Implantation, Conception, Development of Placenta and Establishment of Fetomaternal Circulation

Flashcards covering gametogenesis, spermatogenesis, oogenesis, cleavage, blastocyst formation, implantation, and placental development.

Gametogenesis

Maturation of two highly specialized cells: spermatozoon in males and ovum in females.

Sperm morphology

Adapted for reaching and fertilizing the egg. Head contains DNA and the acrosome with enzymes for penetrating the egg. Midpiece contains mitochondria to form ATP for energy. Tail is flagellum used for locomotion.

Oogonia to Oocytes

Germ cells from yolk sac migrate to ovary and become potential egg cells called oogonia. In fetus, millions of oogonia produced by mitosis but most of them degenerate (atresia). Some develop into immature egg cells called primary oocytes during fetal development. 200,000 to 2 millions present at birth. 40,000 remain at puberty but only 400 mature during a woman’s reproductive life. Each month about 20 primary oocytes become secondary oocytes but usually only one survives to be ovulated from Graffian follicle

Oogenesis

Egg forming cells (oocytes) go through two divisions: primary and secondary. Starts with a 2n=46 primary oocyte that divides, resulting in two n=23 cells, but one is a large secondary oocyte and one is a small 1st polar body that may itself divide. Second division only occurs if secondary oocyte is fertilized. Results in one large n=23 ovum (egg) and one small n=23 2nd polar body. Thus oögenesis results in one large fertilized egg (zygote) and possibly three small polar bodies

Cleavage

Is the process of repeated rapid mitotic cell divisions of the zygote (unicellular structure) to form the Blastula (multicellular structure). The produced cells named Blastomeres. During this stage the size of the embryo does not change, the blastomeres become smaller with each division. The type & pattern of cleavage differ from species to species. Continues divisions to form a ball of 32 cells called the morula. The morula continues divisions to form the hollow blastula with up to several hundred cells. The cavity of the blastula is the blastocoel.

Blastocyst

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

Inner cell mass and amniotic cavity formation

Some cells of the inner cell mass become flattened and come to lie on its free surface and constitute the endoderm. Remaining cells of inner cell mass become columnar and constitute the ectoderm. A space appears between the ectoderm and the trophoblast. This is the amniotic cavity filled by amniotic fluid. The roof of the cavity is formed by amniogenic cells derived from the trophoblast, while its floor is formed by the ectoderm

Implantation

The process by which the developing mass gets embedded within the uterine wall. It is the process by which the Blastocyst penetrates the superficial (Compact) layer of the endometrium of the uterus

Implantation Site and Timing

The normal site of implantation is the posterior wall of the uterus near the fundus. It begins about the 6th day after fertilization and it is completed by the 11th or 12th day.

Blastocyst implantation

By the 5th day the Zona pellucida degenerates and the blastocyst begins implantation by the 6th day. Trophoblast cells penetrate the epithelium of the endometrium .Penetration results from proteolytic enzymes (eg.COX-2) produced by the trophoblast.

Blastocyst nourishment

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

Trophoblast Differentiation

By 7th day, Trophoblast differentiated into 2 layers: Cytotrophblast, inner layer, mononucleated mitotically active cells; Syncytiotrophoblast (outer multinucleated mass, with indistinct cell boundary.

Blastocyst Embedding

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.

Uteroplacental Circulation

Blood-filled Lacunae appear in the Syncytiotrophoblast which communicate forming a network by the 10th or 11th day. Syncytiotrophoblast erodes the endothelial lining of the maternal capillaries which known as sinusoids. Now blood of maternal capillaries reaches the lacunae Uteroplacental circulation is established by 11th or 12th day.

Implantation Detection

Endometrial cells undergo apoptosis (programmed cell death) to facilitates invasion of endometrium by the Syncytiotrophoblast. Syncytiotrophoblast engulf these degenerated cells for nutrition of the embryo. Implantation can be detected by: Ultrasonography and hCG (human chorionic gonadotrophin which is secreted by the Syncytiotrophoblast) about the end of 2nd week

Placenta Formation

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.

hCG

Is an immunosuppressant protein. Secreted by trophoblastic cells. Appears in maternal serum within 24-48 hrs. It is the basis for EPT (Early pregnancy test) in the first 10 days of development

Chorionic Villi Formation

By the 13th day Proliferation of Cytotrophoblast cells produce extension inside the Syncytiotrophoblast to form primary the chorionic villi.

Trophoblast Layers/Function

Trophoblast proliferates rapidly and differentiates into two layers: inner cellular cytotrophoblast or Langhan’s layer. Outer mass of syncytiotrophoblast (multinucleated protoplasm with no cell boundaries) and Finger like processes of syncytiotrophoblast extend through the endometrium and invade the endometrial connective tissue

Placental Villi

The essential functional elements of the placenta are very small finger like processes or villi. These villi are surrounded by maternal blood. In the subustance of the villi, there are capillaries through which fetal blood circulates. Exchanges between maternal and fetal circulations take place through the tissues forming the walls of the villi

Chorionic Villi Formation and Degeneration

The villi are formed as offshoots from the surface of the trophoblast. Chorionic villi are first formed all over the trophoblast and grow into the surrounding decidua. Those related to decidua capsularis are transitory and degenerate and this part of the chorion becomes smooth and is called chorion laevae.

Placenta and Chorion Frondosum

The villi that grow into the decidua basalis undergo considerable development. Along with the tissues of the decidua basalis these villi form a disc shaped mass which is called the placenta. The part of the chorion that helps to form the placenta is called the chorion frondosum.

Maternal Circulation in Intervillous Space

Maternal blood in the intervillous space is constantly in circulation. Both arteries and the vein open into the roof of the cotyledon and that the pressure of the blood in the artery is sufficient to drive blood to the fetal end of the intervillious space.

Ectopic Pregnancy

Implantation outside the uterus. 95 to 97% of ectopic pregnancies occurs in the uterine tube. Most are in the ampulla & isthmus.