The Living World

Fifth Edition

George B. Johnson Jonathan B. Losos Chapter 36 Reproduction and Development Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display

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36.1 Asexual and Sexual Reproduction •

Asexual reproduction

produces offspring that are genetically identical to the parent  mitosis is the process of cell division that is involved  examples are • •

fission

in which one organism splits in two

budding

occurs where part of the parent’s body becomes separated from the rest and differentiates into a new individual

Figure 36.1 Asexual reproduction in protists

36.1 Asexual and Sexual Reproduction •

Sexual reproduction

occurs when a new individual is formed by the union of two cells  these cells are called

gametes

, which are formed by meiosis in the sex organs, or

gonads

 the union of a sperm and an egg produces a fertilized egg, or

zygote

• the zygote will develop by mitosis into a new organism

36.1 Asexual and Sexual Reproduction •

Parthenogenesis

is a special type of reproduction in which offspring are produced from unfertilized eggs  for example, in honeybees, a queen mates only once and stores sperm • if no sperm are released, the eggs develop into drones, which are male • If sperm are released, the eggs develop into other queens or workers, which are female

36.1 Asexual and Sexual Reproduction •

Hermaphroditism

which one individual has both testes and sperm and so can produce both sperm and eggs  is a reproductive strategy in most hermaphroditic organisms require another individual to reproduce • during mating, each switches roles from producing eggs to producing sperm  some hermaphroditic organism change their sex through

sequential hermaphroditism

• •

protogyny

changes from female to male

protandry

changes from male to female

Figure 36.2 Hermaphroditism and protogyny

36.1 Asexual and Sexual Reproduction • In mammals, sex is determined early in development  the reproductive systems of human males and females appear similar for the first 40 days after conception  if the embryo is XY, it is a male and will carry a gene of the Y chromosome whose product converts gonads into testes • the sex-determining gene is

SRY (sex-determining region of the Y chromosome)

 if the embryo is XX, it is a female and the gonads will become ovaries

Figure 36.3 Sex determination

36.2 Evolution of Reproduction Among the Vertebrates • Vertebrate sexual reproduction evolved in the ocean before vertebrates colonized the land  in

external fertilization

, gametes are released into the water • this process is common to most fish  in

internal fertilization

, male gametes are introduced into the female reproductive tract • terrestrial vertebrates and some fish use this process

36.2 Evolution of Reproduction Among the Vertebrates • Vertebrates with internal fertilization may be classified into different groupings 

oviparity

• the eggs are fertilized internally but complete development outside the mother’s body 

ovoviviparity

• the fertilized eggs complete development inside the mother and depend on yolk exclusively for nourishment before being born alive 

viviparity

• the young develop within the mother and obtain nourishment from their mother’s blood before being born alive

Figure 36.4 Viviparous vertebrates carry live, mobile young within their bodies

36.2 Evolution of Reproduction Among the Vertebrates • Most fish and amphibians reproduce by means of external fertilization  in most bony fish, the eggs contain only enough yolk to sustain the developing embryo for a short time • fish mature rapidly but there is high mortality   most cartilaginous fish use internal fertilization amphibians reproduce in the water and have aquatic larval stages before moving to the land • development is longer than in fish but the eggs provide only a little bit additional yolk

Amphibian Reproduction and Development Figure 36.5 Life cycle of the red spotted newt Figure 36.6 The eggs of frogs are fertilized externally Figure 36.7 Different ways young develop in frogs

36.2 Evolution of Reproduction Among the Vertebrates • Most reptiles are oviparous  the eggs are surrounded by a leathery shell that is deposited as the egg passes through the oviduct  other species of reptiles are ovoviviparous or viviparous • All birds are oviparous  as the fertilized egg passes along the oviduct, glands secrete albumin protein (the egg white) and a hard, calcareous shell

Reproduction in Reptiles and Birds Figure 36.8 The introduction of sperm by the male into the female’s body is called copulation Figure 36.9 Egg formation and incubation in birds

