Chapter 46Animal Reproduction

Nudibranchs (sea slugs) – they are hermaphroditic

Types of Reproduction

• Asexual = no cell fusion

• Sexual– Combining genetic info from parents

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e.g., budding, fission, fragmentation &

regrowth and parthenogenesis

Important Features of Sexual Reproduction

Parent ≠ offspring

siblings ≠ siblings

New Combinations!

Sexual Reproduction

• Prior to 1770’s…– Sperm contained a

little human that needed incubation in a female.

Sexual Reproduction

• Sperm contained a little human that needed incubation in a female.

• Sperm and egg combine to form a zygote.

X

The Cost of Sex

Fig. 46-3

Asexual reproduction

Female

Sexual reproduction

Female

Generation 1

Male

Generation 2

Generation 3

Generation 4

What are the Disadvantages?What are the Advantages?

Remember only females can

reproduce in this example

Always produce 2 offspring!

The Cost of Sex

Fig. 46-3

Asexual reproduction

Female

Sexual reproduction

Female

Generation 1

Male

Generation 2

Generation 3

Generation 4

Disadvantage – Numbers (cost)Advantage - new combos of genes

- get rid of harmful genes

Population size stays the same!

The Human Life CycleOnly one set of chromosomes

Two sets of homologous chromosomes

• Meiosis and mitosis - many similarities, however…– Meiosis reduces the chromosome number from

diploid to haploid– Results in production of sperm and egg– Occurs in the gonads

• Testes• Ovaries

Meiosis vs. Mitosis

Copy once; divide twice– Meiosis I = reduction

division• separates homologous

chromosomes

– Meiosis II = equal division• separates sister chromatids

Meiosis – Reduction of Chromosome #...How?

Review Meiosis for the Exam!

Why do we need meiosis?

• Maintains chromosome # generation to generation

• Increases diversity within a species!

Animal Reproduction: Gametogenesis

• Gametogenesis:– Production of eggs and sperm

– Regulated by complex interplay of hormones

– Based on meiosis, but differs in males and females• Spermatogenesis (male)• Oogenesis (female)

Fertilization• Fertilization – union of egg and sperm• Two types:

– External – Internal

• External – Timing is critical (eggs & sperm)– Moist environment (prevent drying out and sperm swim

to egg)

– More gametes produced– Less parental care

Male Reproductive System

Fig. 46-11a

Seminal vesicle(behind bladder)

Urethra

Scrotum

(Urinarybladder)

Prostate gland

Bulbourethral Gland

Vas deferens

Epididymis

Testis

Erectile tissue of penis

Sperm…form in testes

(in seminiferous tubules)

mature in epididymis

stored in vas deferens

exit body via urethra / penis

Sperm within seminiferous tubules

Know name & function of all structures

Sperm complete their maturation in the epididymis (in about 3 weeks). From there, sperm flow to the vas deferens,

ejaculatory duct and urethra

• 60% volume of the semen fluid is produced by the seminal vesicles: mucus, fructose (energy), buffers and regulators

• The prostate gland contributes anticoagulant and citrate, a nutrient for sperm

• Secretions from the bulbourethral gland neutralize the pH in the urethra

Seminal fluid contains

secretions from accessory

glands

Speratogenesis

Fig. 46-12

Primordial germ cell in embryoMitotic divisions

Spermatogonialstem cell

Mitotic divisions

Spermatogonium

Mitotic divisions

Primary spermatocyte

Meiosis I

Secondary spermatocyte

Meiosis II

Differentiation

Sperm

2n

2n

2n

n n

n n n n

n n n n

Early spermatid

Spermatogonium

Primary spermatocyte

Secondary spermatocyte

Spermatids(two stages)

Lumen ofseminiferous tubule

SpermSingle sperm maturation = 7 weeks!

Fig. 46.11

2n

n

2n

n

n

Germ cell

Spermatogonal stem cell

2n

2n

Spermatids differentiate into

mature sperm

Each primary spermatocyte undergoes meiosis to produce

four spermatids

Fig. 46-12d

Tail

Neck

Midpiece Head

MitochondriaNucleus

Acrosome

Structure of SpermHead contains the haploid nucleus

At tip the acrosome

Lots of mitochondria

Tail = Flagellum

• Sperm forms in seminiferous tubules• The wall of the tubule contains Sertoli cells which

nourish and protect developing sperm• Surrounding the tubules are Leydig cells that

produce testosterone and other androgens

Testis

Maturation

Sperm are motileMust pass from male to female

Takes about 7 weeks!

Spermatogenesis vs. Oogenesis• Only in spermatogenesis do all four products of

meiosis develop into mature gametes. Females – do an unequal division

• Spermatogenesis occurs throughout adolescence and adulthood. Females - before birth

• Males – continuous females – long periods of interruptions

Hormones and Reproduction

Fig. 46-13-revised

Hypothalamus

GnRH

FSH

Anterior pituitary

LH

Neg

ativ

e fe

edb

ack

– –

–

TestesOvaries

Neg

ativ

e fe

edb

ack

Sex Hormones

Gonadotropin-releasing hormone

Follicle-stimulating hormone

Luteinizing hormone

Memorize this!

