Coition fertilisation and blastocyst development Flashcards

Question

what happens to the sperm in the epididymis?

Answer 1

* **Loss of the cytoplasmic droplet.** * **Increased concentration of sperm** due to fluid absorption in vas efferentia and epididymis * **Nuclear condensation** (replacement of histones with protamines) and **acrosomal remodeling** * **Metabolic changes** including the selective metabolism of cholesterol and phospholipids, and * **increased dependence on external fructose** for glycolytic energy production * **Motility** is achieved **due to a rise in cAMP** content in the tail flagellum, which becomes more rigid due to increased disuphide bonds. * The **membrane surface becomes coated with glycoproteins** (aid in sperm-oocyte interactions).

Answer 2

Motility is achieved due to a rise in cAMP content in the tail flagellum, which becomes more rigid due to increased disuphide bonds.

Answer 3

When they reach the cauda epididymis, sperm have the potential to be motile and fertilise oocytes.

Answer 4

Dry orgasm – normal erection and orgasm but impaired emission

Answer 5

* eSpermatocrit (100x) * Final morphological changes * Capacity to move (but immotile): tail flagellumèrigid (écAMP) • Metabolic changes (fructose, cholesterol, phospholipids) • Stabilising glycoproteins

Answer 6

Seminal fluid is derived from major accessory sex glands (e.g., seminal vesicle, prostate gland, Cowper’s gland) with only a small contribution of fluid from the epididymis.

Answer 7

The Cowper’s gland (bulbourethral gland) secretes lubricant in the pre-ejaculate and the ejaculate

Answer 8

bad - no fertility

Answer 9

Activity of accessory glands depends on **androgens** (e.g., 5-DHT) since *castration* (removal of the testicles) and *hypophysectomy* (removal of the pituitary gland) inhibit normal function of the prostate and seminal vesicle (see practical notes).

Answer 10

Seminal plasma is **not** essential for sperm function but rather provides; a vehicle for sperm transport, nutritional factors (e.g., fructose), a buffering agent to counteract the acid pH of the vaginal fluids, and anti-oxidants (e.g., ascorbate, hypoteurine).

Answer 11

It also contains prostaglandins, which might stimulate vaginal muscle contractions.

Answer 12

it may include potentially infectious agents (e.g., hepatitis B or C virus, HIV, HPV) and leucocytes. T

Answer 13

The presence of leucocytes is normal but high levels may indicate a urinary tract infection or infertility.

Answer 14

With further stimulation, emission occurs when the **prostate, vas deferens and seminal vesicle muscles contract** and components of the seminal plasma with spermatozoa are expelled into the urethra.

Answer 15

With further stimulation, emission occurs when the prostate, vas deferens and seminal vesicle muscles contract and components of the seminal plasma with spermatozoa are expelled into the urethra. **Noradrenergic sympathetic fibres** via the **hypogastric plexus** mediate this process.

Answer 16

Passage of semen back into the bladder is prevented by **contraction of the vesicular urethral sphincter.**

Answer 17

Retrograde ejaculation – failure of the urethral sphincter causing ejaculate to enter the bladder.

Answer 18

Pre-ejaculate 􏰀 Cowper’s gland Early ejaculate – prostate Mid-ejaculate – vas deferens Late ejaculate – seminal vesicle

Answer 19

The early fraction is rich in acid phosphatase from the prostate, the mid-fraction is rich in spermatozoa from the vas deferens, and the late fraction is rich in fructose from the seminal vesicles

Answer 20

Ejaculate **coagulates** when the components are mixed together in the vagina to ensure that sperm is retained, and liquefies again within 20-60 min.

Answer 21

Vaginal lubrication occurs by **transudation** of fluid through the vaginal wall as the vagina expands and the labia majora become engorged with blood.

Answer 22

With increased stimulation, the width and length of the vagina increase further and the uterus elevates upwards into the false pelvis, lifting the cervical os to produce the so-called tenting effect.

Answer 23

Successful sperm likely enter the cervix through their own propulsion and with the help of ciliated cells of the cervical os that waft the sperm towards the cervical canal

Answer 24

Vaginal and uterine muscle contractions during orgasm are not required to propel sperm into the cervix.

Answer 25

Mostly closed to prevent infection **Oestrogen in follicular phase:** muscles relax = opening epithelium secrete watery mucus allows easy entry for sperm **during progesterone / luteal phase:** cervix firmer closed os thick mucus - preventing spermies

Answer 26

First sperm in fallopian tube after ~5 min

Answer 27

Viable spermatozoa can survive between 28-48 hours in the female genital tract.

