1. Animal Development
Chapter 50

2. Learning Objective 1
Analyze the relationship between cell determination and cell differentiation, and between pattern formation and morphogenesis

3. Cell Differentiation
Process by which a cell becomes specialized to carry out specific functions
Cell determination
molecular events leading to cell differentiation
Stem cells
relatively undifferentiated cells

4. Morphogenesis The development of form In pattern formation, cells may
occurs through pattern formation
In pattern formation, cells may
communicate by signaling
migrate
undergo changes in shape
undergo apoptosis (programmed cell death)

5. KEY CONCEPTS
The development of form requires not only cell division and growth but also cell determination and cell differentiation, and pattern formation and morphogenesis

6. Learning Objective 2
Relate differential gene expression to nuclear equivalence

7. Gene Expression Principle of nuclear equivalence
Usually no genetic changes occur in cell determination and cell differentiation
Differential gene expression
Various types of differentiated cells express different subsets of their genes

8. Learning Objective 3
Describe the four processes involved in fertilization

9. Fertilization 1. Contact and recognition 2. Sperm entry is regulated
between noncellular egg coverings and sperm
2. Sperm entry is regulated
prevents interspecific fertilization
prevents polyspermy (fertilization of egg by more than one sperm)

10. Fertilization 3. Fertilization activates egg
triggering events of early development
4. Sperm and egg pronuclei fuse
initiates DNA synthesis

11. Fertilization SF SM FC SN 1 µm Fig. 50-1, p. 1082
Figure 50.1: Fertilization.
In this TEM, a fertilization cone (FC) forms as a sperm enters a sea urchin egg. (SN, sperm nucleus; SM, sperm mitochondrion; SF, sperm flagellum)
1 µm
Fig. 50-1, p. 1082

12. KEY CONCEPTS
Fertilization includes contact and recognition between egg and sperm, regulated sperm entry, activation of the fertilized egg, and fusion of egg and sperm pronuclei

13. Learning Objective 4 Describe fertilization in echinoderms
Point out some ways in which mammalian fertilization differs

14. Egg Coverings Echinoderms In mammals vitelline envelope jelly coat
zona pellucida

15. Acrosome Reaction Facilitates penetration of egg coverings
when sperm first contacts egg
In mammals, acrosome reaction is preceded by capacitation
maturation process
results in ability of sperm to fertilize egg

16. Effects of Capacitation on Sperm
Increased rate of metabolism
Flagellum beats more rapidly;
Result: Sperm are more motile (hyperactivated)
Changes in sperm glycoproteins
Allow sperm-egg binding
Pro-Acrosin (inactive) is converted to acrosin (active)
Able to digest zona pellucida proteins

17. Capacitation
These are monitor screen images from an instrument which records the movement paths of the sperm cells heads (white points) during a certain time span and displays them with a green line.
UPPER PANEL: Before capacitation the majority of the lines are straight.
LOWER PANEL: After capacitation almost all the sperm cells have now gone over to swinging their heads strongly as indicated by the jagged lines.

18. Polyspermy Echinoderms In mammals
sea urchin fertilization is followed by a fast block to polyspermy (depolarization of plasma membrane) and a slow block to polyspermy (cortical reaction)
In mammals
changes in zona pellucida prevent polyspermy

20. Slow Block to Polyspermy
The left image shows the approach of the sperm at about 2 o'clock and the rising of the vitelline membrane.
Intracellular Ca++ is monitored by an indicator that becomes more fluorescent when it binds Ca++.
Slow Block

21. Cortical Reaction

22. Learning Objective 5
Trace the generalized pattern of early development of the embryo from zygote through early cleavage and formation of the morula and blastula

23. The Zygote Undergoes cleavage
a series of rapid cell divisions without a growth phase
partitions zygote into many small blastomeres

24. Cleavage
Morula
a solid ball of cells
Blastula
a hollow ball of cells

25. KEY CONCEPTS
Cleavage, a series of rapid cell divisions without growth, provides cellular building blocks for development

26. Learning Objective 6
Contrast early development, including cleavage in the echinoderm (or in amphioxus), the amphibian, and the bird, paying particular attention to the importance of the amount and distribution of yolk

27. Invertebrates and Simple Chordates
Have isolecithal eggs (evenly distributed yolk)
undergo holoblastic cleavage (division of entire egg)

