3. HUMAN REPRODUCTION
PREGNANCY
AND EMBRYONIC DEVELOPMENT
-
After
implantation, finger-like projections (chorionic
villi) appear on the trophoblast.
-
They are
surrounded by uterine tissue and maternal blood.
-
The chorionic
villi & uterine tissue are interdigitated to form placenta. It is a structural and functional unit b/w embryo (foetus)
and maternal body.
-
Placenta is
connected to the embryo by an umbilical
cord. It transports substances to
and from the embryo.
Functions of placenta
· Acts
as barrier between the foetus and mother.
· Supply O2,
nutrients etc. from mother to
foetus.
· Remove
CO2 and excretory wastes from foetus.
· Acts as an endocrine gland. It secretes Human chorionic gonadotropin (hCG), human
placental lactogen (hPL), oestrogens, progesterone & relaxin. Relaxin is also secreted by ovary.
-
During pregnancy, levels of estrogens,
progestogens, cortisol, prolactin, thyroxin etc. are also increased in
maternal blood. They support the fetal growth, metabolic changes in the mother
and maintain pregnancy.
-
The germ layers give rise to all tissues
(organs). The stem cells in inner cell mass have the potency to give
rise to all the tissues and organs.
-
Human pregnancy (gestation period) lasts
9 months (cats: 2 months, dogs: 2 months, elephants: 21 months).
Changes in embryo during pregnancy
·
After one month: Heart is formed.
·
End of second
month: Limbs and digits are developed.
·
End of 12 weeks
(first trimester): Major organs (limbs,
external genital organs etc.) are well developed.
·
During 5th month:
First movement of foetus and appearance of hair on the head.
· End of 24 weeks (end of 2nd
trimester): Body is covered with fine hair, eyelids
separate and eye lashes are formed.
· End
of 9 months: Ready for delivery.
PARTURITION
AND LACTATION
-
Parturition (labour): Process
of giving birth to young ones.
-
Parturition is induced by neuroendocrine
mechanism.
- The signals originating
from the foetus and placenta induce mild uterine contractions (fetal
ejection reflex). This causes the release of oxytocin from maternal
pituitary.
-
Oxytocin causes stronger uterine muscle
contractions which in turn stimulate further secretion of oxytocin. This
process is continued leading to expulsion of the baby out of the uterus through
the birth canal.
-
After parturition, the umbilical cord is cut off.
-
The placenta & remnants of umbilical
cord are expelled from the maternal body after parturition. It is called “after birth”.
-
The mammary
glands produce milk towards the end of pregnancy. It is called lactation.
- The yellowish milk produced during the initial few days of lactation is called colostrum. It contains several antibodies essential to develop resistance for the new born babies.