Revised Ch45 Ch46.pptx

Chapter 45

Overview: The Body’s Long-Distance Regulators

• Animal hormones are chemical signals that are secreted into the circulatory system and

communicate regulatory messages within the body

• *Hormones reach all parts of the body, but only target cells have receptors for that hormone

• *Two systems coordinate communication

throughout the body: the endocrine system and the nervous system

• The endocrine system secretes hormones that coordinate slower but longer-acting responses including reproduction, development, energy metabolism, growth, and behavior

• The nervous system conveys high-speed

electrical signals along specialized cells called neurons; these signals regulate other cells

Concept 45.1: Hormones and other signaling

molecules bind to target receptors, triggering

specific response pathways

• Endocrine signaling is just one of several ways that information is transmitted between animal cells

Intercellular Communication

• The ways that signals are transmitted between animal cells are classified by two criteria

– The type of secreting cell

– The route taken by the signal in reaching its target

Endocrine Signaling

• Hormones secreted into extracellular fluids by endocrine cells reach their targets via the

bloodstream

• Endocrine signaling maintains homeostasis,

mediates responses to stimuli, regulates growth and development

Figure 45.2

(a) Endocrine signaling Blood

vessel Response

Response

Response

Synapse

Response

Response (b) Paracrine signaling

(c) Autocrine signaling

Neuron

(d) Synaptic signaling

Neurosecretory cell

Blood vessel

Paracrine and Autocrine Signaling

• _{Local regulators are molecules that act over} short distances, reaching target cells solely by diffusion

• In paracrine signaling, the target cells lie near the secreting cells

• In autocrine signaling, the target cell is also the secreting cell

Synaptic and Neuroendocrine Signaling

• In synaptic signaling, neurons form specialized junctions with target cells, called synapses

• At synapses, neurons secrete molecules called

neurotransmitters that diffuse short distances and bind to receptors on target cells

• In neuroendocrine signaling, specialized

neurosecretory cells secrete molecules called

neurohormones that travel to target cells via the bloodstream

Endocrine Tissues and Organs

• In some tissues, endocrine cells are grouped together in ductless organs called endocrine glands

• Endocrine glands secrete hormones directly into surrounding fluid

• These contrast with exocrine glands, which have ducts and which secrete substances onto body surfaces or into cavities

Figure 45.4

Chemical Classes of Hormones

• Three major classes of molecules function as hormones in vertebrates

– Polypeptides (proteins and peptides) – Amines derived from amino acids

– Steroid hormones

• *Lipid-soluble hormones (steroid hormones) pass easily through cell membranes, while

water-soluble hormones (polypeptides and amines) do not

• The solubility of a hormone correlates with the location of receptors inside or on the surface of target cells

Cellular Response Pathways

• Water- and lipid-soluble hormones differ in their paths through a body

• Water-soluble hormones are secreted by

exocytosis, travel freely in the bloodstream, and bind to cell-surface receptors

• Lipid-soluble hormones diffuse across cell

membranes, travel in the bloodstream bound to transport proteins, and diffuse through the

membrane of target cells

Pathway for Water-Soluble Hormones

• *Binding of a hormone to its receptor initiates a

signal transduction pathway leading to

responses in the cytoplasm, enzyme activation, or a change in gene expression

• The hormone epinephrine has multiple effects in mediating the body’s response to short-term stress

• Epinephrine binds to receptors on the plasma membrane of liver cells

• This triggers the release of messenger

molecules that activate enzymes and result in the release of glucose into the bloodstream

Figure 45.7-2

Epinephrine

G protein

Adenylyl cyclase

G protein-coupled receptor

GTP

ATP

cAMP Second messenger

Inhibition of glycogen synthesis

Promotion of glycogen breakdown

Pathway for Lipid-Soluble Hormones

• The response to a lipid-soluble hormone is usually a change in gene expression

• Steroids, thyroid hormones, and the hormonal form of vitamin D enter target cells and bind to protein receptors in the cytoplasm or nucleus • Protein-receptor complexes then act as

transcription factors in the nucleus, regulating transcription of specific genes

Figure 45.8-2

EXTRACELLULAR FLUID

Hormone (estradiol)

Estradiol (estrogen)

receptor Plasma

membrane

Hormone-receptor complex

NUCLEUS

DNA

CYTOPLASM

Vitellogenin mRNA

Multiple Effects of Hormones

• *The same hormone may have different effects on target cells that have

– Different receptors for the hormone

– Different signal transduction pathways

Different receptors Same receptors but different

intracellular proteins (not shown)

Different cellular

responses Different cellularresponses

Epinephrine Epinephrine Epinephrine

 receptor  receptor  receptor Glycogen

deposits

Vessel

dilates. Vessel_constricts. Glycogen

breaks down and glucose is released from cell.

