2nd Week of Development (2024)

Implantation, Cleavage,2 nd Week of Development Formation of Bilaminar Disc Dr Vijay Paul Samuel [email protected] Learning Outcomes Outline the steps involved in the process of cleavage and blastula formation. Elucidate the process of implantation. Enumerate...

Implantation, Cleavage,2 nd Week of Development Formation of Bilaminar Disc Dr Vijay Paul Samuel [email protected] Learning Outcomes Outline the steps involved in the process of cleavage and blastula formation. Elucidate the process of implantation. Enumerate the changes during the Bilaminar germ disc formation and correlate with clinical conditions.  Polar body Cleavage Mitotic division results in the Zona pellucida blastomeres A Blastomere B 2-cell stage 4-cell stage Morula has outer trophoblasts Till 8 cell stage-the & inner cell mass cells are totipotent Morula (12-16 cells) or Blastula reaches uterus C Zona pellucida D by 3rd day 8-cell stage Morula becomes Morula blastocyst- 5th day Inner cell mass By day 5-6 zona Degenerating zona pellucida disappears, Early blastocyst implantation zona present Blastocyst cavity begins E F Trophoblast Later blastocyst Early blastocyst Posterior wall Normal site of implantation of uterus Blastocysts Eight-cell Four-cell Two-cell Zygote Morula stage stage stage Fertilization Follicle Oocyte Growing Secondary approaching maturity Mature in tube follicle follicle Early primary follicle follicle Oocyte Blood ity vessels Epithelium Corpus albicans Released oocyte Mature corpus luteum Ruptured follicle Atretic (degenerating) follicle Connective tissue Developing Endometrium corpus Coagulated blood luteum Posterior wall of body of uterus close to the fundus in the compact layer of endometrium  Division of trophoblasts Endometrial gland Endometrial capillary Endometrial Embryonic epithelium pole Inner cell mass Blastocyst cavity Trophoblast Trophoblasts proliferate& form Syncytiotrophoblasts secrete 2 layers:1. Syncytiotrophoblasts 2. Cyto(cellular) trophoblasts proteolytic enzymes & HCG  Endometrial Uterine gland capillary Completion of implantation Syncytiotrophoblasts erode the endometrium & the blastocyst burrows into the compact layer of endometrium Syncytio- trophoblast Starts on the 5th day and finishes on the 10th day Amniotic cavity Amnion Epiblast Endometrial Exocoelomic cavity or Primary yolk sac epithelium Cytotrophoblast Exocoelomic membrane or Hueser’s membrane Hypoblast ( endoderm)  Syncytiotrophoblasts The syncytial layer shows villous projections and lacunar spaces The syncytiotrophoblast secrete: 1. Proteolytic enzymes which erode the endometrium to implant 2. HCG which sustains the corpus luteum  Blastocyst & Bilaminar disc Study objectives: Formation of blastocyst from morula Differentiation of hypoblasts and epiblasts Development of amniotic cavity & primary yolk sac Formation of bilaminar embryonic disc Appearance of extraembryonic (primary) mesoderm Development of extraembryonic coelom (chorionic cavity) Formation of secondary yolk sac Subdivisions of extraembryonic mesoderm. Appearance of body stalk & prochordal plate Chorionic villi & embryotroph  Differentiation of inner cell mass Endometrial gland Endometrial capillary Endometrial Embryonic epithelium pole Inner cell mass Blastocyst cavity Trophoblast Hypoblasts (primary endoderm) differentiate by day 7 Epiblasts (ectoderm) appear next  Uterine gland Endometrial capillary Development of amniotic cavity & primary yolk sac A small cavity appears within the epiblast. This cavity enlarges to become the amniotic cavity. Syncytio- trophoblast Flat- tened cells probably originating from the hypo- blast form a thin membrane, the exocoelomic (Heuser) membrane Amniotic that lines the inner surface of cavity Amnion the cytotrophoblast Epiblast Endometrial Exocoelomic cavity or Primary yolk sac epithelium Cytotrophoblast Exocoelomic membrane or Hueser’s membrane Hypoblast ( endoderm)  Bilaminar embryonic disc The circular area of contact of amniotic cavity & the primary yolk sac forms the bilaminar embryonic disc Hueser’s membrane  Syncytiotrophoblast Bilaminar embryonic disc Exocoelomic (Hueser’s) membrane Bilaminar Disc Exocoelomic cavity (primary yolk sac) Amnion Cytotrophoblast Bilaminar embryonic disc is made of 2 layers 1. Epiblast- Uterine gland 2. Hypoblast Maternal blood in lacunae Bilaminar embryonic disc Primary Endometria l epithelium yolk sac  Formation of extraembryonic mesoderm Amnion Syncytiotrophoblast Primary or extra- Endometrial embryonic mesoderm capillary fills the space between the cytotrophoblast & amnion and primary Lacunar network yolk sac Epiblast Cytotrophoblast Source: 1.Yolk sac endoderm Closing Hypoblast Primary Primary yolk sac plug mesoderm Eroded Maternal Lacunar Amniotic Formation of gland blood network cavity extraembryonic coelom Uterine gland Clefts appear in the extra- embryonic (primary) mesoderm Extra- embryonic endodermal lining of yolk sac interstitial Embryonic disc Cytotrophoblast  Extraembryonic Maternal Lacunar Development of chorion sac somatic mesoderm sinusoid network Primary villus The clefts in the extraembryonic (primary) mesoderm fuse to form the extraembryonic coelom or chorionic cavity The chorionic cavity splits the Endometrium extraembryonic (primary) mesoderm into somatic & splanchnic layers Chorion Connecting Stalk Primary yolk sac starts shrinking to form the secondary (definitive) yolk sac Extraembryonic Extraembryonic coelom splanchnic mesoderm or chorionic cavity Primary Yolk sac Development of Chorion and Chorionic Sac Extraembryonic somatic mesoderm Formed during 2nd week End of 2nd week: primary villi appear 3rd week: secondary & tertiary villi appear Chorion 1. Syncytiotrophoblast 2. Cytotrophoblast 3. Somatopleuric layer of EEM Extraembryonic Primary Extraembryonic splanchnic mesoderm Yolk sac coelom / chorionic cavity Primary Chorionic Villi Maternal Primary Trophoblastic lacuna blood chorionic villus Syncytiotrophoblast somatic EEM Cytotrophoblast Primary Chorionic Chorion Villus central core of cytotrophoblast & outer syncytio- Embryo Chorionic trophoblast cavity Secondary Chorionic Villi Secondary chorionic villus Syncytiotrophoblast Cytotrophoblast Developing blood vessel Wall of EEM chorionic sac Secondary villus Endometrium Tertiary Chorionic Vili Cytotrophoblastic shell Connective tissue EEM Tertiary chorionic villus Intervillous space Maternal blood Capillaries Maternal sinusoid Chorion frondosum – covered with multiple villi → related to decidua basalis. On the rest of chorion wall villi degenerate Maternal blood in Chorionic villus intervillous space Spiral arteries Decidua basalis Umbilical cord Chorionic plate Amniochorionic membrane Uterine Amnion cavity Mucous Smooth chorion plug Decidua parietalis Vagina  Placenta Yolk sac Amnion Chorionic sac (smooth chorion) Amnio- chorionic Decidua parietalis membrane Mucous plug Chorion frondosum and decidua basalis fuse to form the placenta