fertilization, implantaion and embryology

Fertilization, Implantation and Embryology

Dr.Jhansi,senior resident

Department of Obstetrics & Gynaecology,Mahatma Gandhi Medical College & Research Institute,Pondicherry

• Development

• Fertilization

• Gestation

• Cleavage

• Implantation

• Embryonic Development: Gastrula to Fetus

Development - the gradual modification of anatomical structures during the period from fertilization to maturity

• A single cell becomes an individual human being containing trillions of cells

• Differentiation - the process whereby all the different cell types are produced through selective changes in genetic activity

• Major Phases of Development

fertilization - fusion of the male and female gametes

embryological development - first two months after fertilization

fetal development - from the beginning of the 9th week until birth

postnatal development - commences at birth

Fertilization

• Involves the fusion of two haploid gametes with 23 chromosomes each to produce a zygote that contains 46 chromosomes

• fertilization generally occurs in the uterine tube and generally within one day of ovulation

• oocyte - the ovulated secondary oocyte is in metaphase of meiosis II; it is surrounded the corona radiata which is a protective layer of cells; the oocyte is traveling through the uterine tube toward the uterus

• sperm - are introduced into the female reproductive tract; of the 200 million spermatozoa ejaculated into the vagina, about 10,000 enter the uterine tube, and fewer than 100 sperm reach the egg

the acrosomal cap of sperm contains the enzyme hyaluronidase which breaks down the bonds between the cells of the corona radiata

dozens of sperm are needed to penetrate the corona radiata

Relationship between capacitation, acrosome reaction and fertilization

CAPACITATION

Potential to undergoacrosome reaction

Zona penetration

Spermatozoon Binding to ZP3ACROSOME REACTION

Changes in tailChanges in head

Hyperactivation

Fertilization

1. Capacitation

…… changes that enable the spermatozoa to undergo the acrosome reaction and hyperactivation. Normally, this occurs when semen interacts with the vaginal fluids.

Human Cervical mucus plays a role in regulating sperm entry into the uterus.

Periovulatory mucus is watery facilitating sperm movement through the open External os.

Luteal mucus is thick and resistant

As a result of capacitation, spermatozoa acquire the

capacity to bind to the zona pellucida. This event is

believed to result from the “unmasking” of a species

specific ZP-3 receptor in the sperm membrane.

2. ACROSOME REACTION

Spermatozoon Head

Fertilization

• Accomplishing Fertilization

a single sperm makes contact with the oocyte membrane

the oocyte is activated and completes meiosis II, which results in the production of the second polar body (Meiosis I produced the first polar body); note: the oocyte has a single set of 23 chromsomes

the sperm is absorbed into the much larger oocyte cytoplasm

a male pronucleus and female pronucleus form, and then fuse

the result is the formation of a zygote (single cell) with the 2 complimentary pairs of 23 chromosomes (46 chromosomes)

Zona penetrationAs a consequence of the

acrosome reaction, the zona pellucida is locally hydrolyzed. This together with hyperactation, allows the spematozoon to penetrate the zona pellucida.

Tail hyperactivity

Proteases

Zona pellucida

The steps in fertilization

Acrosome reacted sperm penetrating zona pellucida

egg

ZP Sperm

Sperm/oocyte contact triggers the cortical granule reaction that sweeps around the egg.

Cortical Granules

Sperm-egg contact Cortical Granule Reaction

Cortical reaction blocks sperm binding to fertilized egg.

Capacitated sperm do not bind to fertilized egg

Capacitated sperm bind to unfertilized egg

The key events that occur during fertilization

1. Capacitated sperm binds to zona pellucida protein-3 (ZP-3) induces the acrosome reaction.

2. The acrosomal process facilitates ZP penetration.

3. Sperm-egg contact induces the cortical granule reaction and the block to polyspermy

The oviduct and uterus are critical components of the female reproductive tract.

Ampulla is the site of fertilization

Time course of the development of the human embryo from fertilization to implantation.

Gestation - the time spent in prenatal development

• Can be divided into 3 three month trimesters:

first trimester - embryological development and early fetal development; this trimester begins with fertilization; the basic structure of all the major organ systems appear

second trimester - development of organs and organ systems; by the end of this trimester the fetus looks human

third trimester - characterized by rapid fetal growth; by early in this trimester most of the organ systems become fully functional; this trimester ends in birth.

• An infant born 1 or even 2 months prematurely has a reasonable chance of survival

Histology of the Endometrium

Cleavage - is a series of cell divisions that subdivide the cytoplasm of the zygote

• The first division is completed roughly 30 hours after fertilization

• The following divisions occur at 10-12 hour intervals:

morula - solid ball of cells that results after 3 days of cleavage; all of the cells are identical; the morula reaches the uterus on day 4

blastocyst - hollow ball of cells after 3 more days of cleavage (6th day)

the cells are no longer identical

the inner cavity is called the blastocele (=blastocyst cavity); the outer layer of cells is called the trophoblast

the inner cell mass is a group of cells that will become the embryo; the trophoblast protects the inner cell mass from the outside environment

During the preimplantation period, the fertilized egg develops into a blastocyst while remaining encapsulated by the zona pellicudia.