(a) Liver cell (b) Skeletal muscle

Concept 45.2: Feedback regulation and antagonistic

hormone pairs are common in endocrine systems

• Hormones are assembled into regulatory pathways

Simple Hormone Pathways

• Hormones are released from an endocrine cell, travel through the bloodstream, and interact with specific receptors within a target cell to cause a physiological response

• For example, the release of acidic contents of the stomach into the duodenum stimulates endocrine cells there to secrete secretin

• This causes target cells in the pancreas, a gland behind the stomach, to raise the pH in the

duodenum

Pathway Example



Stimulus Low pH in duodenum

Endocrine cell

S cells of duodenum secrete the hormone secretin ( ).

Hormone

Blood vessel Target

cells Pancreas

Response Bicarbonate release

• In a simple neuroendocrine pathway, the stimulus is received by a sensory neuron, which stimulates a neurosecretory cell

• The neurosecretory cell secretes a

neurohormone, which enters the bloodstream and travels to target cells

Pathway  Example Stimulus Suckling Sensory neuron P o si ti ve f ee d b ac k Hypothalamus/ posterior pituitary Neurosecretory cell Neurohormone Blood vessel Target cells Response Posterior pituitary secretes the neurohormone oxytocin ( ).

Smooth muscle in breasts

Feedback Regulation

• *A negative feedback loop inhibits a response by reducing the initial stimulus, thus preventing excessive pathway activity

• *Positive feedback reinforces a stimulus to produce an even greater response

• For example, in mammals oxytocin causes the release of milk, causing greater suckling by

offspring, which stimulates the release of more oxytocin

*Insulin and Glucagon: Control of Blood

Glucose

• _{Insulin (decreases blood glucose) and glucagon} (increases blood glucose) are antagonistic

hormones that help maintain glucose homeostasis

• The pancreas has clusters of endocrine cells called pancreatic islets with alpha cells that produce glucagon and beta cells that produce insulin

Body cells take up more glucose.

Insulin

Beta cells of pancreas

release insulin into the blood. Liver takes

up glucose and stores it as glycogen. Blood glucose level declines. Blood glucose level rises. Homeostasis: Blood glucose level (70–110 mg/m100mL)

STIMULUS:

Blood glucose level rises (for instance, after eating a

carbohydrate-rich meal). Liver breaks down glycogen and releases glucose into the blood.

Alpha cells of pancreas release glucagon into the blood.

Glucagon

STIMULUS: Blood glucose level falls (for instance, after

skipping a meal).

Target Tissues for Insulin and Glucagon

• *Insulin reduces blood glucose levels by

– Promoting the cellular uptake of glucose – Slowing glycogen breakdown in the liver – Promoting fat storage, not breakdown

• *Glucagon increases blood glucose levels by

– Stimulating conversion of glycogen to glucose in the liver

– Stimulating breakdown of fat and protein into glucose

Diabetes Mellitus

• Diabetes mellitus is perhaps the best-known endocrine disorder

• It is caused by a deficiency of insulin or a

decreased response to insulin in target tissues • It is marked by elevated blood glucose levels

• _{Type 1 diabetes mellitus (insulin-dependent) is an} autoimmune disorder in which the immune

system destroys pancreatic beta cells

• Type 2 diabetes mellitus (non-insulin-dependent) involves insulin deficiency or reduced response of target cells due to change in insulin receptors

Concept 45.3: The hypothalamus and

pituitary are central to endocrine regulation

the nervous system, including the brain

Coordination of Endocrine and Nervous Systems in Vertebrates

• The hypothalamus receives information from the nervous system and initiates responses through the endocrine system

• Attached to the hypothalamus is the pituitary gland, composed of the posterior pituitary and anterior pituitary

• The posterior pituitary stores and secretes hormones that are made in the hypothalamus • The anterior pituitary makes and releases

hormones under regulation of the hypothalamus

Posterior Pituitary Hormones

• The two hormones released from the posterior pituitary act directly on nonendocrine tissues

