Lecture # 18: Early Human Development

Human Development (Chapter 29)

Objectives

1- Describe the process of fertilization.

2- Explain how the egg prevents fertilization by more than one sperm.

3- Describe the major events that transform a fertilized egg into an embryo.

4- Describe the implantation of the embryo in the uterine wall.

6- Describe the formation and functions of the embryonic membranes.

7- Explain how the conceptus is nourished.

5- Identify the major tissues derived from the primary germs layers.

OOGENESIS

MITOSIS

OogoniaDiploid

Diploid

Before birth

MEIOSIS I

After puberty

It stops inprophase

MEIOSIS ICompleted

Primaryoocyte

Haploid

Secondaryoocyte

It stops inmetaphase Before ovulation

After ovulationMEIOSIS IICompleted

Haploid

Secondaryoocyte

If fertilizationoccurs

MEIOSIS II

Oogenesis

First polarbody

1st & 2nd

polarbody

Oocyte at Ovulation

If not fertilized

It completes meiosis II

Zygote

Embryo

Dies

Secondary oocyte(arrested in metaphase

of meiosis II)

Second polarbody (dies)

If fertilized

Ovulation of mature(graafian) follicle

First polar bodyEgg

It is a layer of glyco-protein gel secreted by granulosa cells around the oocyte.

It is composed of several layers of granulosa cells.

Zona pellucida

Corona radiata

Head

Acrosome

Nucleus

Mitochondrion

Axoneme

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Basal body

Principal piece of tail

Endpiece of tail

It contains enzymes that break down material surrounding the oocyte.

It contains one set of 23 chromosomes (haploid).

It is the first part of the flagellum nestled in an indentation in the base of the nucleus.

They provide the ATP needed for the beating of the tail.

It is the core of the flagellum consisting of microtubules.

Mature Spermatozoon

Tail

Midpiece of tail

It propels the sperm during migration in the female reproductive system.

It 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

There are two mechanisms to prevention of polyspermy (fertilization by two or more sperm):

1- Fast block: Binding of the sperm to the egg opens Na+ channels in egg membrane. Inflow of Na+ depolarizes the membrane and inhibits the attachment of any more sperm.

2- Slow block : Sperm penetration releases an inflow of Ca2+, which stimulates the cortical granules to release their secretion beneath the zona pellucida.

The secretion swells with water, pushes any remaining sperm away and creates an impenetrable fertilization membrane between the egg and the zona pellucida.

Cortical reaction2

Fertilization membrane3

Rejected sperm4

Cortical granules

Fertilization

Spermpronucleus

Eggpronucleus

2-celled stage(30 hours)

1-Cleavage:It refers to the mitotic divisions that occurs in the first 3 days, while the conceptus migrates down the uterine tubes.

Blastomeres It 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.

Migration of the Conceptus

The Preembryogenic Stage

It comprises the first 16 days of development, culminating with the existence of an embryo.

1- Cleavage 2- Implantation

3- Embryogenesis

4-celled stage

8-celled stage

Morula(72 hours)

Blastocyst

Implanted blastocyst(6 days)

Fertilization(0 hours)

Zygote

(8 days)

2- Implantation

Endometrium:

Blastocyst:

(6-7 days)

Blastocoel

TrophoblastEmbryoblast

EpitheliumEndometrial gland

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

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 syncytiotrophoblast.

Syncytiotrophoblast

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

Cytotrophoblast

Embryoblast

Trophoblast:

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

2- Implantation:

3- Embryogenesis

It is the arrangement of the embryoblast into three primary germ layers: ectoderm, mesoderm, and endoderm.

Endoderm

Mesoderm

Ectoderm

The embryoblast 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.

Zygote Morula Blastocyst Embryo

Embryogenesis:

The mesoderm is a more loosely organized tissue which differentiates into a loose fetal connective tissue called mesenchyme.

The ectoderm and endoderm are epithelia composed of tightly joined cells.

