Reproduction and Development

Reproduction and Development
36 Reproduction and Development

Do now: Explain how these organisms are reproducing

Different types of reproduction!
Some organisms: Change sex over lifetime Coral reef fish Are both male and female Ex: sea slugs Can fertilize their own eggs Earth worms Can reproduce without sex Bacteria Only have certain members of their population reproduce bees

Discuss: Differences between sexual and asexual reproduction
Different types of asexual reproduction

Concept 36.1: Both asexual and sexual reproduction occur in the animal kingdom
Sexual reproduction is the creation of an offspring by fusion of a male gamete (sperm) and female gamete (egg) to form a zygote Asexual reproduction is the creation of offspring without the fusion of egg and sperm Binary fission Parthogenesis (egg develops without fertilization eg: ants) Budding Fragmentation and Regeneration 5

Mechanisms of Asexual Reproduction
Many invertebrates reproduce asexually by budding, in which new individuals arise from outgrowths of existing ones Also common among invertebrates is fission, the separation of a parent organism into two individuals of approximately equal size Video: Hydra Budding 6

Figure 36.5 In asexual reproduction, courtship and mating behavior still take place, with one female of each pair mimicking male mating behavior Ovary size Ovulation Ovulation Progesterone Estradiol Hormone level Time Behavior Female Male- like Female Male- like Figure 36.5 Sexual behavior in parthenogenetic lizards (a) A. uniparens females (b) The sexual behavior of A. uniparens is correlated with the cycle of ovulation. 7

If each parent produced 2 offspring, which type of reproduction would produce more offspring?
Asexual reproduction Sexual reproduction Female Generation 1 Female Generation 2 Male Figure The “reproductive handicap” of sex (step 1) 8

Asexual reproduction Sexual reproduction Generation 1 Female Female
Figure Asexual reproduction Sexual reproduction Female Generation 1 Female Generation 2 Male Generation 3 Figure The “reproductive handicap” of sex (step 2) 9

Asexual reproduction Sexual reproduction Generation 1 Female Female
Figure Asexual reproduction Sexual reproduction Female Generation 1 Female Generation 2 Male Generation 3 Figure The “reproductive handicap” of sex (step 3) Generation 4 10

Sexual Reproduction: An Evolutionary Enigma
Sexual females have half as many daughters as asexual females; this is the “twofold cost” of sexual reproduction Despite this, almost all eukaryotic species reproduce sexually Why??? 11

genetic recombination Variation allows for greater chances of survival
12

Discuss: What challenges might organisms that are isolated have regarding : sexual reproduction? How might we overcome them?

Variation in Patterns of Sexual Reproduction
For many animals, finding a partner for sexual reproduction may be challenging One solution is hermaphroditism, in which each individual has male and female reproductive systems Any two hermaphrodites can mate, and some hermaphrodites can self-fertilize 14

Other solutions: Individuals of some species undergo sex reversals
For example, in a coral reef fish, the bluehead wrasse, a lone male defends a group of females If the male dies, the largest female in the group transforms into a male Within a few weeks, this individual can begin to produce sperm instead of eggs 15

Fertilization Can occur internally or externally.
Compare and contrast the two methods. What might be the advantages? Disadvantages?

External and Internal Fertilization
Sexual reproduction requires fertilization, the union of sperm and eggs In external fertilization, eggs shed by the female are fertilized by sperm in the external environment In internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract 17

Characteristics : Internal External
Environment does not need to be moist Need to have reproductive structures which will allow for proper meeting of sperm and egg Usually fewer are produced, but more likely to survive Environment must be moist to prevent drying out Need to time so both are released at the same time Lots of zygotes created Ex: aquatic organisms

Figure 36.6 Figure 36.6 External fertilization 19

However fertilization occurs, the mating animals may use pheromones, chemicals released by one organism that can influence physiology and behavior of another individual of the same species 20

