Detailed Notes on Embryonic Development

Zygote Transformation

• The zygote undergoes transformation as it moves down the oviduct.
• It goes through cleavage, dividing from two to four to eight cells.
• At 72 hours (3 days later), it becomes a morula.
• It then undergoes blastulation, becoming a blastocyst.
• The blastocyst, still in the oviduct, consists of three sections:
  • A fluid-filled area that will become amniotic fluid.
  • Trophoblast cells, which have three main functions.

Trophoblast Cells

The three main functions of trophoblast cells:

• Part of the Placenta: They eventually become part of the placenta on the baby side, contributing to the chorion layer.
• Embedding in Endometrium: The blastocyst, after leaving the oviduct and entering the uterus, uses enzymes released by the trophoblast cells to embed itself in the endometrium lining. This prevents it from simply rolling around and getting stuck in the cervix. The enzymes digest into the endometrium, helping the blastocyst secure itself. This embedding process starts around day 5-7 after fertilization.
• HCG Production: After implantation, the trophoblast cells produce and secrete the pregnancy hormone, human chorionic gonadotropin (HCG). This hormone is detectable in pregnancy tests, indicating a medical pregnancy.
  • From fertilization to actual implantation takes about 10-14 days.

Ectopic Pregnancies

• Ectopic pregnancies occur when the blastocyst embeds itself outside of the uterus, most commonly in the oviduct.
  • Ectopic pregnancies are not salvageable, always requiring medical intervention (surgical removal of part of the oviduct or cervix).
• Reasons are due to the the cilial function slowing down, not working their best.
• Women that smoke cigarettes with nicotine actually slow down the cilial function making them two times more likely to have ectopic pregnancies.
• Removal of part of the oviduct reduces future pregnancy chances since ovulation doesn't occur every month in the remaining ovary.

Role of HCG Hormone

• The HCG hormone has the same effect as luteinizing hormone (LH).
• LH triggers the making of the corpus luteum during the luteal phase.
• The goal is to maintain the endometrial lining to prevent menstruation(miscarriage).

Hormone Levels & Corpus Luteum

• During the first two months of pregnancy, the corpus luteum, needs to be maintained to keep producing progesterone and estrogen.
• After about two months, the placenta fully forms and takes over the function of producing estrogen and progesterone.
  • HCG tells the corpus luteum to keep doing its job, which is to produce progesterone and estrogen to maintain the endometrium lining within the first two months.
  • Once the placenta is fully formed, it will take over the role of the corpus luteum.

Graphical Representation of Hormone Levels

• Graph shows months of pregnancy with corresponding hormone levels (HCG, progesterone, and estrogen).
• Hormones suppress menstruation, follicle development, and ovulation to maintain the uterine lining.
• During the first two months, trophoblast cells produce high quantities of HCG, directing the corpus luteum to produce progesterone and estrogen.
• After two months, the fully formed placenta takes over the role of the corpus luteum, producing progesterone and estrogen (same goal, different structure).

Placenta's Role & HCG Levels

• The placenta doesn't need direction from the hypothalamus to keep producing progesterone and estrogen.
• HCG levels drop when the placenta is fully formed, but never go down to zero (pregnant women test positive regardless of the month).
• The placenta continues producing higher levels of progesterone and estrogen as the pregnancy progresses.
• High progesterone and estrogen levels can cause fatigue, emotional changes, breast tenderness, and water retention.

Progesterone and Estrogen

• High levels of progesterone and estrogen help suppress the ability to produce breast milk during pregnancy (energy conservation).
• In high-risk pregnancies, breastfeeding is discouraged as oxytocin production can cause uterine contractions and potential miscarriage or stillbirth.
• The trophoblast layers eventually become part of the placenta, which secretes HCG, progesterone, and estrogen.
• Concerns about placental detachment due to critical role

Pluripotent Cells

• As a morula, any of the cells can be taken because the cells are undifferentiated, unspecialized, and are known as pluripotent cells.
• Pluripotent cells are very different from the totipotent cells that some other organisms have.

Embryonic Stem Cells

• If you remove pluripotent cells, you can get embryonic stem cells.
• These can can be used to take over the role and correct for extreme syndromes
• Stem cells can be harvested from the blastocyst.
• Umbilical Cord & Banking Process
  • Stem cells can also be harvested from the umbilical cord/blood.
  • The umbilical cord itself as stem cells.
  • There are banks where you can cryogenically freeze your blood cords if you pay for them. The bodies won't reject it if you use their own blood cord.
  • The stories of parents becoming pregnant to tap the umbilical cord and use it on their children that have cancer comes from this.

Gastrulation

• After blostulation, the body will undergo gastrulation and now be called the gastrula.
• During gastrulation the blastocoal will open up more and will be called the amniotic cavity.
  • This is what releases the fluid when a woman breaks her water.
• The primary germ layers are created:
  • The little cell mass begins to flatten itself by squeezing over.
  • The inner cell mass then has a name know as the embryonic disc.

Primary Germ Layers

The 3 primary germ layers are:

• Outer Layer (Ectoderm)
• Middle Layer (Mesoderm)
• Inner Layer (Endoderm)

Ectoderm

• The outer layer that is closest to the amniotic fluid.
• This will form the nervous system, skin, teeth, hair, and certain glands.

Mesoderm

• The middle layer that connects the endoderm.
• This will form connective tissue, muscles, bones, gonads, and the circulatory system

Endoderm

• The inner layer that is closest to the wall of the uterus itself, the endometrium lining.
• This will form the gastrointestinal system, including endocrine glands, the gastrointestinal tract, pancreas, liver, and gallbladder.
• The ectoderm produces the nervous system, but the mesoderm produces the skeletal structure.
• Primary germ layers tells you that any of the cells are located in each layer and that is what the cells are going to become.
  • Germinate means to germinate a seed in biology.

Morphogenesis

• Morphogenesis is when an organism morphs.
• Larvae morphing into a moth or butterfly
• Germ layers starts to actually develop and morph into an actual human shape.

Sequence Of Terms

• Zygote, cleavage, morula, blostulation, blastula, gastrulation, gastrula, morphogenesis

Differentiation

• Structures have a different functions.
• Structure will always = function.

Development

• After 4, 6, 10 days of being implanted in the uterus we have the gastroloft in the second week.
• One, two, three layers of ectoderm, mesoderm, and endoderm.
• During week 3 is when we get the nervous system and skeletal structure framework set up.
• What happens is that the middle germ layer cells, the mesoderm cells, begin to form the nodal cord.
• After gastrulation and morphogenesis is when we undergo the process known as neurulation.
• Neurulation leads to the ectoderm producing the nervous system and just about the nodal cord.
  • The spinal cord encases the vertebrae's.
• Skeletal structure and central nervous system get created during this time from fertilization.

Groupings

• Week 3: nervous system development and the neurological system
• Neurulation is the creation of the neural tube.
• Formation of extraembryonic membranes begin in the third week and will fully form by the eighth week. These four membranes take five weeks to form.

Chorion Layer

• Cells will eventually produce and lead to the chorion layer and this will eventually become the fetal side of the placenta which is the wall that joins the mother and a baby.

Amniotic Sac

• Amnotic fluid exits out of the body when the water breaks.
• Serves as a membrane for the embryo.

Amniotic Sac Functions

• Air bag
• The protection from any physical trauma or fluids.
• Helps to regulate any of the temperature fluctuations.