Implantation and placenta formation

Alatise O.I

Implantation

• The morula and blastocyst formed from the fertilized egg is enclosed by Zona pellucida

• The ZP prevented the implantation of the sticky blastomeres during the tubal passage to the uterus

• As the blastocyst move toward the uterus, its thin out and eventually disappears

Implantation

• Prior to the disappearance of the ZP, the blastomeres had been arranged into – The outer cell mass – which eventually become the

trophoblast– The inner cell mass

• The outer cell mass make a direct contact with the endometrium and attached to it with the disappearance of the ZP

• This attachment is called Implantation which occurred about 4 to 6 days after fertilization.

Implantation

• Implantation is a sequel of integrated processes involving three principal mechanisms which are – Muscular – Adhesive– Invasive

• The implantation in man is interstitial with a direct contact between the fetal membrane and the maternal blood – Haemo-chorial

Implantation

• Prior to implantation the uterus is in the secretory phase during which the uterine glands and arteries become coiled and the tissue becomes succulent.

• The endometrium is also arranged into three layers namely– Compact– Spongy – Basal

Implantation

• The human blastocyst normally implants in the endometrium along the posterior or the anterior wall of the body of the uterus, where it becomes embedded between the openings of the gland.

• More of the posterior than the anterior wall of the uterus.

• Occationally, it can implant low in the uterus- Placenta previa

Implantation

• It adheres like a parasite to the uterine mucous membrane and it destroys the epithelium over the area of contact, and excavates for itself a cavity in the mucous membrane in which it becomes imbedded.

• The site of implantation is temporarily seal by fibrin and blood clot but later the site is replaced by the the mucous membrane

Implantation

• The structure actively concerned in the process of excavation is the trophoblast of the ovum, which possesses the power of dissolving and absorbing the uterine tissues.

• The trophoblast differentiate into two part– Cytotrophoblast-Langhans layer (which is the

inner layer of mononucleated cells– Syncytiotrophoblast- syncytium (an outer

multinucleated zone without cell boundaries

Implantation

• Two important process occurred at this stage– In the syncytiotrophoblast, there is formation of

vacuoles which fuse together to form lacunae, this stage of trophoblast development is called the Lacunar stage

– The cell of the endometrium become polyhedral and loaded with glycogen and lipids and the tissue are highly edematous; this reaction is known as Decidual reaction. This reaction is initially confined to the site of implantation but later spread to other part of the endometrium

Implantation

• As the syncytial cells penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries.

• These capillaries are congested and dilated and are known as sinusoids

• The syncytial lacunae become continuous with the sinusoids and the maternal blood enters the lacunar system

• As the trophoblast continues to erode more and more sinusoids, maternal blood begins to flow thru the trophoblastic system, thus establishing the uteroplacental circulation

• At the beginning of the 3rd week, the cytotrophoblast form columns which is surrounded by the syncytium. This column is called the primary villi

• During further development, the mesodermal cells penetrate core of the primary villi and grow in the direction of the decidual. This newly formed columns is called the secondary villi

• By the end of the 3rd week , the mesodermal core differentiate into blood cells and blood small vessels, thus, forming the villous capillary system. The villus is now called tertiary villus or definitive placental villus (Chorionic frondosum)

• The capillaries of the tertiary villi make contact with the capillaries in the chorionic plate and in the connecting stalk.

• These vessels in turn establish contact with the intraembronic circulatory system, thus connecting the placenta and the embroyo

• The cytotrophoblast in the villi continue to grow until it makes contact with maternal endometrium and it spread over the endometrium. The portion is called the outer cytotrophoblast shell. And this attached the chorionic sac firmly to the maternal endometrial tissue.

• This shell disappeared by the fouth month when the placenta is fully developed

• The villi that extend from the chorionic plate to the decidual plate is called the anchoring villi, while the branching part is called the free(terminal) villi. The free villi ensure the exchange of nutrients, gases, drugs etc.