Concept 36.2: Reproductive organs produce and transport gametes
Sexual reproduction in animals relies on sets of cells that are precursors for eggs and sperm 21

Variation in Reproductive Systems
Many (but not all) animals have gonads, organs that produce gametes Some simple systems do not have gonads, but gametes form from undifferentiated tissue More elaborate systems include sets of accessory tubes and glands that carry, nourish, and protect gametes and sometimes developing embryos 22

Most insects have separate sexes with complex reproductive systems
In many insects, the female has a spermatheca in which sperm is stored during copulation 23

A cloaca is a common opening between the external environment and the digestive, excretory, and reproductive systems A cloaca is common in nonmammalian vertebrates; mammals usually have a separate opening to the digestive tract 24

Label structures A-E and hypothesize their functions
(Urinary bladder) Seminal vesicle (Urinary duct) (Rectum) (Pubic bone) D Erectile tissue Ejaculatory duct Figure 36.8 Reproductive anatomy of the human male Urethra Prostate gland E D Glans Bulbourethral gland C B Prepuce A 25

Human Male Reproductive Anatomy
The male’s external reproductive organs are the scrotum and penis The internal organs are gonads that produce sperm and reproductive hormones, accessory glands that secrete products essential to sperm movement, and ducts that carry sperm and glandular secretions Animation: Male Reproductive Anatomy 26

Testes The male gonads, or testes, produce sperm in highly coiled tubes called seminiferous tubules Production of normal sperm cannot occur at the body temperatures of most mammals The scrotum, a fold of the body wall, maintains testis temperature at about 2oC below the core body temperature 27

Ducts From the seminiferous tubules of a testis, sperm pass into the coiled duct of the epididymis During ejaculation, sperm are propelled through the muscular vas deferens and the ejaculatory duct and then exit the penis through the urethra 28

Accessory Glands Semen is composed of sperm plus secretions from three sets of accessory glands The two seminal vesicles contribute about 60% of the total volume of semen The prostate gland secretes its products directly into the urethra through several small ducts The bulbourethral glands secrete a clear mucus before ejaculation that neutralizes acidic urine remaining in the urethra 29

Penis The human penis is composed of three cylinders of spongy erectile tissue During sexual arousal, erectile tissue fills with blood from the arteries, causing an erection The head of the penis, or glans, has a thinner skin covering than the shaft The prepuce, or foreskin, is a fold of skin covering the glans; it is removed when a male is circumcised 30

Label structures A-D and name the function
(Urinary bladder) (Pubic bone) (Rectum) (Urethra) B Body Figure 36.9 Reproductive anatomy of the human female Glans Clitoris A Prepuce Major vestibular gland Labia minora Labia majora 31

Human Female Reproductive Anatomy
The female external reproductive structures include the clitoris and two sets of labia The internal organs are a pair of gonads and a system of ducts and chambers that carry gametes and house the embryo and fetus Animation: Female Reproductive Anatomy 32

Ovaries The female gonads, a pair of ovaries, lie in the abdominal cavity Each ovary contains many follicles, which consist of a partially developed egg, called an oocyte, surrounded by support cells 33

Oviducts and Uterus Upon ovulation, a mature egg cell is released
It travels from the ovary to the uterus via an oviduct Cilia in the oviduct convey the egg to the uterus, also called the womb The uterus lining, the endometrium, has many blood vessels The neck of the uterus is the cervix, which opens into the vagina 34

Vagina and Vulva The vagina is a muscular but elastic chamber that is the repository for sperm during copulation and serves as the birth canal The vagina opens to the outside at the vulva, which consists of the labia majora, labia minora, hymen, and clitoris 35

Mammary Glands The mammary glands are not part of the reproductive system but are important to mammalian reproduction Within the glands, small sacs of epithelial tissue secrete milk 36

Do now: In mammals, which set of chromosomes determine whether an organism will be male? Female? Describe the experiment Alfred Jost conducted in order to determine whether hormones from gonads are necessary in order to determine sex.

