EXTRA - EMBRYONIC MEMBRANES
Four extra-embryonic membranes (or embryonic membranes or foetal membranes) are formed in amniotes (reptiles, birds and mammals) outside the body of embryo by the cells of presumptive ectoderm, mesoderm and endoderm.
These are -
(iv) Yolk sac.
(i) Chorion (serosa) - The outermost covering, formed by ectoderm and mesoderm as a protective layer head fold and tail fold of ectoderm and mesoderm emerge from respective parts of embryo, start growing and folding upon the dorsal side where both fuse (= sero-amniotic connection) to form outer chorion and inner amnion.
(ii) Amnion - It forms private (closest) chamber of embryo filled with amniotic fluid, isotonic to the body fluid. The aquatic medium for embryo to float and grow, denotes the aquatic origin of life. This fluid having cells of embryo is used (amniocentesis) to test its sex and genetic disorders.
(iii) Allantois - It develops from the inner endoderm and outer mesoderm. It comes out from posterior end of embryo within the chorio-amniotic tail fold. In chick embryo this stores the metabolic (excretory) wastes provides surface for exchange of respiratory gases. Its mesodermal part after combining with chorionic mesoderm forms allantochorion that forms placenta in mammals
(iv) Yolk sac - It also develops from the endoderm and mesoderm. The fold grows out from gut towards the anterior end of embryo. In chick it grows bigger to surrounding the yolk, develops li like structure with vitelline blood vessels within the yolk mass. In mammals during early part of development it forms yolk sac placenta (e.g. in metatherians) but degenerates later on.
The average duration of human pregnancy is about 9 months which is called the gestation period. Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus. This process of delivery of the foetus (childbirth) is called parturition. Parturition is induced by a complex neuroendocrine mechanism.
FOETAL EJECTION REFLEX
The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex. This striggers release of oxytocin from the maternal pituitary. Oxytocin acts on
the uterine muscle and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin. The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions. This leads to expulsion of the baby out of the uterus through the birth canal – parturition
Lactation is the production of milk. The mammary glands, or breasts, contain milk glands. In the glands, special epithelial cells line small sacs called alveoli. These cells secrete milk. The alveoli are surrounded by a layer of tissue containing smooth muscle fibres. When the muscle contracts it causes milk to be released. Milk enters a series of ducts (tubes), and each duct has an expanded space called a sinus which stores milk. The ducts eventually pass to separate opening in the nipple.
The breasts increase in size during pregnancy due to the development of the milk, glands, controlled by progesterone, and ducts, controlled by oestrogen. Human placental lactogen, another hormone, is also involved. However, for milk to be produced, the hormone prolactin must be present. This is secreted by the anterior pituitary gland. throughout pregnancy the presence of oestrogen and progesterone inhibits the secretion of prolactin and therefore the formation of milk. At birth, when the oestrogen and progesterone levels fall due to this of the placenta, prolactin is no longer inhibited and it stimulates the alveoli to secrete milk.
The milk produced during the initial few days of lactation is called colostrum which contains several antibodies absolutely essential to develop resistance for the new-born babies.