28. Cleavage in Sea Stars

29. Nucleus 100 µm 50 µm 50 µm (a) Unfertilized egg (b) 2-cell stage (c)
Figure 50.3: LMs showing sea star development.
(a) The isolecithal egg has a small amount of uniformly distributed yolk. (b–e) The cleavage pattern is radial and holoblastic (the entire egg becomes partitioned into cells). (f, g) The three germ layers form during gastrulation. The blastopore is the opening into the developing gut cavity, the archenteron. The rudiments of organs are evident in the sea star larva (h) and the young sea star (i). All views are side views with the animal pole at the top, except (c) and (i), which are top views. Note that the sea star larva is bilaterally symmetrical, but differential growth produces a radially symmetrical young sea star.
100 µm
50 µm
50 µm
(a) Unfertilized egg
(b)
2-cell stage
(c)
4-cell stage
Fig (a-c), p. 1084

30. Blastocoel Archenteron Blastopore 50 µm 50 µm 50 µm (f) Early gastrula
Figure 50.3: LMs showing sea star development.
(a) The isolecithal egg has a small amount of uniformly distributed yolk. (b–e) The cleavage pattern is radial and holoblastic (the entire egg becomes partitioned into cells). (f, g) The three germ layers form during gastrulation. The blastopore is the opening into the developing gut cavity, the archenteron. The rudiments of organs are evident in the sea star larva (h) and the young sea star (i). All views are side views with the animal pole at the top, except (c) and (i), which are top views. Note that the sea star larva is bilaterally symmetrical, but differential growth produces a radially symmetrical young sea star.
Archenteron
Blastopore
50 µm
50 µm
50 µm
(f) Early gastrula
(d)
16-cell stage
(e) Blastula
Fig (d-f), p. 1084

31. Archenteron Mouth Anus Stomach Blastopore 1 mm 50 µm 50 µm (h)
Figure 50.3: LMs showing sea star development.
(a) The isolecithal egg has a small amount of uniformly distributed yolk. (b–e) The cleavage pattern is radial and holoblastic (the entire egg becomes partitioned into cells). (f, g) The three germ layers form during gastrulation. The blastopore is the opening into the developing gut cavity, the archenteron. The rudiments of organs are evident in the sea star larva (h) and the young sea star (i). All views are side views with the animal pole at the top, except (c) and (i), which are top views. Note that the sea star larva is bilaterally symmetrical, but differential growth produces a radially symmetrical young sea star.
1 mm
50 µm
50 µm
(h)
Sea star larva
(i)
Young sea star
(g)
Middle gastrula
Fig (g-i), p. 1084

32. Cleavage in Amphioxus

33. Polar body
Figure 50.4: Cleavage and gastrulation in amphioxus.
As in the sea star, cleavage is holoblastic and radial. The embryos are shown from the side. (a) Mature egg with polar body. (b–e) The 2-, 4-, 8-, and 16-cell stages. (f) Embryo cut open to show the blastocoel. (g) Blastula. (h) Blastula cut open. (i) Early gastrula showing beginning of invagination at vegetal pole. (j) Late gastrula. Invagination is completed, and the blastopore has formed.
Fig (a-d), p. 1085

34. Blastocoel Fig. 50-4 (e-g), p. 1085
Figure 50.4: Cleavage and gastrulation in amphioxus.
As in the sea star, cleavage is holoblastic and radial. The embryos are shown from the side. (a) Mature egg with polar body. (b–e) The 2-, 4-, 8-, and 16-cell stages. (f) Embryo cut open to show the blastocoel. (g) Blastula. (h) Blastula cut open. (i) Early gastrula showing beginning of invagination at vegetal pole. (j) Late gastrula. Invagination is completed, and the blastopore has formed.
Fig (e-g), p. 1085

35. Archenteron Ectoderm Endoderm Blastopore Fig. 50-4 (h-j), p. 1085
Figure 50.4: Cleavage and gastrulation in amphioxus.
As in the sea star, cleavage is holoblastic and radial. The embryos are shown from the side. (a) Mature egg with polar body. (b–e) The 2-, 4-, 8-, and 16-cell stages. (f) Embryo cut open to show the blastocoel. (g) Blastula. (h) Blastula cut open. (i) Early gastrula showing beginning of invagination at vegetal pole. (j) Late gastrula. Invagination is completed, and the blastopore has formed.
Endoderm
Blastopore
Fig (h-j), p. 1085

36. Amphibians Have moderately telolecithal eggs
concentration of yolk at vegetal pole slows cleavage (only a few large cells form)
large number of smaller cells form at the animal pole