Experiment: Interpret: Data/Results
#1. what was blocked in the experiment? What inference can you make about gonads in controlling development of genitalia? #2.some aspect of surgery other than gonad removal results in said data. How to test validity? #3. what result to be seen if female development also required a signal from a gonad?

Gametogenesis Gametogenesis is the production of gametes
What are characteristics of gametes in terms of chromosome #? Through what process are gametes made? Where does this happen in males? Females? There is a close relationship between the structure and function of the gonads Video: Flagella in Sperm Video: Sperm Flagellum 39

Figure 36.10a Epididymis Seminiferous tubule Testis Primordial germ cell in embryo Cross section of seminiferous tubule Mitotic divisions Spermatogonial stem cell 2n Sertoli cell nucleus Mitotic divisions Spermatogonium 2n Mitotic divisions Primary spermatocyte 2n Meiosis I Figure 36.10a Exploring human gametogenesis (part 1: spermatogenesis) Secondary spermatocyte n n Lumen of seminiferous tubule Meiosis II Spermatids (two stages) Early spermatid n n n n Tail Neck Differentiation (Sertoli cells provide nutrients) Plasma membrane Midpiece Head Sperm cell n n n n Acrosome Nucleus Mitochondria 40

Primary spermatocyte Secondary spermatocyte
Figure 36.10aa Epididymis Seminiferous tubule Spermatogonium Sertoli cell nucleus Primary spermatocyte Testis Secondary spermatocyte Cross section of seminiferous tubule Spermatids (two stages) Figure 36.10aa Exploring human gametogenesis (part 1a: spermatogenesis, location) Sperm cell Lumen of seminiferous tubule 41

Primordial germ cell in embryo
Figure 36.10ab Primordial germ cell in embryo Mitotic divisions Spermatogonial stem cell 2n Mitotic divisions Spermatogonium 2n Mitotic divisions Primary spermatocyte 2n Meiosis I Secondary spermatocyte n n Meiosis II Figure 36.10ab Exploring human gametogenesis (part 1b: spermatogenesis, cell divisions) Early spermatid n n n n Differentiation (Sertoli cells provide nutrients) Sperm cell n n n n 42

Structure enables function: what is the purpose of the flagella tail
Structure enables function: what is the purpose of the flagella tail? Mitochondria? Tail Neck Plasma membrane Midpiece Head Acrosome Nucleus Figure 36.10ac Exploring human gametogenesis (part 1c: anatomy of sperm) Mitochondria 43

Role of feedback loops in human male and female reproductive systems:
Discuss: Role of FSH and LH on male reproductive system (refer to muscle man hw) Difference between positive and negative feedback loop

Hormonal Control of the Male Reproductive System
FSH and LH act on different cells in the testes to direct spermatogenesis Sertoli cells, found in the seminiferous tubules, respond to FSH by nourishing developing sperm Leydig cells, connective tissue between the tubules, respond to LH by secreting testosterone and other androgens, which promote spermatogenesis Animation: Male Hormones 45

Spermatogenesis = making of sperm (gametes)
1. Interpreting a negative feedback diagram: Which structure releases GnRH? Which structure responds to GnRH? How does this structure respond? Which structure responds to FSH? LH? What do sertoli cells do? What do leydig cells do?

Spermatogenesis = making of sperm
Analyzing a negative feedback diagram Which is the stimulus that causes the release of GnRH? What stimulus causes the release of FSH ? LH? Which two hormones ultimately regulate the negative feedback loop?

Spermatogenesis = making of sperm
3. Making predictions of a negative feedback diagram If testosterone levels are too high will the following hormones get released? GnRH, LH, FSH If inhibin levels are too low which of the following hormones will get released? GnRH, FSH, LH If testosterone levels are too low but the leydig cells are defective, will sperm get made? What if the sertoli cells were defective?

