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Dr. Zeenat Zaidi Results of Fertilization Stimulates the secondary oocyte to complete its second meiotic division Restores normal diploid number (46) of chromosomes in zygote Results in variation in human beings Determines the chromosomal sex of the embryo Causes metabolic activation of zygote & initiates cleavage (cell division of zygote) Cleavage Repeated mitotic division of zygote Begins about 30 hours after fertilization There is rapid increase in number of cells. The cells, blastomeres, become smaller with each division Normally occurs as the zygote passes along the uterine tube to the uterus During cleavage, zygote lies within the zona pellucida Cleavage cont’d After nine-cell stage, the cells become compactly arranged..compaction 12-16 cell stage is called morula. It is formed about 3 days after fertilization and enters the uterus Internal cells of the morula, inner cell mass, are surrounded by a layer of cells that form the outer cell mass Cleavage cont’d Fluid filled space called the blastocyst cavity (blastocele) appears inside morula Blastomeres are separated into: Outer cell layer, the trophoblast, which gives rise to embryonic part of placenta Centrally located, inner cell mass (embryoblasts) which gives rise to embryo Cleavage cont’d At this stage, the conceptus is called Blastocyst. It has two poles: embryonic & abembryonic Zona pellucida gradually degenerates and disappears Blastocyst takes its nourishment from uterine secretions and enlarges in size. It is ready to get attached and implanted to the uterine wall Embryonic pole Abembryonic pole The process by which the developing mass gets embedded within the uterine wall Implantation Begins 6 days after fertilization: The blastocyst attaches to the endometrial epithelium, usually adjacent to the embryonic pole Implantation cont’d Trophoblast proliferates rapidly and differentiates into two layers: inner cellular cytotrophoblast, outer mass of syncytiotrophoblast (multinucleated protoplasm with no cell boundaries) Finger like processes of syncytiotrophoblast extend through the endometrium and invade the endometrial connective tissue Implantation cont’d By the end of 7th day, the blastocyst gets implanted in the superficial compact layer of endometrium and derives its nourishment from the eroded endometrium Implantation cont’d The blastocyst gradually embeds deeper in the endometrium By 10th day it is completely buried within the ‘Functional layer’ (stratum compactum + stratum spongiosum) of the endometrium Implantation cont’d The defect in the endometrial epithelium is filled by closing plug (day 10) The defect gradually disappear as the endometrial epithelium is repaired (day 12 & 13) by the proliferation of the surrounding cells Implantation cont’d Small cavities, the lacunae appear in syncytiotrophoblast, and get filled with maternal blood, establishing primitive uteroplacental circulation Normal Implantation Sites The implantation site determines the site of formation of the placenta Normally it occurs in the upper part of the body of uterus, more often on the posterior wall Abnormal Implantation Sites Uterine: Implantation in the lower segment leads to placenta praevia Extrauterine: leading to ectopic pregnancies Fallopian tube Ovary Abdominal cavity Formation of the Bilaminar Embryonic Disc The Second Week Formation of Amniotic Cavity As implantation of the blastocyst progresses, changes appear in the inner cell mass (embryoblast) A cavity, amniotic cavity appears separating embryoblast from the trophoblast, which soon becomes lined by amnioblasts derived from inner cell mass The cavity gradually increases in size and is filled with amniotic fluid Formation of Embryonic Disc The inner cell mass becomes flattened forming a circular bilaminar plate, the embryonic disc, consisting of two layers: Epiblast, thicker layer, consisting of high columnar cells, related to amniotic cavity Hypoblast (primary endoderm), consisting of small cuboidal cells adjacent to the blastocyst cavity Formation of Primitive Yolk Sac The blastocyst cavity becomes lined with exocelomic membrane and is called Exocelomic cavity The hypoblast (endodermal cells) soon replace the exocelomic membrane and the cavity is then named as the Primitive (Primary) Yolk Sac Embryonic Disc Lies between the amniotic cavity and the primitive yolk sac. Epiblast forms floor of the amniotic cavity Hypoblast lies in the roof of the primitive yolk sac gives rise to the germ layers that form all the tissues and organs of embryo Formation of Extraembryonic Mesoderm Endoderm of the yolk sac gives rise to a layer of loosely arranged connective tissue, extraembryonic mesoderm (EEM), which surrounds the amniotic cavity and the yolk sac. Formation of Extraembryonic Celome Isolated spaces appear in the EEM These spaces rapidly fuse to form a large fluid filled, C-shaped cavity, extraembyonic celome surrounding the amniotic cavity and the yolk sac Formation of Connecting Stalk The region where no cavity has appeared, forms the connecting stalk that connects the amniotic cavity, yolk sac and the embryonic disc to the outer wall (later chorion) The site of the connecting stalk determines the caudal pole of the embryonic disc By this stage, a circular thickening appears in the hypoblast near the cranial end, in the midline, to form the prechordal plate Formation of Amnion, Chorion & Secondary Yolk Sac With the formation of extraembryonic celome: The EEM is splitted into two layers: • an outer extraembryonic parietal (somatic) mesoderm • an inner extraembryonic visceral (splanchnic) mesoderm The primary yolk sac decreases in size and becomes secondary (definitive) yolk sac Wall of the yolk sac, amnion & chorion are formed: • Amnion: Two layers; amnioblasts & extraembronic splanchnic mesoderm • Wall of the yolk sac: Two layers; endoderm & extraembronic splanchnic mesoderm • Chorion: Three layers: Extraembryonic somatic mesoderm, cytotrophoblast & syncytiotrophoblast