M, Embry, Lecture II

Lecture IILecture II

Cleavage (Segmentation):Cleavage (Segmentation): Steps.Steps. Applied.Applied.Implantation:Implantation: Steps.Steps. Sites.Sites.

Cleavage (Segmentation)Cleavage (Segmentation)@@ Cleavage (Segmentation):Cleavage (Segmentation): is repeated mitosis of is repeated mitosis of zygotezygote leading to leading to increaseincrease inin cell numbercell number without increase without increase in totalin total size (due to persistence of zona pellucida). size (due to persistence of zona pellucida). @@ It leads to formation of It leads to formation of morulamorula and and blastocystblastocyst..

Steps:Steps:1) Formation 1) Formation ofof morula: morula:

## During its journey in uterine tube towards uterine cavity, During its journey in uterine tube towards uterine cavity,

zygote divides (zygote divides (withinwithin zona pellucida) by repeated mitotic zona pellucida) by repeated mitotic divisions to form divisions to form 22, , 44, , 8 8 cells called cells called blastomeresblastomeres (after 36 (after 36 hours).hours).

## Repeated mitosis → Repeated mitosis → morulamorula (after 72 hours) which enter (after 72 hours) which enter uterine cavity by 4th day after fertilization.uterine cavity by 4th day after fertilization.

# # Morula is a mass of Morula is a mass of 1616 cells which differentiate cells which differentiate intointo inner cell inner cell mass (mass (formative massformative mass) and outer cell mass () and outer cell mass (trophoblasttrophoblast).).

2) Formation 2) Formation ofof blastocyst: blastocyst: ## Fluid; absorbed from uterine cavity and/or secreted by trophoblast; Fluid; absorbed from uterine cavity and/or secreted by trophoblast;

accumulates in morula. Many spaces appear between central blastomeres accumulates in morula. Many spaces appear between central blastomeres of morula and fuse together to form single cavity called of morula and fuse together to form single cavity called blastocoleblastocole..

## Now, morula is transformed into Now, morula is transformed into blastocystblastocyst, formed of 50-60 blastomeres , formed of 50-60 blastomeres and it lies in contact with uterine endometrium at 5and it lies in contact with uterine endometrium at 5thth- 6- 6thth day of day of fertilization.fertilization.

## Blastocyst loses its zona pellucida and has following features Blastocyst loses its zona pellucida and has following features:: 1) Two cell groups1) Two cell groups separated by blastocele separated by blastocele:: ** OuterOuter cell layer, cell layer, trophoblasttrophoblast, small cells forming wall of blastocyst , small cells forming wall of blastocyst (nutritive part of blastocyst).(nutritive part of blastocyst). ** InnerInner cell mass, cell mass, embryoblastembryoblast large cells on one pole of trophoblast large cells on one pole of trophoblast (embryo forming part of blastocyst).(embryo forming part of blastocyst). 2) Two poles:2) Two poles: ** Embryonic Embryonic pole overlies inner cell mass and is adjacent to uterinepole overlies inner cell mass and is adjacent to uterine endometrium.endometrium. ** Abemberyonic Abemberyonic pole is a away from uterine endometrium.pole is a away from uterine endometrium.

3) Differention 3) Differention ofof trophoblast trophoblast intointo two layers: two layers:## By 7 By 7th th day, trophoblast begins to split into an outer day, trophoblast begins to split into an outer syncytiotrophoblastsyncytiotrophoblast and and

an inner an inner cytotrophoblastcytotrophoblast..## Syncytiotrophoblast (over embryonic pole) begins to present many finger-like Syncytiotrophoblast (over embryonic pole) begins to present many finger-like

processes (processes (villivilli) , which invade uterine mucosa to begin process of ) , which invade uterine mucosa to begin process of implantation.implantation.

Applied:Applied:@ Embryonic coloning:@ Embryonic coloning: if two blastomeres are if two blastomeres are

separatedseparated from each other from each other in-vitroin-vitro, each one can grow , each one can grow to form a to form a separate separate embryoembryo..

@ Genetic coloning:@ Genetic coloning: a nucleus is isolated from a a nucleus is isolated from a somatic somatic cellcell (e.g. from buccal mucosa) of an individual. This (e.g. from buccal mucosa) of an individual. This nucleus (which contains all genetic characters of this nucleus (which contains all genetic characters of this individual) is implanted into an ovum in-vitro. Resulting individual) is implanted into an ovum in-vitro. Resulting cell is stimulated to divide by electric stimulation to form cell is stimulated to divide by electric stimulation to form a morula, which is implanted into uterus to grow into an a morula, which is implanted into uterus to grow into an embryo, which carries exactly genetic characters of embryo, which carries exactly genetic characters of individual from which somatic nucleus was taken. individual from which somatic nucleus was taken.

ImplantationImplantation@ Implantation @ Implantation is a process by which blastocyst is a process by which blastocyst penetratespenetrates

endometrium and becomes buried completely in it.endometrium and becomes buried completely in it.

@ Onset:@ Onset: It begins at end of first week and is completed at It begins at end of first week and is completed at end of second week of pregnancy.end of second week of pregnancy.

@ Steps:@ Steps:## Trophoblastic cells covering Trophoblastic cells covering embryonic poleembryonic pole adhereadhere to to

endometrium and secrets proteolytic enzymes, which create endometrium and secrets proteolytic enzymes, which create a a holehole through which blastocyst penetrates. through which blastocyst penetrates.

## Last part to penetrate is Last part to penetrate is abembryonic poleabembryonic pole..## By 10 By 10thth day, hole in endometrium is closed by a day, hole in endometrium is closed by a fibrin plugfibrin plug..

Sites:Sites:A) A) NormalNormal site site of of implantation: implantation: In upper part of posterior wall of uterus, near fundus.In upper part of posterior wall of uterus, near fundus.B) B) AbnormalAbnormal site ofsite of implantation: implantation:

1) 1) InIn uterus: uterus: Placenta praeviaPlacenta praevia, in lower uterine segment, near internal , in lower uterine segment, near internal

os.os.2) 2) OutsideOutside uterus (ectopic pregnancy): uterus (ectopic pregnancy):

* Tubal * Tubal pregnancypregnancy:: implantation in Fallopian tube, occurs due implantation in Fallopian tube, occurs due to early disappearance of zona pellucida. Tube usually to early disappearance of zona pellucida. Tube usually ruptures within 1-2 months leading to internal haemorrhage.ruptures within 1-2 months leading to internal haemorrhage.

* Ovarian * Ovarian pregnancypregnancy:: implantation in ovary. implantation in ovary.* Abdominal * Abdominal pregnancypregnancy:: implantation in peritoneal cavity. It implantation in peritoneal cavity. It

may be either primary or secondary (after repture of tubal may be either primary or secondary (after repture of tubal pregnancy).pregnancy).

Changes Changes inin blastocyst blastocyst, , during implanation in 2during implanation in 2ndnd week of week of

pregnancy)pregnancy)::@ Trophoblast @ Trophoblast completes its differentiation into: completes its differentiation into: ** Outer Outer syncytiotrophoblast. syncytiotrophoblast. ** Inner Inner cytotrophoblastcytotrophoblast..@ Embryoblast@ Embryoblast differentiates into: differentiates into: * Epiblast * Epiblast ((ectoderm). ectoderm). * Hypoblast* Hypoblast ( (endoderm).endoderm).

@ Two cavities @ Two cavities are formedare formed:: * * Amniotic cavity (dorsal to ectoderm). Amniotic cavity (dorsal to ectoderm). * * Primary yolk sac (ventral to Primary yolk sac (ventral to

endoderm).endoderm).