Lesson # 19 Early DevelopmentChapter 29Objectives:

1- To describe the process of fertilization.2- To list the stages of prenatal development, and describe the major events of each.3- To describe how the three germ layers participate in the formation of the extraembryonic membranes.4- To discuss the importance of the placenta as an endocrine organ.5- To describe the fetal circulation.

DevelopmentDevelopment is the gradual modification of anatomical structures and physiological characteristics from fertilization to maturity.

Differentiation It is the creation of different types of cells required in development.Differentiation occurs through selective changes in genetic activity: As development proceeds, some genes are turned off, others are turned on.

Fertilization or ConceptionIt is when the male and female gametes fuse.

Development begins at fertilization or conception.

Development Begins at start of ninth week and continues until birth.

Occurs during first two months after fertilization. Study of these events is called embryology.Prenatal

Development

Postnatal Development

Embryological Development

Fetal Development

Begins at birth and continues to maturity, the state of full development or completed growth

The Oocyte at Ovulation

Ovulation releases a secondaryoocyte (haploid) and the first polar body; both are surrounded by the

corona radiata. The oocyte is suspended in metaphase of

meiosis II.

Zonapellucida

First polarbody

Coronaradiata

It is a layer of glycoprotein gel secreted by granulosa cells around the oocyte.

It is composed of several layers of granulosa cells. Atrophic cell

from meiosis I

Metaphase of Meiosis II

The acrosomal reaction is the exocytosis of the acrosome, releasing the enzymes needed to penetrate the egg.

Hyaluronidase, which digests the hyaluronic acid that binds granulosa cells together.When a path has been cleared, a sperm binds to the zona pellucida.

Two acrosomal enzymes are released:

Acrosin, a protease similar to trypsin.

Acrosomal reaction1

Fertilization and Oocyte Activation

The first step is the binding of the spermatozoon to sperm receptors in the zona pellucida.

Sperm penetration releases an inflow of Ca2+, which stimulates the cortical granules to release their secretion beneath the zona pellucida.The cortical granules release enzymes that inactivate sperm receptors and harden the zona pellucida.

Oocyte Activation2Fertilization membrane

Rejected sperm

Cortical granules

- Cortical Reaction

The cortical reaction prevents the polyspermy (fertilization by two or more sperm).

- Completion of Meiosis II and Formation of the Second Polar Body.

- Activation of Enzymes that Cause a Rapid Increase in the Cell’s Metabolic Rate.

Acrosomal enzymes from multiplesperm create gaps in the corona

radiata. A single sperm then makescontact with the oocyte membrane,

and membrane fusion occurs,triggering oocyte activation and

completion of meiosis.

Fertilizingspermatozoon

Second polarbody

Fertilization and OocyteActivation

Pronucleus FormationBegins

The sperm is absorbed intothe cytoplasm, and the female

pronucleus develops.

Nucleus offertilizing

spermatozoonFemale

pronucleus

Spindle Formation andCleavage Preparation

The male pronucleusdevelops, and spindle fibersappear in preparation for the

first cleavage division.

FemalepronucleusMale

pronucleus

Amphimixis:It is the process by which the male and female pronucleus fuse to form a zygote that contains 46 chromosomes.The formation of the zygote is the “moment of conception”.

Amphimixis Occurs andCleavage Begins

Metaphase of firstcleavage division

Cleavage:It is a series of cell divisions (mitotic divisions) that produce an ever-increasing number of smaller and smaller daughter cells.

The first cleavage divisionnears completion roughly30 hours after fertilization.

Cleavage Begins

Blastomeres

The First Trimester1- Cleavage

2- Implantation

It is a sequence of cell divisions (mitosis) that begins immediately after fertilization. During cleavage, the zygote becomes a pre-embryo, which develops into multicellular blastocyst.

It begins with attachment of the blastocyst to the endometrium of the uterus.

It is the formation of viable embryo. This process establishes the foundations for all major organ systems.

3- PlacentationIt occurs as blood vessels form around the periphery of the blastocyst and the placenta develops.

4- Embryogenesis

Cleavage ends when the blastocyst first contacts the uterine wall.

Spermpronucleus

Eggpronucleus

Zygote2-celled stage

DAY 1

BlastomeresIt is a solid ball of 16 cells that resemble a mulberry.

The morula lies free in uterine cavity for 4-5 days and divides into a 100 cells or so.

