EMBROLOGYProf .dr mohsin jahmbbs, frcs, fcps

WEEKLY DEVEOLMENT

Ovulation Occurs in Two Phases

THE FIRST PHASE of the ovulation cycle is the follicular phase. It begins the first day of the last menstrual period (LMP) and stops at the next ovulation. This phase varies greatly for each woman.

THE SECOND PHASE is the luteal phase. It begins the day of ovulation and lasts until the next menstrual cycle begins. Governed by hormone release, it follows a more regular timeline of between 12 to 16 days following ovulation.

Where are the Stem Cells?

The blastocyst embryo with its two cell types: the inner cells that can initiate all cells of the body, and the outer cells destined to become the placenta, is the source of embryonic stem cells (esc).

Embryonic Stem Cells (esc) have the potential to become any type of human body cell. Stem cells are the only cells of the body which divide into two unique entities. Typically when cells divide they produce an identical cell or "sister" cell. But stem cells make a new stem cell AND a new body type cell. This property makes them extremely unique.

Blastocysts only become available for use as stem cells when donated by couples using in-vitro fertilization (IVF) techniques.

A woman in an IVF program who has multiple eggs fertilized in a petri dish, can choose to donate extra blastocysts to research. These stem cells are useful to children and adults needing interventions beyond conventional medicine.

WEEK 1

Fertilization Cleavage Formation of the blastocyst Differentiation into the

Cytotrophoblast / Syncytiotrophoblast

Coitus & Sperm Transfer

2-3 phases– Erection/engorgement:

Parasympathetic reflex– Plateau– Orgasm: Sympathetic reflex

Heart rate, contractility increase (pulse & BP)

Intense pleasure Ejaculation by male

Embryology Review

Week 1– Fertilized Egg– Day 1.25: Two Cells

Cleavage Compaction

– Day 3: Morula Inner Cell Mass Trophoblasts

– Day 5: Blastocyst Cavitation Hatching

– Day 6: Implantation

PHASES OF FERTILIZATION DEFINITIONSSets

1.penetrate corona radiata,

2. penetrate zona pellucida,

3.penetrate oocyte membrane

.1 passage of sperm through corona radiata 2. penetration of zona pellucida 3. fusion of plasma membranes of oocyte and sperm 4. completion of second meiotic division of oocyte, formation of female pronucleus 5. formation of male pronucleus 6. pronuclei fuse to form zygote

Fertilization

Contact of sperm & secondary oocyte occurs in uterine tube– Sperm

penetration– Oocyte completes

meiosis II– Nuclear fusion

Fertilization: Sperm produces

hyaluronidase to penetrate the follicular cell layer of the corona radiata (see diagram below). It will then interact with only one of many receptors.

At a ZP3 receptor, the head of the sperm releases its contents (acrosin) and burrows through the zona pellucida and perivitelline space - the acrosomal reaction.

Fertilization

Sperm penetration

Why so many?

Passage Into the Fallopian Tube

Once the egg is released from the ovary, it travels into the fallopian tube where it remains until a single sperm penetrates it during fertilization

The Laborious Journey of the Sperm

An average ejaculate discharges 40-150 million sperm which eagerly swim upstream toward the fallopian tubes on their mission to fertilize an egg. Fast-swimming sperm can reach the egg in a half an hour, while other may take days.

The sperm can live up to 48-72 hours. Only a few hundred will even come close to the egg, due to the many natural barriers and hurdles that exist in the female reproductive tract.

Fertilization: Sperm Penetrates Egg

If a sperm cell meets and penetrates an egg, it will fertilize the egg. The fertilization process takes about 24 hours. When fertilization happens, changes occur on the surface of the egg to

prevent other sperm from penetrating it. At the moment of fertilization, the genetic makeup is complete, including the sex of the infant.

Once a receptor has been activated, a series of reactions to prevent polyspermy (the entrance of more than one sperm).will be initiated. –First, the cell surface will be

depolarized. –Then, cortical granules

(lysosomes) released into the perivitelline space will hydrolyze the other receptors.