The Placenta

• The placenta connects the fetus to the uterine wall, and is the organ by means of which the nutritive, respiratory, and excretory functions of the fetus are carried on. It is composed of fetal and maternal portions. The fetal part is formed by the chorion frondosum while the maternal part is formed by the stratum compactum and the intervillous spaces of the decidua basalis 

The Placenta

• The fetal portion of the placenta consists of the villi of the chorion frondosum; these branch repeatedly, and increase enormously in size.

• These greatly ramified villi are suspended in the intervillous space, and are bathed in maternal blood, which is conveyed to the space by the uterine arteries and carried away by the uterine veins.cells.

The Placenta

• A branch of an umbilical artery enters each villus and ends in a capillary plexus from which the blood is drained by a tributary of the umbilical vein.

• The vessels of the villus are surrounded by a thin layer of mesoderm consisting of gelatinous connective tissue, which is covered by two strata of ectodermal cells derived from the trophoblast: the deeper stratum, next the mesodermic tissue, represents the cytotrophoblast or layer of Langhans; the superficial, in contact with the maternal blood, the syncytiotrophoblast. After the fifth month the two strata of cells are replaced by a single layer of somewhat flattened cells

The Placenta

• The maternal portion of the placenta is formed by the decidua basalis containing the intervillous space which contain blood

The Placenta

• The fetal and maternal blood currents traverse the placenta, the former passing through the blood vessels of the placental villi and the latter through the intervillous space. The two currents do not intermingle, being separated from each other by the delicate walls of the villi. Nevertheless, the fetal blood is able to absorb, through the walls of the villi, oxygen and nutritive materials from the maternal blood, and give up to the latter its waste products.

The Placenta

• The blood, so purified, is carried back to the fetus by the umbilical vein. It will thus be seen that the placenta not only establishes a mechanical connection between the mother and the fetus, but subserves for the latter the purposes of nutrition, respiration, and excretion. In favor of the view that the placenta possesses certain selective powers may be mentioned the fact that glucose is more plentiful in the maternal than in the fetal blood.

Separation of the Placenta

• After the child is born, the placenta and membranes are expelled from the uterus as the after-birth.

• The separation of the placenta from the uterine wall takes place through the stratum spongiosum, and necessarily causes rupture of the uterine vessels.

Separation of the Placenta

• The orifices of the torn vessels are, however, closed by the firm contraction of the uterine muscular fibers, and thus postpartum hemorrhage is controlled.

• The epithelial lining of the uterus is regenerated by the proliferation and extension of the epithelium which lines the persistent portions of the uterine glands in the unaltered layer of the decidua.

The Placenta

• The expelled placenta appears as a discoid mass which weighs about 450 gm. and has a diameter of from 15 to 20 cm.

• Its average thickness is about 3 cm., but this diminishes rapidly toward the circumference of the disk, which is continuous with the membranes.

The Placenta

• Its uterine surface is divided by a series of fissures into Iobules or cotyledons, the fissures containing the remains of the septa which extended between the maternal and fetal portions.

• Overall there are about 15-30 cotyledon

Nutrition of the ovum and the embroyo

• Oocyte blood vessel of the ovary via the theca interna, granulosa cell and Zona pellucida

• After ovulation secretion by the lining of the tube

Nutrition of the ovum and the embroyo

• In the uterus Uterine secretion (uterine milk which is rich in glycogen, mucopolysaccharides, and the lipid

• During implantation cytolytic products from the breakdown of stroma cells, blood cells, grandular epithelium, capillary walls and glandular secretions

• From the placenta which is fully developed by the 4th month

The Placenta

• Function of the placenta– Respiration– Nutrition – Water, inorganic salts, carbohydrates,

fats, protein and vitamin– Excretion- metabolic waste (kidneys)– Protection- barrier to injurious substances– Endocrine

Placental circulation

• The total capacity of the intervillous space is 175ml of maternal blood

• About 500ml of maternal blood flow thru the placenta per min

• About 400ml of fetal blood flow thru the placenta/ min