Download chapter49

Chapter 49
DEVELOPMENT
FERTILIZATION
Fertilization involves four steps.
1. Contact and recognition
A thin vitelline membrane surrounds plasma membrane of the egg and outside this, a thick glycoprotein
layer called the jelly coat or zona pellucida.
Contact between the zona pellucida and the sperm causes the acrosomal reaction.




Membranes surrounding the acrosome fuse.
Pores enlarge and Ca ions move into the acrosome.
Acrosome releases proteolytic enzymes and digests its path through the zona pellucida to vitelline
membrane.
Species-specific proteins called bindin, located on the acrosome adheres to species-specific
bindin on the vitelline membrane.
2. Sperm enters the egg.
After recognition, enzymes dissolve the area of contact between the acrosome and the vitelline
membrane.
The egg's plasma membrane has microvilli, which elongate to surround the head of the sperm forming the
fertilization cone.
Then the plasma membrane of the egg and sperm fuse.
At the moment of fusion channel ions open and Ca2+ pass into the cell. Depolarization that occurs
prevents other sperms from entering.
Depolarization causes Ca granules beneath the plasma membrane to release Ca2+.
Cortical reaction. These granules also release enzymes by exocytosis into the area between the plasma
membrane and the vitelline membrane.





Proteins linking the two membranes dissolve.
Water passes into the space between the membranes.
Vitelline membrane becomes elevated and hardens in some animals.
In mammals, enzymes prevent other acrosomes to be recognized by the zona pellucida.
Polyspermy is prevented.
3. Sperm and egg nuclei fuse.
Microtubules probably guide the sperm nucleus toward the egg nucleus.
Both nuclei swell and are called pronuclei.
They fuse to form the diploid nucleus of the zygote.
4. Fertilization activates the egg.
Release of Ca2+ into the cytoplasm is necessary for the cortical reaction and it triggers metabolic changes.
A burst of protein synthesis occurs a few minutes after sperm entry.
CLEAVAGE
The zygote is totipotent.

It has the potential to produce all the cell types of the new individual.
Cleavage is a series of rapid mitotic divisions. These cells are called blastomeres.
At about 32-cell stage the embryo is a solid ball called morula.
Eventually a hollow ball of cells forms the blastula with a fluid filled blastocoel.
Cells at these stages are capable of amoeboid movement.
Surface proteins allow cells to recognize one another and determine which ones to adhere to form tissues.
Cytoplasm is not homogeneous and helps determine the course of development.
The amount of yolk determines the pattern of cleavage.
Yolk is a mixture of proteins, phospholipds and fats.
Yolk is evenly distributed in the eggs of most invertebrates and simple chordates: isolecithal eggs.


Entire egg divides producing cell of even size (holoblastic cleavage).
Cleavage may be radial (deuterostomes) or spiral (protostomes).
Many vertebrate eggs have the concentrated at one end of the cell, known as the vegetal pole: telolecithal
eggs. The other pole is more metabolically active and is called the animal pole.
Bony fish and amphibians have telolecithal eggs.


The blastula consists of many small cells in the animal hemisphere and very few large cells in the
vegetal hemisphere.
The blastocoel is displaced toward the animal pole.
In telolecithal reptiles, birds and some fish there is large amount of yolk and only a small amount of
cytoplasm at the animal pole.



Cell division takes place only in the blastodisc.
The blastodisc is the small amount of cytoplasm at the animal pole.
This is called meroblastic cleavage.
In birds and reptile, the blastomeres form two layers or cells, the epiblast and the hypoblast.
Both layers are separated by the blastocoel.
In some species sperm penetration of the egg initiates the rearrangement of the cytoplasm
In amphibians, the formation of the gray crescent occurs opposite to the point of penetration and it
contains growth factors that determine the course of development of the embryo.
GASTRULATION
Gastrulation is the embryological process by which the blastula becomes a three-layered gastrula.
Zygote  morula  blastula  gastrula.
The three germ layers are the endoderm, mesoderm and ectoderm.
Each layer gives rise to specific structures.
There are different patterns of gastrulation.
In amphioxus and sea star, cells from the blastula wall invaginate and eventually meet the opposite wall.


The cavity formed is called the archenteron.
This is the developing gut and the opening is called the blastopore.
In amphibians, the yolk-laden cells obstruct invagination.

Cells from the animal pole move down over the yolk rich cells and invaginate forming the
dorsal lip of the blastopore.
In birds, epiblast cells migrate toward the midline to form a thickened cellular region, known as the
primitive streak.


There is narrow furrow at its center, the primitive groove.
Cells migrate in from the epiblast, sink down and move out laterally and anteriorlly in the
interior. These cells form the notochord.
ORGANOGENESIS
Organogenesis is the process of organ formation.
Brain, notochord and spinal cord are among the first organs to develop
First the notochord, in all chordate embryos, develops as a cylindrical rod of cells on the dorsal side
The developing notochord induces the overlying ectoderm to thicken and form the neural plate.

Induction: certain cells stimulate or influence the differentiation of neighboring cells.
Cells from the neural plate move downward and form the neural groove flanked by the neural folds.
The ridges of the neural folds increase and eventually meet forming the neural tube.
The neural tube is formed beneath the surface. Its anterior portion will form the brain and the rest will
differentiate into the spinal cord.
The neural crest consists of cells that lie near the neural tube and will differentiate into sensory neurons
(dorsal root ganglia and postganglionic sympathetic neurons.
The digestive track is formed as a separate foregut and hindgut.
These two simple tubes are lined with endoderm.
Outpocketings give rise to the liver, pancreas and trachea.
The trachea as it grows downward gives rise to the lung buds.
A series of small outpocketings of the pharynx are the pharyngeal pouches. There are corresponding
inpocketings from the ectoderm, the branchial grooves.
EXTRAEMBRYONIC MEMBRANES
Terrestrial vertebrates have four extraembryonic membranes: chorion, allantois, yolk sac and amnion.
Chorion and amnion enclose the entire embryo.
The amniotic cavity is the space between the amnion and the embryo. It becomes filled with the amniotic
fluid.
In humans the allantois is nonfunctional except that its blood vessels form part of the umbilical cord.
In reptiles and birds, the allantois stores nitrogenous wastes.
The yolk sac in higher vertebrates contains little or no yolk.
GESTATION PERIOD
Development begins in the oviduct.
The embryo enters the uterus on about the fifth day and floats there. Its cells form the blastula or
blastocyst.
The outer layer of the embryo, the trophoblast, will eventually form the chorion and amnion.
The embryo implants in the endometrium on about the seventh day.
Once implanted, the trophoblast produces the human chorionic gonadotropin hormone (hCG) that
signals the corpus luteum that pregnancy has begun.
The corpus luteum responds by releasing large amounts of progesterone and estrogen.
Without the hCG, the corpus luteum will degenerate and the embryo would be aborted and flushed out
with the menstrual flow.
The placenta is the organ of exchange between the mother and the embryo.


It provides nutrients and oxygen to the fetus and removes wastes.
It functions as an endocrine organ that maintains pregnancy.
The chorion invades the endometrium and develops the chorionic villi, which become vascularized.
The umbilical cord has two umbilical arteries and one umbilical vein.
Blood flows from the embryo through the arteries to the villi and returns to the embryo through the
umbilical vein.
After 2 months of development, the embryo is referred as a fetus.
The blood of the fetus and the mother do not mix.
The duration of pregnancy averages 280 days (40 weeks) from the time of the mother's last menstrual
period to the birth of the baby or 266 days from the time of conception.