Several accessory organs develop along side the embryo: the placenta, the umbilical cord and four embryonic membranes (amnion, yolk sac, allantois, and chorion)

16 days

28 days

Allantois

Yolk sac

Amniotic cavity

Amnion Chorionic

villi

Chorion

Placental sinus

Uterus

Chorionicvillus

Developingplacenta

Yolk sac

Amnion

Amniotic cavity

Chorion

Umbilicalblood vessels

Allantois

They are extensions of syncytiotrophoblast into the endometrium by digestion and growth of “roots” of tissue.

They are lacunae filled with maternal blood that merge and surround villi.

Placental sinus

12 weeks

AmnionIt is the outermost membrane enclosing all the rest of the membranes and the embryo.

Chorion

It is a transparent sac that grows to completely enclose the embryo and penetrated only by the umbilical cord fills with amniotic fluid.

It begins as an out- pocketing of the yolk sac. It forms the foundation for the umbilical cord becomes part of the urinary bladder.

Allantois

Umbilical cord PlacentaYolk sac

It contains two umbilical arteries and one umbilical vein.

It contributes to formation GI tract, blood cells, and future egg or sperm cells.

12 weeks

Amniotic cavity fills with amniotic fluid.

Functions of the Amniotic Fluid:

1- It protects the embryo from trauma, infections, and temperature fluctuations.

2- It allows freedom of movement important to muscle development.

3- It enables the embryo to develop symmetrically.

4- It prevents body parts from adhering to each other.

5- It stimulates lung development as the fetus‘ breathes’ fluid.

The fetus swallows amniotic fluid at the same rate. At term, the amnions contains about 700 to 1000 mL of fluid.

At first, amniotic fluid is formed from filtration of mother’s blood plasma.

Beginning at 8 to 9 weeks, the fetus urinates into the amniotic cavity about once per hour contributing substantially to fluid volume.

The Amniotic Fluid

During gestation the conceptus is nourished in three different, over-lapping ways:

Prenatal Nutrition

1- Uterine milk2- Trophoblastic nutrition3- Placental nutrition

1- Uterine milk

It is a glycogen-rich secretion of the uterine tubes and endometrial glands.

The conceptus absorbs this fluid as it travels down the tube and lies free in the uterine cavity before implantation (6, 7 days).

2- Trophoblastic nutrition

The conceptus consumes decidual cells of the endometrium.

Progesterone from corpus luteum stimulates decidual cells to proliferate and accumulate a store of glycogen, proteins, and lipids.

It is the only mode of nutrition for the first week after implantation and remains the dominant source through the end of 8 weeks.

3- Placental nutrition

Nutrients diffuse from the mother’s blood through the placenta into the fetal blood.

The placental phase is the period beginning the 9th week and it is the sole mode of nutrition from end of 12th week until birth.

As the placenta grows:

Placenta and Umbilical Cord

The villi grow and branch and their surface area increases.

The membrane becomes thinner and more permeable.

The placental conductivity (the rate at which substances diffuse through the membrane) increases.

Materials diffuse from the side of the membrane where they are more concentrated to the side where they are less concentrated.

Oxygen and nutrients pass to the fetal blood. Fetal wastes pass the other way and are eliminated by the mother.

The placenta is also permeable to nicotine, alcohol, and most other drugs that may be present in the maternal blood stream.

Right atrium It receives O2 poor blood returning to the heart through the superior and inferior vena cava and the coronary sinus.

Right ventricle It pumps O2 poor blood to the lungs through the pulmonary arteries.

Superior vena cava Left atrium It receives O2 rich blood returning from the lungs through the pulmonary veins.

Pulmonary veins

Left ventricleIt pumps O2 rich blood through the aorta artery to every organ of the body.

Pulmonary trunk

Inferior vena cava

Aorta

Blood Circulation in the Adult

Right atrium

Pulmonary trunk

Aorta

Inferior vena cava

Foramen oval

1

It receives O2 rich blood returning from the placenta, mixed with O2 poor returning through the inferior vena cava.

1

Blood bypasses the lungs by flowing directly from the right atrium through the foramen oval into the left atrium.

2

2

Ductus arteriosus

Blood also bypasses the lungs by flowing from the pulmonary trunk through the ductus arteriosus to the aorta.

3 3

Blood Circulation in the Fetus