Fusion of the plasma membranes of the oocyte and sperm occurs; the sperm nucleus is released into the cytoplasm of the oocyte; the rest of the sperm degenerates

Entrance of sperm into the oocyte causes the secondary oocyte to complete its second meiotic division (2 polar bodies at this point)

Male pronucleus forms and swells; pronuclei membranes become porous

Both the male and female pronuclei are essential for normal development. Evidence for this exists when one or the other is absent, as in the case of a hydatiform mole.

Anatomy – Corona radiata – Cone of attraction and Vitelline

membrane – Acrosome reaction – Zona pellucida

Cortical reaction

Fusion – Cell membranes – Transformations – Replication – Mitosis

Diseases

CORONA RADIATA The corona radiata surround an ovumor unfertilized egg cell, and consist of two or three strata (layers) of follicular cells. They are attached to the outer protective layer of the ovum, the zona pellucida, and their main purpose in many animals is to supply vital proteins to the cell.

They are formed by follicle cells adhering to the oocyte before it leaves the ovarian follicle, and originate from the squamous granulosa cells present at the primordial stage of follicular development. The corona radiata is formed when the granulosa cells enlarge and become cuboidal, which occurs during the transition from the primordial to primary stage.These cuboidal granulosa cells, also known as the granulosa radiata, form more layers throughout the maturation process, and remain attached to the zona pellucida after the ovulation of the Graafian follicle.

Cone of attraction and Vitelline membrane

Where the spermatozoon is about to pierce, the yolk (ooplasm) is drawn out into a conical elevation, termed the cone of attraction.

Once the spermatozoon has entered, the peripheral portion of the yolk changes into a membrane, the vitelline membrane, which prevents the passage of additional spermatozoa.

Acrosome reaction

The acrosome reaction must occur to mobilise enzymes within the head of the spermatozoon to degrade the zona pellucida. example: hyaluronidase.

Cortical reaction Once the sperm cells find

their way past the zona pellucida, the cortical reactionoccurs: cortical granules inside the secondary oocyte fuse with the plasma membrane of the cell, causing enzymes inside these granules to be expelled by exocytosis to the zona pellucida. This in turn causes the glyco -proteins in the zona pellucida to cross-link with each other—that is, the enzymes cause the ZP2 to hydrolyseinto ZP2f—making the whole matrix hard and impermeable to sperm. This prevents fertilization of an egg by more than one sperm.

Fusion

After the sperm enters the cytoplasm of the oocyte, the cortical reaction takes place, preventing other sperm from fertilizing the same egg. The oocyte now undergoes its second meiotic division producing the haploid ovum and releasing a polar body.

The sperm nucleus then fuses with the ovum, enabling fusion of their genetic material.

capacitation To become competent to accomplish these tasks, ejaculated

mammalian sperm must normally be modified by conditions in the female reproductive tract, a process called, which requires about 5–6 hours in humans.

Capacitation is triggered by bicarbonate ions (HCO3–) in the vagina, which enter the sperm and directly activate a soluble adenylyl cyclase enzyme in the cytosol.

The cyclase produces cyclic AMP, which helps to initiate the changes associated with capacitation.

Capacitation alters the lipid and glycoprotein composition of the sperm plasma membrane, increases sperm metabolism and motility, and markedly decreases the membrane potential (that is, the membrane potential moves to a more negative value so that the membrane becomes hyperpolarized).

Once a capacitated sperm has penetrated the layer of follicle cells, it binds to the zona pellucida The zona usually acts as a barrier to fertilization across species, and removing it often eliminates this barrier.

Cell membranes

The cell membranesof the secondary oocyte and sperm fuse.

Transformations In preparation for the fusion of their genetic

material both the oocyte and the sperm undergo transformations as a reaction to the fusion of cell membranes.

The oocyte completes its second meiotic division. This results in a mature ovum.