Answer 28

About 20% of women experience bleeding into the peritoneal cavity caused by ovulation leading to Mittelschmerz pain

Answer 29

The attainment of full fertilising capacity of sperm is achieved within the female reproductive tract and is called sperm capacitation.

Answer 30

Hyperactivated motility defined by whiplash beats of the sperm tail (compared to wave-like beats). This is promoted by the opening of Ca2+ channels to activate PKA. Changes in the surface membrane properties to allow the sperm to become sensitive to local signals sent out by the oocyte culminating in sperm activation. This increases the fusogenicity with the oocyte.

Answer 31

The oestrogen-primed uterus or oviductal isthmus is optimal for sperm capacitation

Answer 32

capacitation involves the **stripping of glycoproteins that coat the spermatozoal surface.** This is associated with a change in the surface charge and lipid structure of the sperm membrane, especially the loss of cholesterol. In response to an influx of Ca2+, cAMP is generated and PKA is activated followed by the phosphorylation of tyrosines on a number of proteins.

Answer 33

Remarkably, capacitation is experimentally reversible (decapacitation) by re-incubating the sperm in epididymal fluid or seminal plasma. In a capacitated state, the sperm is unstable and will die if it does not find an oocyte fairly rapidly.

Answer 34

the terminal phase of capacitation and only occurs once the sperm comes into contact with the cumulus cells that surround the oocyte or the oocyte itself.

Answer 35

Activation is induced by binding directly to the zona pellucida (ZP) proteins on the oocyte. The acrosome swells and its membrane fuses with the overlying plasma membrane, resulting in the exteriorization of the contents of the acrosomal vesicle (e.g., hyaluronidase that is required to digest the matrix between the cumulus cells to expose the zona pellucida) and the inner acrosomal membrane including acrosin (enzyme that digests zona pellucida).

Answer 36

Sperm tail movement changes from wave motion to hyperactive whiplash promoted by the activation of PKC.

Answer 37

proteins on zona pellucida induce acrosome reaction 4 ZPs in humans - esential for blocking corss species fertilisation ZP3 is essential for sperm binding

Answer 38

* sperm binds zona pellucida * penetrates zona pellucida * can take 5-20 minutes * At this point, the sperm enters the perivitelline space.

Answer 39

**Izumo1** on the sperm surface binds to **Juno** on the oocyte allowing **CD9** to accumulate in the binding area to become a binding partner of **Juno**

Answer 40

* after fusion * big rise in intracellular calcium * last 2-3 minutes - occurs in waves - starting at where sperm entered * critical for subsequent events * egg called a zygote now

Answer 41

the elevated calcium levels resulting from oocyte activation lead to the **cortical reaction:** * a release of the contents of subcortical granules into the perivitelline space, which prevent other sperm from binding the zona pellucida. * The release of ovastatin allows for the cleavage of ZP3/4 and ZP2, thus removing the ZP sperm binding properties. * ZPs also become cross-linked to make the zona pellucida impenetrable. * Juno is also exocytosed off the oocyte membrane.

Answer 42

At ovulation, oocyte arrested in **2nd meiotic metaphase**

Answer 43

The oocyte must also complete its second meiotic division. Remember that the oocyte is ovulated while it is arrested in its second meiotic division and therefore, has **two haploid** sets of chromosomes. To transmit only one set to the next generation, it must complete its second meiotic division and extrude the **second polar body.** The failure to do this leads to **gynogenetic triploidy.**

Answer 44

The sperm nuclear membrane breaks down and the highly condensed chromatin starts unwind as **protamines** are replaces with **histones**.

Answer 45

The sperm nuclear membrane breaks down and the highly condensed chromatin starts unwind as protamines are replaces with histones. This process is actively induced by oocyte factors. *The each set of haploid chromosomes becomes surrounded by distinct membranes to form the paternal and maternal pronuclei.*

Answer 46

Syngamy is the final phase of fertilisation involving the coming together of gametic chromosomes occurs. Immediately, the first cell division occurs so that the one-cell zygote becomes a two-cell conceptus (embryo).

Answer 47

yes - in preparation for the first mitotic division

Answer 48

the sperm contributes the centriole and pericentriolar material to make up the centrosome, which is important for cytokinesis at mitosis and without it cellular division fails. Recent research has shown that the sperm also contribute non-coding RNA to the zygote.

