Chapter 28- Pregnancy and human development Flashcards
Zygote
A diploid cell resulting from the fusion of two haploid gametes. Sometimes called a single celled embryo
Embryo
Stage of development from soon after the fertilization of the ovum to week 8 of development. No longer single celled at this point
Fetus
week 8 to birth, called an infant after birth
Fertilization definition
The process in sexual reproduction that involves the union of a sperm and an egg to form a diploid zygote
What is the time window for fertilization?
An oocyte is only viable for 24 hours at most after ovulation, and sperm are most viable 24-48 hours in the female reproductive system. General rule- in order for fertilization to occur- intercourse must occur no more than 2 days before ovulation and no more than 1 day after ovulation. Y chromosome sperm swim faster but are smaller, so they break down more quickly, but there’s not a large difference between Y and X in terms of lifespan
Why do only a fraction of sperm that enter the vagina reach the uterus? (2)
- Millions lost immediately by leaking from the vagina
2. Millions more lost by acidic environment of vagina- glycogen fermenting bacteria
How can sperm be lost in the uterus?
Phagocytes kill off many more sperm. Only a few thousand “survivors”- reverse peristaltic waves push these sperm toward the uterine tubes as the uterus contracts toward the fundus.
How do sperm travel in the right direction?
Sperm have various receptors (chemical, temperature, fluid flow) to ensure they travel in the correct direction
Sperm capacitation
Sperm are incapable of fertilizing the oocyte immediately after entering the vagina- want digestive enzymes to be weakened in the female anatomy but strong in the male anatomy. Before fertilization occurs- sperm must be “capacitated”- motility is enhanced and there is a weakening of membranes- allows easier release of enzymes that help with penetrating the egg. Can take hours after ejaculation
2 protective structures that surround the oocyte
- Zona pellucida- innermost layer
2. Corona radiata- outer layer
Zona pellucida
Innermost layer produced by primary oocyte and follicle. Protects secondary oocyte, is necessary for fertilization to occur
Corona radiata
Outer layer, contains cells that allows communication between cells and oocyte. Protects egg after it has been ovulated
Which structures must the sperm cross for fertilization to occur?
For fertilization to occur, sperm must be able to cross both structures in the egg. Sperm burrow through cells of the corona radiata
Acrosomal reaction
Sperm bind zona pellucida- calcium levels in sperm rise. Acrosomal enzymes released from the acrosome of the sperm- digest holes through the zona pellucida to the plasma membrane of the oocyte. One sperm’s enzymes are never going to be enough to allow fertilization to occur- to get one sperm through, the other sperm have to release all of their enzymes. The first sperm won’t fertilize the egg- neither will the first thousand.
What happens once the acrosomal reaction is initiated and the sperm gets through the layers? (2)
- The plasma membrane of the sperm binds to the sperm binding receptors on the plasma membrane of the oocyte
- Once bound, the two membranes fuse together- sperm contents enter the oocyte
Polyspermy
The entry of more than one sperm cell into the oocyte- results in incorrect chromosome number. Resulting embryo cannot mitotically divide- dies
2 ways the oocyte can block polyspermy
- Oocyte membrane blocks- sperm binding receptors are shed from oocyte surface. Sperm unable to bind oocyte surface and fertilize eggs with those receptors
- Cortical reaction
Cortical reaction
Endoplasmic reticulum of oocyte releases calcium in response to binding of single sperm cell. Granules release enzymes to destroy binding receptors and causes the zona pellucida to harden
How many fertilizing events spontaneously abort?
About 70% of fertilizing events spontaneously abort- unclear why, but likely due to chromosomal mismatch. People often spontaneously abort without knowing that they’re pregnant
Completion of meiosis 2 and fertilization (4 steps)
- Sperm nucleus must travel to the oocyte nucleus. As it travels, sperm nucleus swells in size to form male pronucleus
- Surge in calcium from cortical reaction causes release of zinc from oocyte. Meiosis 2 is completed as zinc leaves cell- forms 2nd polar body and mature ovum
- After meiosis 2- female pronucleus forms
- Pronuclei membranes rupture
What happens when the male pronuclei membranes rupture?