General chart for both males and females

FSH & LH are gonadotopins =

Act on male and female gonads

Sperm production is under hormonal control

Gonadotropin-releasing hormone

Follicle-stimulating hormone

Luteinizing hormone

Nourish developing

sperm

Testosterone level is 10 times higher in males!

Female Reproductive System

Fig. 46-10b

Ovaries Oviduct

Follicles

Uterine wallUterus

Cervix

Endometrium

Vagina

Corpus luteum

Eggs…develop in ovaries (gonads)

are fertilized in oviduct (fallopian tubes)

implanted in uterus (womb) if fertilized

Unlike primary spermatocytes which produce four sperm, each primary oocyte

produces only one viable egg

Each meiotic division involves unequal

cytokinesis. One nucleus gets no cytoplasm and is

called a polar body

Follicle!

Corpus Luteum

Know both sides of the story!

OogenesisOvary

Primary oocyte within follicle

Growing follicle

Mature follicle

Ruptured follicle

Ovulated secondary oocyte

Corpus luteum

Degenerating corpus luteum

Fig. 46-12

Primordial germ cell

Mitotic divisions

Oogonium

Mitotic divisions

Primary oocyte(present at birth), arrestedin prophase of meiosis I

Completion of meiosis I and onset of meiosis II

Secondary oocyte,arrested at metaphase of meiosis II

1st polar body

Ovulation, sperm entry

Completion of meiosis II

2nd polar body

Fertilized egg

2n

2n

n n

n

n

In embryo

At pubertyFSH starts

process!

Eggs are produced

within follicles in the ovary, usually only

one at a time

The timing of events…

Prophase I

Oogenesis vs. Spermatogenesis• Oogenesis

– Unequal distribution of cytoplasm • 1 oogonium

produces 1 functional egg and 3 polar bodies.

– Begins before birth and ceases at about age 50

– Discontinuous process• long “resting”

periods

• Spermatogenesis– Equal distribution

of cytoplasm • 1 spermatogonium

produces 4 functional sperm.

– Begins before birth, resumes in adolescence, and continues throughout adult life

– Continuous process• sperm are

produced continuously throughout a male’s life

Female Reproductive Cycles

• Two different types in mammals:– Menstrual (humans and other primates)– Estrous (other mammals)

Estrous vs. Menstrual Cycle• Estrous Cycle

– Animal other than humans and certain primates

– Endometrium reabsorbed in absence of pregnancy

– Sexual activity at time of ovulation

• Menstrual Cycle

– Humans and certain primates kkkkkkkkkkkk

– Endometrium shed

in absence of pregnancy k

– Sexual activity at any point during cycle

Thickening of endometrium prior to ovulations

The hypothalamus regulates the

anterior pituitary by releasing hormones

of it’s own

Releasing hormone

Inhibiting hormoneEx. Gonadotropin-releasing hormone

(GnRH)

Hormones and Reproduction

Fig. 46-13-revised

Hypothalamus

GnRH

FSH

Anterior pituitary

LH

Neg

ativ

e fe

edb

ack

– –

–

OvariesNeg

ativ

e fe

edb

ack

Sex Hormones Estradiol and Progesterone

Gonadotropin-releasing hormone

Human Female Reproductive SystemHormonal Control

Fig. 46-14a

Control by hypothalamusInhibited by combination of estradiol and progesterone

Stimulated by high levelsof estradiol

Inhibited by low levels of estradiol

Hypothalamus

GnRH

Anterior pituitary

FSH LH

(a)

–

–

+

Human Female Reproductive Cycles

• Uterine/menstrual cycle– Cyclic changes in the uterus– Average length 28 days

• Ovarian cycle– Cyclic changes in the ovaries

Hormones link and synchronize two cycles

The hormonal control of the ovaries and

uterus is cyclical

Whole picture – will break it

down!

GnRHTriggers a

Small release of FSH and

LH

Gonadotropin-releasing hormone

FSH and LH stimulate the

follicle to grow!

The low levels of esterdiol

keep secretion of FSH and LH

low

The growing follicle starts to release low

levels of esterdiol

(only)

2

1

Maturing follicle

produces increasing amounts of

estradiol

As estradiol increases so does the level of LH and

FSHIt has the opposite

effect as before(positive feedback)

LH cells in the pituitary are more

sensitive to the signal

1

2

3

Ovulation – about a day after the LH

surge

End of the Follicular phase and start of the

Luteal phase

LH hormone triggers the

formation of the corpus luteum from

the remaining follicle tissue

The corpus luteum produces both estradiol and progesterone

The combination of estradiol and

progesterone stops the formation of

GnRH – this reduces the flow of LH and FSH

1

2

3

Negative feedback from estradiol and progesterone prevent a new follicle from

maturing.

The low levels of LH and FSH causes the corpus luteum to disintegrate

This lowers the level of estradiol and progesterone secreted by the corpus

luteum

This triggers an increase in GnRH and this triggers the start

of the process all over again

1

2

3

If fertilization does not occur, the endometrium is sloughed off and the

cycle begins again.

Once the egg is released, it travels down the oviduct (AKA Fallopian tube) to the uterus.

If it isn’t fertilized, it will be lost during menstruation

If the egg is fertilized and the embryo implants in the uterus, the embryo releases human chorionic

gonadotropin (hCG), which maintains the corpus luteum and endometrium. Pregnancy tests detect

hCG.