Answer 49

These RNAs are sensitive to environmental stressors (e.g., diet) and have the potential to influence gene expression in the early embryo and disease risk in the adult

Answer 50

Paternal proteins from the sperm are ubiquinated and broken down.

Answer 51

Male spindle retrieves female pronucleus and brings both pronuclei to equator

Answer 52

Sperm mitochondrial (mt) DNA is susceptible to mutation through oxidative stress (theory) * Natural exposure to reactive oxygen species * No histones in mtDNA for protection * No efficient DNA repair mechanisms in sperm Poor quality sperm: mutated mtDNA

Answer 53

Other aneuploidy conceptuses may develop as tumors, such as hydatidiform mole (benign trophoblastic tumor) A hydatidiform mole is a growing mass of tissue inside your womb (uterus) that will not develop into a baby. It is the result of abnormal conception. It may cause bleeding in early pregnancy and is usually picked up in an early pregnancy ultrasound scan. It needs to be removed and most women can expect a full recovery

Answer 54

Disorders of ploidy are likely to result from failure of polar body formation, polyspermy, or failure of one early cleavage division.

Answer 55

A parthenogenetic conceptus (parthenote) contains chromosomes from only one parent.

Answer 56

Human parthenogenetic embryos do not survive to birth because development fails during the first cleavage due to a **lack of centriole inherited from the sperm**. In mice, gynogenetic conceptuses only form embryonic lineages but die early due to a lack of placental tissue.

Answer 57

down the fallopian tube into the uterus with the help of smooth muscle contractions and ciliated cells on the fallopian tube. Embryo transport requires oestrogen and progesterone, and may be obstructed by infection or scarring of the fallopian tube

Answer 58

Therefore, pregnancy is not initiated with fertilisation, but only when the conceptus has signaled its presence to the mother.

Answer 59

in fallopian tube: ## Footnote (ICM versus trophectoderm)

Answer 60

compaction

Answer 61

increased cell-cell adhesion that transforms cells from non-polar (radially symmetrical) to highly polarized some cells become localized to the inside of the embryo and some remain on the outside. This polarization and positioning initiates the first differentiation event and lineage segregation.

Answer 62

During pre-implantation development, the embryo is surrounded by the **zona pellucida.**

Answer 63

It protects the developing embryo by ensuring that the cells do not fall apart and to prevent premature implantation into the fallopian tube or uterus. When the blastocyst matures, it hatches out of the zona pellucida and is free to interact with the uterus.

Answer 64

* Tight junctions on outer cells create diffusional ion gradient * Cause influx of water into embryo to form blastocoel cavity

Answer 65

1) an outer rim of trophoblast cells called the trophectoderm surrounding the blastocoelic cavity, and 2) an inner group of pluriopotent cells that form the inner cell mass.

Answer 66

yes Instead they will form the trophoblast cells of the placenta and are concerned with the nutrition and support of the embryo/fetus.

Answer 67

For example, the genes OCT4, NANOG, and GATA6 are transcription factors whose expression is restricted to the inner cells of the compact morula and early blastocyst.

Answer 68

the expression of transcription factors, such as CDX2, GATA3, and EOMES, is restricted to the outer cells and are required to specify the trophectoderm cell lineage.

Answer 69

initally all cells of 8 cell embryo ## Footnote This restriction is, in part, dependent upon the degree of cell-cell contact. For example, the kinase Hippo is regulated by **degree of cell-cell contact**. In non-polarized cells, Hippo is active and phosphorylates Yap (a transcription factor) preventing it from entering the nucleus. Since a main function of Yap is to activate Cdx2 gene expression, Cdx2 remains off in these cells (i.e., inner cell mass). In polarized cells (the trophectoderm), Hippo expression is suppressed and therefore, Yap can enter the nucleus to initiate Cdx2 expression and trophectoderm cell fate.

Answer 70

The pre-implantation embryo obtains nutrients from secretions from the fallopian tubes and the endometrial glands of the uterus

Answer 71

By the 8-cell stage, just prior to compaction, the embryo depends on glucose and essential amino acids for energy.

Answer 72

hatch from the zona pellucida

Answer 73

Hatching requires proteolytic enzymes secreted from the trophectoderm and possibly from the uterine secretions to dissolve a hole in the zona pellucida for the blastocyst to squeeze out.

Answer 74

near the fundus of the uterus ## Footnote This can occur in a posterior (55% of pregnancies) or anterior (44% of pregnancies) position

Answer 75

Abnormal implantation occurs when the blastocyst implants on or very close to the cervix resulting in placenta praevia (0.5% of pregnancies).

