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1. The process involved in the maturation of the
two highly specialized cells, spermatozoon in
male and ovum in female before they unite to
form zygote, is called gametogenesis.
The process involved in the development of a
mature ovum is called oogenesis.
Oocyte contains haploid number of chromosomes
23, X
Fundamentals of Reproduction

2. Sperm capacitation and
acrosome reaction
Capacitation is the physiochemical
change in the sperm by which it
becomes hypermotile and is able to
bind and fertilize a secondary
oocyte.
Capacitation takes place in the
genital tract and takes about 2–6
hours.
Activation of acrosomal
membranes causes release of
hyaluronidase, hydrolytic enzymes,
proacrosin, acrosin, that help the
sperm to digest the zona pellucida
and to enter into the oocyte.

3. Fertilization is the process of fusion of the
spermatozoon with the mature ovum.
Almost always, fertilization occurs in the
ampullary part of the uterine tube.

4. APPROXIMATION OF THE GAMETES
The ovum, immediately following ovulation is
picked up by the tubal fimbriae which partly
envelope the ovary, especially at the time of
ovulation.
The pick up action might be muscular or by a kind of
suction or by ciliary action or by a positive
chemotaxis exerted by the tubal secretion.
The ovum is rapidly transported to the ampullary
part.
Fertilizable life span of oocyte
ranges from 12 to 24 hours whereas that of sperm
is 48 to 72 hours.

5. CONTACT AND FUSION OF THE GAMETES
Complete dissolution of the cells of the corona
radiata occurs by the chemical action of the
hyaluronidase liberated from the acrosomal cap of the
hundreds of sperm present at the site
Penetration of the zona pellucida is facilitated by the
release of hyaluronidase from the acrosomal
cap. More than one sperm may penetrate the zona
pellucida.
Out of the many sperms, one touches the oolemma. Soon
after the sperm fusion, penetration of
other sperm is prevented by zona reaction (hardening)
and oolemma block.
This is due to release of
cortical granules by exocytosis from the oocyte.

6. Sex of the child is determined by the
pattern of the sex chromosome supplied by
the spermatozoon.
If the spermatozoon contains ‘X’
chromosome, a female embryo (46, XX) is
formed
if it contains a ‘Y’ chromosome, a male
embryo (46, XY) is formed.

7. After the zygote formation, typical mitotic
division of the nucleus occurs by producing two
blastomeres.The two cell stage is reached
approximately 30 hours(more then one day)
after fertilization. Each contains equal
cytoplasmic volume and chromosome numbers.
The blastomeres continue to divide by binary
division through 4, 8, 16 cell stage until a
cluster of cells is formed and is called morula.
Morula after spending about 3 days in the
uterine tube enters the uterine cavity through
the narrow uterine ostium (1 mm) on the 4th day
in the 16-64 cell stage.

8. The transport is a slow process and is controlled
by muscular contraction and movement of the cilia.
The central cell of the morula is known as inner
cell mass which forms the embryo proper and the
peripheral cells are called outer cell mass which will
form protective and nutritive membranes of the
embryo.

10. Implantation occurs in the endometrium of the
anterior or posterior wall of the body near the
fundus on the 6th day which corresponds to the
20th day of a regular menstrual cycle.
Implantation occurs through four stages e.g.
apposition, adhesion, penetration and invasion.
ENDOMETRIUM AT THE IMPLANTATION SITE :
The endometrium is in the secretory phase
corresponding to 20–21 days of cycle.

12. As previously mentioned, the cells of the blastocyst differentiate into
an outer trophectoderm and an inner cell mass.
Just before implantation, the trophectoderm is further differentiated
into an inner mononuclear cellular layer called cytotrophoblast or
Langhans’ layer and an outer layer of multinucleated syncytium
called syncytiotrophoblast.
Placenta and the fetal membranes are developed from the
trophoblast.
The decidua is the endometrium of the pregnant uterus. It is so
named because much of it is shed following delivery.

