Fertilization

Learning Objective Describe the four processes involved in fertilization

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)

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

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

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.

Steps of Fertilization Recognition and contact

Steps of Fertilization Sperm Entry

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

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

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.

http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html

Very good animation http://bcs.whfreeman.com/thelifewire/content/chp43/4301s.swf

Acrosomal reaction in mammals http://www.youtube.com/watch?v=41qQTEhoNjYhttp://www.youtube.com/watch?v=41qQTEhoNjY

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

Fast block polyspermy

http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html

Cortical Reaction

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

Steps of Fertilization (continue) 4. Egg activation

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

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

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

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

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

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

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

Cleavage in Sea Stars

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. 50-3 (a-c), p. 1084

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. 50-3 (d-f), p. 1084

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. 50-3 (g-i), p. 1084

Cleavage in Amphioxus

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. 50-4 (a-d), p. 1085

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. 50-4 (e-g), p. 1085

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. 50-4 (h-j), p. 1085

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

Cleavage in Frogs Animal pole Vegetal pole

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

Cleavage in Birds

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

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