– Oxytocin regulates milk secretion by the mammary glands

– Antidiuretic hormone (ADH) regulates physiology and behavior

Neurosecretory cells of the

hypothalamus

Neurohormone

Posterior pituitary

Hypothalamus

Axons

Anterior pituitary

HORMONE

TARGET

ADH Oxytocin

Kidney tubules

Anterior Pituitary Hormones

• Hormone production in the anterior pituitary is controlled by releasing and inhibiting hormones from the hypothalamus

• For example, prolactin-releasing hormone from the hypothalamus stimulates the anterior pituitary to secrete prolactin (PRL), which has a role in milk production

Tropic effects only: FSH

LH TSH ACTH

Nontropic effects only: Prolactin

MSH

Nontropic and tropic effects:

GH Hypothalamic releasing and inhibiting hormones Posterior pituitary Neurosecretory cells of the hypothalamus

Portal vessels

Endocrine cells of the anterior pituitary

Pituitary hormones

HORMONE FSH and LH TSH ACTH Prolactin MSH GH

TARGET Testes or Thyroid Melanocytes

ovaries Adrenalcortex Mammaryglands other tissuesLiver, bones,

Thyroid Regulation: A Hormone Cascade Pathway

• A hormone can stimulate the release of a series of other hormones, the last of which activates a nonendocrine target cell; this is called a hormone cascade pathway

• The release of thyroid hormone results from a hormone cascade pathway involving the

hypothalamus, anterior pituitary, and thyroid gland

• Hormone cascade pathways typically involve negative feedback

Figure 45.17 _{Pathway Example} Stimulus Cold Sensory neuron   Hypothalamus Neurosecretory cell Releasing hormone Blood vessel Anterior pituitary Tropic hormone Endocrine cell Hormone Target cells Response N e g at iv e f e ed b a c k Hypothalamus secretes thyrotropin-releasing hormone (TRH ).

Anterior pituitary secretes thyroid-stimulating

hormone (TSH, also known as thyrotropin ).

Thyroid gland secretes thyroid hormone

(T₃ and T₄ ).

Body tissues

Tropic and Nontropic Hormones

• A tropic hormone regulates the function of endocrine cells or glands

• Three primarily tropic hormones are

– _{Follicle-stimulating hormone (FSH)} – Luteinizing hormone (LH)

– _{Adrenocorticotropic hormone (ACTH)}

• Growth hormone (GH) is secreted by the anterior pituitary gland and has tropic and nontropic actions

• It promotes growth directly and has diverse metabolic effects

• It stimulates production of growth factors

• An excess of GH can cause gigantism, while a lack of GH can cause dwarfism

Concept 45.4: Endocrine glands respond to

diverse stimuli in regulating homeostasis,

development, and behavior

• Endocrine signaling regulates homeostasis, development, and behavior

Parathyroid Hormone and Vitamin D: Control of

Blood Calcium

• Two antagonistic hormones regulate the

homeostasis of calcium (Ca2+) in the blood of mammals

– Parathyroid hormone (PTH) is released by the

parathyroid glands

– Calcitonin is released by the thyroid gland

Active vitamin D Increases Ca2

uptake in intestines

Stimulates Ca2

uptake in kidneys

Stimulates Ca2 _release

from bones

Parathyroid gland (behind thyroid)

PTH

Blood Ca2

level rises.

Homeostasis: Blood Ca2 _level

(about 10 mg/100 mL)

STIMULUS: Falling blood

Ca2 _level

• PTH increases the level of blood Ca2+

– It releases Ca2+ from bone and stimulates

reabsorption of Ca2+ in the kidneys

– It also has an indirect effect, stimulating the kidneys to activate vitamin D, which promotes intestinal uptake of Ca2+ from food

• Calcitonin decreases the level of blood Ca2+

– It stimulates Ca2+ deposition in bones and

secretion by kidneys

Adrenal Hormones: Response to Stress

• The adrenal glands are adjacent to the kidneys • Each adrenal gland actually consists of two

glands: the adrenal medulla (inner portion) and

adrenal cortex (outer portion)

Catecholamines from the Adrenal Medulla

• The adrenal medulla secretes epinephrine

(adrenaline) and norepinephrine (noradrenaline) • These hormones are members of a class of

compounds called catecholamines

• They are secreted in response to stress-activated impulses from the nervous system

• They mediate various fight-or-flight responses

• Epinephrine and norepinephrine

– Trigger the release of glucose and fatty acids into the blood

– Increase oxygen delivery to body cells

– Direct blood toward heart, brain, and skeletal muscles and away from skin, digestive system, and kidneys

• The release of epinephrine and norepinephrine occurs in response to involuntary nerve signals

