Pregnancy and Human Development

Chapter 28

Fertilization

The process in sexual reproduction that involves the union of a sperm and an egg to form a diploid zygote

Oocyte Fertilization Time Window

Only viable for 24 hours (at most) after ovulation

Sperm Fertilization Window

Can survive at most 5 days in female reproductive tract

Sperm Capacitation

Because sperm are incapable of fertilizing the oocyte immediately after entering vagina, it must have its motility enhanced and have a weakened cell membrane

• Rupture of cell membrane release chromosomes

• Speeds up fertilization

Corona Radiata

Outer layer of the protective structures for an oocyte

• Protects and nourishes the oocyte after it has been ovulated

Zona Pellucida

Inner layer of the protective structures for an oocyte

• Protects secondary oocyte and is necessary for fertilization to occur

Acrosomal Reaction

At the zona pellucida, the sperms’ acrosomes release digestive enzymes to weaken the zona pellucida for fertilization to occur

• Sperm bind to zona pellucida → Ca2+ levels rise in sperm

Polyspermy

Potential problem where the entry of more than one sperm cell into the oocyte

• You don’t want extra chromosomes

Oocyte Membrane Blocks

Sperm-binding receptors are shed from oocyte surface

• Sperm unable to bind oocyte surface and fertilize oocyte

Cortical Reaction

Oocyte releases Ca2+, which causes release of zinc

• Causes whatever is left of the zona pellucida to harden

• Completion of meiosis II

Male Pronucleus

As the sperm nucleus travels towards the oocyte nucleus, it swells in size to form this

Meiosis II Completion

Forms 2nd polar body and mature ovum

• Isn’t done until sperm contents are inside the egg

Female Pronucleus

After meiosis II, this haploid nucleus forms, contributing to the zygote during fertilization

Monozygotic (Identical) Twins

Occurs when a single oocyte is fertilized and splits into 2 identical embryos (reason is unknown)

• Usually share a placenta → can lead to competition between the two fetuses

Dizygotic (Fraternal) Twins

Occurs when 2 oocytes are ovulated and both are fertilized

• Have their own placenta

Zygote

A diploid cell resulting from the fusion of two haploid gametes

• “Single-celled embryo”

• Divides mitotically at fertilization

Cleavage

Rapid divisions of the zygote

• Creates multicellular organism

Blastomere Cells

Individual cells that result from the early cell divisions (cleavage) of a zygote

• Two get produced from first mitotic division, which undergo their own mitotic divisions

Morula

Solid ball of cells that develops from a zygote after 72 hours after fertilization

• Contains ~16 cells

Embryo

Stage of development form soon after the fertilization of the ovum to week 8 of development

• No longer single-celled

Fetus

Stage of development from week 8 to birth

Infant

After birth has occurred

Blastocyst

Cluster of dividing cells made by a zygote

• Morula cells continue to divide into this

• Has two individual sets of cells that have their own form and function

Trophoblast

External layer of the blastocyst that aids in:

• Embryo implantation

• Chorion formation/function

• Immunosuppressive effects

Ensure that the developing embryo is protected and not destroyed by the parent’s immune system

Embryoblast

Internal layer of the blastocyst that eventually forms the embryo proper and extra embryonic membranes

• Inner cell mass

Blastocyst Cavity

Space for all of the inner cell mass to continue to proliferate/grow

Human Chorionic Gonadotropin (hCG)

Once implantation is complete, the embryo releases this to maintain the corpus luteum for ~12 weeks (first trimester)

• Suppresses female immune system to prevent damage to the embryo

Placenta

A temporary organ originating from embryonic and maternal tissues

• Major exchange organ between parent and fetus

• Maintains pregnancy, exchange respiratory gases, provide nutrients to embryo/fetus, dispose of waste, etc.

Chorion

Outermost extraembryonic membrane that surrounds the embryo of a fetus

• Embryo contribution to the formation of the placenta

• Allows exchange of gases, nutrients, and wastes

• Encloses all other extraembryonic membranes

Chorionic Villi

Finger-like projections of the outermost membrane surrounding a developing fetus → important for gas exchange and diffusion

• Inside contains fetal blood, outside is bathed in maternal blood

Umbilical Vessels

The blood vessels that extend from chorionic villi form these

• Serve as the life-support system for the developing fetus, connecting it to the placenta

• Blood transportation

Lacunae

Blood-filled cavity or space that the functional layer of endometrium develops

• Parental contribution to the placenta

Decidua Basalis

Endometrium that lies underneath the embryo becomes this, and forms the placenta with chorionic villi

