Hi everyone,how are you all? I am good as you are.i would like to tell about placenta in this post.lets learn about it.
The human placenta is a Fetomaternal organ containing tissues from two different sources -
Maternal component - contributed by Decidua basalis and is also called
Decidua serotina or Decidual plate
Fetal component - contributed by Chorion frondosum
You will understand what are all these things after reading the post.
The human placenta is called Discoid because it is disc-like shaped and Deciduate because as it sheds off at the time of delivery and Hemichorial as it directly comes in contact with maternal blood.
DEVELOPMENT OF PLACENTA
Before understanding how the placenta is developed please look at the below diagram that i have drawn.when couple do intercourse,then one of the sperms from male partner fertizes the secondary oocyte or egg of female partner and it becomes Zygote.fertilization happens in fallopian tube.it is single cell and it is covered by Zona pellucida which protects it and doesn't allow other sperms to enter into it.
Look at the diagram again,the zygote then divides into 2 celled stage and then 4 celled stage and then to 8 celled stage and then to 16 celled stage.the 16 celled stage is called Morula
Then the fluid in the uterus enters into the morula and some cells moves to one side and it is called Blastocyst and the fluid present in the blastocyst is Blastocoele and the cells divide differentiate. the outer cells present are called Outer cell mass and inside cells are called Inner cell mass.you can observe in the image that i labelled with different colours to make it easy to understand.
Then the blastocyst is implanted on the endometrium .you can see in the below image and i labelled the layers of uterus wall Endometrium,Myometrium and Perimetrium. The outer cell mass is called Trophoblast and the inner cell mass is called Embryoblast.the blastocyst gets implanted on the endometrial wall.remember it should not go deep into decidua basalis or beyond that layer.if it gets implanted deeply decidua basalis and beyond it then the condition is called Placenta accreta ,Placenta increta and Placenta percreta depending on how much depth the placeta gets attached or implanted in the uterus wall.you can understand what i said by below images.
Trophoblast contributes to placenta formation and Embryoblast is developed into Fetus or baby
The trophoblastic cells divide and differentiate into Syncytiotrophoblast and Cytotophoblast.
The syncytiotrophobastic cells divide and forms villi like projections and the space between these villous projections is Intervillous space and in this intervillous space the blood vessels from mother will grow and these blood vessels are called Spiral arteries you can see in the below diagram.the spiral arteries gives nutition,oxygen,immunoglobulins to the baby.you want to know how lets read more.the embryoblast then divide and differentiate into Epiblast and Hypoblast and these will develop into Ectoderm,Endoderm and Mesoderm.two sacs are formed Yolksac and Amniotic sac you can see in the below image that you can understand clearly.
In between cytotrophoblast and embryoblast there forms another layer called Mesoderm it is formed from yolksac layer and hypoblast.you can see in the below image.
Next you can the mesoderm splits into two layers one is Splanchnic Mesoderm and Somatopleuric mesoderm and in between these two layers is Extra embryonic mesoderm there is a connecting stalk between these two layers that i did not labelled and but you can find that stalk in image.it forms the umbilical cord during development.
What is primary villus?what is secondary villus?and what is tertairy villus?
| Villi | structures which forms it |
|---|---|
| Primary villus | Syncytiotrophoblast+Cytotrophoblast |
| Secondary villus | Syncytiotrophoblast+Cytotrophoblast+Mesoderm |
| Tertiary villus | Synctiotrophoblast+Cytotrophablast+Mesoderm+Fetal vessels |
By above table,you have understand which structures the villi.villi are projections so imagine these structures involved in forming villi.now look at the below two diagrams carefully then you can understand clearly.
The cells will divide further and the embryoblast becomes fetus and you can see the fetal vessels from fetus will go through the umbilical cord and present in tertiary villi.these fetal vessels receive nutrition,oxygen and immunoglobulins from mother and give carbon dioxide waste products to mother.so here the exchange will happen.in this way baby receives nutrition from mother and infections also spread through this way.the spiral arteries flow in one direction and fetal blood vessels flow in opposite direction so it is called Counter current flow.you can see in the image and you will understand what is tertiary villi clearly.
