1. Fertilization

2. Learning Objective
Describe the four processes involved in fertilization

3. Fertilization 1. Recognition and contact 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)

4. Fertilization 3. Sperm and egg pronuclei fuse initiates DNA synthesis
4. Egg becomes activated and developmental changes begin

5. 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

6. KEY CONCEPTS
Fertilization includes contact and recognition between egg and sperm, regulated sperm entry, and fusion of egg and sperm pronuclei, Egg becomes activated and developmental changes begin.

7. Steps of Fertilization
Recognition and contact

8. Steps of Fertilization
Sperm Entry

9. 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

10. 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

11. 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.

14. Very good animation

15. Acrosomal reaction in mammals

16. 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

17. Fast block polyspermy

19. Cortical Reaction

20. Steps of Fertilization (Continue)
3. Fusion of sperm and egg pronuclei

21. Steps of Fertilization (continue)
4. Egg activation

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

23. 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

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

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

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

27. 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

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

29. Cleavage in Sea Stars

30. 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

31. 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

32. 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

33. Cleavage in Amphioxus

34. 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

35. 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

36. 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

37. 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

38. Cleavage in Frogs
Animal pole
Vegetal pole

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

40. Cleavage in Birds

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

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