The Attachment of blastocyst with endometrium, Implantaion and Placenta

 

Implantaion and Placenta in Human

• As the sperm reaches the ovum  in the fallopian tube ,the process of fertilization takes place. As a result the zygote is formed.

• Now zygote starts to move  toward the uterus by peristalsis movement and  ciliary movement .

• During its journey to the uterus, the zygote undergoes mitotic cell divisions. This division of zygote by mitosis is called cleavage.

• The mitotic division starts as the zygote moves through the isthmus of the oviduct called cleavage towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres

• The embryo with 8 to 16 blastomeres is called a morula . The morula continues to divide and transforms into blastocyst  as it moves further into the uterus. 

• Due to the cleavage, more cells are formed but it does not increase the total volume of the zygote.

• Due to continuous cleavage , the sixteen blastomere stage is formed and moves towards the uterus.

• These blastomeres are  now compactly arranged and assume  a solid Structure like ball. This ball-like  structure is called the morula. This morula divides and forms a ball with  more than hundred cells.

• The ball is  firmly attached to the uterus  and organizes itself in a fluid-filled cavity termed the blastocoel.

• The combined  structure of morula and blastocoel  is referred to as a blastocyst.

• The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. 

• In blastocyst, a group of cells projected  toward the inside  forms an  inner cell mass. This inner cell mass forms three germinal layers that develop  the embryo after further development .

• The cells of inner mass are totipotent in nature. It means each cell has the potential to differentiate into any cell or tissue in the human body.

• The cells of inner mass  lose  the totipotency once germinal layers are developed and the fate of the embryo is determined

• The outer layer  of  cells in blastocyst  is called trophoblasts. The cells of trophoblast develop the finger  like  chorionic villi. These villi are embedded into the endometrium of the uterus and establish a connection between uterine tissue and foetus to form the placenta.

• The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta 

• Placenta is the organ that derives the nutrition for the foetus , performing the  gaseous exchange as well as removal of nitrogenous waste from the foetus.

• Placenta also act as endocrine tissue because it releases several hormones to maintain the pregnancy like human chorionic gonadotropin (hCG), Human placental lactogen ( hPL), estrogen and Progesterone.

• Ovary in the later phase of pregnancy, also release relaxin at the end of pregnancy that act on pubic symphysis to make easy expulsion of foetus through the birth canal.

• As the blastocyst forms, the trophoblast degrades the zona pellucida  for implantation.  

• The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. 

• After attachment, the uterine cells divide rapidly and covers the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus.  This is called implantation and it leads to pregnancy.

• The blastocyst typically implants  on the posterior wall of the uterus.

• But sometimes time  endometrium is not fully ready to receive the blastocyst, then blastocysts fail to implant. This is the reason  for the failure of the  pregnancy. 

• When implantation occurs the blastocyst is firmly attached to the endometrium. The cells of the trophoblast fuse with each other and form syncytiotrophoblasts.

• As the implantation is settled then the trophoblast also releases human chorionic gonadotropin (hCG).

• The hCG hormone plays a role to develop a healthy condition  for the developing embryo. As a result hCG accumulates in the maternal blood and is excreted through the urine. 

👌👌Remember - Just a few days after implantation, the trophoblast has secreted  hCG at large amounts and it may be detected in urine  to confirm the pregnancy test.

• Generally  an embryo implants within the uterus in a fixed location that can support growth and development of the embryo.

• However in some cases, the embryo implants either outside the uterus. This creates a complication condition for foetus as well as for the mother. It may be an ectopic pregnancy.