EARLY DEVELOPMENT & GERM LAYERS Dr. Saleem Shaikh

Introduction  Development starts once the ovum and sperm fuse to each other to form Zygote  It goes into multiple rounds of cell division (cleavage) 2,4,6, and 16 cell stage where its called as morula.  takes about 4 days for the zygote to reach the uterus and till then this division continues.

 As the cleavage proceeds to 16 cell stage it is known as morula.  If we cut a section through the morula – it shows an inner cell mass which is surrounded by an outer layer of cells.  The outer layer of cells give rise to trophblast  Inner cell mass gives rise to embryo proper hence called as embryoblast  Some fluid enters into the morula and seperates the cells of the inner cell mass from the trophoblast.  Throphoblast cells flattened and inner cell mass gets attached to one end of trophoblast  The MORULA becomes a BLASTOCYST  The cavity seen inside is called as blastocoele

Formation of Germ layers  The blastocyst gives rise to all the tissues and organs of the body, it also forms many structures which support the embryo and help it to acquire nutrition.  This blastocyst converts to a three layered disc…known as the embryonic disc.  The three layers are  Endoderm  Ectoderm  Mesoderm All the tissues of the body are derived from these three layers

 Some cells of the inner cell mass become flattened and line the lower surface this is known as HYPOBLAST  The remaining cells become columnar and called as EPIBLAST  A space is seen between the epiblast and the trophoblast – this is known as amniotic cavity, filled with amniotic fluid  The hypoblast cells spread and line inside the blastocystic cavity to form the primary yolk sac.  The cells of the trophoblast proliferate to give rise to a mass of cells known as extra embryonic mesoderm.

 Chorion, amnion and extra embryonic coelom are formed.  The primary yolk sac becomes smaller and is known as secondary yolk sac, and the cells become cubical  The cubical cells of the hypoblast become tall (columnar) near the margin of the disc. This area is called as the prochordal plate.  This helps to identify the head end of the embryo  Next some of the epiblast cells near the tail end proliferate and bulge into the amniotic cavity. This elevation is called as PRIMITIVE STREAK

 The cells of the primitive streak proliferate and push themselves between the epiblast and hypoblast. These cells form the intra embryonic mesoderm.  Some of these proliferating cells displace the hypoblast and form the endoderm.  The remaining cells of the epiblast form the ectoderm  This process of formation of the primitive streak, intraembryonic mesoderm and endoderm is called as Gastrulation.  The cells of the primitive streak do not spread to the area of the prochordal plate. Hence no mesoderm is seen in this region and forms the bucco-pharyngeal membrane

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