1. Implantation, placenta formation, fetal membranes
Dr Nandor Nagy

2. By the 5th to 6th day of development, the embryo is a hollow ball of about 100 cells called a blastocyst. At this point, it enters the uterine cavity and begins to implant into the endometrial lining of the uterine wall.

3. CLEAVAGE pronuclei 8 cell-stage morula early compact morula
hatching: before hatching zona pellucida prevents the implantation into the uterine tube
late morula
blastocyst
hatching

6. DECIDUAL REACTION
Very soon after arriving in the uterus, the blastocyst becomes tightly adherent to the uterine lining. The adjacent cells of the endometrial stroma respond to its presence and to the progesterone secreted by the corpus luteum by differentiating into
metabolically active, secretory cells called decidual cells.

7. HUMAN CHORIONIC GONADOTROPIN (hCG)
the trophoblast produce the
hormone human chorionic gonadotropin (hCG),
which supports the corpus luteum and thus maintains
the supply of progesterone (maternal recognition of pregnancy).

8. Implantation in abnormal site results in ectopic pregnancy
Occasionally, a blastocyst implants in the peritoneal cavity, on the surface of the ovary, within the oviduct, or at an abnormal site in the uterus. The epithelium at these abnormal sites responds to the implanting blastocyst with increased vascularity and other supportive changes, so that the blastocyst is able to survive and commence development. These ectopic pregnancies often threaten the life of the mother because blood vessels that form at the abnormal site are apt to rupture as a result of growth of the embryo and placenta. Typically, ectopic pregnancy is revealed by symptoms of abdominal pain and/or vaginal bleeding. Drug (methyltrexate, which blocks rapid division) or surgical intervention is usually required to interrupt the pregnancy.

11. Between days 6 and 9, the embryo becomes fully implanted in the endometrium
Proteolytic enzymes, including several metalloproteinases, are secreted by the cytotrophoblast to break down the extracellular matrix between the endometrial cells. Active fingerlike processes extending from the syncytiotrophoblast
then penetrate between the separating endometrial cells and pull the embryo into the endometrium of the uterine
wall. As implantation progresses, the expanding syncytiotrophoblast gradually envelops the blastocyst. By day 9, the syncytiotrophoblast blankets the entire blastocyst, except for a small region at the abembryonic pole (see Fig. 2-3). A plug of acellular material, called the coagulation plug, seals the small hole where the blastocyst implanted, temporarily marking
this point in the endometrial epithelium.

12. 4 extraembryonic membranes:
Amnion, Yolk Sac, Chorion, Allantois….

13. Amnion
Proliferation of hypoblast cells, followed by two successive waves of cell migration, is believed to form the yolk sac membranes, which extend from the hypoblast into the blastocyst cavity. The first wave of migration begins on day 8 and forms the primary yolk sac (the exocoelomic membrane, or Heuser’s membrane)
At the beginning of the 2nd week, the embryoblast splits into two layers, the epiblast and the hypoblast, or primitive endoderm.

14. Although the amniotic cavity is at first smaller than the blastocyst cavity, it expands steadily. By the 8th week, the amnion encloses the entire embryo.

15. Chorion
extraembryonic coelom, or chorionic cavity—develops as the extraembryonic mesoderm splits into two layers.
The extraembryonic mesoderm forms, filling the remainder of the blastocyst cavity with loosely arranged cells (see Fig. 2-4). This early extraembryonic mesoderm is believed to originate in humans from the hypoblast/primary yolk sac, in contrast to the mouse embryo, where it arises from the caudal end of the incipient primitive streak; in addition, the trophoblast may contribute cells as well.

16. Yolk sac
By day 12, the primary yolk sac is displaced (and eventually degenerates) by the second wave of migrating hypoblast cells, which forms the secondary
yolk sac.

17. End of the 2nd week
The syncytiotrophoblast, cytotrophoblast, and associated extraembryonic mesoderm, together with the uterus, initiate formation of the placenta.
During this process, the fetal tissues establish outgrowths, the chorionic villi, which extend into maternal blood sinusoids.

18. By day 13, the embryonic disc with its dorsal amnion and ventral yolk sac is suspended in the chorionic cavity solely by a thick stalk of extraembryonic mesoderm called the connecting stalk.
By day 12, the primary yolk sac is displaced (and eventually degenerates) by the second wave of migrating hypoblast cells, which forms the secondary yolk sac.

19. yolk sac functions:
-Extraembryonic mesoderm forming the outer layer of the yolk sac is a
major site of hematopoiesis.
-Primordial germ cells can first be identified in humans in the wall of the yolk sac.
After the 4th week, the yolk sac is rapidly overgrown by the developing embryonic disc.
The yolk sac normally disappears before birth, but on rare occasions it persists in the form of a digestive tract anomaly called Meckel’s diverticulum.

20. The portion of the primitive hindgut tube lying just deep to the cloacal membrane forms an expansion called the cloaca. A slim diverticulum of the cloaca called the allantois extends into the connecting stalk.
It helps the embryo exchange gases and handle liquid waste.
Allantois

21. Placenta formation
During the 1st week of development, the embryo obtains nutrients and eliminates wastes by simple diffusion. Rapid growth of the embryo makes a more efficient method of exchange imperative. This need is filled by the uteroplacental circulation—the system by which maternal and fetal blood flowing through the placenta come into close proximity and exchange gases and metabolites by diffusion. This system begins to form on day 9 as vacuoles called trophoblastic lacunae open within the syncytiotrophoblast.

22. rapidly anastomose with the trophoblastic lacunae.
Maternal vessels near the syncytiotrophoblast expand to form maternal sinusoids that
rapidly anastomose with the trophoblastic lacunae.
Between days 11 and 13, as these anastomoses continue to develop, the cytotrophoblast
proliferates locally to form extensions that grow into the overlying syncytiotrophoblast.
The growth of these protrusions is thought to be induced by the underlying newly formed extraembryonic
mesoderm. These extensions of cytotrophoblast grow out into the blood-filled lacunae, carrying with thema covering of syncytiotrophoblast. The resulting outgrowths are called primary chorionic stem Villi.

23. By the end of the 3rd week, this villous mesoderm has given rise to blood vessels that connect with the vessels forming in the embryo proper, thus establishing a working uteroplacental circulation. Villi containing differentiated blood vessels are called tertiary chorionic stem villi.

25. Placenta fetal blood - maternal blood barrier
The gases, nutrients, and wastes that diffuse between the maternal and fetal blood must cross four tissue layers:
the endothelium of the villus capillaries
the loose connective tissue in the core of the
villus (extraembryonic mesoderm)
a layer of cytotrophoblast
a layer of syncytiotrophoblast
Mesenchyme
Trophoblast
fetal vessels
Precisely:
Syntitiotrophoblast
(Cytotrophoblast)
Membrana basalis
Mesenchyme
Endothel
The endothelial lining of the maternal sinusoids does not invade the trophoblastic lacunae, so a maternal layer
does not need to be crossed.
Intervillous space
maternal blood

27. Umbilical cord Umbilical arteries Amnion Wharton- jelly
Left umbilical vein

28. Decidua basalis, parietalis, capsularis

29.  hydatidiform mole, which is characterized by the overdevelopment of trophoblastic tissues and the extreme underdevelopment of the embryo. This condition can result from the fertilization of an egg by two spermatozoa and the consequent failure of the maternal genome of the egg to participate in development or from the duplication of a sperm pronucleus in an "empty" egg.