Spermatogenesis : making of sperm
Using a real life application of the negative feedback loop Explain why taking steroids (chemicals which mimic testosterone) will cause shrinkage of the testes Explain why removal of one testicle can cause the enlargement of the other

Hypothalamus GnRH Anterior pituitary FSH LH Negative feedback
Figure 36.12 Hypothalamus GnRH Anterior pituitary FSH LH Negative feedback Negative feedback Sertoli cells Leydig cells Figure Hormonal control of the testes Inhibin Spermatogenesis Testosterone Testis 50

What would happen to sperm production if…
The receptors on the anterior pituitary were not able to bind GnRH? The receptors on sertoli cells were not able to bind FSH? You took a supplement which acts like testosterone in the feedback loop? You took a supplement which acts like FSH in a feedback loop?

Figure 36.10b Ovary Primary oocyte within follicle Growing follicle In embryo Primordial germ cell Mitotic divisions Mature follicle 2n Oogonium Mitotic divisions Primary oocyte (present at birth), arrested in prophase of meiosis I 2n Ruptured follicle Completion of meiosis I and onset of meiosis II First polar body n n Secondary oocyte, arrested at metaphase of meiosis II Figure 36.10b Exploring human gametogenesis (part 2: oogenesis) Ovulated secondary oocyte Ovulation, sperm entry Completion of meiosis II Corpus luteum Second polar body n Fertilized egg n Degenerating corpus luteum 52

Ovary Primary Ruptured oocyte follicle within follicle Ovulated
Figure 36.10ba Ovary Primary oocyte within follicle Ruptured follicle Ovulated secondary oocyte Growing follicle Mature follicle Corpus luteum Figure 36.10ba Exploring human gametogenesis (part 2a: oogenesis, ovulation) Degenerating corpus luteum 53

(present at birth), arrested in prophase of meiosis I 2n
Figure 36.10bb In embryo Primordial germ cell Mitotic divisions 2n Oogonium Mitotic divisions Figure 36.10bb Exploring human gametogenesis (part 2b: oogenesis, meiosis I) Primary oocyte (present at birth), arrested in prophase of meiosis I 2n 54

Completion of meiosis I and onset of meiosis II First polar body n n
Figure 36.10bc Completion of meiosis I and onset of meiosis II First polar body n n Secondary oocyte, arrested at metaphase of meiosis II Ovulation, sperm entry Figure 36.10bc Exploring human gametogenesis (part 2c: oogenesis, meiosis II) Completion of meiosis II Second polar body n Fertilized egg n 55

Gametogenesis: how are they similar?different?
Male Female

Oogenesis, the development of a mature egg, is a prolonged process
Spermatogenesis, the development of sperm, is continuous and prolific (millions of sperm are produced per day); each sperm takes about 7 weeks to develop Oogenesis, the development of a mature egg, is a prolonged process Immature eggs form in the female embryo but do not complete their development until years or decades later 57

Spermatogenesis differs from oogenesis in three ways
All four products of meiosis develop into sperm, while only one of the four becomes an egg Spermatogenesis occurs throughout adolescence and adulthood Sperm are produced continuously without the prolonged interruptions in oogenesis 58

Sex hormones regulate gamete production both directly and indirectly
They serve many additional functions including sexual behavior and the development of primary and secondary sex characteristics Androgens are responsible for male vocalizations and courtship displays 59

Hormonal control of the female reproductive cycle:
Hormones closely link the two cycles of female reproduction Changes in the uterus define the menstrual cycle (also called the uterine cycle) Purpose of the uterus: Changes in the ovaries define the ovarian cycle Purpose of the ovary: Animation: Ovulation Animation: Post-Ovulation 60

