What is embryology?

The study of developmental events that occur during the prenatal stage .

The branch of biology and medicine concerned with the study of embryos and their development.

Embryonic period vs. Fetal periodEmbryonic – first 8 weeks Development of the

three primary germ layers give rise to all structures and Basic body plan takes shape

Fetal period – remaining 30 weeks. Structures

and organs continue to grow and develop.

8 weeks

Developmental History of a Human

Stages of Development1. Fertilization

2. Cleavage

3. Gastrulation Embryogenesis

4.Organogenesis

5.Maturation

Fertilization

The process of fusion or union of the spermatozoon with the mature ovum is known as conception / fertilizaiton/impregnantation.

Which produced the fertilized single mono-nucleated cell called the zygote.

Embryogenesis: The formation

and development of an embryo.

1. Cleavage is a series of rapid mitotic divisions (without cell growth)

2. Gastrulation : is a phase early in the

embryonic development of most animals/human being, during which the single-layered blastula is reorganized into a trilaminar ("three-layered") structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.

3. Organogenesis: The production and

development of the organs of an animal or plant.

How fertilization occurs???? Following ovulation, the ovum, which is about 0.15 mm in

diameter is picked up by the tubal fimbriae and is moved along by the cilia and by peristaltic movement of the tube.

At the time the cervix under the influence of estrogen, secretes a flow of alkaline mucus that deposited in the vagina, only thousands capacitated spermatozoa enter the uterine tube while 300-500 reach the ovum, and remainder are destroyed by the acid medium of the vagina.

It takes about 1 hour for sperm to reach the site.

The sperm release the enzyme, Hylluronidasewhich allows penetration of the zona pellucida and the cell membrane surrounding the ovum.

Many sperm are needed for this to take place but only one will enter the ovum.

After this the membrane is sealed to prevent entry of any further sperm and the nucluei of the two cell fuse.

The sperm and ovum contribute half the complement of chromosomes to make a total of 46.

The sperm and ovum is known as the male and female gametes and the fertilized ovum as the zygote

. Fertilization

Normal site for Conception

The most common site of conception is the ampullary part (Ampulla ) of the fallopian tube which is the widest part located closed to the ovary

Usual Time for Conception Neither sperm nor ovum can survive longer than 2-3

days and the fertilization is most likely to occur when intercourse takes place not more than 48 hours before and 24 hours after ovulation.

So the conception will take place about 14 days before the next period is due.

The sex of the new individual at the time of conception is determined by sex chromosomes.

Every human cell contains 46 chromosomes, which are made up of 44 autosome chromosome and 2 sex chromosomes.

The sex chromosome are X and Y .

Woman have no Y chromosome and male has Y chromosome (male 44+X+Y) (female 44+X+X).

Therefor e, sex of child is always determined by father

Morula1. After fertilization, the Zygote divides into 2 cells

(blastomere) (mitosis division)in about 30 hours after fertilization.

2. The blastomeres continue to divide by binary division through 4, 8, and 16 cell stage until a cluster of cells is formed– Morula, resemblibga mulberry

The morula after spending about 3 days(72 hours) in the uterine tube enters the uterine cavity through the narrow uterine ostium (1mm) on the 4th day .

Morula

Morula

Morula

72 hours post-

fertilization

entering uterus

4. Blastocyst Morula, once entering the uterine cavity, floats

freely(next 2 days) and is covered by endometrial fluid and mucus.

This fluid is absorbed through the canaliculi of the zona pellucida and Morula begins to accumulate fluid and forms a cavity between its cells.

Once cavity appears, it is now called a blastocyst.

Fertilization and the Events of the First 6 Days of Development

Blastocyst The zona pellucida

becomes stretched, thinned and gradually disappear soon prior to implantation.

The cell of the outer cell mass forms the wall of the blastocyst and is known as trophoblast.

The inner cell mass is concerned with the development of the embryo.

Two Distinct Cell Types1. Trophoblasts – will form the invading

placenta

2. Inner cell mass – will form the embryo

Trophoblasts

Blastocyst

Trophoblast Inner cell mass

Placenta Chorion Fetus Amnion umbilical cord

hCG is produced hCG is human chorionic gonadotropin

hormone

It is produced by the trophoblasts starting on day 6

hCG causes endometrium of uterus to grow and proliferate

hCG prevents the menstrual cycle from occuring

This is why a female misses her periods when she is pregnant

Implantation (Week 2)

Implantation occurs about 6th day after fertilization (approx the 20th day of a regular menstrual cycle).

As the zona pellucida surrounding the blastocystdisappear, there is increased adhesiveness of the trophoblastic cells to the endometrium.

The trophoblastic cells invade the stromal cells lying between the secretary endometrial glands by histolytic action of the blastocyst bringing about deeper penetration in the decidua.

The syncytial cells ( multinucleate cell) ultimately penetrate the endothelial coat of the maternal decidual capillaries establishing communication with the syncytial lacunar system

Thus maternal blood circulates through the syncytiallacunae providing nourishment to the blastocyst.

