Embryo pt.4 Flashcards
What is fertilization?
The process in which gametes (an egg and a sperm cell)
fuse to form a zygote
What type of cell is a zygote?
- Totipotent stem cell, the only one able to give rise to any
cell type present in our body - It will preserve its totipotency till it starts to divide forming blastomeres, which will have a lower capability of differentiation
What is the blastocyst composed of?
Inner cell mass (embryoblast) and outer cells mass (trophoblast)
What does the trophoblast differentiate into?
Syncytiotrophoblast and cytotrophoblast
Which part of the blastocyst invades the uterus?
- Syncytiotrophoblast, the invading portion of the blastocyst
- It is able to produce digestive enzyme, which will be used to extend finger-like processes in the endometrium.
At which part of the cycle does implantation occur?
- Secretory phase
- Days 19-21 considered implantation window
- 6-12 days after ovulation, aka 8-9 days after conception
How long does it take from fertilization to implantation?
About one week (8-9 days)
What does gastrulation give rise to?
Gives rise to three layers, endoderm, mesoderm, and ectoderm
What are structures formed from mesoderm?
- Skeletal muscle
- Blood cells
- Lining of blood vessels
- Cardiac muscle
- Ducts and organs of the reproductive and excretory system
- Serosal lining of all body cavities
- ONLY IN TRUNK AND TORSO (excluding head, neck, and limbs): Source of all connective tissues, including cartilage, bones, tendons, dermis, and stroma (connective tissue) of internal organs
What are endoderm derivatives?
- Epithelial lining of:
- Respiratory tract
- Digestive tract (including glands opening to GI tract)
- Urinary bladder & urethra
-. Tympanic cavity & auditory tube
2. Parenchyma of thyroid, parathyroids, liver & pancreas
- Reticular stroma of tonsils & thymus
Ectoderm derivatives
- CNS and PNS (from neural crest cells)
- Epidermis
- Many connective tissues of the head
- Pigment cells
What does the epiblast form?
The epiblast gives rise to the three primary germ layers (definitive ectoderm, definitive/embryonic endoderm, and definitive mesoderm) and to the extraembryonic mesoderm of the visceral yolk sac, the allantois, and the amnion, and the ectoderm of amnion
What does the hypoblast form?
- Endoderm of umbilical vesicle which then becomes extraembryonic mesoderm
- Give rise to the tiny membrane covering the exocoelomic membrane
What is an important reaction in the mother that occurs during implantation?
Decidual reaction
What is decidualization?
Changes in the structure and function of the endometrium that facilitate implantation and growth of embryo
What are some changes that occur during decidualization and what’s their significance?
- Some cells accumulate glycogen (and some lipids), which the syncytiotrophoblast during invasion phagocytosis some of them, represent the first nutrition source for the embryo, before the development of the placenta and nutritional circulation
- There’s also a lot of infiltration of leukocytes, the decidual leukocytes play a role in suppressing the immune response of the mother to prevent treating the fetus as genetically foreign
What are the extraembryonic membranes? List them
- Temporary structures that will disappear with the end of pregnancy
- There are 4 extraembryonic membranes: Chorion , amnion, allantois, umbilical vesicle
What is the role of extraembryonic membranes?
IImportant for protection, nutrition and for removal of waste from the embryo
Which is the innermost and outermost extraembryonic membrane?
- Chorion is outermost
- Amnion is innermost
What is the amnion filled with? What produces it?
Amniotic fluid secreted by specific secretory cells of the amion
Where is the allantois located?
Between amnion and chorion
What is the first extraembryonic structure to develop?
Umbilical vesicle
What is the allantois derived from?
Yolk sac
What is the function of the allantois?
Has a limited function for waste removal.
What does the umbilical vesicle contribute to?
Very important during organogenesis as it contributes to the formation of the digestive system.
What is chorion made of?
Mesodermal germ cells
What is amion made of?
Ectoderm and mesoderm germ layers, so the primary epiblast cells are also found within the amnion
What gives rise to the umbilical cord?
Allantois and umbilical vesicle will become part of the umbilical cord
What is the chance of conception per cycle?
- In humans, the chance of conception per cycle is very low, about 30%
- More or less ⅔ of lost pregnancies occur because of problems during implantation.