36.2 Evolution of Reproduction Among the Vertebrates • Some mammals are seasonal breeders   the females generally undergo the reproductive cycle, whereas the males are more constant in their reproductive cycles most females are “in heat” or sexually receptive to males, only around the time of ovulation • this period of sexual receptivity is called

estrous

– the reproductive cycle in females is called the

estrous cycle

36.2 Evolution of Reproduction Among the Vertebrates • In the estrous cycle of most mammals, changes in the secretion of FSH and LH by the anterior pituitary causes changes in egg cell development and hormone secretion in the ovaries  rabbits and cats differ from most other mammals in that they are

induced ovulators

• females ovulate only after copulation as a result of a reflex stimulation of LH • this makes them extremely fertile

36.2 Evolution of Reproduction Among the Vertebrates • The most primitive mammals, the

monotremes

, are oviparous  the duck-billed platypus and the echidna incubate their eggs in a nest and, because they lack nipples, the young lick milk off their mother’s skin • All other mammals are viviparous

36.2 Evolution of Reproduction Among the Vertebrates • Viviparous mammals are divided into two subcategories based on how they nourish their young 

marsupials

developed give birth to fetuses that are incompletely • they complete their development in a pouch of their mother’s skin, where they obtain nourishment from nipples of her mammary glands 

placental mammals

mother’s uterus retain their young within the • the fetus is nourished by a placenta which allows the fetus to obtain nutrients by diffusion from the mother’s blood

Figure 36.10 Reproduction in mammals

36.3 Males

•

Sperm

is the male gamete that is highly specialized for its role as a carrier of genetic information  sperm do not successfully complete their development at 37 °C • the sperm-producing organs, called

testes

, are found in a sac called the

scrotum

, which hangs between the legs and maintains the two testes at a temperature about 3  cooler than the rest of the body

36.3 Males

• The testis is composed of several hundred compartments  each compartment is packed with large numbers of tightly coiled tubes called

seminiferous tubules

• these are the sites for

spermatogenesis

– the process of spermatogenesis begins in germinal cells toward the outside of the tubule – as the cells undergo meiosis, they move toward the lumen of the tubule

Figure 36.12 The testis and formation of sperm

36.3 Males

• After a sperm cell is manufactured within the testis, it is delivered to a long, coiled tube called the

epididymis

 the sperm cell is not motile when it first arrives at the epididymis and must remain there for at least 18 hours before motility develops  from the epididymis, the sperm is delivered to another long tube, the

vas deferens

 when sperm is ejaculated, it travels from the vas deferens to the

urethra

Figure 36.11 The male reproductive system

Figure 36.13 Human sperm cells

36.3 Males

• The

penis

is an external tube containing two long cylinders of spongy tissue  below and between them is a third cylinder of spongy tissue that contains in its center a tube called the urethra  the penis is designed to inflate • when nerve impulses from the CNS cause the arterioles leading into this tissue to expand, blood collects within these spaces • continued stimulation by the CNS is required for erection to continue

Figure 36.14 Structure of the penis

36.3 Males

• While erection can be achieved without any physical stimulation of the penis, physical stimulation is required for semen to be delivered  stimulation, such as by repeated thrusts into the vagina of a female, leads first to the mobilization of sperm • muscles encircling the vas deferens contract, moving the sperm along the vas deferens to the urethra  the stimulation finally results to the strong contraction of muscles at the base of the penis •

ejaculation

is the forceful ejection of 2 to 5 ml of semen

36.3 Males

• Semen contain sperm and a collection of secretions from glands  these secretions, such as from the prostate gland, provide metabolic energy sources for the sperm  there are several hundred million sperm in the small volume of semen ejaculated • males with fewer than 20 million sperm per ml are considered sterile

36.4 Females

• In females, eggs develop from cells called

oocytes

 these are located in the outer layer of compact masses of cells called

ovaries

 all the oocytes needed for a lifetime are already present at birth  during each reproductive cycle, one or a few of these oocytes are initiated to continue their development • this process is called

ovulation

36.4 Females

• At birth, a female’s ovaries contain some 2 million oocytes, all of which have begun the first meiotic division  at this stage, they are referred to as

primary oocytes

 each primary oocytes waits to receive the proper developmental signal to continue on with meiosis • the signal is FSH and very few of the oocytes will receive it

36.4 Females

• With the onset of puberty, females mature sexually  at this time, the release of FSH initiates the resumption of meiosis in a few oocytes • In humans, only a single oocyte continues to mature to become an

ovum

(plural,

ova

) or egg – the other oocytes will regress  only about 400 of the approximately 2 million oocytes a woman is born with are actually ovulated

