This document summarizes key aspects of human conception, placental development, and fetal circulation. It describes how fertilization occurs through the union of an egg and sperm, forming a zygote. Implantation in the uterus follows cell division. The placenta then develops from the chorion and decidua, establishing the maternal-fetal interface through which nutrients and waste are exchanged. The umbilical arteries and veins connect the developing fetus and placenta in a circuit that allows for gas and nutrient transfer between mother and fetus without blood mixing.

1. Conception and Placental
Development
By: Dr. Noshirwan P. Gazder

2. • Human development begins at fertilization when a male gamete or
sperm unites with a female gamete or oocyte to form a single cell, a
zygote.
• This highly specialized cell marks the beginning of each of us as a
unique individual.
• The zygote, contains chromosomes and genes that are derived from
the mother and father.
• The unicellular zygote divides many times and becomes progressively
transformed into a multicellular human being through cell division,
migration, growth, and differentiation.

5. Gametogenesis
• Gametogenesis is the process of formation and development of
specialized generative cells, gametes(sperm or oocyte).
• The sperm and oocyte,are highly specialized sex cells . Each of these
cells contains half the number of chromosomes (haploid number)
that are present in somatic (body) cells.
• This process, involving the chromosomes and cytoplasm of the
gametes, prepares the sex cells for fertilization.

6. Spermatogenisis
• Primodial germ cells undergo mitosis in seminiferous tubules to develop
into spermatogonia
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• Spermatogonia differenciates into primary spermatocytes (46 XY) which
remain in the stage of prophase of the first meiotic division
↓
• With completion of first meiotic division – two secondary spermatocytes
are formed( 23 X or 23 Y )
↓
• Secondary spermatocytes undergo a second meiotic division to form four
hapolid spermatids
↓
• Spermatids undergo extensive morphological changes to convert them into
spermatozoa(spermiogenisis)

8. • The entire process of spermatogenesis, which includes spermiogenesis,
takes approximately 2 months. When spermiogenesis is complete, the
sperms enter the lumina of the seminiferous tubules.
• Sertoli cells lining the seminiferous tubules support and nurture the germ
cells and may be involved in the regulation of spermatogenesis.
• Sperms are transported passively from the seminiferous tubules to the
epididymis, where they are stored and become functionally mature.
• The epididymis is the elongated coiled duct along the posterior border of
the testis. It is continuous with the ductus deferens (vas deferens), which
transports the sperms to the urethra.

10. Oogenesis
• Germ cells undergo rapid mitotic division and differentiate into oogonia.
↓
• Oogonia enter into prophase of first meiotic division forming Primary oocytes.
(primary oocytes do not finish the first meiotic division until puberty is reached)
↓
• At puberty the primary oocyte undergoes the first meiotic division, forming a
secondary oocyte and the first polar body.
↓
• Ovulation occurs soon after the formation of the secondary oocyte.
↓
• Secondary oocyte completes the second meoitic division after fertilization by the
sperm in falliopian tube- ovum and second polar body. In the absence of
fertilization the secondary oocyte does not complete the second meiotic divison
and degenerates.

12. Fertilization
• Is the process of fusion of spermatzoon with the mature ovum.
• Fertilization occurs in the ampullary part of the uterine tube.
• The ovum following ovulation is picked by tubal fimbriae and is
transported to the ampullary part of the fallopian tube.
• Out of the hundreds of millions sperm deposited in the vagina after a
single ejaculation,only a thousand capacitated spermatozoa enter the
tube while only 300-500 reach the ovum.

13. • The fertilisable life span of oocyte ranges from 12-24 hours where as that
of sperm is 48-72 hours
• Complete disolution of the cells of the corona radiata occurs by the
chemical action of the hyaluronidase liberated from the acrosomal cap of
the sperms.
• Penetration of the zona pellucida is facilitated by the release of
hyaluronidase from the acrosomal cap.
• After the one spermatozoon has entered the ovum , others are prevented
from entering by zona reaction

14. • Completion of the second meiotic division of the oocyte immediately
follows resulting in the female pronucleus (23 X) and 2nd polar body.
• At the same time the head of the spermetazoa separates from middle
piece and tail and transforms into male pronucleus (23Xor 23Y).
• Male and female pronucleus unite at the center resulting in
formation of zygote (46 XX or 46XY).
• Sex of the child will depend on the pattern of sex chromosome
supplied by the sperm.

