1

Chap 47: Animal Development

Brain

Heart

2

Figure 47.2 The acrosomal and cortical reactions during sea urchin fertilization

3

Sea Urchin as Our Model

Acrosomal Reaction

1) Acrosome releases hydrolytic enzymes allows actin filaments to extend and bind with receptors on the vitelline layer

2) This is one prezygotic barrier: no match between protein and receptors, no sperm penetration

3) Sperm and egg cell membranes fuse

4) Sperm nucleus enters the egg

5) Fusion causes sodium ions to enter the egg cell which acts as a fast block to polyspermy

4

Cortical Reaction

6) Calcium ions are released upon fusion of sperm with egg

7) This triggers the cortical granules to fuse with plasma membrane and release enzymes that separate the vitelline layer from the plasma membrane. Water enters into this space, separating these two layers

8) Vitelline layer becomes the fertilization envelope which acts as the slow block to polyspermy.

Sea Urchin Time Lapse

5

Activation of the Egg

9) Rate of metabolism increases greatly in the egg; protein synthesis increases rapidly.

10) Sperm contributes nothing to activation. Eggs can be activated by calciuim ions alone.

11) After about 20 minutes the sperm nucleus merges with egg nucleus producing 2n zygote.

12) DNA synthesis occurs along with cell division in about 90 minutes.

6

Figure 47.3 A wave of Ca2+ release during the cortical reaction

7

Figure 47.4 Timeline for the fertilization of sea urchin eggs

8

Figure 47.6 Cleavage in an echinoderm (sea urchin) embryo

Occurs at about 45-90 minutes after fertilization 4 cell stage

Cleavage

9

Figure 47.6x Sea urchin development, from single cell to larva

10

Figure 47.5 Fertilization in mammals

Capacitation

11

Capacitation occurs to the sperm cells

Vaginal secretions alter molecules on the sperm’s surface

Sperm motility also increases

Sperm migrates through follicle cells and penetrates zona pellucida

Within the ZP is a glycoprotein that functions as a sperm receptor

Sperm then goes through its acrosomal reaction, releasing its hydrolytic enzymes and sperm reaches plasma membrane

A depolarization or charge change occurs across the egg’s membrane (fast block)

Cortical granules release contents

Sperm enters the egg.

12

Cleavage Partitions The Zygote Into Many Smaller Cells

Cleavage: a bunch of quick divisions after fertilization. All the cells resulting from cleavage are called blastomeres.

Blastomeres: each can contain different cytoplasmic components because the cytoplasm itself is not made up of evenly distributed substances. This type of division creates different cells with different components or polarity which affect development.

•Vegetal Pole: area where stored nutrients or yolk is present

•Animal Pole: lower concentration of yolk

13

Figure 32.1 Early embryonic development (Layer 3)

14

Figure 32.7 A comparison of early development in protostomes and deuterostomes

15

An Amphibian’s Polarity

1. Start with the zygote and it has an animal pole and vegetal pole.

2. The animal pole contains pigment granules, melanin, giving it a gray color and the vegetal pole with yolk is yellowish.

3. At fertilization things change.

• Cytoplasm is rearranged

• Plasma membrane rotates towards where the sperm cell entered and this exposes cytoplasm (light gray) which is called the gray crescent

• Opposite to where the sperm entered is a region which will become the dorsal or back side of the embryo.

16

4. Yolk blocks cell division so most cellular division occurs at the animal pole. Cells are larger here than near the vegetal pole.

5. Cleavage occurs to produce a solid ball of cells-the MORULA.

6. A BLASTOCOEL or fluid filled cavity forms within the morula.

7. This fluid filled hollow ball of cells is the BLASTULA.

17

The Impact of Yolk on Cleavage

Birds, reptiles, insects and many fishes have lots of yolk in their egg cells

•Meroblastic Cleavage: when the yolk takes up so much of the fertilized egg and subsequent cell division occurs in a small area of the animal pole.

Sea Urchins, frogs, mammals demonstrate Holoblastic Cleavage where here is little yolk and division of the egg is relatively complete.

18

Figure 47.7 The establishment of the body axes and the first cleavage plane in an amphibian

19

Figure 47.8x Cleavage in a frog embryo

20

Figure 47.8d Cross section of a frog blastula

21

Figure 47.9 Sea urchin gastrulation (Layer 1)

Gastrulation: rearrangement of the blastula cells into 3 germ layers, ectoderm, mesoderm and endoderm

•Cells undergo changes in motility, adhesion and shape

•The three layered end-product is called a gastrula from which tissues and organs will develop.

The invagination continues inward and forms a pocket called the archenteron which will develop into the gut or digestive tube. Depending on the organism, one end becomes the mouth, the other the anus.

The opening is called the blastopore.

