Early Development
The Developmental Period
Clinically, pregnancy is considered to be 40 weeks; beginning from the time of the woman's last menstrual period. This timing is significant because it is based on information the patient can provide, i.e. date of her last period. This 40 week time frame can be divided into three equal segments called trimesters.
Since ovulation generally occurs in the second week of a normal 28 day cycle, human development from an academic perspective, is considered to be approximately 38 weeks; beginning with fertilization and ending at birth or parturition.
Table 1: The divisions of the developmental period.
The first eight weeks constitute the embryonic period in which the basic body plan is established and the building blocks of the organ systems are laid down.
The remaining thirty weeks constitutes the fetal period, characterized by growth and maturation of the fetus in preparation for life outside the womb.
Much of the discussion to follow is focused on the events of the embryonic period as that is the period of development that is most perilous in terms of susceptibility to congenital anomalies caused by teratogens or genetic abnormalities.
Table 2: Details of the embryonic period. The first four weeks are concerned with several events that are highlighted by pink boxes. The stages of development that are related to these events are highlighted by blue boxes. Beginning at 5 weeks, the details of development of each specific organ system is considered.
The first four weeks are occupied by:
- fertilization
- implantation
- gastrulation
- mesoderm differentiation
- neurulation
- establishment of a body plan
- folding of the embryonic disc to form a "tube within a tube structure"
The last four weeks are occupied by:
- organogenesis
- organ placement
- differentiation
- growth
G = gravida and indicates the number of pregnancies
P = para to indicate the number or live births
A = aborta to indicate the number of terminated pregnancies, whether spontaneous or surgical
L = living offspring.
If a patient's chart reads G3, P2, A1, L2 it indicates they had 3 pregnancies, 2 live births, 1 miscarriage and 2 living children.
I'm mentioning it here because this notation is often used in clinical stems for examination questions.
Week 1
Two events must precede fertilization; ovulation & ejaculation
Ovulation results in the release of a secondary oocyte from a mature ovarian follicle. This oocyte is guided into the ampulla of the uterine tube by the action of fimbriae that surround its orifice..
If intercourse has occurred with ejaculation of sperm into the vagina, sperm may navigate through the female reproductive tract to reach the secondary oocyte and fertilization may occur.
Week One of development begins with fertilization a process resulting in the generation of a new individual, the zygote.
In the first week following fertilization, the zygote undergoes cleavage, a series of mitotic divisions, in which each cell division results in a doubling of the cell population without an increase in cytoplasmic mass.
Fig 1. Animated gif image depicting the major events of Week 1: ovulation, fertilization, cleavage and implantation.
As the zygote divides, it moves through the uterine tube toward the uterus. It is a journey of about 4 to 6 days. By the time it reaches the uterus, is has become a hollow ball of cells called the blastocyst that begins to invade into the wall of the uterus in a process known as implantation.
Week 2
Fig 2. Drawing of a day nine concepts as it implants into the endometrium showing the subdivisions of the embryonic disc and trophoblast as well as the amniotic cavity and yolk sac.
During week 2 of development, the blastocyst is busy implanting into the wall of the uterus while also undergoing differentiation.
Week Two of development is often referred to as the "week of two's" because of the appearance of "pairs" of structures.
As the blastocyst invades the endometrium during the second week post fertilization, the following "pairs" of structures make their appearance:
- Trophoblast has divided into cytotrophoblast (turquoise) and syncytiotrophoblast (green).
- Embryo consists of two layers, epiblast (blue layer) and hypoblast (yellow layer)
- Two cavities are present, amniotic cavity and yolk sac
Two layers of extraembryonic mesoderm form later in the week:
- extraembryonic splanchnic mesoderm
- extraembryonic somatic mesoderm
Weeks 3 & 4
During weeks 3 and 4 of the embryonic period several transformative events occur, setting the stage for development of organs and organ systems. These generally occur in overlapping succession and include:
- Gastrulation
- Notochord Formation
- Mesoderm Differentiation
- Neurulation
- Development of a Body Plan
- Embryonic Folding
Fig 3. A cross section of an embryo at the end of week 4 shows some derivatives of and relationships between ectoderm, mesoderm & endoderm, colored blue, red, yellow respectively. At this point, the embryonic disc is ready to fold in the transverse plane and to assume a 'tube within a tube' body form.
By the end of week four, the embryo has folded upon itself to transform from a flat disc to a "tube within a tube" configuration.
A tube shaped body containing a neural tube, heart tube, gut tube and a tubular body cavity. Note the outside of the tube, ectoderm is the layer that interacts with the environment. The inner lining of the gut tube, endoderm extracts essentials from the environment for sustenance while the middle layer, mesoderm makes up the tissues needed to be animate in the environment.
Fig 4. Illustration of the tube within a tube concept. Color coded boxes list the major derivatives from each of the three germ layers of the embryo.