Fetal development - 2

Embryonic period

The embryonic period in humans begins at fertilization (12-24hrs after ovulation, generally between the 2nd and 3rd week of gestational age) and continues until the end of the 10th week of gestation (8th week by embryonic age).

The zygote spends the next few days traveling down the Fallopian tube. Meanwhile it divides several times to form a ball of cells called a morula. Further cellular division is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst. Up to this point there is no growth in the overall size of the embryo, so each division produces successively smaller cells.

The blastocyst reaches the uterus at roughly the fifth day after fertilization. It is here that lysis of the zona pellucida, a glycoprotein shell, occurs. This is required so that the trophectoderm cells, which give rise to extra-embryonic structures such as the placenta, of the blastocyst can come into contact with the luminal epithelial cells of the endometrium. (Contrast this with zona hatching, an event that occurs in vitro by a different mechanism, but with a similar result). It then adheres to the uterine lining and becomes embedded in the endometrial cell layer. This process is also called implantation. In most successful pregnancies, the conceptus implants 8 to 10 days after ovulation (Wilcox et al. 1999). The inner cell mass forms the embryo, while the outer cell layers form the membranes and placenta. Together, the embryo and its membranes are referred to as a conceptus, or the "products of conception".

Rapid growth occurs and the embryo's main external features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, or miscarriage, in the first trimester of pregnancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the first trimester), the developing embryo is also susceptible to toxic exposures, such as:

Alcohol, certain drugs, and other toxins that cause birth defects, such as Fetal alcohol syndrome

Infection (such as rubella or cytomegalovirus)

Radiation from x-rays or radiation therapy

Nutritional deficiencies such as lack of folate which contributes to spina bifida

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny." For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain, develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.

Gestational age vs. embryonic age

Gestational age is the time that has passed since the onset of the last menstruation, which generally or as standard occurs 2 weeks before the actual fertilization. Embryonic age, in contrast measures the actual age of the embryo or fetus from the time of fertilization. Nevertheless, menstruation has historically been the only means of estimating embryonal/fetal age, and is still the presumed measure if not else specified. However, the actual duration between last menstruation and fertilization may in fact differ from the standard 2 weeks by several days.

Thus, the first week of embryonic age is already week three counting with gestational age.

Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization.

The following table summarizes the various expression systems during week number x of gestation.

Week

A GREAT JOURNEY, PREGNANCY PROGRAM!

number Reached age

(whole weeks)

Gestational x x-1

Embryonic x-2 x-3

Week 1-2

Gestational age: 0 to 1 (whole) weeks old. 1–14 days from last menstruation.

Embryonic age: -2 to -1 weeks old. That is, week 1-2 of gestational age are merely theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.

Week 3

Gestational age: 2 (whole) weeks old. 15–21 days from last menstruation.

Embryonic age: Week nr 1. 0 (whole) weeks old. 1–7 days from fertilization.

Fertilization of the ovum to form a zygote. (day 1 of fert. )

The zygote undergoes mitotic cellular divisions, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage. (day 1.5-3 of fert. )

The blastocyst contains only a thin rim of trophoblast cells and a clump of cells at one end known as the "embryonic pole" which include embryonic stem cells.

The embryo hatches from its protein shell (zona pellucida) and performs implantation onto the endometrial lining of the mother's uterus. (day 5-6 of fert. )

If separation into identical twins occurs, 1/3 of the time it will happen before day 5.[6]

Week 4

Gestational age: 3 weeks old. 22–28 days from last menstruation.

Embryonic age: Week nr 2. 1 week old. 8–14 days from fertilization.

Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes. The blastocyst is fully implanted day 7-12 of fert.

Formation of the yolk sac.

The embryonic cells flatten into a disk, two cells thick.

If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined.

Primitive streak develops. (day 13 of fert. )

Primary stem villi appear. (day 13 of fert. )

Week 5

Gestational age: 4 weeks old. 29–35 days from last menstruation.

Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization.

A notochord forms in the center of the embryonic disk. (day 16 of fert. )

Gastrulation commences. (day 16of fert. )

A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear. (day 18 of fert. )

Somites, the divisions of the future vertebra, form. (day 20 of fert. )

Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fert. )

Embryo at 4 weeks after fertilization. The crown-rump length is about 0.2 inches.[7]

A 10mm embryo from an ectopic pregnancy, still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age). Week 6

Gestational age: 5 weeks old. 36-42 days from last menstruation.

Embryonic age: Week nr 4. 3 weeks old. 22-28 days from fertilization.

The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.

The heart bulges, further develops, and begins to beat in a regular rhythm. Septum primum appear.

Branchial arches, grooves which will form structures of the face and neck, form.

The neural tube closes.

The ears begin to form as otic pits.

Arm buds and a tail are visible.

Pulmonary primordium, the first traits of the lung appear.

Hepatic plate, the first traits of the liver appear.

Buccopharyngeal membrane ruptures. This is the future mouth.

Cystic diverticulum, which will become the gallbladder, and dorsal pancreatic bud, which will become the pancreas appear.

Urorectal septum begins to form. Thus, the rectal and urinary passageways become separated.

Anterior and posterior horns differentiate in the spinal cord

Spleen appears.

Ureteric buds appear.

This embryo is also from an ectopic pregnancy, this one in the cornu (the part of the uterus to which the Fallopian tube is attached). The features are consistent with a developmental age of seven weeks (reckoned as the ninth week of pregnancy).

Week 7

Gestational age: 6 weeks old. 43–49 days from last menstruation.

Embryonic age: Week nr 5. 4 weeks old. 29–35 days from fertilization.

The embryo measures 8 mm (1/4 inch) in length.

Lens pits and optic cups form the start of the developing eye.

Nasal pits form.

The brain divides into 5 vesicles, including the early telencephalon.

Leg buds form and hands form as flat paddles on the arms.

Rudimentary blood moves through primitive vessels connecting to the yolk sac and chorionic membranes.

Week 8

Gestational age: 7 weeks old. 50–56 days from last menstruation.

Embryonic age: Week nr 6. 5 weeks old. 36–42 days from fertilization.

The embryo measures 13 mm (1/2 inch) in length.

Lungs begin to form.

The brain continues to develop.

Arms and legs have lengthened with foot and hand areas distinguishable.

The hands and feet have digits, but may still be webbed.

The gonadal ridge begins to be perceptible.

The lymphatic system begins to develop.

Week 9

Gestational age: 8 weeks old. 57–63 days from last menstruation.

Embryonic age: Week nr 7. 6 weeks old. 43–49 days from fertilization.

The embryo measures 18 mm (3/4 inch) in length.

Fetal heart tone (the sound of the heart beat)can be heard using doppler.

Nipples and hair follicles begin to form.

Location of the elbows and toes are visible.

Spontaneous limb movements may be detected by ultrasound.

All essential organs have at least begun formation.