The nucleus of the oocyte is called a pronucleusin this process, to distinguish it from the nuclei that are the result of fertilization.

The sperm's tail and mitochondria degenerate with the formation of the male pronucleus.

This is why all mitochondria in humans are of maternal origin.

Replication

The pronuclei migrate toward the centre of the oocyte, rapidly replicating their DNA as they do so to prepare the embryo for its first mitotic division.

Mitosis

The male and female pronuclei don't fuse, although their genetic material do.

Instead, their membranes dissolve, leaving no barriers between the male and female chromosomes.

During this dissolution, a mitotic spindle forms between them.

The spindle captures the chromosomes before they disperse in the egg cytoplasm.

Upon subsequently undergoing mitosis (which includes pulling of chromatids towards centrioles in anaphase) the cell gathers genetic material from the male and female together. Thus, the first mitosis of the union of sperm and oocyte is the actual fusion of their chromosomes.

Each of the two daughter cells resulting from that mitosis has one replica of each chromatid that was replicated in the previous stage. Thus, they are genetically identical.

Spindle microtubules are stained in green with anti- tubulin antibodies, and DNA is labeled in blue with a DNA stain.

(A) A meiotic spindle in a mature, unfertilized oocyte.

(B) This fertilized egg is extruding its second polar body and is shown about 5 hours after fusion with a sperm. The sperm head (left) has nucleated an array of microtubules.

(C) The two pronuclei have come together.

D) By 16 hours after fusion with a sperm, the centrosome that entered the egg with the sperm has duplicated, and the daughter centrosomes have organized a bipolar mitotic spindle. The chromosomes of both pronuclei are aligned at the metaphase plate of the spindle. As indicated by the arrows in (C) and (D), the sperm tail is associated with one of the centrosomes.

Summary

Mammalian fertilization begins when the head of a sperm binds in a species-specific mannerto the zona pellucida surrounding the egg.

This induces the acrosome reactionin the sperm, which releases the contents of its acrosomal vesicle, exposing enzymesthat help the spermto digest its way through the zonato the egg plasma membranein order to fuse with it.

The fusion of the sperm with the egg induces a Ca2+ signal in the egg. The Ca2+ signal activates the egg to undergo the cortical reaction, in

which cortical granules release their contents, including enzymes that alter the zona pellucida and thereby prevent the fusion of additional sperm.

The Ca2+ signal also triggers the development of the zygote, which begins after sperm and egg haploidpronuclei have come together, and their chromosomes have aligned on a single mitotic spindle, which mediates the first division of the zygote.

Diseases

Various disorders can arise from defects in the fertilization process.

Polyspermy results from multiple sperm fertilizing an egg.

However, some researchers have found that in rare pairs of fraternal twins, their origin might have been from the fertilization of one egg cell from the mother and two sperm cells from the father. This possibility has been investigated by computer simulations of the fertilization process.

Early Development

Cleavage in uterine tube

cleavage In embryology, cleavage is the division of cells in the

early embryo. The zygotes of many species undergo rapid cell cycles with no

significant growth, producing a cluster of cells the same size as the original zygote.

The different cells derived from cleavage are called blastomeres and form a compact mass called the morula.

Cleavage ends with the formation of the blastula. Depending mostly on the amount of yolk in the egg, the cleavage can be

holoblastic (total or entire cleavage) or meroblastic (partial cleavage).

The pole of the egg with the highest concentration of yolk is referred to as the

vegetal pole while the opposite is referred to as the animal pole.

Cleavage differs from other forms of cell division in that it increases the number of cells without increasing the mass. This means that with each successive subdivision, the ratio of nuclear to cytoplasmic material increases.

The Cells Begin to Divide The fertilized egg

begins dividing rapidly, growing into many cells. It leaves the fallopian tube and enters the uterus three to four days after fertilization. Rarely, the fertilized egg does not leave the fallopian tube; this is called a tubal pregnancy or ectopic pregnancy and is a danger to the mother.