Endoderm forms the archenteron

22

Figure 47.9 Sea urchin gastrulation (Layer 2)

Continued invagination of archenteron

23

Figure 47.9 Sea urchin gastrulation (Layer 3)

Digestive tube is formed from endoderm

Ectoderm forms outer surface

Mesoderm forms some of internal skeletal

24

Table 47.1 Derivatives of the Three Embryonic Germ Layers in Vertebrates

25

Figure 47.10 Gastrulation in a frog embryo

26

Figure 47.11 Organogenesis in a frog embryo

27

Figure 47.12 Cleavage, gastrulation, and early organogenesis in a chick embryo

28

Figure 47.13 Organogenesis in a chick embryo

29

Figure 47.14 The development of extraembryonic membranes in a chick

30

Mammalian Development

Egg of mammals is quite small

Divisions of the fertilized egg

•1st Within about 36 hours

•2nd at about 60 hours

•3rd at about 72 hours

Blastocyst: 100 cells arranged around the blastocoel

•There is an inner cell mass that will become the embryo

•Trophoblastic layer will form the fetal contribution to the placenta

•It is the blastocyst that implants itself into the uterus (7 days)

31

Inner cell mass becomes a flattened disc with two layers.

•Epiblast: which will form embryo

•Hypoblast: which will form the yolk sac but contains no yolk

Yolk sac will make blood cells for the embryo

Trophoblast

•Expands into endometrium

•Develops the chorion: contributes to placenta; surrounds all other (3) extraembryonic membranes

32

Allantois: this extraembryonic membrane becomes part of the umbilical cord and forms blood vessels to transfer oxygen and nutrients from placenta to embryo; also carries carbon dioxide and wastes to placenta.

So the four extraembryonic membranes are:

•Chorion Yolk Sac Amnion Allantois

Epiblast: forms the amnion which contains the amniotic fluid that cushions the developing fetus

•Gastrulation occurs in the epiblast

33

Figure 47.15 Early development of a human embryo and its extraembryonic membranes

34

Figure 47.16 Change in cellular shape during morphogenesis

35

Figure 47.17 Convergent extension of a sheet of cells

36

Figure 47.18 The extracellular matrix and cell migration

37

Figure 47.19 The role of a cadherin in frog blastula formation

38

Figure 47.20 Fate maps for two chordates

39

Figure 47.21 Experimental demonstration of the importance of cytoplasmic determinants in amphibians

40

Figure 47.22 The “organizer” of Spemann and Mangold

41

Figure 47.23 Organizer regions in vertebrate limb development

42

Figure 47.24 The experimental manipulation of positional information

43

How is Sex Determined?HHMI Holiday Lecture Series 2001: Lecture 1 of 4

HHMI Holiday Lecture Series 2001: Lecture 1 of 4

How is the sex of the human embryo determined?

• At about 6 weeks after fertilization, there is no anatomical difference.

• At the 7th week, the gonads are bipotential, that is, that can either differentiate into testes or ovaries.

• Hormonal secretions determine the sexual fate of the reproductive structures; the masculinization or feminization of other structures also occurs, including the brain.

In humans, the presence of the Y chromosome is sex determining but . . . There are XX males)

44

XX Males

• 1 out of every 20,000 males is XX

• They have a penis, scrotum, testes, but will not produce sperms or eggs

• The reason why they are XX males is because they have a portion of the Y chromosome that was crossed over during meiosis in making sperm cells. And the X chromosome from the father that fertilized the egg of the mother had this small portion of the Y chromosome.

• This portion is the testes determining portion so this has the critical gene or genes.

• The gene is called the SRY gene: This codes for a DNA binding protein that affects many other genes.

• SRY causes development of the testes so if you are +SRY you follow the testicular path and if you are – SRY you follow the ovarian path.

45

XY Females

• Well, if you’ve been able to follow my description of the loss of this SRY portion from the Y chromosome, then you probably have

realized there is a sperm cell with a Y chromosome without the SRY gene because of this crossing over.

• 1 out of every 20,000 females is an XY female.

• She has a clitoris, labia, ovaries, fallopian tubes, uterus but no sperm or egg production.

• Yes, she has a Y chromosome but not the SRY portion that would make her a guy. So “Dude looks like a lady?” (Aerosmith, 19_ _?)

• XY females do not produce testosterone but do have female hormone levels.

46

Transgenic mice research to prove the role of SRY gene

• In vitro, researchers took an XX fertilized mouse egg and injected the SRY gene

• The SRY integrated into the host genome

• Then the implanted into the uterus of the female mouse.

• Development for 20 days

• Produced an XX +SRY transgenic mouse that had testes and is male but no sperm production.

• So: SRY is a sex determining gene

• Two X chromosomes are incompatible with producing sperm even in the presence of SRY gene. Another portion of the Y chromosome is responsible for this also.