Oral Biology

EmbryologyMaryann MacNeil

Summer 2008

Lecture Outline

• Terminology review

• Fertilization• First week of

development• Week 2 of

development• Week 3-4 of

development

Some Terminology• Gamete – highly specialized sex cell

that are derived from primordial germ cells. These cells undergo mitosis and meiosis (gametogenesis). The oocyte (female gamete) is produced in the ovaries, the spermatozoon (male gamete) is produced in the testes

• Zygote – single cell that results from the union of the oocyte and the spermatozoon

• Conceptus – the embryo and associated membranes

• Fertilization age- The age of the embryo counting from the actual time of fertilization (usually day 14-16 of the menstrual cycle)

• Gestational age -How physicians calculate the age of the embryo- from the first day of the last normal menstrual period (LNMP).

Gestational age is 2 weeks longer than fertilization age.All dates given in the upcoming lectures are fertilization age of the

embryo.

• Embryo- developing human up to 56 days or the end of the 8th week. All major structures are present at this point

• Fetus- developing human from the 9th week until birth. This is a period of rapid growth and weight gain

• Differentiation- to change characteristics and establish a new population of cells – cells undergo a change to a more specialized form or to carry out a new function

• Induction- the process that initiates differentiation,- one part of the embryo causes another part to differentiate

• Competence- the cells must be competent to respond to the induction process- the cells vary their response- from no response to max response

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Lecture Outline

• Terminology review

• Fertilization• First week of

development• Week 2 of

development• Week 3-4 of

development

Fertilization and the First Week of Development

During ovulation (day 14),the secondary oocyte, the zona pellicuda and the corona radiata are expelled into the peritoneal cavity

The fimbraie of the oviduct move over the ovary and sweep the secondary oocyte into the oviduct, and the oocyte moves to ampulla of the uterine tube via peristaltic movement of the wall.

Spermatozoa are unable to fertilize the oocyte until they undergo the following 2 events:

1. Capacitation:• Glycoprotein coat

removed• Occurs in the

uterus or uterine tubes

• No morphological changes- but more active

• Completion allows acrosome reaction

2. The Acrosome Reaction:• Acrosome binds with a glycoprotein on the zona

pellucida• Molecular changes result in perforations in the

acrosome• The membrane breaks down and releases

enzymes that facilitate fertilization

• The female oocyte degenerates 24 hours after ovulation.

• The usual time frame is 12 hours.

• The male sperm survives for 48 hours

• The usual site is the ampulla.

• Fertilization is defined a a sequence of events, forming the zygote.

Sequence of events involved in fertilization:1. Penetration of the corona radiata. Only about 200 sperm will make it to the site of

fertilization. Do you remember how many are ejaculated?

Corona radiata Zona pellucida

Secondaryoocyte

2. Penetration of the zona pellucidaEnzymes (acrosin) released from the acrosome result in the breakdown of the ZP. The zona reaction prevents polyspermy.

3. Fusion of the plasma membranes of the oocyte and sperm- head and tail enter the cytoplasm of the oocyte- plasma membrane stays behind.

Plasmamembrane

Resumption of meiosis II- oocytes completes the second meiotic division, gives rise to definitive oocyte and a second polar body. The nucleus of the definitive oocyte is the female pronucleus

5. The male pronucleus forms – nucleus enlarges, the tail degenerates.

**Each pronucleus must replicate its DNA (so when the zygote begins mitotic division each cell has the normal amount of DNA)

6. The pronuclei fuse into a single diploid number of chromosomes, forming the zygote.

The chromosomes organize on a cleavage spindle in preparation for mitotic cleavage of the zygote.

1. Restoration of the diploid number of chromosomes.After stimulating the female oocyte to complete meiosis II the zygote contains a diploid number of chromosomes with a new combination of genetic material.

2. Determination of the sex of the embryo. An X carrying sperm will produce a female, Y carrying sperm a male

3. Metabolic activation of the egg and initiation of cleavage (cell division)

Without fertilization the oocyte degenerates in 24 hours.

Results of Fertilization

Lecture Outline

• Terminology review

• Fertilization• First week of

development• Week 2 of

development• Week 3-4 of

development

CleavageThe repeated mitotic divisions of the zygote that

result in a rapid increase in the number of cells. The daughter cells are blastomeres and become

smaller and smaller with each division.

The cleavage division begins about 30 hours post-fertilization. The zygote divides into 2, 4 and 8 blastomeres. They are still surrounded by the zona pellucida.

CompactionAfter the zygote has reached the 8 cell stage the

blastomeres form a ball and closely adhere to one another. The process is called compaction, and is a prerequisite of the segregation of internal cells (inner cell mass) from external cells (outer cell mass).

At approximately 3 days PF there are 12-32 blastomeres and the developing conceptus is called a morula. Now the internal cells are surrounded by the outer cell layer.

The morula enters the uterus on day 3-4 PF.

• As the morula enters the uterine cavity, a fluid filled space called the blastocyst cavity forms. The developing embryo is now a blastocyst.

• The blastomeres are clearly separated into 2 parts.

Day 4

1. The thin, outer cell layer, the trophoblast, contributes to the placenta.

2. The more centrally located cells, the inner cell mass or embryoblast, gives rise to the tissues of the embryo.

The cells of the embryoblast are attached to the trophoblast at the embryonic pole of the blastocyst.

Outer Cell Mass

“Hatching of the blastocyst”

The zona pellucida provides early protection for the conceptus during cleavage and transport along the uterine to the uterus. After floating in the uterus for approximately 2 days the ZP degenerates allowing the “hatched” blastocyst to grow rapidly.

The ZP degenerates and disappears

On day 6-7 PF (day 20 of the menstrual cycle) the blastocyst attaches to the endometrial epithelium adjacent to the embryonic pole. This triggers changes in the trophoblast and the connective tissue beneath the uterine epithelium

The trophoblast begins to rapidly make more cells and differentiate into two layers.

A. Outer mass- some trophoblasts fuse to make a large multinucleated cell -the syncytiotrophoblast

B. Inner layer of cells- some trophoblast cells remain unfused and are called the cyotrophoblast.

Fingerlike processes of the syncytiotrophoblast extend through the endometrial epithelium and releases enzymes that allow it to borrow into the endometrium. The blastocyst receives nutrition from the eroded maternal tissues.

We are on day 21 of the menstrual cycle- the endometrium has proliferated and is full of blood vessels and glands.

Endometrium

The normal site for implantation is the posterior (or anterior) wall of the body of the uterus (X).

An implantation outside of the uterus is called in ectopic pregnancy.

95% of ectopic pregnancies occur in the uterine tube- primarily in the ampulla.

X

Placenta PreviaAbnormal

implantation within the uterus.

The blastocyst implants close to the cervix (internal os).

Causes the placenta to separate early and life threatening bleeding.

• What 2 events must spermatozoa undergo in order to fertilize the oocyte?

• What is the usual timeframe for fertilization (how many hours post ovulation?)?

• Recall the sequence of events leading to fertilization.

• What are the immediate results of fertilization?• Understand cleavage-is it mitosis or meiosis?• What is the role of the zona pellucida?• What is the trophoblast? The embryoblast?• What day post-fertilization does implantation

occur?• Understand cytotrophoblast and

syncytotrophoblast- how these cells come about-