37. Cleavage in Frogs
Animal pole
Vegetal pole

38. Reptiles and Birds Have highly telolecithal eggs
large concentration of yolk at one end
undergo meroblastic cleavage (restricted to the blastodisc)

39. Cleavage in Birds

40. Blastodisc Yolk Fig. 50-7a, p. 1086
Figure 50.7: Cleavage in a bird embryo.
Fig. 50-7a, p. 1086

41. Epiblast Hypoblast Blastocoel Yolk Fig. 50-7b, p. 1086
Figure 50.7: Cleavage in a bird embryo.
Fig. 50-7b, p. 1086

42. Cytoplasm Distribution
Animals whose zygotes have relatively homogeneous cytoplasm
exhibit regulative development (embryo develops as a self-regulating whole)
Animals with unequal distribution of cytoplasmic components
have relatively rigid developmental patterns (mosaic development)

43. The Gray Crescent Of an amphibian zygote
determines body axis of embryo

44. Cytoplasmic Determinants First cleavage (control)
in Frogs
First cleavage (control)
(a) The first cleavage is allowed to occur normally, and each separated blastomere includes half of the gray crescent.
Figure 50.8: Cytoplasmic determinants in frog development.
Fig. 50-8a, p. 1087

45. First cleavage (experimental)
(b) The plane of cleavage is altered by the experimenter such that only one blastomere contains the gray crescent.
Figure 50.8: Cytoplasmic determinants in frog development.
Fig. 50-8b, p. 1087

46. Learning Objective 7
Identify the significance of gastrulation in the developmental process, and compare gastrulation in the echinoderm (or in amphioxus), the amphibian, and the bird

47. Germ Layers
During gastrulation, basic body plan is laid down as 3 germ layers
1. outer ectoderm
2. middle mesoderm
3. inner endoderm

48. The Archenteron Forerunner of digestive tube Blastopore in some groups
opening to the exterior

49. Gastrulation 1 In sea star and amphioxus
cells from blastula wall invaginate
eventually meet opposite wall, forming archenteron

50. Gastrulation 2 In the amphibian
invagination at vegetal pole obstructed by large, yolk-laden cells
cells from animal pole move down over yolk-rich cells and invaginate, forming dorsal lip of the blastopore

51. Gastrulation 3 In the bird invagination occurs at the primitive streak
no archenteron forms

52. Learning Objective 8 Define organogenesis
Summarize the fate of each of the germ layers

53. Organogenesis 1 The process of organ formation Ectoderm becomes
nervous system
sense organs
outer layer of skin (epidermis)

54. Organogenesis 2 Mesoderm becomes Endoderm becomes notochord skeleton
muscles
circulatory system
inner layer of skin (dermis)
Endoderm becomes
lining of the digestive tube

55. KEY CONCEPTS
Gastrulation establishes ectoderm, mesoderm, and endoderm; each gives rise to specific types of tissues

56. Learning Objective 9
Trace the early development of the vertebrate nervous system

57. Early Vertebrate Nervous System Development
Developing notochord is responsible for induction
causes ectoderm to differentiate and form central nervous system
Brain and spinal cord develop from the neural tube

58. Human Nervous System Development

59. KEY CONCEPTS
Neurulation, the origin of the central nervous system, is one of the earliest events in organogenesis (organ development)

60. Learning Objective 10
Give the origins and functions of the chorion, amnion, allantois, and yolk sac

61. Extraembryonic Membranes 1
Chorion
derived from ectoderm and mesoderm
used in gas exchange
Amnion
fluid-filled sac that surrounds the embryo
keeps embryo moist
acts as a shock absorber

62. Extraembryonic Membranes 2
Allantois
derived from endoderm and mesoderm
stores nitrogenous wastes
Yolk sac
makes food available to the embryo

63. 4 day-old chick Yolk Chorion Yolk sac Fig. 50-13a, p. 1091
Figure 50.13: Extraembryonic membranes.
The formation of the extraembryonic membranes of the chick is illustrated at (a) 4 days, (b) 5 days, and (c) 9 days of development. Each of the membranes develops from a combination of two germ layers. The chorion and amnion form from lateral folds of the ectoderm and mesoderm that extend over the embryo and fuse. The allantois and the yolk sac develop from endoderm and mesoderm.
Chorion
Yolk sac
Fig a, p. 1091