Figure 36.13 The reproductive cycles of the human female
Control by hypothalamus Inhibited by combination of estradiol and progesterone Hypothalamus Stimulated by high levels of estradiol 1 GnRH Anterior pituitary Inhibited by low levels of estradiol 2 FSH LH (b) Pituitary gonadotropins in blood 6 LH FSH 3 FSH and LH stimulate follicle to grow LH surge triggers ovulation (c) Ovarian cycle 7 8 Growing follicle Corpus luteum Maturing follicle Degenerating corpus luteum Follicular phase Ovulation Luteal phase Estradiol secreted by growing follicle in increasing amounts Progesterone and estradiol secreted by corpus luteum 4 (d) Ovarian hormones in blood Peak causes LH surge (see ) 5 6 Figure The reproductive cycles of the human female 10 Estradiol 9 Progesterone Estradiol level very low Progesterone and estradiol promote thickening of endometrium (e) Uterine (menstrual) cycle Endometrium Menstrual flow phase Proliferative phase Secretory phase Days 5 10 14 15 20 25 28 61

Control by hypothalamus Inhibited by combination of
Figure 36.13a (a) Control by hypothalamus Inhibited by combination of estradiol and progesterone Hypothalamus Stimulated by high levels of estradiol 1 GnRH Anterior pituitary Inhibited by low levels of estradiol 2 FSH LH (b) Pituitary gonadotropins in blood 6 Figure 36.13a The reproductive cycles of the human female (part 1: FSH and LH levels) LH FSH FSH and LH stimulate follicle to grow LH surge triggers ovulation 3 Days 5 10 14 15 20 25 28 62

Check your understanding..
1. Which hormone is directly part of positive and negative feedback? 2. What effect does GnRH have? 3. FSH and LH are going to go affect which organ?

Corpus luteum Degenerating corpus luteum Maturing follicle
Figure 36.13b (b) Pituitary gonadotropins in blood 6 LH FSH 3 FSH and LH stimulate follicle to grow LH surge triggers ovulation (c) Ovarian cycle 7 8 Figure 36.13b The reproductive cycles of the human female (part 2: ovarian cycle) Corpus luteum Degenerating corpus luteum Growing follicle Maturing follicle Follicular phase Ovulation Luteal phase Estradiol secreted by growing follicle in increasing amounts Progesterone and estradiol secreted by corpus luteum 4 Days 5 10 14 15 20 25 28 64

Check your understanding…
4.What effect does FSH and LH have on the ovary? 5.Which hormone does the follicle secrete as it matures? 6.Which organ is going to contain the target cells of this hormone?

Corpus luteum Degenerating corpus luteum Maturing follicle
Figure 36.13c (c) Ovarian cycle 7 8 Corpus luteum Degenerating corpus luteum Growing follicle Maturing follicle Follicular phase Ovulation Luteal phase Estradiol secreted by growing follicle in increasing amounts Progesterone and estradiol secreted by corpus luteum 4 (d) Ovarian hormones in blood Peak causes LH surge (see ) 6 5 Figure 36.13c The reproductive cycles of the human female (part 3: ovarian hormone levels) 10 9 Estradiol Progesterone Estradiol level very low Progesterone and estradiol promote thickening of endometrium Days 5 10 14 15 20 25 28 66

7.What effect does estradiol have on the uterine lining? 8.Why do you think this response is appropriate? 9.What effect does increased estradiol have on the hypothalamus? 10.What eventually causes the rupture of the follicle?

11.Which hormones are secreted by the corpus luteum? 12.Why are these hormones appropriate for this time period? 13.What happens to these hormone levels as the corpus luteum degenerates? 14.Why is this action appropriate?

Putting it all together..
What effect do low estrogen levels have on the hypothalamus? What effect do high estrogen levels have on the hypothalamus? Why are high estrogen levels associated with thickening of the uterine lining? What causes the corpus luteum to disintegrate? What effect does this have on the feedback system? If you wanted to prevent pregnancy by stopping egg production, what hormone would your birth control want to mimic?