Steroid Hormones from the Adrenal Cortex

• The adrenal cortex releases a family of steroids called corticosteroids in response to stress

• These hormones are triggered by a hormone cascade pathway via the hypothalamus and anterior pituitary (ACTH)

• Humans produce two types of corticosteroids: glucocorticoids and mineralocorticoids

Gonadal Sex Hormones

• The gonads, testes and ovaries, produce most of the sex hormones: androgens, estrogens, and

progestins

• All three sex hormones are found in both males and females, but in significantly different

proportions

• *The testes primarily synthesize androgens, mainly testosterone, which stimulate

development and maintenance of the male reproductive system

• Testosterone causes an increase in muscle and bone mass and is often taken as a supplement to cause muscle growth, which carries health risks

• *Estrogens, most importantly estradiol, are responsible for maintenance of the female reproductive system and the development of female secondary sex characteristics

• In mammals, progestins, which include

progesterone, are primarily involved in preparing and maintaining the uterus

• Synthesis of the sex hormones is controlled by FSH and LH from the anterior pituitary

Melatonin and Biorhythms

• The pineal gland, located in the brain, secretes

melatonin

• Light/dark cycles control release of melatonin

• Primary functions of melatonin appear to relate to biological rhythms associated with reproduction

Chapter 46

Concept 46.1: 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 creation of offspring without the fusion of egg and sperm

• Sexual reproduction results in genetic recombination, which provides potential advantages

– An increase in variation in offspring, providing an increase in the reproductive success of parents in changing environments

– An increase in the rate of adaptation

– A shuffling of genes and the elimination of harmful genes from a population

Reproductive Cycles

• _{*Ovulation is the release of mature eggs at the} midpoint of a female cycle

• 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

Concept 46.2: Fertilization depends on mechanisms

that bring together sperm and eggs of the same species

• The mechanisms of fertilization, the union of egg and sperm, play an important part in sexual

reproduction

• 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

• Internal fertilization requires behavioral

interactions and compatible copulatory organs • All fertilization requires critical timing, often

mediated by environmental cues, pheromones, and/or courtship behavior

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

Gamete Production and Delivery

• To reproduce sexually, animals must produce gametes

• In most species individuals 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 developing embryos

Concept 46.3: Reproductive organs produce and transport gametes

• The following section focuses on the human reproductive system

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

Ovaries

• The female gonads, the 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

• *Once a month, an oocyte develops into an ovum (egg) by the process of oogenesis

• Ovulation expels an egg cell from the follicle, the cells of which produce estradiol prior to ovulation • The remaining follicular tissue grows within the

ovary, forming a mass called the corpus luteum

• The corpus luteum secretes estradiol and

progesterone that helps to maintain pregnancy • If the egg is not fertilized, the corpus luteum

degenerates

Oviducts and Uterus

• The egg cell travels from the ovary to the uterus via an oviduct, or fallopian tube

• Cilia in the oviduct convey the egg to the uterus, also called the womb

• The uterus lining, the endometrium, has many blood vessels

• The uterus narrows at the cervix, then opens into the vagina

Vagina and Vulva

• The vagina is a thin-walled 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

• The clitoris has a head called a glans covered by the prepuce

• The vagina, labia minora, and clitoris are rich with blood vessels; the clitoris also has many nerve

endings

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

Male Reproductive Anatomy

• The male’s external reproductive organs are the scrotum and penis

• Internal organs are the gonads, which produce sperm and hormones, and accessory glands

Seminal vesicle (behind bladder) Urethra Scrotum (Urinary bladder) Prostate gland Bulbourethral gland Erectile tissue of penis Vas deferens Epididymis Testis Seminal vesicle (Rectum) Vas deferens Ejaculatory duct Prostate gland

Testes

• The male gonads, or testes, consist of highly coiled tubes surrounded by connective tissue • Sperm form in these seminiferous tubules

• Leydig cells produce hormones and are scattered between the tubules

• Production of normal sperm cannot occur at the body temperatures of most mammals

• The testes of many mammals are held outside the abdominal cavity in the scrotum, where the

temperature is lower than in the abdominal cavity

Ducts

• From the seminiferous tubules of a testis, sperm pass into the coiled tubules 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

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

Penis

• The human penis is composed of three cylinders of spongy erectile tissue

• During sexual arousal, the erectile tissue fills with blood from the arteries, causing an erection

• The head of the penis has a thinner skin covering than the shaft and is more sensitive to stimulation

Gametogenesis

• _{Gametogenesis, the production of gametes,}

differs in male and female, reflecting the distinct structure and function of their gametes