Cleavage

•Following mitosis, two blastomeres form, each one totipotent (capable of forming its own organism) •Continuing along the uterine tube, the zygote divides to four, then eight cells •Size of individual blastomeres decreases with progressive divisions as zygote maintains size

•A ball of 12 or more cells(32), the morula, enters the uterus around day 3

 

Mechanism

Types of cleavage – Determinate – Indeterminate – Holoblastic – Meroblastic

Mammals

Mechanism The rapid cell cycles are facilitated by

maintaining high levels of proteins that control cell cycle progression such as the cyclins and their associated cyclin-dependent kinases (cdk).

The complex CyclinB /cdc2 a.k.a. MPF (maturation promoting factor) promotes entry into mitosis.

The processes of karyokinesis (mitosis) and cytokinesis work together to result in cleavage. The mitotic apparatus is made up of a central spindle and polar asters made up of polymers of tubulinprotein called microtubules.

The asters are nucleated by centrosomes and the centrosomes are organized by centrioles brought into the egg by the sperm as basal bodies. Cytokinesis is mediated by the contractile ring made up of polymers of actinprotein called microfilaments. Karyokinesis and cytokinesis are independent but spatially and temporally coordinated processes. While mitosis can occur in the absence of cytokinesis, cytokinesis requires the mitotic apparatus.

The end of cleavage coincides with the beginning of zygotic transcription. This point is referred to as the midblastula transition and appears to be controlled by the nuclear: cytoplasmic ratio (about 1/6).

Types of cleavageDETERMINATE

Determinate is the form of cleavage in most protostomes.

It results in the developmental fate of the cells being set early in the embryo development. Each cell produced by early embryonic cleavage does not have the capacity to develop into a complete embryo.

INDETERMINATE

A cell can only be indeterminate if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is characteristic of deuterostomes - when the original cell in a deuterostome embryo divides, the two resulting cells can be separated, and each one can individually develop into a whole organism.

HOLOBLASTIC In the absence of a large concentration of yolk, four major

cleavage types can be observed in isolecithal cells (cells with a small even distribution of yolk) or in mesolecithal cells (moderate amount of yolk in a gradient) –– bilateral holoblastic, – radial holoblastic, – rotational holoblastic, and– spiralholoblastic, cleavage.T

These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis. Coeloblastula is the next stage of development for eggs that undergo these radial cleavaging.

In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms.

MEROBLASTIC

In the presence of a large amount of yolk in the fertilized egg cell, the cell can undergo partial, or meroblastic, cleavage.

Two major types of meroblastic cleavage are discoidal and superficial.– DISCOIDAL

In discoidal cleavage, the cleavage furrows do not penetrate the yolk. The embryo forms a disc of cells, called a blastodisc, on top of the yolk. Discoidal cleavage is commonly found in monotremes, birds, reptiles, and fish that have telolecithal egg cells (egg cells with the yolk concentrated at one end). Superficial

– I SUPERFICIAL CLEAVAGE, mitosis occurs but not cytokinesis, resulting in a polynuclear cell. With

the yolk positioned in the center of the egg cell, the nuclei migrate to the periphery of the egg, and the plasma membrane grows inward, partitioning the nuclei into individual cells. Superficial cleavage occurs in arthropods that have centrolecithalegg cells (egg cells with the yolk located in the center of the cell).

Cleavage

BLASTOCYST

The is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula.

It possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid-filled blastocyst cavity known as the blastocoele or the blastocystic cavity. The human blastocyst comprises 70-100 cells.

Blastocyst formation begins at day 5 after fertilization in humans, when the blastocoele opens up in the morula, a process known as hatching.

