10.3: Pre-Embryonic Period
After fertilization (around 24 hours after fertilization), the zygote begins a process of dividing by mitosis in a process called cleavage . It divides until it reaches 16 cells. It is now referred to as a morula . As the morula floats freely within the uterus, it starts to bring nutrients into the cells. The morula fills with fluid and the cells inside start to form two separate groups. At this stage it is now a blastocyst . The inner layer of cells is called the embryoblast, and will become the fetus. The outer layer is called a trophoblast, which will develop into part of the placenta.
Implantation
The blastocyst preserves itself by secreting Human chorionic gonadotropin (hCG) , a hormone that indirectly stops menstruation. The trophoblast cells secrete hCG hormones that help maintain the corpus luteum, which would normally regress. In turn, the corpus luteum continues to secrete progesterone, which maintains the endometrium, the mucous membrane lining of the uterus, in the secretory phase. This helps the blastocyst to continue to grow and stay embedded within the endometrium. The fetal life support system and the placenta begin to form, and eventually the placenta will take over the job of producing progesterone.
The embryoblast within the blastocyst forms 3 primary germ layers: the ectoderm, mesoderm, and endoderm, which make up the developing fetus.
Ectoderm
The ectoderm forms the nervous tissue and the epithelium covering the outer body surface. The ectoderm includes. The epidermis of skin, including hair and nails, glands of skin, linings of oral cavity, nasal cavity, anal canal, vagina, brain, spinal cord, sensory organs, lens of eye and epithelium of conjunctiva (a membrane that covers the sclera and lines the inside of the eyelids), pituitary gland, adrenal medulla, and enamel of teeth.
Mesoderm
The mesoderm forms all of the muscle tissue and the connective tissue of the body, as well as the kidneys and the epithelium of the serous membranes and blood vessels. The mesoderm includes all muscle tissue (skeletal, smooth, cardiac), all connective tissue (fibrous connective tissue, bone, blood, cartilage), dentin of teeth, adrenal cortex, kidneys and ureters, internal reproductive viscera, epithelium lining vessels, joint cavities, and the serous body cavities.
Endoderm
The endoderm forms the lining epithelium and glands of the visceral body systems. The endoderm include the lining epithelium and glands of digestive, respiratory, and parts of urogenital systems, thyroid and parathyroid glands, and thymus.
Formation of the Placenta
As changes to the endometrium occur, cellular growth and the accumulation of glycogen cause fetal and maternal tissue to come together. This formation creates the functional unit called the placenta. The placenta does not mix blood between mother and fetus, but allows nutrients and waste products to diffuse between the two blood systems. The placenta provides protection by filtering out many, but not all harmful substances that the mother comes in contact with. The placenta cannot protect against some teratogens including but not limited to:
- Thalidomide
- Heroin
- Cocaine
- Aspirin
- Alcohol
- Chemicals in cigarette smoke
- Propecia, also known as Finasteride, which can cause birth defects simply by handling a broken pill during pregnancy.
Amniotic Fluid
Attached to the placenta is the membranous sac which surrounds and protects the embryo. This sac is called the amnion. It grows and begins to fill, mainly with water, around two weeks after fertilization. This liquid is called Amniotic fluid , it allows the fetus to move freely, without the walls of the uterus being too tight against its body. Buoyancy is also provided here for comfort. After a further 10 weeks, the liquid contains proteins, carbohydrates, lipids and phospholipids, urea and electrolytes, all of which aid in the growth of the fetus. In the late stages of gestation much of the amniotic fluid consists of fetal urine. The fetus swallows the fluid and then voids it to prepare its digestive organs for use after birth. The fetus also "breathes" the fluid to aid in lung growth and development.
Developing Fetus
The womb is expanding and the baby is growing while taking all needed nourishment from the mother. What started as a microscopic two-celled egg will be formed into a baby in just twelve weeks. The baby develops from conception to term, in a month-to-month progress.