Progesterone and estradiol promote thickening of endometrium
Figure 36.13d (d) Ovarian hormones in blood Peak causes LH surge (see ) 6 5 10 9 Estradiol Progesterone Estradiol level very low Progesterone and estradiol promote thickening of endometrium (e) Uterine (menstrual) cycle Endometrium Figure 36.13d The reproductive cycles of the human female (part 4: menstrual cycle) Menstrual flow phase Proliferative phase Secretory phase Days 5 10 14 15 20 25 28 70

The Ovarian Cycle The sequential release of GnRH then FSH and LH stimulates follicle growth Follicle and oocyte growth and an increase in the hormone estradiol characterize the follicular phase of the ovarian cycle Estradiol causes GnRH, FSH, and LH levels to rise through positive feedback The final result is the maturation of the follicle 71

At the end of the follicular phase, the follicle releases the secondary oocyte
The follicular tissue left behind transforms into the corpus luteum in response to LH; this is the luteal phase The corpus luteum disintegrates, and ovarian steroid hormones decrease by negative feedback mechanisms 72

The Uterine (Menstrual) Cycle
Hormones coordinate the uterine cycle with the ovarian cycle Thickening of the endometrium during the proliferative phase coordinates with the follicular phase Why? Secretion of nutrients during the secretory phase coordinates with the luteal phase Shedding of the endometrium during the menstrual flow phase coordinates with the growth of new ovarian follicles 73

Exit Ticket: Female Reproductive System:
Name the two hormones which control the ovarian cycle, explain what triggers their release, where they are released from and what effect they have Name the two hormones which controls the uterine cycle, explain what triggers their release, where they are released from and what effect they have.

Menopause After about 500 cycles, human females undergo menopause, the cessation of ovulation and menstruation Menopause is very unusual among animals What might be the evolutionary benefit of menstruation? 75

Menstrual Versus Estrous Cycles
Menstrual cycles are characteristic only of humans and some other primates The endometrium is shed from the uterus in a bleeding called menstruation Estrous cycles are characteristic of most mammals The endometrium is reabsorbed by the uterus Sexual receptivity is limited to a “heat” period The length and frequency of estrus cycles vary from species to species Sexual receptivity is not limited to a time frame 76

Concept 36.4: Fertilization, cleavage, and gastrulation initiate embryonic development
Across animal species, embryonic development involves common stages occurring in a set order First is fertilization, which forms a zygote During the cleavage stage, a series of mitoses divide the zygote into a many-celled embryo The resulting blastula then undergoes rearrangements into a three-layered embryo called a gastrula 77

EMBRYONIC DEVELOPMENT Sperm Zygote
Figure 36.14 EMBRYONIC DEVELOPMENT Sperm Zygote Adult frog Egg FERTILIZATION CLEAVAGE Metamorphosis Blastula GASTRULATION Figure Developmental events in the life cycle of a frog ORGANO- GENESIS Larval stages Gastrula Tail-bud embryo 78

Basal body (centriole) Sperm head Acrosome Jelly coat Vitelline layer
Figure Basal body (centriole) Sperm head Figure The acrosomal and cortical reactions during sea urchin fertilization (step 1) Acrosome Jelly coat Vitelline layer Sperm- binding receptors Egg plasma membrane 79

Basal body (centriole) Sperm head Acrosome Hydrolytic enzymes
Figure Basal body (centriole) Sperm head Figure The acrosomal and cortical reactions during sea urchin fertilization (step 2) Acrosome Hydrolytic enzymes Jelly coat Vitelline layer Sperm- binding receptors Egg plasma membrane 80

Sperm nucleus Acrosomal process Basal body (centriole) Actin filament
Figure Sperm nucleus Acrosomal process Basal body (centriole) Actin filament Sperm head Figure The acrosomal and cortical reactions during sea urchin fertilization (step 3) Acrosome Hydrolytic enzymes Jelly coat Vitelline layer Sperm- binding receptors Egg plasma membrane 81

Sperm plasma membrane Sperm nucleus Acrosomal process Basal body
Figure Sperm plasma membrane Sperm nucleus Acrosomal process Basal body (centriole) Actin filament Sperm head Fused plasma membranes Figure The acrosomal and cortical reactions during sea urchin fertilization (step 4) Acrosome Hydrolytic enzymes Jelly coat Vitelline layer Sperm- binding receptors Egg plasma membrane 82