13. The well developed decidua differentiates into three
layers
 Superficial compact layer consists of compact mass
of decidual cells, gland ducts and dilated capillaries.
The greater part of the surface epithelium is either
thinned out or lost.
 Intermediate spongy layer (cavernous layer)
contains dilated uterine glands, decidual cells and
blood vessels. It is through this layer that the
cleavage of placental separation occurs.
 Thin basal layer containing the basal portion of the
glands and is opposed to the uterine muscle.
Regeneration of the mucous coat occurs from this
layer following parturition.
Functional
layer

15. After the interstitial implantation of the blastocyst into
the compact layer of the decidua, the different
portions of the decidua are renamed as
 Decidua basalis or serotina — the portion of the
decidua in contact with the base of the blastocyst
 Decidua capsularis or reflexa — the thin superficial
compact layer covering the blastocyst and
 Decidua vera or parietalis — the rest of the decidua
lining the uterine cavity outside the site of
implantation.
Its thickness progressively increases to maximum of 5–10
mm at the end of the second month and thereafter
regression occurs with advancing pregnancy so that
beyond 20th week, it measures not more than 1 mm.

17. The chorion is the outermost layer of the two
fetal membranes (chorion and amnion)
At the beginning of the 3rd week, the
syncytiotrophoblast produces irregular finger
like projections which are lined internally by the
cytotrophoblast.
These finger like buds are called primary stem
villi— surrounded by lacunar spaces which will
later form into intervillous spaces.

19. After the appearance of the primitive mesenchyme and
the development of the chorion, the 1.primary
stem villi are named chorionic villi.
With the insinuation of the primary mesoderm into the
central core of the villi structures, 2.secondary villi are
formed on 16th day.
Later on mesodermal cells in the villi begin to
differentiate into blood cells and blood vessels, thus
forming villous capillary system.These vascularized villi
are called 3.tertiary villi which are completed on 21st
day.
Later on, this extraembryonic circulatory system
establishes connection with the intraembryonic
circulatory system through the body stalk

20. 1 2
3

22. The embryo can be differentiated as human at 8th week.
ECTODERMAL LAYER: Central and peripheral nervous
system, epidermis of skin with its appendages, pituitary
gland, chromaffin organs, salivary glands; mucous lining
of the nasal cavity, paranasal sinus, roof of the mouth
etc.
MESODERMAL LAYER: Bones, cartilage, muscles,
cardiovascular system, kidney, gonads, suprarenals,
spleen,
most of the genital tract; mesothelial lining of
pericardial, pleural and peritoneal cavity etc.
ENDODERMAL LAYER: Epithelial lining of the
gastrointestinal tract, liver, gallbladder, pancreas;
epithelial lining
of respiratory tract and most of the mucous membrane of
urinary bladder and urethra; bulbourethral and greater
vestibular glands etc.

23. ‘0’ hour — Fertilization (day-15 from LMP)
30 hours — 2 cell stage (blastomeres)
40–50 hours — 4 cell stage
72 hours — 12 cell stage
96 hours — 16 cell stage. Morula enters the uterine
cavity
5th day — Blastocyst
4–5th day — Zona pellucida disappears
5–6th day — Blastocyst attachment to endometrial
surface
6–7th day — Differentiation of cyto and
syncytiotrophoblastic layers
10th day — Synthesis of hCG by syncytiotrophoblast
9–10th day — Lacunar network forms
10–11th day —Trophoblasts invade endometrial sinusoids establishing
uteroplacental circulation
Interstitial implantation completed
with entire decidual coverage
13th day — Primary villi
16th day — Secondary villi
21st day — Tertiary villi
21st–22nd day — Fetal heart. Fetoplacental circulation

24. THE PLACENTA Only eutherian mammals possess
placenta. The human placenta is discoid,
because of its shape; hemochorial, because of
direct contact of the chorion with the maternal
blood and deciduate, because some maternal
tissue is shed at parturition.
The Placenta and Fetal Membranes

25. The placenta is attached to the uterine
wall and establishes connection between
the mother and fetus through the
umbilical cord.
The fact that maternal and fetal tissues
come in direct contact without rejection
suggests immunological acceptance of the
fetal graft by the mother.

