Download Cell lineage The genealogical history of blastomeres is known as

CELL LINEAGE
The genealogical history of blastomeres is known as cell lineage. In otherwords, it is the
study of blastomeres with respect to their origin and fate in later development. In 1878, Charls
Otis Whitman traced for the first time the history of the cells formed division in leech, thereby
initiating the study of cell lineage. In a species, during normal development, each blastomere
always gives rise to a definite structure of the adult. By carefully tracing the descent of
blastomeres and by enumerating them, we can determine their ultimate fate.
Cell lineage studies are more easily made in the eggs of certain species because of the
present of natural markings. By using suitable methods, it has been possible to study the cell
histories in a variety of organisms. The cell lineage of many types of invertebrates has been
investigated in a similar manner. In normal development, it may be found that during cleavages,
the successive generations of blastomeres show a progressive differentiation. Earlier or later, the
organ forming materials of the fertilized egg are segregated into different groups of blastomeres
which thus become the organ forming areas of the blastula.
The cell lineage has been thoroughly studied in Planocera. The egg undergoes holoblastic
and spiral pattern of cleavage. For clearly tracing the blastomeres from stage to stage, a system
of notation has been adopted naming each cell by an alphabet with or without an index.
1. The four blastomeres in the four celled stage are labeled a A, B, C and D, of which D is
the largest.
2. The blastomeres derived from the divisions of each of these cells constitute a quadrant.
3. The third cleavage is horizontal and results in the formation of an eight celled stage. Of
the eight resulting blastomeres, those towards the animal pole are smaller than those
towards the vegetal pole. The smaller cells are known as micromeres and labelled as la,
lb, lc and 1d; larger sister cells are known as macromere’s and named 1A, 1B 1C and 1D.
The micromeres 1a, 1b, 1c and 1d are known as the first quartet of micromeres.
4. The fourth cleavage occurs obliquely in a levotropic pattern (spirally twisted to the left)
resulting in a 16 - celled stage. During this division, la gives rise to la1 and 1a2 ; 1b to 1
b1 and 1b2; 1c to 1c1 and 1c2; 1d to 1d1 and 1d2; the macromeres 1A, 1B, 1C and 1D
give rise to four micromeres 2a, 2b, 2c and 2d which form the second quartet and four
macromeres 2A,2B, 2C and 2D.
5. The sixteen - celled stage develops into the thirty - two celled stage by further divisions
of the cells. Each cell of the first two quartets of micromeres divides into two. The
macromeres 2A, 2B, 2C and 2D give rise to the third quartet of micromeres 3a, 3b, 3c
and 3d and four macromeres 3A, 3B, 3C and 3D. By this time, the germ layers are fixed.
6. The first three quartets of micromeres and their derivatives form the ectomeres and give
rise to the whole ectoderm of the larva and the adult. They extend over the greater part of
the embryo forming a single layer of cells leaving a gap, the blastopore, and a gastrula
stage is reached.
7. A further division of the cells results in the formation of the fourth quartet of micromeres
4a, 4b, 4c and 4d and of four macromeres 4A, 4B, 4C and 4D. Of these, 4a, 4b, 4c, 4A,
4B, 4C and 4D become broken into smaller cells which secrete the food material, the
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8. 4d, which is larger than 4D gives rise to the whole of the endoderm and mesoderm .4d
hence known as the mesoblast. This cell divides into 4d1 and 4d2. The derivatives of 4d1,
namely 4d11. 4d12, 4d111. 4d121, etc. give rise to the endoderm of the intestine. 4d2
divides into 4d21 and 4d22. Each of these contributes to a small cell, 4d211 and 4d221.
These cells give rise to the pharyngeal endoderm and the rest form the mesoderm,
developing into muscles, mesenchyme, reproductive system etc. In the following chart,
the descendant cells of quadrant 4d are shown.
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