64. 5 day-old chick Chorion Allantois Yolk Amnion Amniotic cavity Yolk sac
Figure 50.13: Extraembryonic membranes.
The formation of the extraembryonic membranes of the chick is illustrated at (a) 4 days, (b) 5 days, and (c) 9 days of development. Each of the membranes develops from a combination of two germ layers. The chorion and amnion form from lateral folds of the ectoderm and mesoderm that extend over the embryo and fuse. The allantois and the yolk sac develop from endoderm and mesoderm.
Amniotic cavity
Yolk sac
Fig b, p. 1091

65. 9 day-old chick Amniotic cavity Amnion Chorion Allantois Yolk sac
Figure 50.13: Extraembryonic membranes.
The formation of the extraembryonic membranes of the chick is illustrated at (a) 4 days, (b) 5 days, and (c) 9 days of development. Each of the membranes develops from a combination of two germ layers. The chorion and amnion form from lateral folds of the ectoderm and mesoderm that extend over the embryo and fuse. The allantois and the yolk sac develop from endoderm and mesoderm.
Yolk sac
Allantois
Fig c, p. 1091

66. KEY CONCEPTS
Extraembryonic membranes (amnion, chorion, allantois, and yolk sac) have evolved in terrestrial vertebrates as adaptations to reproduction on land

67. Learning Objective 11
Describe the general course of early human development, including fertilization, the fates of the trophoblast and inner cell mass, implantation, and the role of the placenta

68. Early Human Development
Fertilization
occurs in the oviduct
Cleavage
takes place as embryo is moved down the oviduct
Blastocyst
develops in the uterus

69. Cleavage in Humans

70. (a) Male and female pronuclei prior to fusion
Figure 50.14: Cleavage in a human embryo.
(From Lennart Nilsson, Being Born, pp. 14, 15, 17. Putnam Publishing Group, 1992.)
50 µm
(a) Male and female pronuclei prior to fusion
Fig a, p. 1092

71. 24 hours 50 µm (b) 2-cell stage Fig. 50-14b, p. 1092
Figure 50.14: Cleavage in a human embryo.
(From Lennart Nilsson, Being Born, pp. 14, 15, 17. Putnam Publishing Group, 1992.)
24 hours
50 µm
(b) 2-cell stage
Fig b, p. 1092

72. 50 µm (c) 8-cell stage Fig. 50-14c, p. 1092
Figure 50.14: Cleavage in a human embryo.
(From Lennart Nilsson, Being Born, pp. 14, 15, 17. Putnam Publishing Group, 1992.)
50 µm
(c) 8-cell stage
Fig c, p. 1092

73. (d) Cleavage continues giving rise to a morula
Figure 50.14: Cleavage in a human embryo.
(From Lennart Nilsson, Being Born, pp. 14, 15, 17. Putnam Publishing Group, 1992.)
~ 3 days
50 µm
(d) Cleavage continues giving rise to a morula
Fig d, p. 1092

74. Blastocyst Outer trophoblast Inner cell mass
gives rise to chorion and amnion
Inner cell mass
becomes the embryo proper
Blastocyst undergoes implantation
in the endometrium

75. The Placenta Organ of exchange Umbilical cord
between maternal and fetal circulation
derives from embryonic chorion and maternal tissue
Umbilical cord
connects embryo to placenta

76. Uterine epithelium Uterine gland Uterine blood vessel
Trophoblast
Inner cell mass
Fertilization
Inner cell mass
Implantation
Endometrium
Figure 50.15: Implantation and early development in the uterus.
Trophoblast
Uterine gland
Uterine blood vessel
(a) 7 days.
Fig a, p. 1093

77. Healing site of implantation
Uterine epithelium
Yolk sac
Chorion
Figure 50.15: Implantation and early development in the uterus.
Embryonic disc
Maternal vessel
Amniotic cavity
(b) 10 days.
Fig b, p. 1093

78. Site of implantation Epithelium of uterus Embryonic disc 0.1 mm
Figure 50.15: Implantation and early development in the uterus.
0.1 mm
(c) 12 days.
Fig c, p. 1093

79. Chorionic villi (area of future placenta) Maternal blood
Amniotic cavity
Amnion
Embryo
Yolk sac
Figure 50.15: Implantation and early development in the uterus.
Chorionic cavity
Chorionic villi (area of future placenta)
Maternal blood
(d) 25 days.
Fig d, p. 1093

80. Umbilical arteries and vein
Chorion
Amnion
Umbilical arteries and vein
Figure 50.15: Implantation and early development in the uterus.
Umbilical cord
Amniotic cavity
Placenta
(e) 45 days.
Fig e, p. 1093

81. 2nd Month of Development

82. Fetus (12 to 20 Weeks)