Embryonic development
The zygote formed because of syngamy is activated to divide.
Stages of Embryonic development | Germinal layers Ectoderm , Mesoderm, Endoderm
Cleavage
Blastulation
Implantation
Gastrulation
A. Cleavage
Process of early mitotic division of the zygote. To form hollow multicellular blastula
Does not involve the growth of the daughter cells.
The cells formed by cleavage are called blastomeres
Size of blastomeres reduced at cleavage and metabolic rate increases.
So cleavages are thus faster than earlier one.
This requires rapid replication of DNA and high consumption of oxygen.
Process of cleavage
In human, cleavage is holoblastic i.e. The whole zygote gets divided.
The cleavage planes may be longitudinal or meridional and equatorial or horizontal.
It is radial and indeterminate i.e. Fate of each blastomere is not predetermined.
1st cleavage is meridional occurs 30 hours after fertilization.
Divides longitudinally into two blastomeres, one large other is small.
After 30 hours 2nd cleavage is occur .
2nd cleavage also longitudinal but at the right angle to the 1st one
The 3rd cleavage is horizontal and embryo become is 8-cell stage.
After 4th day of fertilization embryo become solid ball 16-32 cell looking like mulberry. This stage is thus called morula
B. Blastulation
Blastulation is formation hollow and multicellular blastocyst.
Blastocyst remains floating in uterine cavity for 2-4 days.
Trophoblast layer is form at outer layer of morula.
Cell of trophoblast absorb the glycogen rich milk uterine milk.
Doubles in size from 0.15 mm to 0.30 mm.
More fluid entering inside the blastocyst cavity is formed
These outer cells become flat and are called trophoblast cells
Help only in absorbing nutrition for the developing embryo
Inner larger cell become embryoblast remain attached to the trophoblasts on only one side(the embryo proper develops from this cells)
Cells of Rauber are trophoblast cells in contact with the embryonal knob
At 7th day the blastocyst is fully formed. Ready for implantation and gastrulation.
In uterus it expose the sticky and phagocytic trophoblast cells at implantation site , hence zona pellucida now rupture
C. Implantation
Blastocyst
embedded into the endometrium of the uterus called Implantation.
Implants at region
of the fundus of uterus.
After 7day of
fertilization Implantation occur.
Trophoblast cells
of the animal pole have the power to stick to the uterine wall, embryo attaches
itself by its embryonic pole.
Rapid division of
the trophoblast cells lying against the embryonal knob takes place.
It results in the formation of two distinct layers-
Syncytiotrophoblast
and cytotrophoblast
Cytotrophoblast
inner cell
Syncytiotrophoblast
outer layer.
Layer of
protoplasm with many nuclei
It gives out
processes which extensively invade the endometrium.
Trophoblasts,
rupture the endometrial cells by secreting lytic enzyme and embryo begins to
get implanted
End of the 10th day process of implantation complete.
C.Gastrulation
It is the process of formation of ‘Gastrula’ from the blastocyst.
In the gastrula stage, there is slowing of the rate of cleavage or divisions but there are two important events that take place actively.
a. Differentiation of blastomeres : This process results in the formation of three germinal layers i.e. ectoderm, mesoderm and endoderm from the cells of the embryoblast.
Ectoderm
Ectoderm gives rise to epidermis of skin, hair, nails, sweat glands, salivary glands, mammary glands, lacrimal glands, sebaceous glands, cornea, lens, retina, conjunctiva, nasal epithelium, enamel of teeth, internal and external ear, foregut, hindgut, adrenal medulla, anterior and posterior pituitary, pineal gland, entire nervous system.
Mesoderm
Mesoderm forms all types of muscles
(except iris muscles and ciliary muscles of eye which originate from ectoderm),
connective tissues, dermis of skin, adrenal cortex, heart, blood, blood
vessels, lymphatic vessels, middle ear, dentine of teeth, urinary and
reproductive ducts, gonads, kidneys, sclera and choroid of eye.
Endoderm
Endoderm develops into epithelium of
mid- gut, glands of stomach and intestine, tongue, tonsils, lungs, trachea,
bronchi, larynx, urinary bladder, vagina, liver, pancreas, thyroid gland,
parathyroid gland, thymus gland, Eustachian tube, epithelium of urethra, lining
of middle ear.
b. Morphogenetic movements : These are different types of movements to reach their definite place in the embryo.
Embryonic development
Stages of Embryonic development
Cleavage| Blastulation | Implantation | Gastrulation
Process of Gastrulation
After the 8th
day fertilization Gastrulation is begins
Hypoblasts
(primitive endoderm) become flattened, start dividing and grow downward towards
the blastocoels.
Endoderm is first to differentiate
grows within the blastocoel forms Yolk
sac
The remaining cells of the inner cell
mass, in contact with cells of Rauber are called epiblasts (primary ectoderm).
Both layers form a flat, bilaminar
embryonal disc.
After formation of
endoderm the second layer to be differentiated is the ectoderm.
Cells of epiblast
divide and redivide and move in such a way that they enclose the amiotic
cavity.
The floor of this cavity has the embryonal disc.
The pyramidal cells of the disc towards the amniotic cavity
form the embryonal ectoderm.
The roof of amniotic cavity is lined by amniogenic cells.
Later, these cells divide and redivide to form the amnion.
Amnion is an extra embryonic membrane that surrounds and
protects the embryo.
As a result of all these changes, the bilaminar embryonic
disc is positioned in between amniotic cavity and Yolk sac.
Actual
gastrulation occurs about 15 days after fertilization, in which the bilaminar
embryonic disc is transformed into trilaminar embryonic disc.
This
transformation occurs by division, rearrangement and migration of cells of
epiblast.
It begins with
formation of primitive streak and a shallow groove on the surface is called
primitive groove.
This streak
progresses from posterior to anterior end of embryo.
From site of a
primitive streak, a third layer of cells called mesoderm extends between
ectoderm and endoderm.
Anterior end of
primitive groove communicates with yolk sac by an aperture called blastopore
(future anus).
The embryonal disc
now has differentiated into three layers-ectoderm, mesoderm and endoderm.
The further
process after gastrulation is called organogenesis
Embryonic development | germinal layers| Ectoderm , Mesoderm, Endoderm
Stages of Embryonic development | germinal layers
Cleavage| Blastulation | Implantation | Gastrulation
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