Maternal blood vassels

Further penetration is arrested by the maternal immunological blocking factor. The point of entry of the blastocyst into the decidual is sealed by a fibrin coat and later by epithelium

The process of implantation is completed by the 10th

or 11th day of fertilization.

This type of deeper penetration of the human blastocyst is called interstitial implantation and the blastocyst is covered on all sides by the endometrium(decidual).

Occasionally, there may be increased blood flow into the lacunar space and cause bleeding. It is called implantation bleeding and This corresponds approx to 13th day after fertilization (at about the expected day of period).

This may produce confusion in determination of the EDD

The outer cell mass of the blastocyst form the trophoblast layer.

The inner cell mass on the 8th day differentiates into bilaminar germ disc consisting of a dorsal ectodermallayer of the tall columnar cells and a ventral endodermal layer of flat polyhydral cells.

The bilaminar germ disc is connected to the trophoblast by a mesenchymal connecting stalk called the body stalk which later forms the umblical cord.

Formation of Amniotic cavity and Yolk sac Two cavities appear on either side of the bilaminar

embryonic disc.

The dorsal cavity between the ectoderm and trophoblastic layers lined by mesenchyme(connective tissue) is called the amniotic cavity.

The cavity on the ventral aspect lined by the primitive mesenchyme on the outside and the endodermal layer of the germ cell disc on the inside is called the yolk sac

Week 2

Inner cell mass divides into epiblast and hypoblast

2 fluid filled sacs Amniotic sac from epiblast within

which the embryo and later the fetus develop until birth

Yolk sac from hypoblast which is one of the structures through which the mother supplies nutrients to the early embryo

Bilaminar embryonic disc: area of contact

(gives rise to the whole body)

Bilaminar to trilaminar disc

Three primary “germ” layers: all body tissues develop from these

Ectoderm

Endoderm

Mesoderm

Week 3

Formation of the 3 “germ” layers

Primitive streak (groove) on dorsal surface of epiblast

Grastrulation: invaginationof epiblast cells

Days 14-15: they replace hypoblast becoming endoderm

Day 16: mesoderm (a new third layer) formed

in between

Epiblast cells remaining on surface: ectoderm

Major derivatives of the

embryonic germ layers

Trophoblast

Small projection begins to appear all over the surface of the blastocyst, becoming most prolific at the area of contact. These trophoblastic cells differentiate into 3 layer.

1. The outer syncytiotrophoblast(syncytium): it erodes the walls of the blood vessels of the decidua, making the nutrient in the maternal blood accessible to the developing fetus.

2. The inner cytotrophoblast: it’s a single layer cells which produces a hormone HCG. This is responsible for informing the corpus luteum that a pregnancy has begun and corpus luteumcontinues to produce estrogen and progesterone, which maintain the integrity of the decidua.

3. In the inner aspect: a layer of mesoderm or primitive mesenchyme, which consists of loose connective tissue.

There is similar tissue in the inner cell mass and the two are continuous at the point where they join in the body stalk. (fig 3.9 1st picture from book tuitui)

The trophoblast is responsible for the formation of the placenta and fetal membrane and sub-serve the important functions of attachment of the fetus to maternal tissues, providing nutrition, oxygenation, and clearing of the fetal metabolic wastes and producing hormones, thus ensuring growth and development of the fetus.

DeciduaThe endometrial lining of the uterus is

called decidua during pregnancy and it is shed after delivery.

Progesterone from the corpus luteum and placenta maintain decidua during pregnancy. It is called the decidual reaction.

Well developed Decidua consists of 3 layers.1. Superficial compact layer: consists of

compact mass of decidual cells, gland, ducts and dilated capillaries. The greater part of the surface epithelium is either thinned out or lost.

2. Intermediate spongy layer (cavernous

or functional layer): contains dilated uterine glands, decidual cells and blood vessesls. Placenta separation occurs in this layer.

3. The basal layer: containing the basal portion of the glands and is opposed to the uterine muscle. Regeneration of the mucous coat occurs from this layer following parturition.

After the implantation of the blastocyst into the compact layer of the decidua, the different portions of the decidua are renamed as:

1. Decidua basalis or serotina: The portion of the decidua in contact with the base of the blastocyst.

2. Decidua capsularies or reflexa: The thin superficial compact layer covering the blastocyst.

3. Decidua vera or parietalis: the rest of the decidua lining the uterine cavity outside the site of implantation.

The ovum bulges into the uterine cavity. The space between the deciduacapsularies and decidua verais called the decidual space.

It progressively diminishes as the ovum enlarges in pregnancy

Until at 16 weeks, the space is obliterate because of the fusion of the deciduacapsularis with the deciduavera.

At term this fused layer is gradually attenuated and its constituent layers can not be identified.

The decidua basalisbecomes the maternal portion of the placenta.

Functions of Decidua

1. To provide place for implantation of the fertilized ovum.

2. To provide nutrition to the growing ovum.

3. Provide a protective action.

4. Provides the basal plate of the placenta.

Important Events Following Fertilization “O” hour - Fertilization

30 hours - 2 cell stage

40-50 hours - 4 cell stage

72 hours - 12 cell stage

96 hours - 16 cell stage morula enter the uterine cavity

5th day - blastocyst

6-7th day - zona pellucida disappear. Interstitial implantation occurs.