Figure 36.16 The ovary and formation of an ovum

36.4 Females

• The

fallopian tubes

(also known as

uterine tubes

, or

oviducts

) transport ova from the ovaries to the

uterus

 in humans, the uterus narrows to a muscular ring called the

cervix

 the uterus is lined with a stratified epithelial membrane called the

endometrium

• the surface of the endometrium is shed during menstruation

Figure 36.15 The female reproductive system

36.4 Females

• Mammals other than primates have more complex female reproductive tracts  part of the uterus divides to form uterine “horns” Figure 36.17 The mammalian uterus —several examples

36.4 Females

• To fertilize an egg successfully, the sperm must make its way far up the fallopian tube  the egg is moved down the fallopian tube by contractions of smooth muscle lining the tube • sperm swim against the current created by these contractions  an egg loses its capacity to develop within a few days  a fertilized egg attaches itself to the endometrial lining to continue development

36.5 Hormones Coordinate the Reproductive Cycle • The female reproductive cycle, called a

menstrual cycle

, is composed of two distinct phases  the

follicular phase

in which an egg reaches maturation and is ovulated  the

luteal phase

where the body prepares for pregnancy  these phases are coordinated by a family of hormones whose production is controlled by the hypothalamus

36.5 Hormones Coordinate the Reproductive Cycle • The

follicular phase

corresponds to days 0 to 14 of the reproductive cycle   it is when the egg develops through the ovary the anterior pituitary starts the phase by secreting FSH which binds to receptors on the surface of cells surrounding the egg (called a

follicle

) and triggers resumption of meiosis  several follicles are stimulated by FSH but only one grows to maturity

36.5 Hormones Coordinate the Reproductive Cycle • FSH levels fall at the end of the follicular phase due to negative feedback in the form of

estrogen

produced by the ovary  a rise in estrogen signals the end of the follicular phase

36.5 Hormones Coordinate the Reproductive Cycle • The

luteal phase

occurs during days 14 through 28 of the reproductive cycle  after shutting down FSH production, the hypothalamus causes the anterior pituitary to secrete LH • this hormone causes ovulation because LH is carried in the bloodstream and causes the wall of the follicle to burst • after the egg’s departure, LH directs the repair of the ruptured follicle so that it fills in to become the

corpus luteum

36.5 Hormones Coordinate the Reproductive Cycle • The corpus luteum secretes

progesterone

which also inhibits FSH  progesterone completes the body’s preparation of the uterus for fertilization, including the thickening of the endometrium  if fertilization does not occur, production of progesterone slows and eventually stops  the decreasing levels of progesterone caused the thickened layer of the endometrium to be sloughed off • this process,

menstruation

, usually occurs about midway between successive ovulations

Figure 36.18 The human menstrual cycle

36.5 Hormones Coordinate the Reproductive Cycle • If fertilization does occur high in the fallopian tube, the zygote undergoes a series of cell divisions, called

cleavage

, while traveling toward the uterus  at the blastocyst stage, it implants in the lining of the uterus  the embryo secretes

human chorionic gonadotropin (hCG)

• this maintains the corpus luteum and prevents menstruation • because hCG comes from the embryo and not from the mother, it is hCG that is tested in all pregnancy tests

Figure 36.19 The journey of an ovum

36.5 Hormones Coordinate the Reproductive Cycle • Two additional hormones, secreted by the pituitary, are important in the female reproductive system 

prolactin

stimulates milk production usually by the third day after delivery 

oxytocin

is released following the infant suckling at the breast and initiates milk release • it also initiates labor and delivery