Chromosomes are released together, and maternal and paternal chromosomes combine to form a diploid zygote- this is fertilization. This is also the first appearance of the zygote
When does development begin?
Development begins at fertilization. Diploid cell (zygote) quickly begins to divide mitotically
Cleavage
Rapid mitotic divisions of the zygote. About 36 hours after fertilization- zygote has completed first division, contains 2 identical blastomere cells. 72 hours after fertilization- morula is formed- 16 cells
What is the benefit of cleavage?
Genetically identical cells are clumped together, which increases the surface area of the embryo- allows for better gas exchange, waste removal, and uptake of nutrients. A lot easier to build a functional person from many cells than from one single large cell- easier to start with many pieces you can put together however you want than with one large piece of stone that you have to carve to make the house
The morula divides to form
Morula continues to divide- forms blastocyst with 100+ cells
2 layers of the blastocyst
- External trophoblast
2. Internal embryoblast
External trophoblast
Single layer of flattened cells. Aids in embryo implantation, contributes to chorion formation/function, has immunosuppressive effects
Internal embryoblast
Cluster of 20-30 rounded cells. Will eventually form the embryo proper (and organs) and extraembryonic membranes
How is the blastocyst implanted in the endometrium?
Endometrium is receptive to implanting embryo due to high estrogen and progesterone levels. Cell adhesion molecules on endometrium bind the trophoblast when it makes contact with the lining, this usually occurs high in the uterus.
Once bound, what happens to the blastocyst?
Now called a trophoblast. Once bound, trophoblast releases digestive enzymes and growth factors on epithelium of endometrium
Trophoblast growth factors
Endometrium thickens as blood vessels enlarge and become leaky. Trophoblast proliferates, forms 2 layers- cytotrophoblast and syncytiotrophoblast.
Cytotrophoblast
Innermost layer. Mass of cells that anchors growing embryo to uterine tissue and release enzymes to facilitate implantation
Syncytiotrophoblast
Outermost layer. Mass of fused cells that produce and release enzymes to facilitate implantation
Trophoblast digestive enzymes
Responsible for erosion of endometrium around blastocyst. Blastocyst buries itself in the blood rich lining, and surrounding endometrial cells proliferate- covers the blastocyst
Human chorionic gonadotropin (hCG)
Released by embryo once implantation is complete. hCG is a hormone that prevents breakdown of the corpus luteum and suppresses female immune system. hCG drops at 4 months, corpus luteum breaks down. By this time, the placenta has formed and is making estrogen and progesterone
The placenta
A temporary organ originating from embryonic and maternal tissues. Functions- maintain pregnancy, exchange respiratory gasses, provide nutrients to embryo/fetus, dispose of waste, etc
When does the placenta become a fully functional organ?
The placenta is a fully functional organ by the 4th month of pregnancy. Will allow oxygen, nutrient, and waste exchange before this time, maternal and fetal blood do not mix
Embryonic contribution to placenta formation
Formation of the chorion membrane that surrounds embryo/fetus. Blood vessels extend from chorionic villi, form umbilical vein and arteries
Parental contribution to placenta formation (3)
- Functional layer of endometrium develops blood filled lacunae, which chorionic villi are immersed in
- Endometrium that lies underneath embryo becomes decidua basalis
- Endometrium that surrounds uterine cavity face of embryo forms decidua capsularis
Lacunae
Chorionic villi has bloods of blood (lacuna) from the parent, where exchange occurs between parent and fetus
Decidua basalis function
Forms the true placenta with the lacuna and chorionic villi immersed in it. Function- forms placenta with chorionic villi
Decidua capsularis
Endometrium that surrounds uterine cavity face of embryo forms decidua capsularis. Function- expands to accommodate the growing fetus. Chorionic villi here degenerate as pregnancy progresses
Vasculature of the placenta
Umbilical vein and 2 umbilical arteries
When do extraembryonic membranes form?
