1. Reproduction and development
Chapter 31

2. Asexual and Sexual Reproduction
Asexual reproduction produces offspring that are genetically identical to the parent.
Mitosis
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.

3. 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.
In vertebrates, the zygote will develop by mitosis into a multicellular organism.

4. 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.

5. Asexual and Sexual Reproduction
Hermaphroditism is a reproductive strategy in which one individual has both testes and sperm and so can produce both sperm and eggs.
Most hermaphroditic organisms require another individual to reproduce.
Some hermaphroditic organisms change their sex through sequential hermaphroditism.
Protogyny is a change from female to male.
Protandry is a change from male to female.

6. Asexual and Sexual Reproduction
In mammals, sex is determined early in development.
If the embryo is XY, it is a male and will carry a gene on 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. Y X
(Follicles do not
develop until
third trimester)
Ovaries
Testes
Zygote XY
Sperm
Ovum
Leydig cells
Seminiferous
tubules
Develop in early
embryo
No SRY
SRY
Indifferent
gonads XX

7. Evolution of Vertebrate Sexual Reproduction
Vertebrate sexual reproduction evolved in the ocean before vertebrates colonized the land.
In external fertilization, gametes are released into the water.
Fish, frogs
In internal fertilization, male gametes are introduced into the female reproductive tract.
Some fish, salamanders, reptiles, birds, mammals

8. Evolution of Vertebrate Sexual Reproduction
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.

9. Evolution of Vertebrate Sexual Reproduction
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.

10. Evolution of Vertebrate Sexual Reproduction
Most cartilaginous fish use internal fertilization

11. Evolution of Vertebrate Sexual Reproduction
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 slightly more yolk.
Adults
mating
Gilled larva
Red eft
Eggs

12. Evolution of Vertebrate Sexual Reproduction
Most reptiles are oviparous
The eggs are surrounded by a leathery shell that is deposited as the egg passes through the oviduct.
Most reptiles abandon their eggs after laying them.

13. Evolution of Vertebrate Sexual Reproduction
Developing ovum
Ovum
Ovary
Mesentery
Rectum
Uterus
(shell gland)
(a)
(b)
Opening to cloaca
Oviduct
b: © Wolfgang Kaehler/Corbis
All birds are oviparous.
Most birds incubate their eggs after they are laid.

14. Evolution of Vertebrate Sexual Reproduction
Some mammals are seasonal breeders, while others have multiple short reproductive cycles.
The females generally undergo the reproductive cycle, whereas the males are more constant in their reproductive activity.
Most females are “in heat,” or sexually receptive to males, only around the time of ovulation.
This period of sexual receptivity is called estrus.
The reproductive cycle in females is called the estrous cycle.

15. Evolution of Vertebrate Sexual Reproduction
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.

16. Evolution of Vertebrate Sexual Reproduction
Viviparous mammals are divided into two subcategories based on how they nourish their young.
Marsupials give birth to fetuses that are incompletely developed.
They complete their development in a pouch of their mother’s skin, where they obtain nourishment from mammary glands.

17. Evolution of Vertebrate Sexual Reproduction
Placental mammals retain their young within the mother’s uterus.
The fetus is nourished by a placenta which allows the fetus to obtain nutrients by diffusion from the mother’s blood.
The young are born in a more developed state
Drink milk after birth.

18. (top): © David M. Phillips/Photo Researchers
Males
Tail
Mitochondrion
Nucleus
Acrosome
Head
Centriole
Flagellum
Body
(top): © David M. Phillips/Photo Researchers
Sperm is the male gamete and 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 maintains the testes at a temperature 3° cooler than the rest of the body.

19. Males
The testis is composed of several hundred compartments 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.

20. 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.

21. Females In females, eggs develop from cells called oocytes.
These are located in the outer layer of female gonads called ovaries.
During each reproductive cycle, one or a few of these oocytes are initiated to continue their development.
This process is called ovulation.

22. Females
The fallopian tubes (also known as uterine tubes, or oviducts) transport eggs from the ovaries to the uterus.
The uterus is lined with a stratified epithelial membrane called the endometrium.
The surface of the endometrium is shed during menstruation.

23. 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 24 hours of ovulation.
Sperm can remain viable for up to 6 days.