Sperm plasma membrane Sperm nucleus Acrosomal process Basal body
Figure Sperm plasma membrane Sperm nucleus Acrosomal process Basal body (centriole) Actin filament Sperm head Cortical granule Fused plasma membranes Figure The acrosomal and cortical reactions during sea urchin fertilization (step 5) Acrosome Perivitelline space Hydrolytic enzymes Jelly coat Fertilization envelope Vitelline layer Sperm- binding receptors Egg plasma membrane 83

Acrosomal reaction: plasma membrane
Figure 36.16 1 2 3 4 6 8 10 20 30 40 50 5 60 90 Binding of sperm to egg Acrosomal reaction: plasma membrane depolarization (fast block to polyspermy) Seconds Increased intracellular calcium level Cortical reaction (slow block to polyspermy) Formation of fertilization envelope complete Figure Timeline for the fertilization of sea urchin eggs Increased protein synthesis Minutes Fusion of egg and sperm nuclei complete Onset of DNA synthesis First cell division 84

Cleavage and Gastrulation
Fertilization is followed by cleavage, a period of rapid cell division without growth Cleavage partitions the cytoplasm of one large cell into many smaller cells The blastula is a ball of cells with a fluid-filled cavity called a blastocoel The blastula is produced after about five to seven cleavage divisions Video: Cleavage of Egg 85

(a) Fertilized egg (b) Four-cell stage (c) Early blastula
Figure 36.17 50 m (a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula Figure Cleavage in an echinoderm embryo 86

During gastrulation, a set of cells at or near the surface of the blastula moves to an interior location, cell layers are established, and a primitive digestive tube forms The hollow blastula is reorganized into a two- or three-layered embryo called a gastrula 87

Digestive tube (endoderm) Anus (from blastopore)
Figure 36.18 Animal pole Blastocoel Mesenchyme cells Vegetal plate Vegetal pole Blastocoel Filopodia Archenteron Mesenchyme cells Blastopore Figure Gastrulation in a sea urchin embryo 50 m Key Blastocoel Future ectoderm Ectoderm Future mesoderm Archenteron Future endoderm Mouth Blastopore Mesenchyme (mesoderm forms future skeleton) Digestive tube (endoderm) Anus (from blastopore) 88

Animal pole Future ectoderm Blastocoel Future mesoderm Mesenchyme
Figure 36.18a-1 Animal pole Future ectoderm Blastocoel Future mesoderm Mesenchyme cells Future endoderm Vegetal plate Vegetal pole Figure 36.18a-1 Gastrulation in a sea urchin embryo (part 1, step 1) 89

Figure 36.18a-2 Animal pole Future ectoderm Blastocoel Future mesoderm Mesenchyme cells Future endoderm Vegetal plate Filopodia Vegetal pole Archenteron Figure 36.18a-2 Gastrulation in a sea urchin embryo (part 1, step 2) 90

Figure 36.18a-3 Animal pole Future ectoderm Blastocoel Future mesoderm Mesenchyme cells Future endoderm Vegetal plate Filopodia Vegetal pole Archenteron Figure 36.18a-3 Gastrulation in a sea urchin embryo (part 1, step 3) Blastocoel Archenteron Blastopore 91

Digestive tube (endoderm) Anus (from blastopore)
Figure 36.18a-4 Animal pole Future ectoderm Blastocoel Future mesoderm Mesenchyme cells Future endoderm Vegetal plate Filopodia Vegetal pole Archenteron Figure 36.18a-4 Gastrulation in a sea urchin embryo (part 1, step 4) Blastocoel Ectoderm Archenteron Mouth Blastopore Mesenchyme (mesoderm forms future skeleton) Digestive tube (endoderm) Anus (from blastopore) 92

