Development Of the Placenta | Embryology🩺

Key Concepts

• Blastomeres: Cells formed during early cleavage of a fertilized ovum.
• Morula: An early-stage embryo consisting of 12-16 cells (blastomeres).
• Inner Cell Mass (ICM): Cells within the morula that will form the embryo proper.
• Outer Cell Mass: Cells within the morula that will form the trophoblast and extraembryonic membranes.
• Blastocyst: A structure formed during early development, possessing an inner cell mass (embryoblast) and an outer layer of cells (trophoblast) surrounding a fluid-filled cavity.
• Saito trophoblast: The outer cell mass of the blastocyst after it reaches the uterine cavity.
• Embryo blast: The inner cell mass of the blastocyst after it reaches the uterine cavity.
• Embryonic Pole: The region of the blastocyst where the embryoblast is located.
• Abembryonic Pole: The region of the blastocyst opposite the embryoblast.
• Selectins: Molecules that mediate the initial attachment of the blastocyst to the endometrium.
• Endometrium: The inner lining of the uterus.
• Secretory Phase: The phase of the menstrual cycle after ovulation, characterized by increased progesterone levels and endometrial changes that prepare the uterus for implantation.
• Zona compacta: The superficial most layer of the endometrium.
• Zona spongiosa: The second layer of the endometrium, characterized by a spongy appearance due to its high vascularity and glandular structure.
• Basal Layer: The deepest layer of the endometrium, which remains intact during menstruation and serves as the source for regeneration of the functional layers.
• Functional Layer: The layer of the endometrium (zona compacta and zona spongiosa) that undergoes cyclical changes and is shed during menstruation.
• Corpus Luteum: A temporary endocrine gland in the ovary that produces progesterone.
• Human Chorionic Gonadotropin (HCG): A hormone produced by the syncytiotrophoblast that maintains the corpus luteum during early pregnancy.
• Syncytiotrophoblast: A multinucleated layer of cells formed by the fusion of cytotrophoblast cells, responsible for implantation and HCG production.
• Decidualization: The process by which the endometrium undergoes changes in preparation for implantation, including increased vascularity, glandular development, and the accumulation of glycogen and lipids in decidual cells.
• Decidua: The endometrium during pregnancy.
• Primary Villi: Initial projections of cytotrophoblast cells extending into the syncytiotrophoblast.
• Lacunar System: Interconnected fluid-filled cavities that appear within the syncytiotrophoblast.
• Extraembryonic Mesoderm: Mesoderm derived from the yolk sac cells, located outside the embryo proper.
• Extraembryonic Coelom: The cavity that forms within the extraembryonic mesoderm, eventually becoming the chorionic cavity.
• Chorionic Cavity: The cavity surrounding the embryo and its membranes, formed from the extraembryonic coelom.

Development of the Placenta

Early Embryonic Development

• The development of the placenta begins with the fertilized ovum (zygote) undergoing cleavage to form blastomeres.
• At the morula stage (12-16 cells), the blastomeres differentiate into the inner cell mass (ICM) and the outer cell mass.
  • The inner cell mass will give rise to the embryo proper.
  • The outer cell mass will form the trophoblast and other extraembryonic membranes, including the placenta.
• The morula reaches the uterine cavity around 3-4 days after fertilization and begins to transform into a blastocyst.
• The blastocyst consists of the inner cell mass (now called the embryoblast), the outer cell mass (now called the cytotrophoblast), and a fluid-filled cavity.
• The zona pellucida disintegrates, and the blastocyst hatches out.
• The cytotrophoblast cells grow, especially on the embryonic pole (where the embryoblast is located).
• Selectins, molecules produced by the cytotrophoblast, facilitate the attachment of the blastocyst to the endometrium.

Endometrial Changes During Implantation

• The endometrium is in the secretory phase under the influence of progesterone produced by the corpus luteum.
• The endometrium becomes highly vascular and glandular, with cells accumulating glycogen and lipids, preparing for implantation.
• The endometrium is divided into three layers:
  • Zona compacta (superficial layer)
  • Zona spongiosa (middle, spongy layer)
  • Basal layer (deepest layer)
• The functional layer (zona compacta and zona spongiosa) is shed during menstruation but is maintained during pregnancy.
• The blastocyst implants itself within the functional layer of the endometrium.

Preventing Menstruation During Pregnancy

• Normally, the corpus luteum regresses 10-12 days after ovulation if pregnancy does not occur, leading to a drop in progesterone levels and menstruation.
• During pregnancy, the syncytiotrophoblast (formed by the fusion of cytotrophoblast cells) produces human chorionic gonadotropin (HCG).
• HCG acts on the corpus luteum, preventing its degeneration and stimulating it to produce more progesterone.
• The corpus luteum becomes the corpus luteum of pregnancy, producing high levels of progesterone.
• The high progesterone levels maintain the functional layer of the endometrium, preventing menstruation.

Decidualization of the Endometrium

• The high progesterone levels induce decidualization of the endometrium.
• Decidualization involves:
  • Increased vascularity and glandular development
  • The appearance of decidual cells, which are enlarged cells loaded with glycogen and lipids.
• The endometrium undergoing decidualization is called the decidua.

Early Placental Development

• As the conceptus develops, the cytotrophoblast cells form columns of cells called primary villi.
• The syncytiotrophoblast surrounds the primary villi.
• Lacunae (fluid-filled cavities) appear within the syncytiotrophoblast.
• The syncytiotrophoblast erodes the walls of maternal arterioles and venules, connecting them to the lacunar system.
• Maternal blood enters the lacunar system from the arterioles and drains into the venules, establishing the beginning of utero-placental circulation.

Formation of the Chorionic Cavity

• The embryoblast develops into a bilaminar disc, consisting of the amniotic sac and the yolk sac.
• Cells from the yolk sac produce extraembryonic mesoderm, a loose connective tissue that surrounds the yolk sac and lies between the cytotrophoblast and the amniotic cavity.
• Cavities develop within the extraembryonic mesoderm, eventually coalescing to form the extraembryonic coelom.
• The extraembryonic coelom divides the extraembryonic mesoderm into an inner layer and an outer layer.
• The extraembryonic coelom eventually becomes the chorionic cavity, which surrounds the embryo and its membranes.

Conclusion

The development of the placenta is a complex process that begins with the differentiation of cells in the early embryo. The outer cell mass gives rise to the trophoblast, which plays a crucial role in implantation, HCG production, and the establishment of utero-placental circulation. The endometrium undergoes decidualization in response to progesterone, creating a supportive environment for the developing embryo. The formation of the chorionic cavity from the extraembryonic coelom further organizes the extraembryonic tissues. These early events are essential for the successful development of the placenta and the support of the growing fetus.