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Cleavage and Blastulation

Cleavage is the process after fertilization when early mitotic cell divisions occur that progressively reduce cell size.
  • During cleavage, the total embryonic mass, however, remains constant.
  • In mammals, when the embryo has about 16 cells, its individual cells begin to adhere to one another and it coalesces to form into a morula.
      A blastocyst (blastocyst cavity) forms in the morula when it enters into the uterus.
        This cavitation is an important transition from homogeneous cells to differentiated cell function.
          This new structure is called a blastocyst which consists of an outer layer, the trophoblast, and an inner cluster of cells referred to as the inner cell mass.
  • Implantation is the process in which the blastocyst attaches to and penetrates into the uterine wall.
      Upon contact with the uterine lining or endometrium during implantation, the trophoblast cells invade the uterine lining to give the embryo access to the deeper layers of the uterine wall.
        These trophoblast cells differentiate into two new cell types referred to as syncytiotrophoblasts and cytotrophoblasts.
        • The syncytiotrophoblasts continue to grow but without cell division and begin to fuse.
        • The cytotrophoblasts remain distinct and invade deeper into the uterine wall.
  • The egg is fertilized in the ampulla of the fallopian tube or first third of the oviduct.
  • The zygote undergoes a series of cleavages until it forms the blastocyst at the time of implantation and invades the endometrium.
  • During the cleavage process there is no increase in cell volume and the zygote cytoplasm is divided into increasingly smaller cells.
    • This is accomplished by abolishing the growth period between cell divisions.
    • In other words, there is no G1 or G2 phase of the cell cycle in this case.
    • The cells continue dividing without growth at a very rapid rate and this cleavage ends at the mid-blastula transition at about the time of implantation.
        At this point G1 and G2 are again added to the cell cycle and the cells begin to grow and embryo volume increases.
There are several different types of cleavage patterns which is determined by the amount and distribution of yolk protein in the cytoplasm and factors influencing the mitotic spindle.
  • When one pole of the egg is yolk-free, the cellular divisions occur there at a faster rate than at the opposite pole.
  • The pole with the lesser yolk concentration is the animal pole and the pole with the greater yolk concentration is the vegetal pole.
      The zygote nucleus is usually located in the animal pole and the yolk in the vegetal pole tends to inhibit cleavage.
        The influence of yolk is a major factor in the type of cleavage that is seen in different species.
Two major types of cleavage are seen and referred to as holoblastic (or complete cleavage) and meroblastic (or incomplete cleavage).
  • Holoblastic cleavage occurs in mammals and cleavage occurs throughout the entire egg due to the presence of little yolk.
  • In organisms such as birds there is a large accumulation of yolk and cleavage occurs primarily in the animal pole of the blasomere (meroblastic cleavage).
  • As examples of this, the frog embryo undergoes holoblastic cleavage with divisions occurring throughout the developing embryo.
  • However, in zebrafish, meroblastic cleavage occurs and cell cleavage is initially confined to the animal (or top) half of the embryo.
The symmetry of cleavage is further divided into subtypes such as radial or spiral, depending on the position of the yolk.
    • The simplest pattern such as occurs in the sea urchin is radial cleavage.
        Here successive symmetric cleavages divide the embryo into equal-sized cells.
    • However, in flatworms, the divisions are unequal and the first cleavage of the egg produces two cells of unequal size.
    • Spiral cleavage is yet another symmetry of cell divisions that occurs in mollusks and roundworms.
        In this case there is also unequal cleavage but the cells arrange in different planes within the embryo which appears as a spiral formation.
In mammals we see a unique type of cleavage process in the early embryo.
  • The eggs of mammals are among the smallest in the animal kingdom and cleavage occurs very slow taking about 12-24 hours.
  • They also undergo what is referred to a rotational cleavage.
    • In the first division the cells divide in half with the plane from top to bottom.
    • However, in the second cleavage, one of the two blastomeres divides the same as the first cleavage and the other divides at the equator.
    • This is referred to as rotational cleavage and is unique to mammals.
  • Also unique to mammalian cleavage is that the cells do not always divide at the same time producing the 2, 4, or 8 cell stages but sometimes divide at different times so that odd numbers of cells may be present such as a 5-cell embryo.
  • One of the most important differences that distinguishes mammalian cell cleavage from those of other organisms is the process of compaction.
    • Up until the 8-cell stage the blastomeres are loosely arranged and have plenty of space between them.
    • After the third cleavage, the blastomeres tighten greatly to form a compacted structure.
    • These changes are the result of changes in cadherin which concentrates at regions of intracellular contact and now acts for the first time as an adhesion molecule.
    • This process of compaction is where the cells at the 8-cell stage are smooth and during compaction the cells increase their contact with one another, flatten, and have more microvilli on their surface.
        This increase in microvilli is caused by the contraction of actin filaments drawing the cortical elements to the surface.
  • The cells of the compacted embryo divide to produce a 16-cell morula.
      These cells are divided into internal and external cells.
      • The external cells of the morula become the trophoblast cells (trophoderm) that do not produce any cell of the embryo but are necessary for implantation of the embryo into the uterine wall.
        • The trophoblast cells eventually produce the chorion or the embryonic portion of the placenta which provides oxygen and nourishment from the mother.
        • The trophoderm also secretes hormones that will regulate the mother's immune system preventing immune rejection of the new embryo.
      • The inner cells of the morula eventually form the embryo itself.
          The cells of the inner cell mass form a separate group consisting of about 13 cells by the time the embryo reaches the 64-cell stage (sixth division).
      • This distinction between the trophoblast and inner cell mass represents the first differentiation event in mammalian development.
The morula at the compacted stage does not have an internal cavity.
  • During the process of cavitation the trophoblast cells secrete a fluid into the morula to produce the blastocoel.
  • The inner cell mass is positioned on one side of what is now termed the blastocyst and the trophoblast cells line the cavity.
  • The position of the cells in the morula either in the internal or external portion of the cell mass is the major determinant of whether a particular cell will become a trophoblast or an embryo.
The blastocyst expands within the zona pellucida (which is the extracellular matrix of the egg) as it travels through the fallopian tubes.
  • This expansion is caused by a sodium pump in the cell membranes of the trophoblast cells.
      Proteins in the cell membrane pump sodium into the central cavity which draws water in osmotically.
  • Eventually the blastocyst will secrete a protease (strypsin) and lyse the components of the zona pellucida to make direct contact with the uterus.
  • The trophoblast cells bind to the uterine cavity and secrete proteases enabling the blastocyst to bury itself within the uterine wall.
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