26. DEVELOPMENT
The placenta is developed from two sources. The
principal component is fetal which develops from the
chorion frondosum and the maternal component
consists of decidua basalis.
When the interstitial implantation is completed on
11th day, the blastocyst is surrounded on all sides by
lacunar spaces around cords of syncytial cells, called
trabeculae. From the trabeculae develops the stem
villi on 13th day which connect 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 system through the body stalk (Fig. 2.9).

27. Simultaneously, lacunar spaces become confluent
with one another and by 3rd–4th week, form a
multilocular receptacle lined by syncytium and
filled with maternal blood.
This space becomes the future intervillous space.
As the growth of the embryo proceeds, decidua
capsularis becomes thinner beginning at 6th
week and both the villi and the lacunar spaces in
the abembryonic area get obliterated, converting
the chorion into chorion laeve.

28. Maternal surface:
The maternal surface is rough and spongy
Maternal blood gives it a dull red color.
A thin grayish, somewhat shaggy layer which is the
remnant of the decidua basalis (compact and spongy
layer) and has come away with the placenta, may be
visible.
The maternal surface is mapped out into 15–20
somewhat convex polygonal areas known as lobes or
cotyledons which are limited by fissures. Each fissure
is occupied by the decidual septum which is derived
from the basal plate. Numerous small grayish spots
are visible. These are due to deposition of calcium in
the degenerated areas and are of no clinical
significance.

29. The maternal portion of the placenta amounts to less than one-fifth
of the total placenta.
Only the decidua basalis and the blood in the intervillous space are
of maternal origin.
Margin:Peripheral margin of the placenta is limited by the fused
basal and chorionic plates and is continuous with the chorion laeve
and amnion. Essentially, the chorion and the placenta are one
structure but the placenta is a specialized part of the chorion.
Attachment:The placenta is usually attached to the upper part of the
body of the uterus encroaching to the fundus adjacent to the
anterior or posterior wall with equal frequency. The attachment to
the uterine wall is effective due to anchoring villi connecting the
chorionic plate with the basal plate and also by the fused decidua
capsularis and vera with the chorion laeve at the margin.
Separation:Placenta separates after the birth of the baby and the
line of separation is through the decidua spongiosum.

30. STRUCTURES
The placenta consists of two plates.
The chorionic plate lies internally. It is lined by
the amniotic membrane. The umbilical cord is
attached to this plate.
The basal plate lies to the maternal aspect.
Between the two plates lies the intervillous
space containing the stem villi with their
branches, the space being filled with maternal
blood

31. AMNIOTIC MEMBRANE:
It consists of single layer of cubical
epithelium loosely attached to the adjacent
chorionic plate. It takes no part in formation
of the placenta.
CHORIONIC PLATE: From within outward, it
consists of (i) primitive mesenchymal tissue
containing branches of umbilical vessels, (ii)
a layer of cytotrophoblast and (iii)
syncytiotrophoblast. The stem villi arise
from the plate. It forms the inner boundary
of the choriodecidual space.

32. Functions of placenta
Nutrition and gas exchange
The placenta intermediates the transfer of nutrients between mother
and fetus. The perfusion of the intervillous spaces of the placenta
with maternal blood allows the transfer of nutrients and oxygen from
the mother to the fetus and the transfer of waste products and
carbon dioxide back from the fetus to the maternal blood. Nutrient
transfer to the fetus can occur via both active and passive
transport.[26] Placental nutrient metabolism was found to play a key
role in limiting the transfer of some nutrients.[27] Adverse pregnancy
situations, such as those involving maternal diabetes or obesity, can
increase or decrease levels of nutrient transporters in the placenta
potentially resulting in overgrowth or restricted growth of the
fetus.[28]