24. Females 2 1 3
Oviducts
Uterus
Cervix
Ovary
Vagina
(a)
(b)
Sperm
First polar body
Egg
Egg membrane
Granulosa cells
Zona pellucida
Nucleus
Acrosome
Extracellular space
Sperm nucleus
fertilizing egg
When the first sperm cell penetrates the oocyte’s protective layers the oocyte blocks the entry of other sperm.
The oocyte completes meiosis II, forming a haploid ovum.
When the female haploid nucleus joins with the male haploid nucleus, the egg is fertilized and becomes a zygote.
A fertilized egg attaches itself to the endometrial lining to continue development.

25. 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.

26. Hormones Coordinate the Reproductive Cycle
The follicular phase corresponds to days 0 to 14 of the reproductive cycle.
The anterior pituitary starts the phase by secreting FSH and LH in small amounts.
Initially, several follicles (a follicle is an oocyte and its surrounding tissue) are stimulated to grow.
The follicle begins to secrete the female hormone estrogen.

27. Hormones Coordinate the Reproductive Cycle
The low but rising levels of estrogen in the bloodstream act as negative feedback.
The output of FSH and LH are reduced.
This ensures that only one oocyte matures at a time.
A rise in estrogen signals the end of the follicular phase.

28. Hormones Coordinate the Reproductive Cycle
The luteal phase occurs during days 14 through 28 of the reproductive cycle.
The higher levels of estrogen begin to have a positive-feedback effect on FSH and LH secretion.
The surge in LH causes ovulation and the wall of the follicle bursts.
The follicle is released into one of the fallopian tubes.
LH directs the repair of the ruptured follicle so that it fills in to become the corpus luteum.

29. Hormones Coordinate the Reproductive Cycle
The corpus luteum secretes progesterone which also inhibits FSH and LH secretion.
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 cause the thickened layer of the endometrium to be sloughed off.
This process, menstruation, usually occurs about midway between successive ovulations.

30. 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. 1 2 3 4
Egg
Fallopian tube
Fertilization
Developing follicles
Ovulation
Ovary
Blastocyst
Implantation
Cleavage
Uterus
Morula
Corpus
luteum

31. Embryonic Development
During cleavage, the mammalian 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.

32. Embryonic Development
The blastocyst contains a fluid-filled cavity called the blastocoel.
Within the blastocyst 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 hCG.

33. 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 uterine tissue to form the placenta.
The placenta connects the developing embryo to the blood supply of the mother.

34. Embryonic Development
Gastrulation occurs 10 to 11 days after fertilization and involves certain groups of cells moving inward from the surface of the inner cell mass.
The lower cell layer of the inner cell mass becomes the endoderm and the upper layer becomes the ectoderm.
The moving cells differentiate into mesoderm
They grow inward along a furrow called the primitive streak.

35. Embryonic Development
Neurulation is the stage of development that begins in the third week of embryonic development.
The three primary germ layers begin their development into the tissues and organs of the body.

36. Embryonic Development
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.

37. Fetal Development
(a)
Tail
Lower limb
bud
Forebrain
Future lens
Pharyngeal
arches
Somites
Neural tube
forming
Upper limb
(b)
Developing
heart
Hand
plate
Future
external
ear
Head
(growth accelerated)
Foot
Umbilical
cord
Retinal
pigment
Organogenesis, the process of forming body organs, begins in the fourth week of human pregnancy.

38. 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 an embryo.

39. Fetal Development The second trimester is a time of growth.
Development is essentially complete.
The developing human is now referred
to as a fetus.
Facial expressions and primitive
reflexes are carried out.
Arms and legs begin
to move.
All of the major body organs
have been established.
Bones actively enlarge.
Mother can feel baby kicking.
Following a period of rapid growth,
the fetus is born.
Neurological growth
continues after birth.
The second trimester is a time of growth.
By the end of the sixth month, the fetus still cannot survive outside the uterus without special medical intervention.

40. 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.
Amnion
Chorion
Placenta
Maternal
vein
artery
Uterine
wall
Umbilical cord

41. Fetal Development
At approximately 40 weeks from the last menstrual cycle, the process of birth begins, including labor and delivery.
Oxytocin and prostaglandins work by positive feedback to stimulate uterine contractions
Uterus
Placenta
(detaching)
Umbilical
cord
(b)
(c)
(d)
(a)
Vagina
Cervix

42. Fetal Development
The mammary glands are prepared to nourish the baby after birth.
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.
Growth continues rapidly after birth.
Different organs grow at different rates.
Neurological growth of the baby continues long after birth.