Membranes formed during the first few weeks of development. Embryoblast divides into epiblast and hypoblast to form the embryonic disc. Extraembryonic membranes and primary germ layers form from the disc and the divisions
Extraembryonic membranes include (4)
- Amnion
- Yolk sac
- Allantois
- Chorion
Amnion
Thin, transparent layer that extends completely around the embryo, except where umbilical vein and arteries enter. Filled with amniotic fluid- amniotic fluid is derived from fetal urine later in pregnancy
Functions of amniotic fluid (4)
- Provides buoyancy and protection- protects embryo from being crushed by its own body and acts as a shock absorber
- Maintains consistent temperature
- Prevents developing parts of embryo from sticking together/fusing
- Allows movement
Yolk sac definition
Sac structure that hangs from ventral surface of the embryo
Yolk sac functions (3)
- Eventually forms digestive tube
- Forms earliest blood cells
- Primordial germ cells move from here to gonads- forms sperm/ova
Allantois
Structural basis for the umbilical cord, will eventually form part of the urinary bladder
Chorion functions (2)
The chorion is the embryonic contribution to the placenta
- Contain chorionic villi- contact maternal blood and allow exchange of gasses, nutrients, and wastes
- Encloses all other extraembryonic membranes
Gastrulation definition
Cellular rearrangements and migrations of cells from 2 layered embryonic disc into a 3 layered embryo
3 primary germ layers
- Endoderm
- Ectoderm
- Mesoderm
Gastrulation process (4 steps)
- Begins with formation of primitive streak in the embryonic disc
- Epiblast cells migrate and enter the primitive streak
- Other cells follow, push between cells of upper and lower layers
- Some cells remain at the dorsal surface
Primitive streak
Primitive streak is a groove with raised edges that will form the long axis of the embryo
Notochord
Mesodermal cells immediately beneath primitive streak form the notochord. The notochord is the first axial support of the embryo
Organogenesis
Formation of body organs and organ systems, begins with gastrulation. Early process- all organ systems are recognizable by week 8
How does organogenesis occur?
When plates of gastrulation begin to form, they fold laterally soon after, like folding paper. Anterior and posterior ends will eventually fold over and close the tube, resulting in an embryo (looks kind of like a tadpole at first). The endoderm is the layer that is enclosed inside, and these cells fuse to seal off the embryo. The yolk sac hangs from the ventral portion of the embryo
What does the endoderm specialize into?
Folding process forms the primitive gut, endoderm becomes epithelial lining of GI tract. The respiratory tract forms from outpocketing of the foregut. Other glands formed from endothelial outpocketings along the tract. Oral and anal ends of the gut will open
What does the ectoderm specialize into?
Undergoes neurulation, remaining portion becomes the epidermis
Neurulation
Formation of the brain and spinal cord, induced by chemicals released by notochord. Results in formation of a neural tube that sits over the notochord. Anterior portion becomes the brain, remaining becomes the spinal cord
How long does it take the preliminary structures of the nervous system to form?
By week 4- forebrain, midbrain, and hindbrain evident. By week 8- cerebral hemispheres evident, brain waves can be recorded. There is some electrical activity in the cells
What structures does the mesoderm develop into?
Forms mesodermal blocks called somites that hug the notochord on either side. Remaining mesoderm forms kidneys and gonads, connective tissue of limbs, heart and blood vessels, dermis of ventral body, etc. If it’s not formed by the endoderm or ectoderm, it’s formed by the mesoderm
3 functional parts of somites
- Sclerotome- produce vertebrae and rib at each associated levels
- Dermatome- forms dermis in dorsal part of the body
- Myotome- forms skeletal muscle of neck, body trunk, and limbs
Cardiovascular modifications seen only during development, not maintained after birth (2)
- Umbilical arteries and vein
2. Vascular shunts
Umbilical arteries and vein function
Although blood is transported via these structures, gas exchange (oxygen and carbon dioxide), waste elimination, and various other processes occur in the placenta. Vein carries oxygenated blood to the fetus, arteries carry oxygen poor blood away from the fetus.