• Sperm are small and motile and must pass from male to female

• Eggs are larger and carry out their function within the female

• 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

• *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

Epididymis

Seminiferous tubule

Testis

Cross section of seminiferous tubule Sertoli cell nucleus Lumen of seminiferous tubule Plasma membrane Tail Neck Midpiece Head Mitochondria Nucleus Acrosome

Primordial germ cell in embryo Mitotic divisions Spermatogonial stem cell Spermatogonium Mitotic divisions Mitotic divisions Primary spermatocyte

Meiosis I

Meiosis II

Spermatids (two stages) Secondary spermatocyte Early spermatid Sperm cell Differentiation (Sertoli cells provide nutrients) 2n 2n 2n n n

n n n n

n n n n

2n 2n n n n n Ovary

Primordial germ cell

Mitotic divisions

Mitotic divisions Oogonium

In embryo

Primary oocyte

(present at birth), arrested in prophase of meiosis I

Completion of meiosis I and onset of meiosis II

Secondary oocyte, arrested at metaphase of meiosis II

First polar body

Ovulation, sperm entry

Completion of meiosis II

Concept 46.4: The interplay of tropic and sex hormones regulates mammalian reproduction

• 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 and LH from the anterior pituitary • FSH and LH regulate processes in the gonads and

the production of sex hormones

• Sex hormones serve many functions in addition to gamete production, including sexual behavior and the development of primary and secondary sex

characteristics

Hormonal Control of the Female Reproductive Cycles

• In females, the secretion of hormones and the reproductive events they regulate are cyclic

• 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

• *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

Figure 46.13

This is Important (a) Control by hypothalamus Hypothalamus

GnRH Anterior pituitary

FSH LH

Inhibited by combination of estradiol and progesterone Stimulated by high levels of estradiol

Inhibited by low levels of estradiol    10 9 8 7 6 5 4 3 2 1

(b) Pituitary gonadotropins in blood

LH FSH FSH and LH stimulate follicle to grow

6

LH surge triggers ovulation Ovarian cycle

Growing follicle _Maturing follicle

Corpus

luteum corpus luteumDegenerating Follicular phase Ovulation Luteal phase

Estradiol secreted by growing follicle in increasing amounts (c)

Progesterone and estradiol secreted by corpus luteum (d) Ovarian hormones

in blood Estradiol

Peak causes LH surge (see ) Progesterone

Progesterone and estra-diol promote thickening of endometrium Estradiol level

very low

(e) Uterine (menstrual) cycle Endometrium

Menstrual flow phase Proliferative phase Secretory phase

Figure 46.13a

Control by hypothalamus (a)

Hypothalamus

GnRH

Anterior pituitary

FSH LH

Inhibited by combination of estradiol and progesterone Stimulated by high levels of estradiol

Inhibited by low levels of estradiol

 



2

Figure 46.13b

(b) Pituitary gonadotropins

in blood

LH

FSH

FSH and LH stimulate

follicle to grow LH surge triggers ovulation

Ovarian cycle

Growing follicle _Maturing

follicle

Corpus

luteum corpus luteumDegenerating

Follicular phase Ovulation Luteal phase

Estradiol secreted by growing follicle in increasing amounts

Progesterone and estradiol secreted by corpus luteum (d) Ovarian hormones

in blood

Estradiol

Peak causes LH surge (see )

Progesterone

Progesterone and estra-diol promote thickening of endometrium

Estradiol level very low

(e) Uterine (menstrual) cycle

Endometrium

Menstrual flow phase Proliferative phase Secretory phase

The Ovarian Cycle

• The sequential release of GnRH then FSH and LH stimulates follicle growth

• Follicle growth and an increase in the hormone estradiol characterize the follicular phase of the ovarian cycle

• The follicular phase ends at ovulation, and the secondary oocyte is released

• Following ovulation, the follicular tissue left behind transforms into the corpus luteum; this is the

luteal phase

• The corpus luteum disintegrates, and ovarian steroid hormones decrease

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

• A new cycle begins if no embryo implants in the endometrium

• Cells of the uterine lining can sometimes migrate to an abnormal, or ectopic, location

• Swelling of these cells in response to hormone stimulation results in a disorder called

endometriosis

Hormonal Control of the Male Reproductive System

• FSH promotes the activity of Sertoli cells, which nourish developing sperm

• LH regulates Leydig cells, which secrete

testosterone and other androgens, which in turn promote spermatogenesis

Hypothalamus    GnRH Anterior pituitary FSH LH

Sertoli cells Leydig cells

Inhibin Spermatogenesis Testosterone

• 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

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)

