Reproduction and Development

Reproduction and Development
36 Reproduction and Development

Overview: Pairing Up for Sexual Reproduction
Each sea slug produces sperm and eggs and thus acts as both mother and father to the next generation Animal reproduction takes many forms A population outlives its members only by reproduction, the generation of new individuals from existing ones 2

Figure 36.1 Figure 36.1 How can each of these sea slugs be both male and female? 3

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 4

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 5

Video: Hydra Budding

Figure 36.2 Figure 36.2 Asexual reproduction of a sea anemone (Anthopleura elegantissima) 7

Fragmentation is breaking of the body into pieces, some or all of which develop into adults
Fragmentation must be accompanied by regeneration, regrowth of lost body parts Parthenogenesis is the development of a new individual from an unfertilized egg 8

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 9

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

Figure Asexual reproduction Sexual reproduction Female Generation 1 Female Generation 2 Male Generation 3 Figure The “reproductive handicap” of sex (step 2) 11

Figure Asexual reproduction Sexual reproduction Female Generation 1 Female Generation 2 Male Generation 3 Figure The “reproductive handicap” of sex (step 3) Generation 4 12

Sexual reproduction results in genetic recombination
The resulting increased variation among offspring may enhance the reproductive success of parents in changing environments 13

Reproductive Cycles Most animals exhibit reproductive cycles related to changing seasons Reproductive cycles are controlled by hormones and environmental cues Because seasonal temperature is often an important cue in reproduction, climate change can decrease reproductive success 14

Figure 36.4 Figure 36.4 Caribou (Rangifer tarandus) mother and calf 15

Reproduction is exclusively asexual, and there are no males
In some species of whiptail lizards, the members of breeding pairs alternate roles Reproduction is exclusively asexual, and there are no males However, courtship and mating behavior still take place, with one female of each pair mimicking male mating behavior The two alternate roles two or three times during the breeding season 16

(a) A. uniparens females (b) The sexual behavior of A. uniparens is
Figure 36.5 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. 17

(a) A. uniparens females
Figure 36.5a Figure 36.5a Sexual behavior in parthenogenetic lizards (part 1: photo) (a) A. uniparens females 18

(b) The sexual behavior of A. uniparens is
Figure 36.5b Ovary size Ovulation Ovulation Progesterone Estradiol Hormone level Time Figure 36.5b Sexual behavior in parthenogenetic lizards (part 2: graph) Behavior Female Male- like Female Male- like (b) The sexual behavior of A. uniparens is correlated with the cycle of ovulation. 19

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 20

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 21

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 22

A moist habitat is almost always required for external fertilization to prevent gametes from drying out and to allow sperm to swim to eggs Many aquatic invertebrates simply shed gametes into the surrounding water In this case, timing of release of gametes is crucial to ensure that sperm and egg encounter one another If external fertilization is not synchronous across a population, courtship behaviors might lead to fertilization 23

Figure 36.6 Figure 36.6 External fertilization 24

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 25

Ensuring the Survival of Offspring
Internal fertilization is typically associated with production of fewer gametes but the survival of a higher fraction of zygotes Internal fertilization is also often associated with mechanisms to provide protection of embryos and parental care of young 26

Some other animals retain the embryo, which develops inside the female
The embryos of some terrestrial animals develop in eggs with calcium- and protein-containing shells and several internal membranes Some other animals retain the embryo, which develops inside the female In many animals, parental care helps ensure survival of offspring 27

Figure 36.7 Figure 36.7 Parental care in an invertebrate 28

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 29

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 30

Most insects have separate sexes with complex reproductive systems
In many insects, the female has a spermatheca in which sperm is stored during copulation 31

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 32

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 33

Animation: Male Reproductive Anatomy
Right click slide / Select play

(Urinary bladder) Seminal vesicle (Urinary duct) (Rectum) (Pubic bone)
Figure 36.8 (Urinary bladder) Seminal vesicle (Urinary duct) (Rectum) (Pubic bone) Vas deferens Erectile tissue Ejaculatory duct Figure 36.8 Reproductive anatomy of the human male Urethra Prostate gland Penis Vas deferens Glans Bulbourethral gland Epididymis Testis Prepuce Scrotum 35

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 36

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 37

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 38

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 39

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 40

Animation: Female Reproductive Anatomy

Oviduct Ovary Uterus (Urinary bladder) (Pubic bone) (Rectum) (Urethra)
Figure 36.9 Oviduct Ovary Uterus (Urinary bladder) (Pubic bone) (Rectum) (Urethra) Cervix Body Figure 36.9 Reproductive anatomy of the human female Glans Clitoris Vagina Prepuce Major vestibular gland Labia minora Labia majora Vaginal opening 42

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 43

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 44

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 45

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 46

Gametogenesis Gametogenesis is the production of gametes
There is a close relationship between the structure and function of the gonads 47

