Blastogenesis

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Cleavage

Zygote

Blastomeres

Morula

Blastocyst

Cleavage

Cleavage – mitotic division without cell growth and without proteosynthesis

Doughter cells (blastomeres) become smaller and embryo does not change its size, being surrounded by zona pellucida

Mitotic division is total and equal

Formation of 2 cells (24 hours)

4 cells (40 hours)

6 – 12 cells (3ED)

16 – 32 cells (4ED). This stage is called morula

Differentiation of blastomeres

Blastomeres start to differentiate on the stageconsisting of 8 cells – expression of adhesive proteins –

association and polarization of cells (compaction)

Tight and junction complex – junction of outer cell. Gap junction – junction of inner cell

Cells in centre – inner cell mass – embryoblast

Outer cells - trophoblast

Blastocyst cavity comprises fluid

Blastocyst – Embryonic pole

Vegetative pole

Blastocyst

Blastocyst cavity – from ED 4

ED5 - loss of zona pellucida – “hatching” - possibility of implantation

Cells of trophoblast express adhesive molecules –L-selectin - adhesion to the epithelial lining of uterus (secretory phase of menstrual cycle) –integrins (= receptors to laminin and fibronectin).

Imprinted genes, that are active from father, cause chorion development

Implantation

Adhesion to the endometrium – embryonic pole Trophoblast – proliferation and cell coalescence – syncytium

Cytotrophoblast – proliferating cells

Syncytiotrophoblast – loss of inter-cellular borders

Implantation – proteolytic enzymes (metalloproteinases) – cytotrophoblast

Syncytiotrophoblast – penetrates among epithelial cells = it pull embryo in the endometrium

Implantation

Implantation

Implantation ED 9 – blastocyst is implanted in mucosa

It is covered by the coagulation plug

Syncytiotrophoblast develops in contact places with maternal tissue, successively surrounds whole blastocyst

Maternal reaction to embryo – decidual reaction – secretion of mucus, glycogen accumulation and oedema

Decidua basalis, marginalis, capsularis and

parietalis

Immune reaction

Endometrium – immunologically preferred tissue

Progesterone – decreases nonspecific immune reaction – (complement)

Secretion of interleukin-2 – decreases specific reaction

Syncytiotrophoblast and cytotrophoblast does not express common antigens – or they are covered -both form barrier between maternal and fetal tissues

Cytotrophoblast shell – cells of cytotrophoblastpenetrate stem villi and form barrier between maternal and fetal connective tissues

Prenatal losses

Implantation tests embryos

Chromosomal abnormalities are the most frequent causes of spontaneous abortion

Immune reason - auto-antibodies

More that 50% embryos are unable to implant

Only 25 – 30% zygotes survive to birth

Development of embryoblast

Inner cell mass differentiates in two layers even before implantation - epiblast and hypoblast

Epiblast – columnar cells

Hypoblast – cuboidal cells

Amnion

Amniotic cavity (ED8) – appears among cells of epiblast –differentiation into amnioblasts and columnar cells of epiblast

Cavity is small, later surrounds whole embryo

Hypoblast

Cell proliferation and migration along the cytotrophoblast - (squamous epithelium) lining of blastocyst cavity – exocoelomic Heuser´s membrane

Blastocyst cavity is also called exocoelomic cavity or primary yolk sac

Extraembryonic reticulum (extracellular matrix) is produced between Heuser´s membrane and cytotrophoblast

ED12 – cells – extraembryonic mesoderm (probably from epiblast)

Exocoelomic vesicles – they fuse in new chorionic cavity – extraembryonic coelom – between yolk sac and cytotrophoblast

Cavity growth separates amnion and yolk sac from chorion – junction –connecting stalk

Mesoderm – visceral (splanchnopleura) and parietal (somatopleura) layers

Yolk sac

ED12 - hypoblast proliferates again – cells migrate along Heuser´s membrane –secondary definitive yolk sac (smaller than primary)

Primary yolk sac – exocoelomic vesicles -disappears

Function

Haematopoesis and development of vessels –vitellinne vasculature (blood islands)

Production of serum proteins, metabolism of nutrients

Germ cells (storage)

Embryonic nutrition Resource in embryo – for first 4 – 8 blastomers,

only

Nutrition – diffusion from fluid in uterine tube and uterus

Growth (proliferation) needs effective transport of gases (oxygen and carbon dioxide) and metabolites into the cells – development of vessels and placenta

Development of chorionic villi, cardiovascular system, contact with maternal blood

Development of chorion -placenta

Development of lacunae in trophoblast (ED9)lacunar phase

Contact with maternal vessels – blood into the lacunae – intervillous spaces

Vascular system needs adaptation for pregnancy – development of utero-placental vessels from spirale arteries (under influence of hormones) –otherwise IUGR develops

Implantation is regulated by hormones – takes place in the lamina functionalis, only

Development of chorion

Chorion frondosum, chorion laeve

Primary stem villi – syncytiotrophoblast and cytotrophoblast (ED 11 to 13)

Secondary stem villi – syncytiotrophoblast, cytotrophoblast, extraembryonic mesoderm (ED16)

Terciary stem villi (definitive) –syncytiotrophoblast, cytotrophoblast, extraembryonic mesoderm and vessels

(ED21)

Placenta

Fetal organ providing nutrition and other function to embryo:

Functions:

Metabolism (synthesis – glycogen)

Transport of gases and nutrients

Excretion of vaste products

Hormone production (hCG)

Structure

Fetal part – chorion

Chorionic plate

Chorionic villi

Maternal part

Endometrium – pars

functionalis-

decidua basalis

Structure

Placental shape – discoid (olliformis) + haemochorial

Placental septa – rests of decidua basalis. They separate placenta from maternalside in lobes - cotyledons

Cotyledons – 15-25) contain 2 and more anchoring villi

Diameter – 15 -20 cm, thickness 2-3 cm, weight 500 to 600 g

Placental villus

Function of trophoblast

Transport of respiratory gases

Transport of metabolites and electrolytes

Transport of maternal antibodies (IgG)

Production of hormones: progesteron, estriol, hCG, somatomammotropin (placental lactogen)

Transport

Many substances including drugs from

maternal blood may transfer placental barrier

Nutrients – glucose, AK, fatty acids, water, vitamins, electrolytes

Hormones – only steroid nonconjugated

Maternal antibodies, transferrin+ iron

Placental transport

Diffusion

Facilitated diffusion

Active transport

Pinocytosis

Other types of transfer:

Damage of placental barrier – blood cells

Own activity – Treponema pallidum

Damage due to infection - toxoplasmosis

Ectopic implantation

Abdominal cavity (Douglas pouch –retro-uterine cavity), ovary (primary ovarian pregnancy), uterine tube (95%) - most frequent in ampulla, intestitial – in uterine horn

Placenta previa

Placenta accreta, percreta – penetrates into the zona basalis and myometrium

Twins

Dizygotic versus monozygotic

Dichorial x monochorial

Diamniotic x monoamniotic

Conjointed twins

Twins