• 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

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

Concept 46.5: In placental mammals, an embryo develops fully within the mother’s uterus

• An egg develops into an embryo in a series of predictable events

Conception, Embryonic Development, and Birth

• Conception, fertilization of an egg by a sperm, occurs in the oviduct

• The resulting zygote begins to divide by mitosis in a process called cleavage

• Division of cells gives rise to a blastocyst, a ball of cells with a central cavity

5 4 1 2 3 Cleavage Fertilization Ovary Ovulation Uterus Cleavage continues Implantation Endometrium (a) From ovulation to implantation

(b) Implantation of blastocyst

• After blastocyst formation, the embryo implants into the endometrium

• The embryo releases human chorionic gonadotropin (hCG), which prevents menstruation

• Pregnancy, or gestation, is the condition of carrying one or more embryos in the uterus

• Duration of pregnancy in other species correlates with body size and maturity of the young at birth

• Pregnancies can terminate spontaneously due to chromosomal or developmental abnormalities

• An ectopic pregnancy occurs when a fertilized egg begins to develop in the fallopian tube

First Trimester

• Human gestation can be divided into three

trimesters of about three months each

• The first trimester is the time of most radical change for both the mother and the embryo

• During implantation, the endometrium grows over the blastocyst

• 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

• Blood from the embryo travels to the placenta

through arteries of the umbilical cord and returns via the umbilical vein

Figure 46.16 Placenta Uterus Umbilical cord Chorionic villus, containing fetal capillaries Maternal blood pool Maternal

arteries Maternalveins

• Splitting of the embryo during the first month of development results in genetically identical twins • Release and fertilization of two eggs result in

fraternal and genetically distinct twins

• 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

• Changes occur in the mother

– Mucus plug to protect against infection – Growth of the placenta and uterus

– Cessation of ovulation and the menstrual cycle – Breast enlargement

– Nausea is also very common

Figure 46.17

Second Trimester

• During the second trimester

– The fetus grows and is very active

– The mother may feel fetal movements

– The uterus grows enough for the pregnancy to become obvious

Third Trimester

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

• A complex interplay of local regulators and hormones induces and regulates labor, the process by which childbirth occurs

Figure 46.18 Estradiol   from ovaries Activates oxytocin receptors on uterus

Oxytocin from fetus and mother’s posterior pituitary Prostaglandins P o si ti ve f ee d b ac k Stimulates uterus to contract Stimulates placenta to make

Stimulate more contractions

• Labor typically has three stages

– Thinning and opening of the cervix, or dilation – Expulsion or delivery of the baby

– Delivery of the placenta

Figure 46.19

Dilation of the cervix

2 1

3

Placenta

Umbilical cord Uterus

Cervix

Expulsion: delivery of the infant

Delivery of the placenta

Uterus

Umbilical cord Placenta

• Delivery of the baby and placenta is brought about by a series of strong, rhythmic uterine contractions • _{Lactation, the production of milk, is unique to}

mammals

Contraception and Abortion

• _{Contraception, the deliberate prevention of}

pregnancy, can be achieved in a number of ways • Contraceptive methods fall into three categories

– Preventing release of eggs and sperm – Keeping sperm and egg apart

– Preventing implantation of an embryo

Figure 46.20

Male Female

Method Event Event Method

Vasectomy Abstinence Condom Coitus interruptus (very high failure rate) Production

of sperm primary oocytesProduction of

Sperm transport down male duct system Oocyte development and ovulation Combination birth control pill (or injection, patch, or vaginal ring) Abstinence Female condom Sperm deposited in vagina

Capture of the oocyte by the

oviduct Sperm movement through female reproductive tract Transport of oocyte in

oviduct Tubal ligation Spermicides; diaphragm; progestin alone (as minipill or injection)

Meeting of sperm and oocyte in oviduct

Union of sperm and egg

Implantation of blastocyst in endometrium

• The rhythm method, or natural family planning, is to refrain from intercourse 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

• 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%

• Sterilization is permanent and prevents the release of gametes

– Tubal ligation ties off the oviducts – Vasectomy ties off the vas deferens

• Abortion is the termination of a pregnancy

• Spontaneous abortion, or miscarriage, occurs in up to one-third of all pregnancies

• The drug RU486 results in an abortion within the first 7 weeks of a pregnancy

Modern Reproductive Technologies

• Recent advances are addressing reproductive problems