Answer 76

Ectopic implantation occurs in regions other than in the uterine lumen including in the ovary, Fallopian tube, Caesarean scar, abdominally, or cervically. T his is painful for the woman and results in embryonic lethality.

Answer 77

The uterus is receptive during the progesterone dominance phase (luteal phase) of the menstrual cycle.

Answer 78

Progesterone (from corpus luteum) and oestrogen (from ovary) are required to prime the uterine lining for implantation

Answer 79

Oestrogen is also responsible for preparing the conceptus for implantation

Answer 80

During the non-receptive phase, the epithelial cells of the uterine endometrium are resistant to embryo adhesion. The cells have a **thick mucin (MUC1) glycoprotein coat**, **long microvilli** and a **negative surface charge** that prevent the blastocyst from adhering.

Answer 81

However, during the receptive window, the blastocysts able to adhere due to a thinner mucin coat, a loss of negative charge, the expression of LIF acting as a ‘come hither’ signal, and shortened microvilli on the endometrial epithelial cell surface.

Answer 82

L-selectin on the surface of the trophectoderm is important for this interaction. It binds to the L-selectin ligand on epithelium. This promotes the secretion of human chorionic gonadotropin by the trophoblast cells, which signals to the epithelium to secrete trophinin and breakdown the MUC1 glycocalyx. blastocyst to more easily interact with molecules on the surface of the uterine epithelial cells (e.g., integrins) required for adhesion.

Answer 83

It causes an up-regulation of HB-EGF (heparin binding epidermal growth factor-like growth factor) on the uterine epithelium, which binds to HSPG (heparine sulfate proteoglycan) on the trophectoderm. LIF also induces the expression of adhesion molecules, such as integrins on the uterine epithelium and the trophectoderm.

Answer 84

1) containment and control to keep the trophoblastic invasion in check (TIMPs); and 2) incitement and encouragement to promote the invasive events (MMPs). There is no evidence of epithelial cell death caused by trophoblastic invasion.

Answer 85

This causes the endometrium to become a nutrient packed, highly vascular tissue called the decidua. This is similar to a ‘proinflammatory endometrial reaction’ because prostaglandins are involved. Maternal stromal cells of the endometrium, which are spindly, are converted to maternal decidual cells that are large and round due to the accumulation of lipids and glycogen. Decidual cells secrete cytokines including IGF binding proteins and prolactin that aid the growth of the conceptus. An important feature of the decidual response is the increased vascular permeability and angiogenesis. Some decidualisation occurs in the absence of an implanted embryo in the secretory phase of the menstrual cycle.

Answer 86

By day 11, the conceptus has completely embedded itself into the uterine wall, which allows it access to uterine glands and uterine spiral arteries as the placenta begins to grow

Answer 87

the **decidua basalis** (‘beneath the conceptus’; closest to the myometrium, and becomes the maternal face of the mature placenta), the **decidua capsularis** (‘over the conceptus’; covers the protruding side of the conceptus and will disintegrate over time), and the **decidua parietalis** (the remainder of the maternal decidua).

Answer 88

At the end of a normal menstrual cycle, progesterone levels drop allowing for the shedding of the endometrial lining. This is because **decidual cells cannot de-differentiate back into stromal cells**, and as a result, the endometrial lining is newly developed with each menstrual cycle.

Answer 89

During implantation in humans, the trophoblast cells secrete a hormone called human chorionic gonadotropin (hCG). This maintains the corpus luteum and therefore, progesterone levels remain suitably high enough to prevent a new menstrual cycle.

Answer 90

By 10 weeks of pregnancy, the placenta secretes sufficient progesterone and the ovarian progesterone is no longer required. Of note, hCG is the chemical detected by most pregnancy tests since it is secreted in the urine and is only present during early pregnancy.

Answer 91

Pregnancy can be defined **biochemically** (based on the detection of hCG levels in urine or blood) or **clinically** after the first missed menstrual period and ultrasound evidence of a conceptus.

Answer 92

While the reason behind pre-clinical and clinical miscarriage is not completely understood, some explanations might be: 1. chromosomal abnormalities, 2. physical factors, 3. atypical uterine environment, 4. and fertilization errors that might be due to separation of coitus from ovulation (old oocytes or sperm). 1-2% of couples experience recurrent pregnancy loss (three or more consecutive miscarriages) likely due to an endometrial defect since the incidence of genetic abnormalities is much lower in these conceptuses.

Answer 93

mamba well done