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