36.6 Embryonic Development

• During

cleavage

, the zygote divides rapidly into a larger and larger number of smaller and smaller cells  the resulting tightly packed mass of about 36 cells is called the

morula

• each individual cell in the morula is called a

blastomere

 further divisions of the blastomeres in the morula lead to a hollow ball of 500-2,000 cells • this is called the

blastocyst

36.6 Embryonic Development

• The

blastocyst

contains a fluid-filled cavity called the

blastocoel

 within the ball is an

inner cell mass

concentrated at one pole that goes on to form the developing embryo  the outer sphere of cells, called the

trophoblast

, releases the hCG

36.6 Embryonic Development

• The implantation of the blastocyst in the uterine lining initiates the formation of membranes  

amnion

encloses the developing embryo

chorion

forms from the trophoblast and interacts with the uterine tissue to form the

placenta

• the placenta connects the developing embryo to the blood supply of the mother

36.6 Embryonic Development

•

Gastrulation

occurs 10 to 11 days after fertilization and involves certain groups of moving inward from the surface of the inner cell mass  the lower cell layer of the blastocyst becomes

endoderm

 the moving cells differentiate into

mesoderm

and

endoderm

• they grow inward along a furrow called the

primitive streak

36.6 Embryonic Development

•

Neurulation

is the stage of development that beings in the third week of embryonic development  it involves the three primary germ layers beginning their development into the tissues and organs of the body • the

notochord

forms first from mesoderm • the

neural tube

forms from ectoderm •

somites

form along the side of the notochord and will become muscles, vertebrae, and connective tissue • between two layers of mesoderm, the

coelom

forms

Table 36.1 Stages of Mammalian Development

36.7 Fetal Development

•

Organogenesis

, the process of forming body organs, begins in the fourth week of pregnancy  this is a crucial time during development because the proper course of events can be interrupted easily • alcohol use during pregnancy is one of the leading causes of birth defects, producing

fetal alcohol syndrome

 most spontaneous abortions (i.e., miscarriages) occur during this period

36.7 Fetal Development

• During the second month of pregnancy, great changes in morphology occur as the embryo takes shape  it begins to look distinctly human • Development is essentially complete at the end of the third month   only the lungs and brain need to develop more the developing human is now referred to as a

fetus

instead of embryo

Figure 36.20 The developing human

Figure 36.20 The developing human

36.7 Fetal Development

• The second trimester is a time of growth  during the fifth month, the head and body become covered with fine hair, called

lanugo

, which will later be lost  by the end of the six month, the fetus cannot survive outside the uterus without special medical intervention

36.7 Fetal Development

• The third trimester is a period of rapid growth  all of the growth is fueled by the mother’s bloodstream, passing into the fetal blood supply within the placenta  the placenta contains blood vessels that extend from the umbilical cord into tissues that line the uterus • these tissues are called the

decidua basalis

Figure 36.21 Structure of the placenta

36.7 Fetal Development

• Growth continues rapidly after birth  different organs grow at different rates •

allometric growth

refers to the fact that different parts of the body grow or cease growing at different times • for example, the lower part of the human jaw grows at a faster rate than the rest of the skull

36.8 Contraception and Sexually Transmitted Diseases •

Birth control

is a way to avoid reproduction without avoiding sexual intercourse  contraception methods differ from one another in their effectiveness and their acceptability to different couples • • • • • •

abstinence prevention of egg maturation prevention of embryo implantation sperm blockage sperm destruction sterilization

Figure 36.23 Three common birth control methods

36.8 Contraception and Sexually Transmitted Diseases •

Sexually transmitted diseases (STDs)

are diseases that spread from one person to another through sexual contact 

gonorrhea

• is caused by a bacterium,

Neisseria gonorrhoeae

• produces symptoms of discharge from the penis or vagina • in women, if left untreated, could lead to

pelvic inflammatory disease (PID)

, which could lead to sterility

36.8 Contraception and Sexually Transmitted Diseases •

Chlamydia

  caused by the bacterium

Chlamydia trachomatis

women usually experience no symptoms until the infection is established  can also lead to PID •

syphilis

 caused by the bacterium

Treponema pallidum

 the disease progresses in four stages following the appearance of a small, painless legion, called a

chancre

, on the penis or hidden in the vagina

36.8 Contraception and Sexually Transmitted Diseases •

Genital herpes

 causes by the herpes simplex virus type 2 (HSV-2)   the most common STD in the US causes red blisters on the penis or on the labia, vagina, or cervix • the blisters rupture and scab over • the lesions heal but the virus travels to the dorsal root ganglion by way of the sensory neurons where they become dormant

Inquiry & Analysis

• How might heightened public awareness explain why the trend in levels of gonorrhea differs from that of chlamydia?

Graph of Trends in STDs in the United States