02 Reproduction in Lower and Higher Animals - part 09 - Embryonic development

02 Reproduction in Lower and Higher Animals - part 09 - Embryonic development


Embryonic development : 
  • The zygote formed as a result of syngamy is activated to divide. 
Cleavage :
  • It is the process of early mitotic division of the zygote into a hollow multicellular blastula. 
  • It does not involve the growth of the daughter cells. The cells formed by cleavage are called blastomeres. 
  • Since, there is no growth phase between the cleavages, the size of blastomeres will be reduced with every successive cleavage. 
  • As the size reduces, the metabolic rate increases. 
  • Subsequent 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.
  • The 1st cleavage in the zygote is meridional and occurs at about 30 hours after fertilization. 
  • It divides longitudinaly into two blastomeres, one slightly larger than the other. 
  • The 2nd cleavage is also longitudinal but at the right angle to the 1st one and occurs after 30 hours of 1st cleavage. 
  • The 3rd cleavage is horizontal. After 3rd cleavage the embryo is in 8-cell stage. 
  • As the cleavages are going on the young embryo is gradually being pushed towards the uterus. 
  • By the end of 4th day after fertilization, embryo is a solid ball of 16-32 cells and externally looking like mulberry. This stage is thus called morula. 
  • The morula shows cells of two types : smaller, clearer cellstowards the outer side and inner cell mass of larger cells. 
  • Cells are compactly arranged. 
  • Till the formation of morula the zona pellucida is retained around the embryo and thus, there is no change in the overall size from zygote to morula. 
  • The morula reaches the isthmus and gains entry into the uterus by the end of day 4.
 Blastulation : 
  • Blastulation is the process of formation of the hollow and multicellular blastocyst. 
  • The embryo (blastocyst) that enters the uterus remains floating in uterine cavity for 2-4 days after its entry. i.e. till the end of 7th day after fertilization. 
  • The outer layer of cells seen in the morula now form the layer called trophoblast. 
  • Cells from the trophoblast begin to absorb the glycogen rich uterine milk. 
  • The blastocyst doubles in size from 0.15 mm to 0.30 mm. 
  • With more fluid entering inside the blastocyst cavity is formed. 
  • These outer cells become flat and are called trophoblast cells (since they help only in absorbing nutrition for the developing embryo). 
  • The inner larger cells form inner cell mass or embryoblast (the embryo proper develops from the embryoblasts). 
  • These remain attached to the trophoblasts on only one side. 
  • The trophoblast cells in contact with the embryonal knob are called cells of Rauber. At this stage, the blastocyst shows polarity. 
  • The side with inner cell mass is called the embryonal end and the side opposite to it is the abembryonal end. 
  • By the end of the 7th day the blastocyst is fully formed. 
  • It is now ready for implantation and gastrulation. 
  • The function of zona pellucida is to prevent the implantation of the embryo at an abnormal site. 
  • It does not expose the sticky and phagocytic trophoblast cells till it reaches the implantation site i.e. within the uterus, hence zona pellucida now ruptures.

Implantation : 
  • The blastocyst after its formation, gets implanted or embedded into the endometrium of the uterus. 
  • This process usually begins on day 7 after fertilization and by end of 10th day, the embryo is completely burried inside the endometrium. 
  • The embryo usually implants in the region of the fundus of uterus. 
  • In the process, the embryo attaches itself by its embryonic pole, close to the endometrium. 
  • The trophoblast cells of the animal pole have the power to stick to the uterine wall. 
  • Rapid division of the trophoblast cells lying against the embryonal knob takes place. 
  • It results in the formation of two distinct layers - 
  1. syncytiotrophoblast and 
  2. cytotrophoblast. 
  • The outer layer, syncytiotrophoblast is syncytium i.e. a layer of protoplasm with many nuclei. 
  • It gives out processes which extensively invade the endometrium. 
  • The lytic enzymes secreted by the trophoblasts, rupture the endometrial cells thereby making a burrow, into which the embryo begins to get implanted. 
  • By the end of the 10th day the whole embryo is deeply embedded into the endometrium, completing the process of implantation.
  • The inner layer of cells is called cytotrophoblast (cells with defined membrane) since, the cells retain their cell boundaries. 
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. 
b. Morphogenetic movements : 
  • These are different types of movements to reach their definite place in the embryo.
  • Gastrulation begins in the embryoblast cells on about 8th day after fertilization. 
  • Cell on the free end of inner cell mass called hypoblasts (primitive endoderm) become flatend, start dividing and grows downward towards the blastocoel, cavity of blastocyst. 
  • This layer called endoderm is first to differentiate. 
  • It grows within the blastocoel and forms a sac called Yolk sac. 
  • The remaining cell 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 treak, 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.
Do you know ?
Stem cells: 

  • These are undifferentiated somatic cells of a multicellular organism. 
  • They are capable of giving rise to many more cells of the same type or they can also differentiate into other type of cells. 
  • Bone marrow cells, blood stem cells, cord cells or umbilical cord cells are examples of stem cells. 
  • They can be used in the treatment of  - 
  1. Parkinson's disease
  2. Alzheimer's disease
  3. Diabetes
  4. Leukemia
  5. Arthritis, etc.

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