Chap 47: Animal Development Brain Heart Brain development begins within one month after conception Hearts, as small as can be, begin beating within the first 4 weeks.

Figure 47.2 The acrosomal and cortical reactions during sea urchin fertilization 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 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. 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.

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

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

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.

Figure 47.3 A wave of Ca2+ release during the cortical reaction The high cytosolic concentration of calcium ions causes the cortical granules to fuse with the plasma membrane. The fertilization membrane is then produced which acts as a slow block to polyspermy.

Figure 47.4 Timeline for the fertilization of sea urchin eggs

Figure 47.6 Cleavage in an echinoderm (sea urchin) embryo Occurs at about 45-90 minutes after fertilization 4 cell stage Cleavage

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

Figure 47.5 Fertilization in mammals Capacitation 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.

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.

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

Figure 32.1 Early embryonic development (Layer 3) Some animals have larval stages: this is an immature adult stage, distinctly different in that the larvae eats different food, may have a different habitat and looks very different. Metamorphosis produces the adult.

Figure 32.7 A comparison of early development in protostomes and deuterostomes

An Amphibian’s Polarity Start with the zygote and it has an animal pole and vegetal pole. The animal pole contains pigment granules, melanin, giving it a gray color and the vegetal pole with yolk is yellowish. 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.

Yolk blocks cell division so most cellular division occurs at the animal pole. Cells are larger here than near the vegetal pole. Cleavage occurs to produce a solid ball of cells-the MORULA. A BLASTOCOEL or fluid filled cavity forms within the morula. This fluid filled hollow ball of cells is the BLASTULA.

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.

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

Figure 47.8x Cleavage in a frog embryo

Figure 47.8d Cross section of a frog blastula

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 Invagination at the vegetal pole Cells rearrange and migrate

Figure 47.9 Sea urchin gastrulation (Layer 2) Continued invagination of archenteron

Figure 47.9 Sea urchin gastrulation (Layer 3) Digestive tube is formed from endoderm Ectoderm forms outer surface Mesoderm forms some of internal skeletal

Table 47.1 Derivatives of the Three Embryonic Germ Layers in Vertebrates

Figure 47.10 Gastrulation in a frog embryo

Figure 47.11 Organogenesis in a frog embryo Organogenesis: the three germ layers will develop into organs First Organs to take shape are the neural tube and notochord. Notochord: will eventually form the vertebrae Neural Tube: eventually forms the brain and spinal cord Neural Plate folds inwards forming the neural tube Originates as ectoderm Notochord: forms from mesoderm Somites: develop from mesoderm: give rise to vertebrae and muscles of back

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

Figure 47.13 Organogenesis in a chick embryo

Figure 47.14 The development of extraembryonic membranes in a chick

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) Egg of mammals is quite small not much need for a food reserve holoblastic cleavage results Divisions of the fertilized egg 1st Within about 36 hours 2nd at about 60 hours 3rd at about 72 hours Blastomeres are about equal size 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)

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

Epiblast: forms the amnion which contains the amniotic fluid that cushions the developing fetus Gastrulation occurs in the epiblast 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

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

Figure 47.16 Change in cellular shape during morphogenesis

Figure 47.17 Convergent extension of a sheet of cells

Figure 47.18 The extracellular matrix and cell migration

Figure 47.19 The role of a cadherin in frog blastula formation

Figure 47.20 Fate maps for two chordates

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

Figure 47.22 The “organizer” of Spemann and Mangold

Figure 47.23 Organizer regions in vertebrate limb development

Figure 47.24 The experimental manipulation of positional information

How is Sex Determined? How is the sex of the human embryo determined? HHMI Holiday Lecture Series 2001: Lecture 1 of 4 How is Sex Determined? 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)

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.

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.

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.