperm
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 the epididymis for 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
The Male Penis
 The penis contains 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
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

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


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
A Comparison of Mammalian Uteruses
 Marsupials, such as opossums, have two unconnected horns, two
cervices and two vaginas
 Male marsupials have a forked penis!
Events of Oogenesis

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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 II and is ovulated
If penetrated by sperm the second
oocyte completes meiosis II, yielding:
 One large ovum (the functional
gamete)
 A tiny second polar body
The Ovary and Formation of an Ovum
 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 called
ova (singular, ovum)
 This cycle is repeated about
every 28 days
Fertilization
 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
Fate of the Zygote
 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
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

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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
Vasectomy in males
Tubal ligation in 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