Embryology
Unit 20 Synopsis
Pregnancy
Gestation period: time from the last menstrual period to the birth of the baby, lasts approximately 280 days
When fertilization occurs, the mother is officially 2 weeks pregnant
Embryonic period: fertilization to week 8
Fetal period: week 9 to birth (parturition)
All controlled by hormones. Reminder
*E4: estradiol
*P2: progesterone
*hCG: human chorionic gonadotropin - acts like LH but is not secreted through the HPG axis (released from CL)
Fertilization
Fertilization occurs within the ampullar isthmic junction within the fallopian tube. Sperm cells must travel through the female reproductive tract to the fallopian tube. The motile sperm can live in the female reproductive tract for up to 5 days. The spermatozoon must first penetrate the corona radiata, the layer of cells surrounding the oocyte. Below the corona radiata is the zona pellucida. This layer is designed to prevent polyspermy by creating a zona block once a sperm has penetrated it. At this point, the one sperm that made it within the zona block will melt into the oocyte cell membrane by releasing digestive enzymes from the acrosome cap. The male genetic information enters the oocyte, forming the male pronuclei that will combine with the female pronuclei to form the complete genetic profile with all 46 chromosomes (23 from mom and 23 from dad). This fertilized ovum is now referred to as a zygote. The cells will undergo mitosis, doubling in number. After the 16 cell stage, it becomes a blastocyst, which is the first step of differentiation. The blastocyst consists of the blastocoel, which is the open area, devoid of cells in the blastocyst, the inner mass and the trophectoderm, which are the cells along the outside of the blastocyst. Mitosis will continue. Eventually, when enough cells are present, they will further differentiate and undergo a process called gastrulation, where tissue layers are formed.
Embryonic Development
Division
The zygote will travel down the Fallopian tube to the uterus for implantation
Cleavage: period of rapid mitotic divisions of the zygote without growth to form a blastocyst
Purpose is produce small cells with a high surface area to volume ratio
To enhance nutrient and oxygen uptake and waste disposal
Morula: clump of 16 or more cells, formed about 72 hours after fertilization
Blastocyst: fluid filled sphere with a single layer of trophoblast cells (that will develop into the placenta) and a cluster of cells known the embryoblast (that forms the embryo)
Implantation
Occurs around day 6 or 7, the blastocyst embeds itself into the endometrium of the uterine wall
Takes 5 days to complete the implantation process (about day 12 post-fertilization and around day 26 of the woman's cycle
*Prior to implantation the blastocyst is being nourished by secretions in the uterine cavity as it floats through
*The blastocyst is able to implant by essentially eroding the wall of the endometrium so that it can burrow in. The endometrial cells continue to proliferate, which will eventually cover and seal off the implanted blastocyst from the uterine cavity
*The endometrium is receptive to the blastocyst because of surging levels of estrogens and progesterone during this time. (One issue infertile women can have is having low progesterone levels, which affects implantation. This is an easy fix though and usually all she needs to do is take progesterone supplements)
*Failure to implant will lead to degradation of the endometrium, resulting in a period of menstruation
*Important note: Implantation fails more often than not! It is estimated that 2/3 of all zygotes fail to implant. Of the 1/3 that do implant, 30% of those will later miscarry due to genetic defects of the embryo, uterine malformation, or other problems.
Differentiation
The embryo secretes hCG, a hormone which prompts the corpus luteum to keep secreting progesterone and estrogens
*This is the hormone that modern pregnancy tests detect as it only should be present in pregnancy
*hCG also helps to prevent the mother’s immune system from rejecting the implanted embryo.
Placenta = temporary organ made from embryonic and maternal tissues, forming during the 2nd month
Takes over as the nutritive, respiratory, excretory, and endocrine organ by the end of the 3rd month
*The placenta takes over hormone production, hCG declines around month 3
*When the placenta takes over, the corpus luteum degenerates and the ovaries officially go inactive for the rest of pregnancy
In the first several weeks, the endometrium nourished the nascent embryo
Weeks 4-12: developing placenta gradually takes over the role of feeding the embryo
The placenta is developed from part of the embryo and part of the endometrium
The placenta is connected to the conceptus via the umbilical cord, where oxygen and nutrients are transported
Maternal and fetal blood do not commngle because blood cells cannot move across the placenta and prevents the cytotoxic T cells from reaching and destroying the fetus
Placentation is complete by weeks 14 to 16
Gastrulation
Gastrulation is the formation of germ layers: the endoderm, ectoderm, and mesoderm.
The endoderm forms the digestive tract, respiratory tract, liver, and pancreas.