ECTODERM (outer layer of embryo)
Figure 36.19 ECTODERM (outer layer of embryo) Epidermis of skin and its derivatives (including sweat glands, hair follicles) Nervous and sensory systems Pituitary gland, adrenal medulla Jaws and teeth Germ cells MESODERM (middle layer of embryo) Skeletal and muscular systems Circulatory and lymphatic systems Excretory and reproductive systems (except germ cells) Dermis of skin Adrenal cortex Figure Major derivatives of the three embryonic germ layers in vertebrates ENDODERM (inner layer of embryo) Epithelial lining of digestive tract and associated organs (liver, pancreas) Epithelial lining of respiratory, excretory, and reproductive tracts and ducts Thymus, thyroid, and parathyroid glands 93

Human pregnancy averages 38 weeks from fertilization
The condition of carrying one or more embryos in the uterus is called gestation, or pregnancy Human pregnancy averages 38 weeks from fertilization Gestation varies among mammals, from 21 days in many rodents to more than 600 days in elephants Human gestation is divided into three trimesters of about three months each 94

(a) From ovulation to implantation
Figure 36.20 3 Cleavage 4 Cleavage continues. Ovary 2 Fertilization Uterus 5 Implantation 1 Ovulation Endometrium (a) From ovulation to implantation Figure Formation of a human zygote and early postfertilization events Endometrium Inner cell mass Cavity Trophoblast Blastocyst (b) Implantation of blastocyst 95

During the first trimester, the embryo secretes hormones that signal its presence and regulate the mother’s reproductive system Human chorionic gonadotropin (hCG) acts to maintain secretion of progesterone and estrogens by the corpus luteum through the first few months of pregnancy Why must we maintain secretion of progesterone and estrogens? What would happen if it were to stop? 96

During its first 2 to 4 weeks, the embryo obtains nutrients directly from the endometrium
Meanwhile, the outer layer of the blastocyst, called the trophoblast, mingles with the endometrium and eventually forms the placenta Materials diffuse between maternal and embryonic circulation to supply the embryo with nutrients, immune protection, and metabolic waste disposal 97

The first trimester is the main period of organogenesis, development of the body organs
All the major structures are present by 8 weeks, and the embryo is called a fetus Video: Ultrasound of Fetus 98

(a) 5 weeks (b) 14 weeks (c) 20 weeks Figure 36.21
Figure Human fetal development (a) 5 weeks (b) 14 weeks (c) 20 weeks 99

Childbirth begins with labor, a series of strong, rhythmic contractions that push the fetus and placenta out of the body Once labor begins, local regulators, prostaglandins and hormones, induce and regulate further contractions of the uterus Which hormones are involved? What kind of feedback? 100

Lactation, the production of milk, is an aspect of maternal care unique to mammals
Which hormone causes milk production of mammary glands? Positive or negative feedback? Which hormone triggers milk release? Positive or negative feedback? 101

Contraception Contraception, the deliberate prevention of pregnancy, can be achieved in a number of ways Contraceptive methods fall into three categories Preventing development or release of eggs and sperm Keeping sperm and egg apart Preventing implantation of an embryo 102

Meeting of sperm and oocyte Implantation of blastocyst
Figure 36.22 Male Female Method Event Event Method Production of sperm Production of primary oocytes Vasectomy Combination birth control pill (or injection, patch, or vaginal ring) Sperm transport down male duct system Oocyte development and ovulation Abstinence Abstinence Condom Female condom Coitus interruptus (very high failure rate) Sperm deposited in vagina Capture of the oocyte by the oviduct Tubal ligation Spermicides; diaphragm; progestin alone (as minipill or injection) Sperm movement through female reproductive tract Transport of oocyte in oviduct Figure Mechanisms of several contraceptive methods Meeting of sperm and oocyte in oviduct Morning-after pill; intrauterine device (IUD) Union of sperm and egg Implantation of blastocyst in endometrium 103

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