Vascular shunts function
Redistributes blood to parts of the body that need it most, mostly bypasses organs not yet used by developing fetus
Vascular shunts (3)
- Ductus venosus
- Foramen ovale
- Ductus arteriosus
Ductus venosus
Shunt that bypasses the liver. This is because the parent’s liver does all the work for the fetus
Foramen ovale
Shunts blood from the right atrium to the left atrium, causes blood to bypass lungs
Ductus arteriosus
Shunts blood from pulmonary trunk to aorta, causes blood to bypass lungs
After birth, what happens to the umbilical blood vessels?
Umbilical blood vessels constrict and fibrose. Umbilical artery gives rise to- portion of artery supplies the bladder and one portion of artery also becomes median umbilical ligaments. Umbilical vein remnant become the round ligament of the liver
What happens to the ductus venosus after birth?
Ductus venosus collapses and is converted to ligamentum venosum
What happens to the foramen ovale after birth?
Foramen ovale closes as pulmonary circulation becomes functional. Edges fuse to septal wall of heart- forms the fossa ovalis
What happens to the ductus arteriosus after birth?
Ductus arteriosus constricts and closes off- forms the ligamentum arteriosum. Helps support the pulmonary artery and the aorta
What happens to the size of the uterus during pregnancy?
At beginning of pregnancy, the uterus is about the size of a pear, at the end of pregnancy, the top of the uterus is even with the xiphoid process. Uterus and fetus push all other organs up and out. Pelvis widens from release of relaxin- eventually makes it difficult to walk normally
How long does pregnancy last?
Pregnancy lasts 38-40 weeks so all organs can develop enough. The lungs are the last to develop and are usually formed by week 38
How much weight is gained during pregnancy?
About 28 pounds- depends on weight at beginning of pregnancy. This is due to fat, extra weight of the fetus, placenta, and uterus. About 300 extra calories per day necessary to maintain a healthy pregnancy- not a huge amount necessary
Which hormones affect the metabolism of the pregnant person? (3)
- Placental growth hormone
- Human placental lactogen (hPL)
- Corticotropin-releasing hormone (CRH)
Placental growth hormone
Replaces growth hormone in pregnant women. Stimulates lipolysis and glucose production for growing fetus
Human placental lactogen (hPL)
Stimulates maturation of breasts for lactation, promotes fetal growth, and is glucose sparing. Glucose sparing- mother’s cells metabolize more fatty acids to save glucose for fetus. Side effect- reduction in insulin sensitivity (gestational diabetes)
Corticotropin-releasing hormone (CRH)
Rises only toward end of pregnancy. As CRH rises, mother’s ACTH and cortisol levels rise- helps during birth
Cortisol functions (2)
- Cortisol leads to maturation of fetal organs- especially the lungs
- Cortisol increases blood glucose which is delivered to the fetus
How can pregnancy affect the GI system?
Surge in HCG, estrogen, and progesterone can result in nausea/vomiting- called “morning” sickness, but can occur at any time of day. Protective measure in an evolutionary sense- often occurs when smelling meat. This is because spoiled meat is very threatening to the pregnancy. Also, the uterus invades the abdomen, pressing on other organs- heartburn and constipation
How can pregnancy affect the urinary system?
Increased metabolic rate, higher blood volume and need to dispose of extra waste from fetus= more urine produced. Uterus pushes on bladder- increases frequency of urination
How can pregnancy affect the respiratory system?
The uterus does not enter the thoracic cavity, but it eventually pushes on the bottom part of the diaphragm- diaphragm can’t flatten out like it should and breathing is difficult by end of pregnancy. Respiratory rate is unaffected, but there is an increase in tidal volume and a decrease in residual volume. Increased carbon dioxide, medullary respiratory centers become more sensitive to carbon dioxide
How does blood volume change during pregnancy?
Total blood volume increases by at least 25%, may be as high as 40% by the end of the 32nd week. Accommodates needs of fetus, protects mother from blood loss during childbirth. Blood pressure remains the same
How does pregnancy affect the cardiovascular system?
