The placenta is formed gradually during the first three months of pregnancy, while, after the fourth month, it grows parallel to the development of the uterus. Once completed, it resembles a spongy disc 20 cm in diameter and 3 cm thick.

1. Development of Placenta

2. Objectives
• Gross anatomy of placenta
• Development of placenta
• Classification of placenta
• Function of placenta
• Clinical correlates
• MCQ’s
• Clinical Vignettes
• References

3. Periods of embryology
Length of human pregnancy 280 days or 40
weeks
Gestation period is subdivided into two stages:-
1. Embryonic period ( first to eight week )
• Germinal period
• Embryonic period
2. Foetal period ( third to termination of pregnancy )

4. Meiosis II complete
Formation of male and
female pronuclei
Decondensation of male
chromosomes
Fusion of pronuclei
Zygote
Fertilization

6. Week 1: days 1-6
• Fertilization, day 1
• Cleavage, day 2-3
• Compaction, day 3
• Formation of blastocyst, day 4
• Ends with implantation, day 6

7. Fertilized egg
2 polar bodies
2 pronuclei
Day 1
0.1 mm
Fertilized egg (zygote)

8. Cleavage
Cleavage = cell division
Goals: grow unicellular
Zygote to multicellular embryo.
Divisions are slow: 12 - 24h
Divisions are not synchronous
Cleavage begins about 24h after
pronuclear fusion

9. 2 Cell Stage
Individual cells = blastomeres
Mitotic divisions maintain
2N (diploid) complement
Cells become smaller
Blastomeres are equivalent (aka totipotent).

10. 4 cell; second cleavage
4 equivalent blastomeres
Still in zona pellucida

11. 8 Cell;
third
cleavage
Blastomeres
still equivalent

12. Embryo undergoes compaction after 8-cell stage:
first differentiation of embryonic lineages
Caused by increased cell-cell adhesion
Cells that are forced to the outside of the morula are destined to
become trophoblast--cells that will form placenta
The inner cells will form the embryo proper and are called the inner
cell mass (ICM).

13. Formation of the blastocyst
Sodium channels appear on the surface of the outer trophoblast cells;
sodium and water are pumped into the forming blastocoele. Note that
the embryo is still contained in the zona pellucida.

16. Early blastocyst
Day 3
Later blastocyst
Day 5
blastocoele
inner cell mass

17. Monozygotic twinning typically occurs
during cleavage/blastocyst stages

18. “Hatching” of the blastocyst:
preparation for implantation
Hatching of embryo from zona pellucida occurs just prior to
implantation. Occasionally, inability to hatch results in infertility, and
premature hatching can result in abnormal implantation in uterine
tube.

19. Week 2: days 7-14
implantation
• Implanted embryo becomes more deeply
embedded in endometrium
• Further development of trophoblast into
placenta
• Development of a bi-laminar embryo,
amniotic cavity, and yolk sac.

20. Development of Placenta

21. Implantation and placentation (day 8)
Trophoblast further differentiates and invades maternal
tissues
– Cytotrophoblast
– Syncytiotrophoblast
– Breaks maternal capillaries, trophoblastic lacunae fill with
maternal blood
Inner cell mass divides into epiblast and hypoblast:

22. Implantation and placentation (day 9)

23. Implantation and placentation (day 12)
.

24. Implantation and placentation (day 13)

25. Stem villi

26. Until beginningof 8th week,
entire chorionic sac is
covered with villi.
Sac grows, only part that
is associated with Decidua
basalis retain itsvilli.
Villi of Decidua capsularis
compressed by developing
sac.
Thus, two types of chorion
are formed:
Chorion frondosum
(villous chorion)
Chorion laeve –bare
(smooth) chorion
About 18weeks old, it
covers 15-30% of the
decidua and weights
about 1 6 of fetus

27. Villous chorion ( increase
in number, enlargeand
branch ) will form fetal
part of placenta.
• Decidua basalis will
form maternal part of
placenta.
By end of 4th month,
decidua basalis is almost
entirely replaced by fetal
part of placenta.

28. All eutherian mammal consist of placenta
Human placenta is
• Discoid
• Chorio- deciduate organ

29. Full term placenta is disc type
• Foetal surface
• Maternal surface
Foetal surface
Maternal surface

30. Foetal surface
Smooth
Covered with amnion
Umblical cord attached close to it centre
Umblical cord

31. Maternal surface
• Rough and irregular
• 15-30 polygonal area
(cotyledons)
Cotyledons

32. Normal Placenta(At term)
Diameter : 15to 22 cm
Thickness : 2.0 ~ 2.5 cm
Weights : approximately 500 g (about 1lb)

33. Placenta consist of

34. Placental Barrier
Haemo-chorial

35. Classification of Placenta
• According to attachment of the umbilical cord
Battle- dore placenta
Velamentous placenta

36. According to site of implantation
• Placenta previa
• Accessory placenta

37. According to degree of adhesion and penetration

38. According to the shape of placenta
• Lobe placenta
• Placenta circumvallate

39. According to distribution of umbilical arteries
• Disperse type
• Magistral type

40. – Placental transfer
– Hormone production
– Haematopoietic
– Immunological

41. RESPIRATORY--
Gaseous exchange[CO2, O2]
– Passive diffusion across a pressure gradient
–assisted by maternal hyperventilation[progesterone
effect] &fetal haemoglobin.
Oxygen supplyto the fetus @8ml/kg/minis achieved with
cordblood flow of 165-330ml/min.