The zona pellucida dis-integrates and releases conceptus: blastocyst.

4-celled stage DAY 2

Early morula DAY 3

Advanced MorulaDAY 4

Blastocyst DAY 6

Implanted blastocyst

Fertilization(0 hours)

2- Implantation DAYS 7-10

Cleavage and Blastocyst FormationCleavage is a sequence of cell divisions (mitosis) that begins immediately after fertilization. During cleavage, the zygote becomes a pre-embryo, which develops into multicellular blastocyst.

Hatching

Blastocoel

Trophoblast

Inner cell mass

Cleavage and Blastocyst Formation

Zygote

(DAY 8)

Implantation

Endometrium:

Blastocyst:

(DAY 7)

Blastocoel

TrophoblastInner cell mass

EpitheliumEndometrial gland

The blastocyst attaches to uterine wall 6 days after ovulation, usually on the fundus or the posterior wall of the uterus.

Implantation: It is the process of attachment to uterine wall that begins when blastocyst adheres to the endometrium.

The trophoblast on the attachment side separates into two layers:

The superficial layer in contact with the endometrium. The plasma membranes break down and trophoblastic cells fuse into a multinucleate mass called syncytial trophoblast.

Syncytial trophoblast

The deep layer, close to embryoblast, retains the individual cells divided by membranes and is called cellular trophoblast.

Cellular trophoblast

Inner cell mass (or embryoblast)

Trophoblast:

The trophoblast secrets human chorionic gonadotropin (HCG), which stimulates the corpus luteum to secret estrogen and progesterone (it suppresses menstruation).

Stages of Implantation

Formation of the Amniotic Cavity

It is the arrangement of the inner cell mass into three primary germ layers: ectoderm, mesoderm, and endoderm

Gastrulation and Germ Layer Formation

Endoderm

Mesoderm

Ectoderm

The inner cell mass separates slightly from the trophoblast and creates a narrow space between them: the amniotic cavity

Amniotic cavity

Once the three primary germ layers are formed, embryogenesis is complete and the individual is considered an embryo. It is about 2 mm long and 16 days old.

Inner cell mass Cellular trophoblast

Syncytial trophoblast

Inner cell mass Gastrulation:

Zygote Morula Blastocyst Embryo FetusCleavage Hatching Gastrulation

Gastrulation and Germ Layer Formation

Inner cell mass

Gastrulation is the arrangement of the inner cell mass into three primary germ layers: ectoderm, mesoderm, and endoderm

Formation of the Extraembryonic Membranes Germ layers also form four extraembryonic membranes: 1- The Yolk Sac 2- The Amnion 3- The Allantois 4- The Chorion

Yolk Sac (Endoderm and mesoderm)It becomes an important site of blood cell production.

(Ectoderm and mesoderm)As development proceeds, the amnion and amniotic cavity continue to enlarge. The amniotic cavity contains amniotic fluid, which surrounds and cushions the embryo.

Amnion

Allantois (Endoderm and mesoderm)The base of the allantois gives rise to the urinary bladder.

Chorion (Mesoderm and trophoblast)

Chorionic villi of placenta

The appearance of blood vessels in the chorion is the first step in the creation of the placenta.

Deciduacapsularis

Deciduabasalis

Deciduaparietalis

The portion of endometrium that participates in the formation of the placenta.

The portion of endometrium that has no contact with the chorion.

Chorionicvilli

Umbilicalvein

Umbilicalarteries

Area filled withmaternal blood

Amnion

Maternalblood vessels

Placentation

Chorionic villi of

placenta

Umbilicalcord

The Endocrine Placenta

Appears in maternal bloodstream soon after implantation.Provides reliable indication of pregnancy.Pregnancy ends if absent.

1- Human Chorionic Gonadotropin (hCG)

Prepares mammary glands for milk production.Stimulatory effects on other maternal tissues comparable to that of growth hormone (GH).Ensures adequate glucose and protein is available for the fetus.

2- Human Placental Lactogen (hPL)

A peptide hormone secreted by placenta and corpus luteum during pregnancy.Increases flexibility of pubic symphysis, permitting pelvis to expand during delivery.Causes dilation of cervix.Suppresses release of oxytocin by hypothalamus and delays labor contractions.

3- Relaxin

4- ProgesteroneProgesterone maintain the endometrial lining and continue the pregnancy.

Estrogens play a role in stimulating labor and delivery. 5- Estrogens

Fetal Circulation