The ectoderm forms the outer layer of skin, hair, lining of the nose and mouth, and the nervous system
The mesoderm forms the muscle and skeleton
Organogenesis: the process of body organ and organ system formation that proceeds gastrulation
Fetal Development
Before 9 weeks of gestation, the conceptus is referred to as an embryo; after 9 weeks, it is now considered a fetus and all the adult organ systems are recognizable. At this time the U/S should show a healthy heartbeat and averages 170 bpm (up from 110 at 5 weeks)
Pregnancy lasts 40 weeks (counting from the first day of the last menstrual cycle) or about 38 weeks (counting from conception)
Embryonic Period
Amniotic sac forms: a fluid-filled sac that contains and protects the embryo inside the placenta
Umbilical cord: connects the embryo to the placenta
Fetal Period
Month 2:
All major brain regions and limbs are present.
Ossification begins
Cardiovascular system is fully functional
Month 3:
Hematopoiesis begins
Lungs begin to develop
Baby can open and close its fist
Lungs
Fetal lungs are nonfunctional. Fetal circulation will bypass the lungs by shifting the blood through a shunt called the foramen ovale which will directly connect the right and left atria
Sexual Differentiation
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Determined by inherited chromosomes (Female - XX; Male - XY)
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Males have an SRY gene on the Y chromosome that will override the female development and begin production of specific hypothalamic hormones and androgens for male gonad development
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Differentiation begins around 9 to 12 weeks into gestation
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All fetuses have bipotential gonads that can develop into either male or female
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Males will develop the Wolffian ducts that will develop into the vas deferens
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Females will develop the Mullerian ducts that will become the uterine tubes and uterus
Month 4:
Baby can blink
Sucks thumb
Yawns
Reproductive organs are fully developed
Bones become distinct
Glands develop in the GI tract
Month 5:
Hair starts to grow and limbs start to reach final proportions
Mother can feel them moving (quickening)
Month 6-7:
Major weight gain
Myelination of the spinal cord
Heading fully develops
Months 8-9:
Fat accumulation
Thickening of the skin
Rapid brain and lung development
Trimesters
Three trimesters that are 13 weeks each
Before week 12, the CL produces progesterone to support pregnancy and helps suppress menstrual cycling)
After week 12, the placenta will begin producing progesterone, as well as relaxin.
Relaxin increases the elasticity of the pubis symphysis and pelvic ligaments; and dilates cervix during labor
No caloric changes need to be made in the first trimester. In the second and third trimester, caloric intake should increase by 300 calories.
Pregnancy Symptoms
"Morning Sickness"
Misnomer - can occur any time of the day
Increased sensitivity to odors
Potentially due to increased levels of estrogen, progesterone, and human chronic gonadotropin or decreased levels of intestinal peristalsis
Typically subsides by week 12
Breasts swell
Later stages of pregnancy
Uterus expands from fist-sized to taking up the entire abdominal cavity (from diaphragm to bladder), in part due to relaxin
Results in:
Gastric reflux (heartburn)
Upward constrictive pressure of growing uterus on stomach
Frequent urination
Downward pressure on bladder
Kidneys must process more waste
Constipation
Second half of pregnancy (after week 19)
Increases air intake by 50%
Dyspnea - shortness of breath from uterus putting pressure on the diaphragm
Dermis Stretches (Stretch marks)
Blood volume increases by 40% by the 32nd week
Pre-parturition
Braxton Hicks contractions: false labor caused by decreasing levels of progesterone that inhibit contractions
Bloody Show: the mucus plug that accumulated in the cervical canal blocking the entrance to the uterus will loosen and be expelled with a small amount of blood 1 to 2 days prior to the onset of true labor
Birth is regulated by a positive feedback mechanism
Posterior pituitary boosts oxytocin levels and increases sensitivity of the myometrium
As labor nears, oxytocin stimulated stronger, more painful uterine contractions, that stimulate the secretion of prostaglandins that enhance uterine contraction strength. At the same time the fetus' pituitary is secreting oxytocin which will increase prostaglandins
Progesterone decreases to allow for uterine contractions
Fetus releases cortisol to allow the placenta to release more estrogen
Fetus secretes oxytocin to allow the placenta to release prostaglandins
Stages of Childbirth
Stage 1: Cervical Dilation
Longest stage of the birthing process, that lasts around 6 to 12 hours
Broken down into three stages:
Early labor: from start to 3 cm
Active labor: 3 to 7 cm
Transition: 7 to 10 cm
Dilate fully to 10 cm and efface (thin) 100%
Contractions push infants head against cervix, causing it to thin and dilate
At the start, uterine contractions are every 3 to 30 minutes and last about 20 to 40 seconds
At the end, contractions are every 1.5 to 2 minutes apart and last about 1 minute
Each contraction sharply reduces the amount of oxygenated blood flow to the fetus, therefore relaxation is critical
Stage 2: Explusion stage
Begins with the fetal head in the birth canal and ends with the birth of the newborn
Typically takes up to 2 hours
Breech babies: incorrect positioning (buttocks first)
Occurs in less than 5% of births
Doctors will generally perform a Caesarean section
When the newborn's head is out, the mucus is aspirated from the mouth and nose.
Stage 3: After birth
Delivery of the placenta and associated membranes