What are the stages of implantation?
Can be divided into three stages:
- Apposition (blastocyst contacts the implantation site of the endometrium, first contact between the two)
- Adhesion (in which we should have high receptice endometrial epithelium that allows the blastocyst to attach, adhesion mechanisms are activated)
- Invasion (Invasive trophoblast cells cross the endometrial epithelial basement, and then digests endometrial stroma, physically invading the endometrium)
When does endometrial receptivity occur?
Restricted to a specific window (days 16-22 of a 28 days cycle, a couple of days following ovulation)
What drives endometrial receptivity?
The rise in progesterone
What are some soluble factors involved in implantation?
- Cytokines, growth factors, prostaglandins
- Leukemia inhibitory factor (LIF) and heparin-binding epidermal growth factor-like growth factor (HB-EGF) (both crucial for uterine receptivity and blastocyst activation)
What can be summarized as the most important factors for implantation?
- Implantation-competent blastocyst
- Uterine receptivity
What is the location of extrauterine implantations?
Most of them (>95%) occur in the fallopian tubes (70% ampulla, 12% Isthmic, 11% fimbrial, 2-3% interstitial) , rest other parts)
- Cervix (<1%), ovary (3%), C-section scar (<1%), intraligamentous or abdominal (1%).
What causes implantation outside of uterus?
- Contraction of smooth muscle cells and the movement of cilia is critical for an embryo to implant in uterus
- Problems that prevent the contraction of the muscle tissue or alterations of the cilia structure (which can be genetic or environmental) will lead to the blastocyst attached to the mucosa of the tubes
What is the best tool for diagnosing ectopic pregnancy?
The best diagnostic tool for ectopic pregnancy is the detection of embryonic heartbeat outside the uterine cavity using the US
What are risk factors for ectopic pregnancy?
- Previous ectopic pregnancy
- Previous fallopian tube surgery
- History of intrauterine device placement
- History of pelvic inflammatory disease
- Endometriosis
- History of smoking
What is the leading cause of first trimester maternal death?
Ectopic pregnancies due to life threatening hemorrhages (rupture of arcuate or uterine artery)
Describe vascularization of normal vs ectopic pregancy
- In the normal situation, the gestational sack implants very far from the main vascular supply of the endometrium
- In ectopic pregnancies (tubal in this scenario), the gestational sac can form around arcuate and uterine artery which can lead to massive hemorrhage if rupture
Describe the change of shape of the embryo from a blastocyst to gastrulation
Embryo transforms from a one-dimensional bilaminar disc)of epithelial cells (BLASTULA) into a multilayered and multidimensional structure called the GASTRULA
What does gastrulation mark the beginning of? What is the first sign of its initiation?
- Morphogenesis, the formation of structures, organs and systems
- The first morphologic sign of gastrulation is the formation of the primitive streak on the surface of the epiblast of the bilaminar embryonic disc
- It is also at this stage that we can distinguish which is the “right hand”, which is the “left hand”, which is the dorsal/ventral portion of the embryo i.e., we can distinguish the axial orientation of the embryo
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What is the order of the layers formed in gastrulation (chronological)?
Endoderm, then ectoderm, then mesoderm
Which direction does the primitive streak elongate? What happens then?
Towards the caudal end, eits cranial end proliferates to form the primitive node which contains a circular depression known as primitive pit
What happens if there is a defect during gastrulation?
Most defects are incompatible with life
How many genetic alterations have been seen during gastrulation?
More than 700 (almost 800) genotypes
What can genetic abnormalities in gastrulation be divided into?
- Abnormal embryo turning (over 400), embryo is not turning, is not folding in the right way
- Abnormal gastrulation movements
- Abnormal germ layer development (Over 200, abnormal ectoderm, endoderm, or mesoderm development)
- Delayed gastrulation (start later than the normal situation)
- Failure to gastrulate (entire failure of gastrulation, this is of course the most crucial situation)
What are examples of abnormal embryo turning?
- Abnormal direction of embryo turning
- Delayed embryo turning
- Failure of initiation of embryo turning (strictly related to the delay)
- Incomplete embryo turning – turning starts but stops prematurely