Cardiac output increases 35-40%. Uterus pushes on blood vessels as it grows- blocks blood flow, forming varicose veins and edema, especially in the legs. Blood tends to pool in the legs if the person stands for too long
How is labor initiated?
The fetus determines its own birth date. Increases in fetal cortisol levels leads to maturation of lung tissue (and increase in surfactant) and surge in estrogen from the placenta
Affects of the surge of estrogen from the placenta to initiate labor (3)
- Increased synthesis of prostaglandins- soften/thin the cervical mucus
- Addition of more actin, myosin, and other contractile elements. Allows uterus to contract more forcefully during labor and as a single unit.
- Increase in oxytocin and prostaglandin receptors in myometrium. Both chemicals lead to strong, rhythmic contractions associated with labor
Why is oxytocin released during labor?
Released by the posterior pituitary gland of mother to stimulate uterine contractions- uterine contractions push head of fetus into the cervix. Stretching of cervix by head stimulates more oxytocin release by the posterior pituitary- positive feedback mechanism. When fetus exits, the mechanism turns off
Stages of labor (3)
- Dilation stage- occurs from onset of labor until cervix is fully dilated
- Expulsion- occurs from full dilation to delivery
- Placental- occurs after delivery of infant
Dilation stage of labor
Highly variable duration of time- dilation caused by force of baby’s head against cervix. Full dilation is about 10 cm in diameter. Contractions begin as weak/short, becomes progressively stronger. Push of head against the cervix will physically break the amnion- “water” breaks
Expulsion stage of pregnancy
Contractions occur every 2-3 minutes, last about 1 minute, urge to push becomes strong. Crowning occurs when the largest portion of infant’s head distends vulva. Ideally, the infant is in a vertex position
What are the advantages of the baby being in the vertex position?
Dilates the cervix for rest of body to pass easier, can suction mucus from oral/nasal passage for breathing to occur sooner
Breech position
Butt first- dangerous for infant and difficult to stretch the cervix enough for the infant to pass through in that position
Placental stage of labor
Occurs about 30 minutes after birth. Mother must also deliver placenta and all of its structures (afterbirth). Uterine contractions compress blood vessels and tear placenta from uterine wall. If not completely removed, can result in infection
Which major adjustments occur in the baby immediately after birth? (2)
- Taking the first breath
2. Closing shunts and fetal blood vessels
What happens for the baby to take their first breath?
Umbilical blood vessels constrict- prevents removal of carbon dioxide from infant. Respiratory acidosis results- stimulation of respiratory centers occurs and is normally followed by the first breath. Also, the lungs must be completely inflated and not collapse. Surfactant prevents lung collapse
Structure of mammary glands
Each gland consists of 15-20 lobes. Lobules contain glandular alveoli- produce milk during lactation, but not before. The glands aren’t functional when a person isn’t pregnant.
Prolactin
During late pregnancy and after birth, rising levels of progesterone, hPl, and estrogens leads to release of prolactin. Glandular alveoli to become active- produce and release milk. Prolactin only produced for a few weeks after birth- milk production stimulated by feeding infant after this
Colostrum
There is a delay in milk production, and during this time, colostrum is secreted by the mammary glands. Contains less lactose than milk, very little fat, but contains much more protein, minerals, vitamin A and certain antibodies (IgA). IgA is resistant to being destroyed by gastric juices, provides a starter immune defense
Benefits of breast milk for the infant (4)
- Various components of breast milk are more easily metabolized by the infant (fat, certain minerals, some amino acids)
- Various protective molecules- IgA, complement, lysozymes, interferon (protect from viruses), interleukins
- Helps build healthy bacterial flora of infant
- Serves as a natural laxative
Why is it beneficial for breast milk to act as a laxative?
Cleanses infant bowels of meconium- contains epithelial cells, bile, other substances. Failure to clear it leads to jaundice- increased absorption of bilirubin from the intestine if there is a large amount in the intestine. Infant’s skin and eyes will turn yellowish orange
