The first cleavage results in bisection of the zygote into left and right halves. The following cleavage planes are centered on this axis and result in the two halves being mirror images of one another. In bilateral holoblastic cleavage, the divisions of the blastomeres are complete and separate.
Radial cleavage is characteristic of the deuterostomes, which include some vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte.
Mammals display rotational cleavage, and an isolecithal distribution of yolk (sparsely and evenly distributed). Because the cells have only a small amount of yolk, they require immediate implantation onto the uterine wall in order to receive nutrients. Rotational cleavage involves a normal first division along the meridional axis, giving rise to two daughter cells.
In spiral cleavage, the cleavage planes are oriented obliquely to the polar axis of the oocyte. At the third cleavage the halves are oblique to the polar axis and typically produce an upper quartet of smaller cells that come to be set between the furrows of the lower quartet.
In discoida cleavage, the cleavage furrows do not penetrate the yolk. The embryo forms a disc of cells, called a blastodis, on top of the yolk. Discoidal cleavage is commonly found in birds, reptiles, and fish which have telolecithal egg cells (egg cells with the yolk concentrated at one end).
In superficial cleavage, mitosis occurs but not cytokinesis, resulting in a polynuclear cell. With the yolk positioned in the center of the egg cell, the nuclei migrate to the periphery of the egg, and the plasma membrane grows inward, partitioning the nuclei into individual cells. Superficial cleavage occurs in arthropods which have centrolecithal egg cells (egg cells with the yolk located in the center of the cell).
- a phase early in the development of most animal embryo, during which the morphology of the embryo is dramatically restructured by cell migration.
GASTRULATION - a phase early in the development of most animal embryos, during which the morphology of the embryo is dramatically restructured by cell migration.
The purpose of gastrulation is :
- to position the three embryonic germ layers, the endoderm, ectoderm and mesoderm.
gastrulation occurs after implantation, around days 14-16 after fertilization in human embryogenesis. (in human )
The process gastrulation (in human) are :
- The formation of the primitive streak and Hensen's node and the ingression of cells through the primitive groove to form the endoderm and the mesoderm.
- Thus, gastrulation creates all three germ layers of the embryo: ectoderm, mesoderm, and endoderm
- Extraembryonic mesoderm forms within the hypoblast or embryonic mesoderm and migrates out to form the blood vessels of the chorion and connect the chorion to the embryo through the umbilical cord.
GATRULATION IN SEA URCHINS
The archenteron is elongated by three mechanisms :
- First, the initial invagination is caused by a differential expansion of the inner layer made of fibropellins and outer layer made of hyalin to cause the layers to bend inward.
- Second, the archenteron is formed through convergent extension.
-Third, secondary mesenchyme pull the tip of the archenteron towards the animal pole.
The process of gastrulation in amphibian is at higher density of yolk in the vegetal half of the embryo results in the blastocoel cavity being placed asymmetrically in the animal half of the embryo.
four kinds of tissue movements that drive gastrulation in Xenopus, that are :
- At the vegetal edge of the dorsal marginal zone, cells change from a columnar shape to become a bottle cell and drive invagination.
- At this invagination, cells begin to involute into the embryo. This initial site of involution is called the dorsal lip.
- Directed cell intercalation within the dorsal mesoderm drives convergent extension. The dorsal cells become the first to migrate along the roof of the blastocoel cavity and form the anterior/posterior axis of the embryo.
- Both prior to and during the involution, the animal cap undergoes epiboly and spread toward the vegetal pole.
GASTRULATION IN FISH
GASTRULATION IN BIRD
EVENTS IN DEVELOPMENT THAT INVOLVED THE MIGRATION OF CELLS WITHIN THE EMBRYO DEVELOPMENT
About 1th week
After fertilization,embryo reaches two-cell stage
The blastula implants into the uterus
Within 2 weeks,many thousand of cells formed(embryo)
About 5 th week
A gestational sac on ultrasound
Embryo at 4 weeks after fertilization.
6 th week
In the beginning of the 6 th week,a small ring called yolk sac on ultrasound
At the end of the 6 th week the fetal pole and perhaps cardiac activity in the embryo
Embryo at 4 weeks after fertilization.
At 7 th week A well defined fetal pole and deinite cardiac cavity By 9 th week A baby is called fetus. At this time the heartbeat with a doptone device about 50% of the time can heard is about five weeks old (or from the seventh week of menstrual age). Fetus at 8 weeks after fertilization. Fetus at 8 weeks after fertilization. This embryo is also from an ectopic pregnancy , this one in the cornu (the part of the uterus to which the Fallopian tube is attached). The features are consistent with a developmental age of seven weeks (reckoned as the ninth week of pregnancy
At end of first trimester(12 th week) Placenta formed and supply the baby with oxygen from mother’s blood supply,and ridding wastes tthrough mom’s blood system At 13 th week Baby growing very quickly Week 16 th – 20 th Mother may feel a fluttering that is baby’s movement(quickening) Fetus at 18 weeks after fertilization 20 th week Baby is half-way fully formed Baby is quite active and moving often 21th week Baby’s eyes still closed,movement is stronger,skin is pink As baby and uterus grow,they are displacing organs that reside normally in the lower abdomen and pelvis Fetus at 18 weeks after fertilization
By 24 th week Uterus having intermitent contractions Baby weights over one and one half pound Baby is consider viable(half babies born is survive at this stage) 26 th to 28 th week Lungs matured Baby starting to store part in the subcutaneous layer of skin and hair growing Baby eyes is open 32th to 33th week Baby weight about 4 ½ pounds and about 16-17 inches along About 34 th week Baby lung start to work well From 36 th week Baby consider fully develop Fetus at 38 weeks after fertilization
NEURULATION AND ORGANOGENESIS
The process involved in the formation of the neural plate and neural folds and closure of the folds to form the neural tube constitute neurulation.
Neurulation is complete by the end of the fourth week.
During neurulation, the embryo may be referred to as neurula.
Neurulation in vertebrates results in the formation of the neural tube, which gives rise to both the spinal cord and the brain.
Neural crest cells are also created during neurulation.
Neural crest cells migrate away from the neural tube and give rise to a variety of cell types, including pigment cells and neurons.
PROCESS OF NEURULATION
Neurulation begins with the formation of a neural plate, a thickening of the ectoderm caused when cuboidal epithelial cells become columnar.
Changes in cell shape and cell adhesion cause the edges of the plate fold and rise, meeting in the midline to form a neural tube.
The cells at the tips of the neural folds come to lie between the neural tube and the overlying epidermis. These cells become the neural crest cells.
Both epidermis and neural plate are capable of giving rise to neural crest cells.
Organogeneis is the period of animal development during which the embryo is becoming a fully functional organism capable of independent survivial.
Organogenesis is the process by which specific organs and structures are formed, and involves both cell movements and cell differentiation.
Organogenesis requires interactions between different tissues. These are often reciprocal interactions between epithelial sheets and mesenchymal.
ORGAN PRODUCED BY THE 3 GERM LAYERS
The endoderm produces tissue within the lungs, thyroid and pancreas.
The mesoderm aids in the production of cardiac muscle, skeletal muscle, smooth muscle, tissues within the kidneys, and red blood cells.
The ectoderm produces tissues within the epidermis and aids in the formation of neurons within the brain, and melanocytes.