Lecture - 02

• Topics covered

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1. Implantation
2. Placentation – classification & function
3. Superfecundation
4. Superfetation

Dr Alok Kumar

Veterinary Gynaecology and Obstetrics

alok9alright@gmail.com

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IMPLANTATION

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• Implantation in mammals refers to a series of highly coordinated interactions that begin with intimate contact between apical plasma membranes of the conceptus trophectoderm and the uterine luminal epithelium and concludes with the formation of a placenta as a means to support embryonic/fetal development throughout pregnancy.

IMPLANTATION

• Classification
• 1. based on the position the blastocyst in the uterus
• Centric: the blastocyst enlarges in size before implantation and remains in the center of the uterus. (eg. carnivores, ruminants, horses, and pigs.
• Eccentric: The blastocyst remains small and implants within the endometrium. (eg. Rodents)
• Interstitial: The blastocyst remains small and erodes through endometrial epithelium into subepithelial connective tissue.
• Interstitial implantation is sometimes termed as Nidation (nestmaking). eg,. primates, including humans, and guinea pigs.

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IMPLANTATION

• 2. based on the degree of uterine invasion by embryo.

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• INVASIVE – Blastocyst penetrates uterine mucosa and reaches uterine stroma by the process of phagocytosis and digestion of uterine luminal epithelium. Now the embryo remains encapsulated under uterine luminal surface.

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• Decidualization (proliferation and transformation of uterine stromal cells) facilitates invasive process in vicinity of developing embryo

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• Eg. – Rodents and Primates

IMPLANTATION

• NON-INVASIVE – Superficial and non penetrating type
• Involves, trophoblast – uterine epithelial cell apposition and adhesion.
• Eg – Ruminants, pig

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Placentation

• The placenta is an apposition or fusion of fetal membranes to the endometrium to permit physiological exchange between fetus and mother. (Hafez and Hafez. 2006)

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• Development of placenta – consist of two basic structures

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• 1. Fetal membrane
• These membranes participate in placental development with or without combination
• Yolk sac, Amnion, Allantois and chorion

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• 2. Chorionic Villi
• Fingerlike projections over chorion
• Structure consist of vascular mesenchymal cones covered by cuboidal trophoblastic cells and giant binucleate cells
• Primary function of villi is facilitate close proximity of allantoic (fetal) with endometrial (maternal) blood supply

Morphological classification of placenta

Diffuse:

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• Approximately entire surface of the allantochorion participates in development of placenta.

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• Eg.  Equine and swine

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• Source – Furukawa et al., 2014

Morphological classification of placenta

Cotyledonary:

• Characterized by presence of several round and raised area called as placentomes
• placentomes facilitates attachment between fetus and dam
• Placentomes are formed by union of The fetal structure (cotyledons) with maternal contact sites (caruncles)
• Eg. Ruminants

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• Source – personal collection

Morphological classification of placenta

Zonary:

characterized by a complete or incomplete band of tissue surrounding the fetus.

eg. Dog, Cat, seals, bears, and elephants.

Discoid:

Entire placenta remains in disc shape.

eg. primates and rodents

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• Source – Furukawa et al., 2014

• Source – Personal collection

Grosser’s classification of Placenta

• Classification based on histological structure – Grosser’s classification
• Based on the histological relationship established between the fetal layers and maternal layers (No. of maternal layers retained)

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• The fetal layers
• Endothelial lining allantoic capillaries
• Connective tissue of chorioallantoic mesoderm
• Chorionic epithelium (trophoblast derived outermost layer of fetal membranes)

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• The maternal layers
• Endothelial lining of blood vessels in endometrium
• Connective tissue of the endometrium
• Epithelial cells of endometrium

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Classification of Placenta

Based on involvement of maternal layers

PLACENTAL FUNCTIONS

1. Exchange of substances

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• The placental regulates variety of substances between dam and fetus.
• Transfer includes following process

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• Simple diffusion

• Active transport – Most significant way of transfer

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A₁. Gaseous exchange - placenta acts as fetal lung, exchange occurs via following ways

1. Countercurrent – Most efficiant in oxygen transfer
2. Multivillous - Moderate
3. Pool – Less than multivillous
4. Concurrent – Least efficiant

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PLACENTAL FUNCTIONS

• A₂. Water exchange
• More than 75% water is readily absorbed by fetus
• Water molecule moves from dam to fetus against osmotic gradient which is facilitated via reduced plasma protein concentration

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. A₃. Transport of nutrients

• Transport of macro minerals i.e. calcium, phosphorus, sodium and iron
• Transport of trace minerals i.e. copper etc.
• Transport of glucose/fructose, fatty acids, glycerol

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A_4. Transport of Hormones – eg. Gonadotrophins and steroid hormones

• A₅. Exchange of metabolic waste products – Kidney like function

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Superfecundation

• Superfecundation is the condition in which offspring from two sires are conceived contemporaneously.

(Veterinary Reproduction and Obstetrics., 9^th ed)

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• Condition is more common in dogs due to following reason

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• Longer estrous period
• More no of ova/estrous
• Higher longevity of ova
• promiscuous mating behavior in bitches

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• Condition can be identified by different colour, conformation and sizes of fetus, however , it only happens when both sires were pure breed.

Superfetation

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• Superfetation is the condition that arises when an animal already pregnant mates, ovulates and conceives a second fetus or second litter.

(Veterinary Reproduction and Obstetrics., 9^th ed)

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• Superfetation can be expected with following condition

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• Fetuses of significantly different size are born together.
• If two fetuses are born at widely separated times.
• If litters are born at widely separated times.

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Condition can be seen in dogs and cats

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