Formation of the Blastocyst

Compaction: Blastomeres clump together; the amount of cytoplasm is reduced

Adhesion: E- cadherin gene is expressed; these proteins facilitate intercellular adhesion and communication

Zona pellucida is shed, allowing for cell growth and for uterine fluid to infiltrate

A fluid-filled cavity ( blastocoele) forms in the center, pushing cells to the periphery

The outer cell layer, the trophoblast ("tropho" - to nourish), may have a nutritive role as the future embryonic part of the placenta

The inner cell mass, the future embryo, forms as a ball of cells on one side of the spherical blastocyst

Parts of the blastocyst Formation of the blastocyst

Parts of the blastocyst The blastocyst consists of two primary cell

types: the inner cell mass, also known as the

"embryoblast" (this part of the embryo is used in stem cell research)

the trophoblast, a layer of cells surrounding the inner cell mass and the blastocyst cavity (blastocoele)

The former is the source of embryonic stem cells and gives rise to all later structures of the adult organism.

The latter combines with the maternal endometrium to form the placenta in eutherian mammals

Inner cell mass

In early embryogenesisof most eutherian mammals, the inner cell mass (abbreviated ICMand also known as the embryoblast or pluriblast, the latter term being applicable to all mammals) is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus.

This structure forms in the earliest steps of development, before implantation into the endometrium of the uterus has occurred.

The ICM lies within the blastocoele(more correctly termed "blastocyst cavity," as it is not strictly homologous to the blastocoele of anamniote vertebrates) and is entirely surrounded by the single layer of cells called trophoblast

Trophoblasts (from Greek trephein:to feed, and blastos: germinator)

are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. They are formed during the first stage of pregnancyand are the first cells to differentiate from the fertilized egg. This layer of trophoblasts is also collectively referred to as "the trophoblast",or, after gastrulation,the trophectoderm, as it is then contiguous with the ectoderm of the embryo.am. Amniotic cavity.b.c. Blood- clot.b.s. Body-stalk.ect. Embryonic ectoderm.ent. Entoderm.mes. Mesoderm.m.v. Maternal vessels.tr. Trophoblast.u.e. Uterine epithelium.u.g. Uterine glands.y.s. Yolk-sac.

A blastocoel(e) or blastocele (also called blastocyst cavity,cleavage cavity or segmentation cavity)

is the fluid-filled central region of a blastocyst. A blastocoele forms during embryogenesis when a zygote (a fertilized ovum) divides into many cells through mitosis.

A blastocoel can be described as the first cell cavity formed as the embryo enlarges. It is essential for later gastrulation.

Formation of the blastocyst

The morula is a solid ball of about 16 undifferentiated, spherical cells. As cell division continues in the morula, the blastomeres change their shape and tightly align themselves against each other. This is called compaction and is likely mediated by cell surface adhesion glycoproteins.

Blastulation. 1 - morula, 2 - blastula.

First stages of segmentation of a fertilized mammalian ovum. Semidiagrammatic.

z.p. Zona pellucida. p.gl. Polar bodies.

a. Two-cell stage.b. b. Four-cell stage. c. c. Eight-cell stage. d. d, e. Morula stage

Implantation

After entering the uterus, the fertilized egg attaches to the uterine lining, or endometrium. This process is called implantation. The cells continue to divide.

Differentiation into the Cytotrophoblast/Syncytiotrophoblast (day 7) Near the end of the first week, the

blastocyst implants in the posterior wall of the uterus in the presence of high hormone levels; the blastocyst attaches at the embryonic pole (inner cell mass) so the embryo will eventually be attached dorsally.

Occasionally, the blastocyst will implant on another spot in the uterus, potentially causing problems (placenta previa).

The trophoblast layer undergoes mitosis upon contact with the uterine wall and differentiates into the cytotrophoblast ("cellular" layer) and the syncytiotrophoblast ("multinucleated" layer).

Early Development

Implantation– 6-7 days after

fertilization– Trophoblast

forms placenta– “Inner cell

mass” forms embryo

Pregnancy Hormones

Human chorionic gonadotropin (hCG) is a hormone present in the blood within about a week of conception. It is the hormone detected in a blood or urine pregnancy test, but it usually takes three to four weeks for levels of hCG to be high enough to be detected by pregnancy tests. It is secreted by cells that develop into the placenta.