Zygote with Fusion of pronuclei at 24 hrs

Two-cell human embryo at 30 hr

Four cell human embryo at 2 days

Human blastocyst at 5 days

Implantation - seven days after fertilization, implantation begins as the blastocyst connects to the endometrium of the uterus

• The trophoblast erodes a path through the endometrium by secreting hyaluronidase

• The trophoblast continues to enlarge and spread into the surrounding endometrium

• The erosion of uterine glands releases nutrients that are absorbed by the trophoblast and distributed by diffusion to the inner cell mass

• Trophoblastic extensions encircle endometrial capillaries

• Capillary walls are destroyed and maternal blood percolates through trophoblastic channels called lacunae

• Villi - extend from the trophoblast into the endometrium and larger endometrial blood vessels are surrounded and broken down resulting in greater blood flow through the lacunae

• Implantation most often occurs in the fundus or body of the uterus;

• Ectopic pregnancy - when implantation occurs anywhere but the uterus; if implantation takes place in the uterine tubes it can be life-threatening

Implantation phases

A. Hatching phaseB. Adhesion phaseC. Invasive phase

Hatching phase

Site of Zona rupture

Blastocyst escaping from the Zona Pellucida

Invasive phase: After attachment, the trophoblast cells begin to penetrate the uterine epithelium.

Trophoblast cells invading endometrium

Human embryo completely embedded in the endometrium

embryo

endometrium

Secretory glands

• Implantation can also occur in uterine tube, cervix, or the abdominal cavity.

• Implantation anywhere outside the uterus is called an ectopic pregnancy.

• It is possible for fetus to grow in the abdominal cavity, but growth inside the uterine tube causes the tube to rupture, resulting in severe bleeding.

• Inner cell mass forms two cavities:– The yolk sac– Amniotic cavity

• In humans the yolk sac produces blood cells and future sex cells• The amniotic cavity becomes the cavity in which the embryo floats.

Fluid is produced from fetal urine, and secretions from the skin, respiratory tract, and amniotic membranes.

Embryonic Development: Gastrula to Fetus

• Amniotic cavity - formed by the time of implantation in the blastocyst; this cavity is formed when the inner mass separates from the trophoblast

• There are two layers of cells that comprise the inner mass.

• By day 12, a third layer forms through gastrulation in which cells move toward the primitive streak

• In between the yolk sac and the amniotic cavity is the embryonic disc, which gives rise to the primary germ layers

• The 3 Germ Layers - these three layers of cells will differentiate

into all of the structures of the body:

ectoderm - will form the skin, hair, nails, brain, spinal cord, epithelium of nasal passages, and mouth

mesoderm - will form the bones, muscles, heart, blood vessels, kidneys

endoderm - will form the pancreas, thyroid gland, liver, urinary bladder, respiratory epithelium, digestive epithelium


• Four extraembryonic membranes:

yolk sac - important site of blood cell formation

amnion - surrounds the fluid-filled amniotic cavity which protects the embryo

allantois - will give rise to the urinary bladder

chorion - combination of mesoderm and trophoblast that will form the placenta

• Chorionic villi - branch and enlarge within the endometrium; embryonic blood vessels develop within each villus and blood blood flow begins in them by the beginning of the 3rd week; they provide the surface area for active and passive exchange of gases, nutrients, and waste products between fetal and maternal blood streams

• Embryonic phase of development lasts from fertilization until the 8th week of gestation, when it becomes a fetus.

• By day 35 the heart is beating, and eye and limb buds are present.

• By month four, the rudiments of all organ systems are formed and functioning, and from then on, fetal development is primarily a matter of growth.

• By the end of the third month the placenta is functioning.


• Placentation - the enlarging chorion becomes the placenta

by the end of the 4th week a body stalk has formed that connects the embryo and chorion; the body stalk contains blood vessels that carry blood to and from the placenta

the placenta becomes concentrated in a disc-shaped area in the endometrium

near the end of the first trimester the fetus moves farther from the placenta

the fetus and placenta remain connected by the umbilical cord which now contains the placental blood vessels

the placenta provides nutrients to the embryo-fetus, a site for gas exchange between the mother and the embryo-fetus, and a place where the embryo-fetus can get rid of its wastes


• Umbilical arteries - these paired arteries carry blood from the embryo-fetus to the placenta

• Umbilical vein - this vein carries nutrient and oxygen rich blood from the placenta to the embryo-fetus

*Mother and baby's blood do not mix!

This concludes the current lecture topic

THANK YOU

fertilization, implantaion and embryology

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