9th day - lacunar period, endometrial vessels tapped

10-11th day - implantation completed

13th day - primary villi seen

16th day - secondary villi

21st day - tertiary villi

21st -22nd - fetal heart, fetal placental circulation started

The prenatal development of the fetus can be divided into 3 phases.

1. Ovular period or germinal period (It lasts from the stage of fertilization upto 2 weeks after ovulation)

The embryonic period (first 8 weeks Development of the three primary germ layers give rise to all structures and Basic body plan takes shape )

Fetal period (remaining 30 weeks. Structures

and organs continue to grow and develop)

Fetal Length In the earlier weeks, it is expressed as the

measurement from the vertex to the coccyx (crown rump length) while after mid pregnancy (20 weeks onwards), the measurement of the fetus is determined from the vertex to the heel (crown heel length).

Age of the FetusThe length is more reliable criterion than

the weight to calculate the age of the fetus.

Haase’s rule is employed in calculating the age of fetus from its length in centimeters, divided by 5.

1. First lunar month: Fertilization to 2weeks of embryonic growth

Implantation is complete.

Primary chorionic villi forming

Embryo develops into two cell layers (bilaminarembryonic disks)

Amniotic cavity appears.

Note: lunar month is a period that is measured based on the movement of the moon. It is roughly 28 days and it constitutes a month. In that context, pregnancy is 10.

2. Second lunar month: 3 to 6 weeks of embryonic growth At the end of 6 weeks of growth, the embryo is

approximatey 1.2 cm long.

Arms and leg buds are visible; arm buds are more developed with finger ridges beginning to appear.

Rudiments of the eyes, ears and nose appear.

Primitive intestinal tract is developing.

Primitive cardiovascular system is functioning.

Neural tube, which forms the brain and spinal cord closes by the 4th week.

3. Third lunar month: 7 to 10 weeks of growth The middle of this period (8 weeks)marks the end of the

embryonic period and beginning of the fetal period.

At the end of 10 weeks of growth, the fetus is 6.1 cm from crown to rump and weigh 14 gm.

Appearance of external genitalia.

By the middle of this month, all major organs, systems have formed.

The heart has formed four chambers (by 7th week)

The fetus assumes a human appearance

Bone ossification begins

Rudimentary kidney begins to secrete urine.

4. Fourth lunar month: 11 to 14 weeks old fetus

At the end of 14 weeks of growth, the fetus is 12 cm crown to rump length and 110 gm.

Head erect; lower extremities well developed.

Hard palate and nasal septum have fused.

External genitalia of male and female can now be differentiated.

Eyelids are sealed

5.Fifth lunar month: 15 to 18 weeks old fetus At the end of 18 weeks of growth, the fetus is 16 cm

crown rump length and 320 gm.

Ossification of fetal skeleton can be seen on X-ray.

Ears stand out from head.

Fetus makes sucking motions and swallows amniotic fluid.

Fetal movement may be felt by the mother (end of month)

6. Sixth lunar month: 19-22 weeks old fetus At the end of 22 weeks of

growth, the fetus

is 21 cm crown rump length and 630 gm.

Vernix caseosa covers the skin.

Head and body hair (lanugo) visible.

Skin is wrinkled and red.

Brown fat(adipose tissue), an important site of heat production, is present in neck and sternal area.

7. Seventh lunar months: 23-26 weeks old fetus At the end of 26 weeks of

growth, the fetus is 25 cm crown to rump length and 1000 gm.

Fingernails present.

Lean body

Eyes partially open: eyelashes present.

Bronchioles are present: primitive alveoli (terminal sacs) are forming

Skin begins to thicken on hands and feet

Startle reflex present, grasp reflex is strong.

8. Eighth lunar month: 27-30 weeks old fetus At the end of 30 weeks

of growth, the fetus is 28 cm crown rump length and 1,700 gm.

Eyes open

Ample hair on head: lanugo begins to fade

Skin slightly wrinkled

Toe nails presents

Testes in inguinal canal, begin descent to scrotal sac

Surfactant coat in much of the alveolar epithelium

9. Ninth lunar months: 31-34 weeks of old fetusAt the end of 34 weeks of growth, the fetus

is about 32 cm crown –rump length and 2,500 gm

Fingernails reach finger tips

Skin pink and smooth

Testes in scrotal sac

10. Tenth lunar month: 35 to 38 weeks old fetus End of 38 weeks of growth, fetus is about 36 cm

crown –rump length and 3400 gm.

Ample of subcutaneous fat present

Lanugo is decreasing

Toe nails reach upto toe tips

Testes in scrotum

Vernix caseosa mainly on the back

Breasts are firm.

Weeks 3-4

Weeks 5-8 Embryo will develop all structures that an adult has by

the end of week 8

Fetal Period

9-12

13-16

17-20

21-38