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Flashcards in Animal Development Deck (50)
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Development is determined by

zygote’s genome.


Molecules in the egg called

cytoplasmic determinants


Cell differentiation

the specialization of cells in structure and function.



the process by which an animal takes shape.


The first stage of development

gametogenesis (formation of gametes)


Formation of sperm



Formation of eggs



Production of primary oocytes

is complete before birth.


Production of mature gametes

ceases at 50



process is the fusion of the sperm and egg to form a diploid zygote.


fertilization in sea urchins

The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development.


fertilization in sea urchins

1. Acrosomal reaction is stimulated by contact with egg jelly (stimulatory molecules is fuses sulfate).
2.Fuses sulfate binds to sperm and activates the Ca2+ transport channel
3.Elevated Ca2+ triggers fusion of acrosome and cell membranes.4.Proteolytic enzymes digest a path through jelly coat to egg surface.5.Ca2+ influx also stimulates actin polymerization to form the acrosomal process
6.Acrosomal process adheres to vitelline envelope via binding proteins found in egg membrane.
7.Sperm Acrosomal process membrane fuses with egg membrane.
8.The sperm’s nucleus, and centriole enters the egg after fusion occurs.


egg activation in sea urchins

* Ca2+ in the cytosol increases the rate of cellular respiration and protein synthesis by the egg cell.
* Collection of events are referred to as egg activation.
* Proteins and mRNAs needed for activation are already present in the egg.
* The sperm nucleus fuses with the egg nucleus, and cell division begins.


blocking polyspermry in sea urchins

* Ca2+ also triggers the cortical reaction.
* Cortical granules release their content via exocytosis.
* Hyalin and other enzymesseparate the vitelline envelope from the egg.
* Hyalin-sugar-rich molecule that attracts water via osmosis into the space between egg and vitelline envelope.
* Polyspermy-fusion of additional sperm to the fertilized egg.


fertilization in mammals

* Process is similar to sea urchins but much slower (12-36 hours).
* Sperm must travelthrough a layer of follicle cells.
* Egg outer layer is not a jelly coatbut rather a zona pellucida.
* Fertilization is internal.


formation of the fertilisation envelope in mammals

* Cortical granules release enzymes that harden the zona pellucida.
* Enzymes also strip the sperm receptors found in the egg membrane.
* Hyalinattracts water via osmosis.

**In sea urchins, sodium (Na+) influx changes membrane potential of the egg.



* Fertilization is followed by cleavage, a period of rapid cell division without growth.
* Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres.
* The blastula is a ball of cells with a fluid-filled cavity.
* a fluid-filled cavity is called a blastocoel


Cleavage of egg with moderate to little yolk

* In species whose eggs have little or moderate amounts of yolk, such as sea urchins and mammals, cleavage occurs throughout the whole egg.
* This pattern is termed Holoblastic cleavage-complete division of the egg.
* Results in cells of equal size surrounding the blastocoel.


Cleavage in eggs with large yolk

* The yolk in these species slows down cell division and result in:
1. Meroblastic cleavage-incomplete division of the egg.
2. Occurs in species with yolk-rich eggs, such as reptiles and birds


Meroblastic cleavage

incomplete division of the egg


Cleavage patterns in frogs

1. The egg yolk affects cleavage patterns.
2. The eggs and zygotes of many animals, except mammals, have a definite polarity -defined by distribution of yolk:
- Vegetal pole has more yolk.
- Animal polehas less yolk.
3. The polarity causes unequal cell divisions.
4. That results in the blastocoel forming entirely in the animal hemisphere


Establishment of body axes in frogs

1. The three body axes are established by the egg’s polarityand by a cortical rotation following binding of the sperm
2. The animal –vegetal asymmetry dictates where the anterior –posterior axis will form.
3. Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles.
4. Once the anterior-posteriorand dorsal-ventral axis are established, the right and left axis is fixed.


establishment of the body axes in chicks

1. Gravity plays a part in establishing the anterior-posterior axis.
2. pH levels between the two sides of the blastodermestablish the dorsal-ventral axis.



1. rearranges the cells of a blastula into a three-layered embryo called a gastrula.
2. A Gastrulahas a primitive gut.
3. The three layers produced by gastrulation are called embryonic germ layers.


embryonic germ layers

1. The ectodermforms the outer layer.
2. The endodermlines the digestive tract.
3. The mesodermpartly fills the space between the endoderm and ectoderm.


Gastrulation in sea Urchins

1. The blastula consists of a single layer of cells surrounding the blastocoel.
2. Mesenchyme cells migrate from the vegetal pole into the blastocoel.
3. The vegetal plate forms from the remaining cells on the vegetal pole and buckles inward.
4. The endoderm cells form an invagination.
5. The newly formed cavity is called the archenteron
6. This opens through the blastopore, which will become the anus.


Gastrulation in frogs

1. The frog blastula is many cell layers thick.
2. Cells of the dorsal lip originate in the gray crescent.
3. Cells below the dorsal lip invaginate to create the archenteron.
4. Cells continue to move from the embryo surface into the embryo by involution until the blastocoel disappears.
5. These cells become the endodermand mesoderm
6. The blastoporeencircles a yolk plug when gastrulation is complete.
7. The surface of the embryo is now the ectoderm, the innermost layer is endoderm, and the middle layer is mesoderm


Gastrulation in chicks

1. Cleavage forms a small cap called a blastoderm instead of a blastula.
2. Blastodermforms on top of the yolk mass.
3. The upper layer of the blastodermis called the epiblast.
4. The lower layer is the hypoblast.
5. The epiblastmoves toward the midline of the blastodermand then into the embryo toward the yolk.
6. Cell movements result in a pile-up of cells and in the formation of the primitive streak.
7. primitive streak-The thickened midline in the blastoderm.
8. The migration of cells gives rise to the endoderm,mesoderm, and ectoderm.
9. The epiblast cells continues to migrate and sort-a process called convergent extension.
10. This lengthens and narrowsthe primitive streak.
11. The hyploblastcells are displaced to form the sac that surrounds the yolkand the stalk that connect the yolk to the embryo
12. The embryo develops from the epiblastand the hypoblastform the extraembryonic membrane.


Gastrulation in humans

1. Human eggs have very little yolk.
2. Ablastocyst is the human equivalent of the blastula.
3. The inner cell mass is a cluster of cells at one end of the blastocyst.
4. The trophoblastis the outer epithelial layer of the blastocyst and does not contribute to the embryo
5. Instead, the trophoblast initiates implantation.
6. Gastrulation is preceded by implantation.
7. During implantation, the trophoblast secretes enzymes that break the uterine lining allowing invagination of the blastocyst.
8. The cells of the inner cell mass form a flat disc of cells. 9. The inner layer is the epiblastand the outer layer is thehypoblast.
10. The trophoblastalso forms finger-like projections and a structure called Chorion.
11. Cells of the epiblast migrate to form a primitive streak.
12. Some of those cells will gives rise to the endoderm,mesoderm, andectoderm.
13. Four extraembryonic membranes form from the epiblast.
14. At the end of gastrulation, four distinct membranes surround the embryo.
15. The cells of the trophoblast, epiblastand the endometrial tissues will form the placenta


how long does it take for the extra-embryonic membranes to develop in the placenta

2 weeks