Cell lineage in planocera

The genological 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, Char 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 a group of blastomeres always give rise to a definite structure of th 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 bee investigated in a similar manner. In normal development, it may be found that during cleavages, the successive generations of blastomere 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. The four blastomeres in the four celled stage are labeled a A,B,C and D, of which D is the largest. The blastomeres derived from the divisions of each of these cells constitute a quadrant . The third cleavage is horizontal and results in the formation o 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 an 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.

The fourth cleavage occurs obliquely in a levotropic pattern (spirally twisted to the left) resulting in a 16 - celled stage. During this sdivision, 1a gives rise to la1 and 1a2 ; 1b to 1 b1 and 1b2; 1c to 1c1 and 1c2; id to 1d1 and 1d2; the macromeres 1A, 1B, 1C and 1D give rise to four micromeres 2a, 2b, 2c .1d 2d which form the second quartet and four macromeres 2A,28, 2C and 2D.

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, 38, 30 and 3D. By this time, the germ layers are fixed. 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. A further division of the cells results in the formation of the lourth 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 yollk. 4d, which is larger than 4D gives rise to the whole of the endoderm and mesoderm .1d hence known as the mesentoblast. 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, 4d²¹¹and 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 A are shown Similar is the pattern of descent of the other quadrants B,C and D.