Veterinary Physiology-III

IMPLANTATION

• Implantation is the process whereby the new embryo becomes established at a developmental site on the endometrium to develop into a fetus.
• Following fertilization, the new embryo reaches the uterus 4 days post-estrus in cows and ewes. It is slightly sooner in sows and slightly latter in bitches
• Estrogen influences the motility of the oviducts and the passage of zygote from the oviduct into the uterus. The rate of transport is controlled by ratio of estrogen and progesterone.
• Before the passage of the embryos into the uterus, progesterone from the developing CL has been preparing the uterus to retain and nurses the embryos. Progesterone decreases the muscular activity and tonicity of the uterus and promotes the development of a glandular epithelium, which is responsible for producing uterine milk.
• The nutritive requirements of the embryo are supplied by the yolk material and secretions of the oviducts and uterus
• The embryo remains free-floating in the uterine fluid after its entry into the uterus until implantation and derives its nutrients from the endometrial secretions.
• The rate of transport through oviduct is controlled by the estrogen and progesterone ratio.
• After its entry into the uterus the embryo remains free-floating in the uterine fluid until implantation and derives its nutrients from the endometrial secretions.
• The embryo continues to grow by mitotic division and the cells become arranged to one side (known as compaction) and fluid accumulates in the central cavity known as blastocoele and the conceptus is called blastocyst
• The blastocyst has two cell types –
• an outer layer of cells called trophoblast which later forms the placenta and
• group of cells surrounded by trophoblast known as inner cell mass (ICM) which develops into foetus

• When the blastocyst reaches the uterus, it is still covered by zona pellucida. In 6-11 days after fertilization, the zona pellucida breaks down and release the embryo which is referred to as "zona hatching". Zona hatching is contributed by blastocyst expansion and contraction and aided by lytic enzymes
• Zona hatching – cattle 9-11 days, horse-8; sheep-7-8 and swine in 6 days after fertilization
• The blastocyst elongates because of formation and hyperplasia of extra embryonic membranes
• Blastocyst elongation occurs in cows, sheep, and pigs, but not in horses
• Concurrent with blastocyst elongation, gastrulation (ICM differentiate into three distinct layers – ectoderm, mesoderm and endoderm) occurs in the embryo

    

(Picture source: Wikipedia)

• Spacing of embryo in the uterine horn in polytocous animals occurs during this period.
• In litter bearing animals, transuterine migration of zygote helps to distribute the embryos which are affected by uterine muscle contraction in the presence of embryos. The spacing of embryos is important for the normal foetal development. 

Species	Days of implantation
Cow	30 – 35 days
Doe	20 - 25 days
Ewe	15 – 18 days
Sow	14 - 20 days
Bitch	15 days
Queen	13 days
Primates	6 - 8 days