Video: Flagella in Sperm

Video: Sperm Flagellum

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 50

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 51

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 52

Tail Neck Plasma membrane Midpiece Head Acrosome Nucleus Mitochondria
Figure 36.10ac Tail Neck Plasma membrane Midpiece Head Acrosome Nucleus Figure 36.10ac Exploring human gametogenesis (part 1c: anatomy of sperm) Mitochondria 53

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 54

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 55

(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 56

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 57

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 58

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 59

Concept 36.3: The interplay of tropic and sex hormones regulates reproduction in mammals
Human reproduction is coordinated by hormones from the hypothalamus, anterior pituitary, and gonads Gonadotropin-releasing hormone (GnRH) is secreted by the hypothalamus and directs the release of FSH (follicle-stimulating hormone) and LH (luteinizing hormone) from the anterior pituitary 60

FSH and LH regulate processes in the gonads and the production of sex hormones
The major androgen is testosterone and major estrogens are estradiol and progesterone Both males and females produce androgens and estrogens but differ in their blood concentrations of particular hormones For example, males have testosterone levels about 10 times higher than females 61

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 62

Figure 36.11 Figure Androgen-dependent male anatomy and behavior in a lizard 63

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 64

Animation: Male Hormones

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 66

Two negative feedback mechanisms control sex hormone production in males
Testosterone regulates the production of GnRH, FSH, and LH through negative feedback mechanisms Sertoli cells secrete the hormone inhibin, which reduces FSH secretion from the anterior pituitary 67

Hormonal Control of Female Reproductive Cycles
Females produce eggs in cycles Prior to ovulation, the endometrium thickens with blood vessels in preparation for embryo implantation If an embryo does not implant in the endometrium, the endometrium is shed in a process called menstruation 68

Hormones closely link the two cycles of female reproduction
Changes in the uterus define the menstrual cycle (also called the uterine cycle) Changes in the ovaries define the ovarian cycle 69

Animation: Ovulation Right click slide / Select play

Animation: Post-Ovulation

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 72

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 73

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 74

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 75

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 76

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 77

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 78

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 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 79

Menopause After about 500 cycles, human females undergo menopause, the cessation of ovulation and menstruation Menopause is very unusual among animals Menopause might have evolved to allow a mother to provide better care for her children and grandchildren 80

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 Sexual receptivity is not limited to a time frame 81

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 82

Human Sexual Response Two reactions predominate in both sexes
Vasocongestion, the filling of tissue with blood Myotonia, increased muscle tension The sexual response cycle has four phases: excitement, plateau, orgasm, and resolution Excitement prepares the penis and vagina for coitus (sexual intercourse) 83

Direct stimulation of genitalia maintains the plateau phase and prepares the vagina for receipt of sperm Orgasm is characterized by rhythmic contractions of reproductive structures In males, semen is first released into the urethra and then ejaculated from the urethra In females, the uterus and outer vagina contract 84

During the resolution phase, organs return to their normal state and muscles relax
85

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 86

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 87

Fertilization Molecules and events at the egg surface play a crucial role in each step of fertilization Sperm penetrate the protective layer around the egg Receptors on the egg surface bind to molecules on the sperm surface Changes at the egg surface prevent polyspermy, the entry of multiple sperm nuclei into the egg 88

Video: Cortical Granule

Video: Calcium Release of Egg

Video: Calcium Wave of Egg

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 92

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 93

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 94

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 95

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 96

The events of fertilization also initiate metabolic reactions that trigger the onset of embryonic development, thus “activating” the egg Activation leads to events such as increased protein synthesis that precede the formation of a diploid nucleus Sperm entry triggers a release of Ca2+, which activates the egg and triggers the cortical reaction, the slow block to polyspermy 97

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 98

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 99

Video: Cleavage of Egg

(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 101

(a) Fertilized egg 50 m Figure 36.17a
Figure 36.17a Cleavage in an echinoderm embryo (part 1: fertilized egg) (a) Fertilized egg 102

(b) Four-cell stage 50 m Figure 36.17b
Figure 36.17b Cleavage in an echinoderm embryo (part 2: four-cell stage) (b) Four-cell stage 103

(c) Early blastula 50 m Figure 36.17c
Figure 36.17c Cleavage in an echinoderm embryo (part 3: early blastula) (c) Early blastula 104

(d) Later blastula 50 m Figure 36.17d
Figure 36.17d Cleavage in an echinoderm embryo (part 4: later blastula) (d) Later blastula 105

After cleavage, the rate of cell division slows
The remaining stages of embryonic development are responsible for morphogenesis, the cellular and tissue-based processes by which the animal body takes shape 106

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 107

The cell layers produced by gastrulation are called germ layers
The ectoderm forms the outer layer and the endoderm the inner layer In vertebrates and other animals with bilateral symmetry, a third germ layer, the mesoderm, forms between the endoderm and ectoderm 108