Follow the diagrams and colours which i denoted.then you undertsnad what i have explained.how the placenta looks like? See the below image
So i this way the placenta is developed.leta learn a bit about hormones released by placenta
HORMONES PRODUCED BY PLACENTA
There are 4 hormones that are mainly produced by placenta Progesterone,Estrogen,hcg,Human placental lactogen. We have learn about each hormone a bit little
PROGESTERONE
Its main sources during pregnancy is Corpus luteum for 8 weeks and Placenta from 8 weeks onwards.so this is called is Luteal placental shift as hormone synthesis is shifting from corpus luteum to placenta.
To make progestorone,precursors are required so the precursor here is Maternal LDL and cholesterol
The production rate of progesterone is 250mg/day and if it is twins then 600mg/day and two reactions named as decidual reaction and arias stella reactions takes place.these reactions make glands hypersecretory.
Progesterone main action is to relax smooth muscles in pregnancy and it prevent shedding of endothelium(mentruation) in pregnancy.
ESTROGEN
Actually,placenta cannot synthesise estrogen using precursors from mother.the reason is it lacks 17alpha-hydroxylase enzyme.
17OH progesterone is precursor and it is converted to Androstenedione(C19 steroid) by the enzyme 17alpha hydroxylase but here the placenta lacks this enzyme.so androstenedione is not formed which is required to synthesise estrogen.
Fetal Adrenal glands help placenta here.Fetal ACTH hormone acts on fetal adrenal glands and it secreted DHEA-sulphate(C19 steroid) which is alternate to androstenedione.this DHEA-sulhpate is converted to estrogen by the placental aromatase and suphatase enzymes.in this way the estrogen is produced.
In erythroblastosis fetalis and Rh incompatability estrogen levels will rise.
In Anencephaly (where there is absent/hypoplastic adrenal glands seen),Deficiency of placental sulfatase,aromatase,Down syndrome fall is estrogens levels will be seen.
HUMAN CHORIONIC GONANODOTROPIN(hCG)
It is a glycoprotein hormone(hormone with maximum glycogen content)
source - syncitiotrophoblast
It has two subunits -Alpha and Beta
Alpha subunit is non-specific one and it is similar to LH,FSH&TSH.the gene encoding this is oresent on chromosome 6.
Beta subunit is specific one and is gene encodinh it is present on chromosome 19.
hCG is functionally similar ro LH hormone and it is detected on 22day or cycle or 8 days after fertilization.it is detected by Fluoroscent immuno-assay
You can see in the below graph that i have drawn shows the hcg levels rise slowly and max level is seen at 8-10 weeks of gestation and minimum is seen at 16 weeks of gestation and this low level of gestation is continued throughout the pregnancy.hcg levels become normal 1-2 weeks after normal delivery and 2-4 weeks after abortion and 7 weeks after partial mole evacuation and 9 weeks after complete mole evacuation.
Half life of hCG is 36 hours(T½ of LH - 2 hrs)
Functions of hCG-
Helps to maintain corpus luteum of pregnancy
Initial stimulus for leydig cells to produce testosterone
Prevents rejection of fetus
| Rise in hCG levels seen in | Decrease in hCG levels seen in |
|---|---|
| Twin pregnancy,molar pregnancy,choriocarcinoma,Down's syndrome,erythroblastosis fetalis | ectopic pregnacy,all trisomies except down's syndrome,abortion |
What is critical titre of hCG?
It means the value of hCG at which 100% of cases of intrauterine pregnancy has a visible gestational sac.
If >1500 IU of hCG levels are seen then transvaginal sonography(TVS) shows gestational sac.
If >6500 IU of hCG levels are seen in transabdominal sonography(TAS) shows gestational sac.
HUMAN PLACENTAL LACTOGEN
It is also known as Human chorionic somatotropin.it is a single chain polypepide hormone and is similar to growth hormone and prolactin hormone.
It is detected at 3 weeks of pregnancy and continues to increase throughout the pregnancy and it reaches maximum at 36 weeks of pregnancy.it is a marker to check placental function.its T½ is 20-30 mins.
Functions-
Promotes lipolysis in mother.so free fatty acids are increased which are used by mother for energy requirement and glucose is spared for fetus
It causes insulin resistance.so there is increase in glucose levels
We learned about development of placenta and hormones produced by it clearly.read again and again see the diagrams that i have drawn and you will undertsand them clearly.stay tuned for the next post.
Thanks for reading,
with regards
