IMPLANTATION

&

PLACENTATIONPranjal Gupta

Bsc(H) Zoology, Ramjas College

OVERVIEW

Human embryo cleavages and blastulation

Human implantation

Human basic gastrulation

Extra-embryonic membranes in birds

Extra-embryonic membranes in mammals

Classification of placenta

Human placental physiology

HUMAN EMBRYO CLEAVAGES

1st cleavage occur in first 30h; Upper oviduct

Next within 40hrs after fertilization

Third about 3days after fertilization, i.e. 8celled

embryo.

By the end of 3rd day, 8-16 celled morula reaches

uterus

Holoblastic cleavage

Enclosed within zona pellucida

BLASTOCYST FORMATION

Uterine milk from endometrium nourishes morula

Growth of embryo increase, outer peripheral cell enlarge and flatten to form trophoblast(Later form extra embryonic membranes.

Secretion of trophoblast create blastocoel

Cell layers separated in trophoblast and embryoblast(inner cell mass).

ICM give rise to embryo hence also called embryonal knob & determines animal pole

Cells of Rauber- trophoblast cell in contact with knob

Zona pellucida till implantation avoids ectopic pregnancies

IMPLANTATION

Anchoring or embedding of the blastocyst into

endometrium of uterus.

7th day post fertilization, and a 3 day long

process.

Zona hatching

Trophoblast not exposed till proper site reached

Implantation allows the ICM region to be

engraved inside endometrium

Local endometrial lysis by trophoblastic enzymes

Formation of Chorionic villi from trophoblast.

Embedded embryo

inside endometrium

Trophoblast’s been divided into syncytiotrophoblast(STB) and

cytotrophoblast(CTB).

Endometrium entirely encloses the embryo now

Implantation involves events of both blastocyst and the uterine

wall.

Decidua of pregnancy formed(nourished,vascularized

endotmetrium)

Regions of decidua

Decidua basalis

Portion of endometrium b/w chorion and myometrium of

uterus. Become maternal placenta

Decidua capsularis

Potion between the embryo and uterine cavity

Decidua parietalis

Portion of modified endometrium that lines entire pregnant uterus,

except of area of placenta

Decidua of pregnancy-three layers

CHANGES IN POSITIONS DURING

PREGNANCY

ECTOPIC PREGNANCIES

GASTRULATION

By morphogenetic movements three germ layers are separated

Trophoblast divided into two, and CTB directly cover the ICM while STB spreads in endometrium.

CTB give rise to extra-embryonic mesoderm(EEM).

EEM diff. to outer or parietal or Somatopleuric EEMand inner Splanchnopleuric EEM.

ICM differentiated to epiblast and hypoblast, both form a disk called embryonic disk.

Epiblast divides to amnion and embryonic ectoderm.

Chorion formed by Somatopleure inside and trophoblast outside

Later chorion becomes essential part of placenta.

Hypoblast give rise to EE endodermal cells lining primary yolk sac.

Yolk sac become smaller to form secondary yolk sac

EXTRA EMBRYONIC MEMBRANES

It is an adaptation to terrestrial mode of life.

Situated outside boundaries of embryo hence so called.

4 major membranes- Chorion. Amnion, Allantois and Yolk sac.

In birds

Amnion and chorion arises from two folds that grow to surround

embryo.

Amnion, inside surround embryo and suspend it in amniotic fluid

Chorion is in contact with the shell and separated from other

membranes through EE coelom

Yolk here is utilized for nourishment unlike mammals

Allantois is derived as out pouch of gut and store uric acid excreted.

Allantois later expands to form a sac that fuses with chorion under

eggshell, a functional unit of embryonic blood vessels inside allantois

is formed.

Allantois now in direct contact with the porous shell act as ‘lung’ for

chick embryo.

In mammals

Embryonic membranes arise from trophoblast.

First prior to implantation trophoblast form chorion

Chorion has great role in placentation. Made of Trophoblast outside and Somatopleure inside

Yolk sac here is not involved in nutrition for long run only initial vitelline circulation. Made of endoderm inside and splanchnopleure outside

Allantois contributes blood vessels to structure that forms the umbilical cord. Endoderm inside and splanchnopleure outside.

Amnion, consist of trophoblast inside and somatopleure outside. It cover the entire embryo. Shock absorber and avoid desiccation.

Umbilical cord is formed of stalk of yolk sac and allantois.

PLACENTATION

3 month

7 week4 week

4 month

PLACENTATION

Zonary placenta

Diffuse placenta

Discoidal placenta

Metadiscoidal placenta

Cotyledonary placenta

PLACENTA WITH TRIPLETS.

FUNCTIONS OF PLACENTA

Formation of twins

and

placenta partitioning