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) 109

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) 110

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) 111

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 112

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) 113

Blastocoel Filopodia Archenteron Mesenchyme cells Blastopore 50 m
Figure 36.18b Blastocoel Filopodia Archenteron Mesenchyme cells Figure 36.18b Gastrulation in a sea urchin embryo (part 2: LM) Blastopore 50 m 114

Sea urchins and other echinoderms are deuterostomes, as are chordates
Cell movements and interactions that form the germ layers vary among species One distinction is whether the mouth develops at the first opening that forms in the embryo (protostomes) or the second (deuterostomes) Sea urchins and other echinoderms are deuterostomes, as are chordates 115

Each germ layer contributes to a distinct set of structures in the adult animal
Some organs and many organ systems derive from more than one germ layer 116

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 117

Human Conception, Embryonic Development, and Birth
Conception, fertilization of an egg by a sperm, occurs in the oviduct The resulting zygote begins cleavage about 24 hours after fertilization and produces a blastocyst after 4 more days A few days later, the embryo implants into the endometrium of the uterus 118

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 119

(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 120

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 121

Occasionally an embryo splits during the first month of development, resulting in identical or monozygotic twins Fraternal, or dizygotic, twins arise when two eggs are fertilized and implant as two genetically distinct embryos Some embryos cannot complete development due to chromosomal or other abnormalities These spontaneously abort, often before a woman is aware of her pregnancy 122

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 123

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 124

Video: Ultrasound of Fetus

(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 126

Figure 36.21a Figure 36.21a Human fetal development (part 1: 5 weeks) (a) 5 weeks 127

Figure 36.21b Figure 36.21b Human fetal development (part 2: 14 weeks) (b) 14 weeks 128

Figure 36.21c Figure 36.21c Human fetal development (part 3: 20 weeks) (c) 20 weeks 129

During the second trimester the fetus grows and is very active
The mother may feel fetal movements as early as one month into the second trimester 130

During the third trimester, the fetus grows and fills the space within the embryonic membranes
131

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 132

Lactation, the production of milk, is an aspect of maternal care unique to mammals
Suckling stimulates the release of prolactin, which in turn stimulates mammary glands to produce milk, and the secretion of oxytocin, which triggers milk release from the glands 133

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 134

A health-care provider should be consulted for complete information on the choice and risks of contraception methods 135

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 136

Male Female Method Event Event Method Production of sperm
Figure 36.22a 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 Figure 36.22a Mechanisms of several contraceptive methods (part 1) Tubal ligation Spermicides; diaphragm; progestin alone (as minipill or injection) 137

Meeting of sperm and oocyte Implantation of blastocyst
Figure 36.22b Male Female Method Event Event Method Sperm movement through female reproductive tract Transport of oocyte in oviduct Meeting of sperm and oocyte in oviduct Morning-after pill; intrauterine device (IUD) Figure 36.22b Mechanisms of several contraceptive methods (part 2) Union of sperm and egg Implantation of blastocyst in endometrium 138

The rhythm method, or natural family planning, is temporary abstinence when conception is most likely; it has a pregnancy rate of 10–20% Coitus interruptus, the withdrawal of the penis before ejaculation, is unreliable Barrier methods block fertilization with a pregnancy rate of less than 10% A condom fits over the penis A diaphragm is inserted into the vagina before intercourse 139

Intrauterine devices (IUDs) are inserted into the uterus and interfere with fertilization and implantation; the pregnancy rate is less than 1% Female birth control pills are hormonal contraceptives with a pregnancy rate of less than 1% 140

Infertility and In Vitro Fertilization
Infertility, the inability to conceive offspring, affects about one in ten couples in the United States and worldwide The causes are varied and equally likely to affect men and women Sexually transmitted diseases (STDs) are the most significant preventable causes of infertility 141

Some forms of infertility are treatable
Hormone therapy can sometimes increase sperm or egg production In vitro fertilization (IVF) mixes oocytes with sperm in culture dishes and returns the embryo to the uterus at the eight-cell stage Sperm may be injected directly into an oocyte as well 142

Figure 36.UN01 Figure 36.UN01 Skills exercise: making inferences and designing an experiment 143

Human gametogenesis Spermatogenesis Oogenesis Primary spermatocyte
Figure 36.UN02 Human gametogenesis Spermatogenesis Oogenesis Primary spermatocyte Primary oocyte 2n 2n Polar body n Secondary spermatocytes Secondary oocyte n n n n n n n Spermatids Sperm n n n n Figure 36.UN02 Summary of key concepts: human gametogenesis n Polar body Fertilized egg n 144

Reproduction and Development

Similar presentations

Presentation on theme: "Reproduction and Development"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Reproduction and Development

Similar presentations

Presentation on theme: "Reproduction and Development"— Presentation transcript:

Similar presentations

About project

Feedback