33. Excretion
Waste products excreted from the fetus such as urea, uric acid, and
creatinine are transferred to the maternal blood by diffusion across the
placenta.
Immunity
The placenta functions as a selective barrier between maternal and fetal
cells, preventing maternal blood, proteins and microbes (including bacteria
and most viruses) from crossing the maternal-fetal barrier.Deterioration in
placental functioning, referred to as placental insufficiency, may be related
to mother-to-child transmission of some infectious diseases. A very small
number of viruses including rubella virus, Zika virus and cytomegalovirus
(CMV) can travel across the placental barrier, generally taking advantage of
conditions at certain gestational periods as the placenta develops. CMV and
Zika travel from the maternal bloodstream via placental cells to the fetal
bloodstream.
Beginning as early as 13 weeks of gestation, and increasing linearly, with the
largest transfer occurring in the third trimester, IgG antibodies can pass
through the human placenta, providing protection to the fetus in utero.This
passive immunity lingers for several months after birth, providing the
newborn with a carbon copy of the mother's long-term humoral immunity to
see the infant through the crucial first months of extrauterine life. IgM
antibodies, because of their larger size, cannot cross the placenta,one
reason why infections acquired during pregnancy can be particularly

34. Endocrine function
The first hormone released by the placenta is called the human chorionic
gonadotropin (hCG) hormone. This is responsible for stopping the process at
the end of menses when the corpus luteum ceases activity and atrophies. If
hCG did not interrupt this process, it would lead to spontaneous abortion of
the fetus. The corpus luteum also produces and releases progesterone and
estrogen, and hCG stimulates it to increase the amount that it releases. hCG
is the indicator of pregnancy that pregnancy tests look for. These tests will
work when menses has not occurred or after implantation has happened on
days seven to ten. hCG may also have an anti-antibody effect, protecting it
from being rejected by the mother's body. hCG also assists the male fetus by
stimulating the testes to produce testosterone, which is the hormone needed
to allow the sex organs of the male to grow.
Progesterone helps the embryo implant by assisting passage through the
fallopian tubes. It also affects the fallopian tubes and the uterus by
stimulating an increase in secretions necessary for fetal nutrition.
Progesterone, like hCG, is necessary to prevent spontaneous abortion
because it prevents contractions of the uterus and is necessary for
implantation.

35. Estrogen is a crucial hormone in the process of proliferation. This involves
the enlargement of the breasts and uterus, allowing for growth of the fetus
and production of milk. Estrogen is also responsible for increased blood
supply towards the end of pregnancy through vasodilation. The levels of
estrogen during pregnancy can increase so that they are thirty times what a
non-pregnant woman mid-cycles estrogen level would be.
Human placental lactogen (hPL) is a hormone used in pregnancy to develop
fetal metabolism and general growth and development. Human placental
lactogen works with growth hormone to stimulate Insulin-like growth factor
production and regulating intermediary metabolism. In the fetus, hPL acts on
lactogenic receptors to modulate embryonic development, metabolism and
stimulate production of IGF, insulin, surfactant and adrenocortical hormones.
hPL values increase with multiple pregnancies, intact molar pregnancy,
diabetes and Rh incompatibility. They are decreased with toxemia,
choriocarcinoma, and Placental insufficiency.

36. Immunological barrier
Further information: Immune tolerance in pregnancy
The placenta and fetus may be regarded as a foreign body inside the mother
and must be protected from the normal immune response of the mother that
would cause it to be rejected. The placenta and fetus are thus treated as
sites of immune privilege, with immune tolerance.
For this purpose, the placenta uses several mechanisms :
It secretes Neurokinin B-containing phosphocholine molecules. This is the
same mechanism used by parasitic nematodes to avoid detection by the
immune system of their host.[40]
There is presence of small lymphocytic suppressor cells in the fetus that
inhibit maternal cytotoxic T cells by inhibiting the response to interleukin
2.[41]
However, the Placental barrier is not the sole means to evade the immune
system, as foreign fetal cells also persist in the maternal circulation, on the
other side of the placental barrier.[42]

37. Other
The placenta also provides a reservoir of blood for the
fetus, delivering blood to it in case of hypotension and
vice versa, comparable to a capacitor