42. EXCRETORY
Waste products from fetus such as urea, uric acid &
creatinine are excreted in maternal blood by simple
diffusion.
NUTRITIVE
Glucose - facilitateddiffusion
Lipids - triglycerides &fatty acidsdirectly transported
from mother to fetus.
Amino acids - active transport (amino acid
concentration is higher in fetal blood than in maternal
blood)

43. Water and Electrolytes
Na, K, Cl - simple diffusion
Ca, P, Fe - active transport
Water soluble vitamins - active transport
Fat soluble vitamins - slow transfer (remains at
low level in fetal blood)

44. HORMONES
 Insulin
 Adrenal steroids
 Thyroxine
Chorionic gonadotrophin cross placenta at a very
slow rate to keep the fetal plasma concentration low.
 Parathormone
 Calcitonin
does not cross the placenta.

45. Some Substances and virus easily crosses placenta
Hormone
• Synthetic progestins
• Synthetics estrogen diethylstilbestrol (DES)
Virus
Rubella
Cytomegalo virus
Coxsackie
Variola
Varicella
Measles and poliomyelitis

46. Protein [polypeptides] Hormones
1. Human Chorionic Gonadotrophin –
- rises in 1st-early 2nd trimester, low levels after ~16 wks
- responsible for fetal adrenal cortex development
2. Human Chorionic Somatotrophin –
- fosters embryonic development by increasing fetal cell
glucose absorption and stimulating lipid and CHO
metabolism.

47. 3. Human Placental Lactogen –
- Rises progressively from ~12 wks upto term
- Possibly useful in preparation for lactation
- Contributes to diabetogenic effects of pregnancy
4. TSH, Melanocyte Stimulating Hormone,
Relaxin, Oxytocin,Vasopressin –
All isolated from placental tissue but most likely are of
maternal orfetal origin.

48. All rise progressively to plateau at term
1.Progesterone – Maintains pregnancy
Maintains uterine quiesence
↑ mammary growth
Antialdosterone effect
2.Oestrogens (oestriol) –↑ uterine growth &vascular supply to
decidua &myometrium
- ↑ metabolism &placental enzyme
systems.
3. Androgens
4. Corticosteroids
STEROID HORMONE

49. Placenta takes up Fe, Vit. B12&Folic acid tendency
towards anaemia inpregnancy.
Fetal erythropoietin may cross placenta to mother since
maternal reticulocyte counts are elevated in presence of fetal
anaemia.

50. •Feto-placental unit is an allograft that defies foreign
body tissue reaction.[Type IV cell-mediated reaction]
•Fetus notantigenically mature.

51. Clinical Correlates

52.  Causes : Trophoblast aging or impairment of
uteroplacental circulation with infarction.
 Deposition of calcium salts is heaviest on maternal surface
in basal plate –
→ further deposition occurs along septa and both
increase as pregnancy progresses.
 Diagnosis : Sonography

53. - Placental Infarctions
- Maternal Floor Infarction
- Placental Vessel Thrombosis

54. Erythroblastosis fetalis &
Fetal Hydrops
• Fetal blood escape placenta barrier
• Elicit antibody response by mother’s immune system
• If maternal response is sufficient, anitbodies will
attack and hemolyze fetal red blood

55.  Gestational Trophoblastic Disease
 Chorioangioma (hemangioma)
 Tumours metastatic to Placenta
 Embolic Fetal BrainTissue

57. MCQ’s
Q1 Which of the hormone is secrete by women
in the urine used as an indicator of
pregnancy?
a) Pregestrone
b) Estriol
c) Human chorionic gonadotropin (hCG)
d) Somatomammotropin
a) Human chorionic gonadotropin (hCG)
langmans-medical-embryology-12th-ed , pg no -107)

58. Q2 which of the hormone, cross placental barrier?
A) Parathormone
B) Calcitonin
C) Heparine
D) Thyroxine
Thyroxine
(langmans-medical-embryology-12th-ed , pg no -107)

59. Clinical Vignettes
Q A foetus of age 23rd week and 4 day born dead with
edema and effusion into the body as shown in image
below , mother diagnosis with D(Rh)negative body
and she says her first delivery is completely normal .
• What would be probable diagnosis and condition on
basis of above sign ?
Fetal hydrops and Isoimmunization
(langmans-medical-embryology-12th-ed , pg no -106)

60. Q2) A 32 year old 10 weeks pregnant women visit gynae
clinic for her routine check-up, during (USG) physician
found abnormality in placental position which produce
serious haemorrhage before parturition and would be a
life threatening to mother.
Name the condition in which blastocyst implanted over
internal os of cervix ?
Ans Placenta previa
(A.K Datta medical-embryology-7th-ed , pg no -67 )