Development

The anatomical and physiological changes from fertilization through birth are referred to as prenatal development.

Note that development (gestational) weeks are not the same as pregnancy weeks. Pregnancy weeks start at the first day of the last menstrual cycle. Gestational weeks start two weeks later, at fertilization. This unit uses gestational weeks.

Fertilization

The secondary oocyte was released from the ovary in the process known as ovulation (day 14 of the ovarian cycle). It is taken into the uterine tube and if sperm are present then the process known as fertilization may occur. Most frequently this happens in the ampulla of the uterine tube. 

1. Sperm has penetrated the corona radiata but hasn't made it through the zona pellucida yet.

2. The sperm fertilizes the secondary oocyte by penetrating the zona pellucida. The head of the sperm is located in the cytoplasm of the secondary oocyte. The zona pellucida hardens, preventing additional sperm from entering the secondary oocyte.

3. The secondary oocyte completes meiosis II to form the ovum.

4. The female pronucleus and the male pronucleus migrate towards one another. (note the polar bodies)

5. The pronuclei combine, forming the zygote. This is the first cell stage of a new individual. This begins day 1 of gestation.

6. The first mitotic division of cleavage occurs.

Cleavage

As the zygote travels from the ampulla to the isthmus of the uterine tubes, it begins a series of mitotic cell divisions known as cleavage. The zygote will divide into two cells, then 2 to 4 cells, 4 to 8 cells, 8 to 16 cells, and 16 to 32 cells. This process takes about three days, and the final product is the 32-cell stage called a morula. Even though it is 32 cells, the morula is about the same size as the original zygote, because the cells do not grow between cell divisions. Each individual cell is called a blastomere. 

The morula will continue to travel from the isthmus of the uterine tube to the uterus. As it enters the uterus, it begins to hollow out and form a fluid-filled ball called a blastocyst. The early blastocyst is formed around 4 days after fertilization, and is free-floating in the uterus.

Blastocyst

The blastocyst stage lasts from day 4  through day 14 after fertilization. The early blastocyst forms two types of cells:

1. trophoblasts - forms the chorion which will eventually forms the fetal placenta. 

2. inner cell mass - these cells will give rise to the embryo itself. The inner cell mass will form two layers (bilaminar disc): 

   • epiblasts are adjacent to the trophoblasts (and later the amnion)

   • hypoblasts are adjacent to the blastocyst cavity (and later the yolk sac)

The embryonic period begins with implantation and continues to week 9. Implantation begins around day 7 after fertilization. The trophoblasts of the blastocysts begin to establish a connection to the endometrium, and this will eventually become the placenta. Once the blastocyst begins to adhere to the endometrium, it is called an implanting blastocyst. 

The blastocyst stage continues until day 14 after fertilization. The late blastocyst is fully implanted into the endometrium, and the endometrium actually grows over the top of the blastocyst. The blastocyst cavity remodels into a extraembryonic coelom. The extraembryonic membranes are forming. This includes:

1.  the chorion - trophoblasts form the chorion and the chorion gives rise to the fetal placenta

2. the yolk sac - in humans this structure does not nourish the embryo, it gives rise to the first blood cells

3. the amnion - forms a protective sac (amniotic sac) around the embryo/fetus; produces amniotic fluid to cushion the developing embryo/fetus

4. the allantois - a protrusion of the yolk sac within the body stalk; origin of the umbilical cord

Gastrulation

Around day 15-16 after fertilization, a third layer of cells begins to form between the hypoblasts and epiblasts. This process is known as gastrulation and it results in the formation of the three germ layers of the embryo, now called a gastrula. The three new layers (trilaminar disc) are known as:

1. ectoderm - This layer is adjacent to the amniotic cavity. 

   • Ectoderm forms the epidermis, accessory skin structures, nervous system, special sense organs, epithelia of the oral cavity, nasal cavity and anal canal, adrenal medulla, pituitary and pineal glands

2. mesoderm - This is the middle layer. 

   • Mesoderm forms the dermis, skeleton, cartilage, most muscle tissue, blood vessels, kidneys, ureters, most lymphoid organs, internal reproductive organs, serous membranes, and the adrenal cortex.

3. endoderm - This layer is adjacent to the yolk sac. 

   • Endoderm forms the epithelial linings of the respiratory tract, GI tract, urinary tract, and reproductive tract. It also forms the liver, gallbladder, pancreas, thymus gland, parathyroid gland, and thyroid gland. 

Embryo

The 4-week embryo has developmentally distinct areas, with cephalic/caudal, ventral/dorsal, and right/left surfaces easily discernable. The beginnings of the eye can be located. The human heart starts beating about six weeks after fertilization. Arm buds and leg buds also appear. 

Placentation is the formation of the placenta. The fetal placenta forms as the trophoblasts of the chorion develop into vascular chorionic villi, which extend into the endometrium. The endometrium directly below the fetal placenta forms the maternal placenta. During pregnancy, the endometrium has regional differences:

1. decidua basalis - the area of endometrium directly below the embryo and fetal placenta - will form the maternal placenta

   • Develops intervillous spaces that interdigitate with the chorionic villi, to allow for exchange of vital substances between mother and fetus.

2. decidua capsularis - endometrium covering the top of the embryo

3. decidua parietalis - endometrium that has no direct contact with the embryo or chorion

Fetus

The fetal period of development starts at the 9th week of gestation. The human form of the fetus is clearly identifiable because all of the major tissues and organs are now in place. The focus of development during this period is on continued differentiation and growth of tissues, organs, and organ systems.

Placenta

The placenta is a temporary organ that permits the exchange of oxygen, carbon dioxide, nutrients and waste between the fetus and the mother. The fetal blood does not mix with maternal blood. Instead, exchange occurs via diffusion between the chorionic villi of the fetal placenta and intervillous spaces of the maternal placenta. The fetal side of the placenta is shiny and smooth. The maternal side is ragged and dull.

Fetal blood travels to and from the placenta via the umbilical cord. Inside the cord, two (2) umbilical arteries carry carbon dioxide and wastes away from the fetus. This fetal blood enters chorionic villi where the wastes diffuse into the intervillous spaces, which contain maternal blood. Simultaneously, oxygen and nutrients diffuse from the intervillous spaces into the chorionic villi. This recharged blood will then travel back to the fetus via the single (1) umbilical vein.