16. Cleavage of the Zygote
• Cleavage consists of repeated mitotic divisions of the zygote ,
resulting in a rapid increase in the number of cells.
• Cleavage normally occurs as the zygote passes along the uterine tube
toward the uterus
• Division of the zygote into blastomeres begins approximately 30 hours
after fertilization.
• Subsequent cleavage divisions follow one another, forming
progressively smaller blastomeres .
• When there are 12 to 32 blastomeres, the developing human is
called a morula.

18. • The morula is rapidly propelled along the Fallopian tube to enter the
uterine cavity.
• During its passage, fluid passes through canaliculae in the zona pellucida to
create a central fluid-filled cavity in the morula, forming a blastocyst.
• On reaching the uterine cavity the zona pellucida becomes distended and
thin.
• It soon disappears, leaving the surface cells of the blastocyst in contact
with the endometrial stroma.
• About 50% of blastocysts adhere to the endometrium. The surface
trophoblastic cells of the adhering blastocyst differentiate into an inner
cellular layer, the cytotrophoblast, and an outer syncytiotrophoblast.

21. Implantation
• Begins 6 days after fertilization:
• The blastocyst attaches to the
endometrial epithelium, usually
adjacent to the embryonic pole.
• The blastocyst goes deeper and
deeper into the uterine mucosa
till it comes to lie within the
thickness of the endometrium.
This is called interstitial
implantation

22. • 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.

23. • 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.

24. • After the implantation of the embryo, the uterine endometrium is
called the decidua.
• When the morula reaches the endometrium , it is in the secretory
phase.
• After implantation , features of the endometrium in secretory phase
are intensified- stromal cells enlarged , become vacuolated and store
glycogen and lipids. This change in stromal cells is called the decidual
reaction.
• The portion of the decidua where the placenta is to be formed ( deep
to the developing blastocyst ) is called the decidua basalis.

25. Formation of Placenta
Developed from two sources:
• Chorion frondosum: Fetal Component.
• Maternal component: Decidua Basalis.
• The chorion frondosum and the decidua basalis form the discrete
placenta. It begins at 6th week and is completed by 12th week.
• By the end of 16th week, the placenta grows both in thickness and
circumference.

26. • When the interstitial implantation is completed on 10th day, the blastocyst
is surrounded on all sides by lacunar spaces around cords of syncytial cells,
called trabaculae.
• From the trabaculae develops the stem villi on 13th day which connects
the chorionic plate with the basal plate.
• Primary, secondary and tertiary villi are successively developed from the
stem villi.
• Arterio-capillary-venous system in the mesenchymal core of each villus is
completed on 21st day.
• This ultimately makes connection with the intraembryonic vascular systems
through the body walls.

27. • The placenta is attached to the uterine wall and establishes
connection between the mother and fetus through the umbilical
cord.
• The placenta at term is a circular disc with a diameter of 15-20 cm
and thickness of about 2.5 cm at its center.
• Its spongy and weighs approximately 500 gm.
• It has two surfaces, fetal(smooth) and maternal(rough).

28. Fetoplacental Circulation
• The two umbilical arteries enter the chorionic plate underneath the
amnion, each supplying one half of the placenta.
• The arteries break up into small branches which enter the stems of
the chorionic villi.
• Each in turn divides into primary, secondary and tertiary vessels of
the corresponding villi.
• The blood flows into the corresponding venous channels either
through the terminal capillary networks or through the shunts.

29. • Maternal and fetal blood streams flow side by side, but in opposite
directions.
• This counter current flow facilitates material exchange between the
mother and fetus.
• Inspite of close proximity, there is no mixing of the maternal and fetal
blood. The two are separated by tissues called the placental
membrane.

30. Function of the Placenta

31. Thank You