• Most parental contribution to the placenta

Decidua Capsularis

Formed from the surrounding endometrium of the uterine cavity face of embryo

• Expands the accommodate growing fetus

• Chorionic villi will degenerate as pregnancy progresses here

Extraembryonic Membranes

Membranes formed during the first few weeks of development that support and nourish developing embryo/fetus during gestation

Amnion

Layer of the extraembryonic membrane that extends AROUND the embryo

Amniotic Fluid

Fills the layer that extends around the embryo

• Provides buoyancy and protection

• Maintains consistent temperature

• Prevents developing parts of embryo from sticking together/fusing

• Allows movement of embryo/fetus

Yolk Sac

Sac-like structure of the extraembryonic membranes that spermatogonia arise from

• Is a nutrient source in the early stages, but not much later on

• Eventually forms digestive tube, earliest blood cells, and precursor gametes

Allantois

Extraembryonic membrane that helps form the umbilical cord alter in fetal development

• Allows for gas exchange, waste disposal, and nutrient exchange

Gastrulation

Early developmental process where the blastocyst is reorganized into a 3-layered embryo

• Gives rise to every single part that we have

Primitive Streak

Process of gastrulation begins with this

• Groove that will eventually form the long axis of the embryo

Endoderm

Formed by cells migrating and entering the primitive streak

• Inferior layer — forms the epithelial lining of GI tract

• Respiratory tract forms outpocketings, in which other glands are formed

• Opening of opposite ends form the mouth and the anus

Mesoderm

Cells that enter the primitive streak that do not form the endoderm, but get pushed up between upper and lower layers—“middle” layer

• Forms somites for the notochord

• Remaining forms kidneys and gonads, connective tissues of limbs, heart, and blood vessels, dermis of ventral body region, etc.

Notochord

Formed from the mesodermal cells immediately beneath the primitive streak

• First axial support of the embryo (does NOT form the spinal chord or vertebral column)

• Helps keep cells in place so embryo doesn’t develop crookedly

Ectoderm

Cells that remain at the surface form this embryo layer

• Where neurulation takes place

• Remaining forms the epidermis

Organogenesis

Formation of body organs and organ systems

• Begins with gastrulation

  • Cells in 3 primary germ layers will differentiate to form organs and organ systems

Neurulation

Formation of the brain and the spinal cord

• Induced by chemicals released by notochord

Neural Tube

Formed as a result from neurulation, sits over the notochord

• Develops into the central nervous system, formed during the first month of pregnancy

Anterior Neural Tube

Portion of the neural tube that becomes brain

• Branches form the cranial nerves

• By week 8 → cerebral hemispheres evident, brain waves can be recorded

Remaining Neural Tube

Portion of neural tube that becomes the spinal cord

Somites

Mesodermal blocks that hug the notochord on either side

Sclerotome

Functional portion of semite that produce vertebrae and rib at each associated levels

Dermatome

Functional portion of somites that forms the dermis in the dorsal part of body

Myotome

Functional portion of the somite that forms skeletal muscle of the neck, body trunk, and limbs

• Most of the skeletal muscle tissue comes from here

Umbilical Veins

Blood vessels that carry OXYGENATED blood TO the fetus

Umbilical Arteries

Blood vessels that carry OXYGEN-POOR blood AWAY from fetus

• Supply bladder and anchors bladder to the umbilicus

Vascular Shunts

Redistributes blood to parts of body that need it most

• Mostly bypasses organs not yet used by developing fetus

Ductus Venosus

Vascular shunt that bypasses the liver

• Parent’s liver does all the work for the fetus

Foramen Ovale

Vascular shunt that shunts blood from right atrium to the left atrium

• Causes blood to bypass lungs (fetus doesn’t use lungs)

Ductus Arteriosus

Vascular shunt that shunts blood from pulmonary trunk to aorta

• Causes blood to bypass lungs (fetus doesn’t use lungs)

Round Ligament

Umbilical vein remnant becomes this of the liver

Ligamentum Venosum

Ductus venous collapses and is converted into this → doesn’t have a function, but can be used as a market for important structures during surgery

Fossa Ovalis

Indentation in the right atrium → foramen ovale closes as pulmonary circulation becomes functional

Ligamentum Arteriosum

Ductus arteriosus constricts and closes off, forming this → doesn’t have a function, but is slightly supportive for the pulmonary artery and the aorta (helps hold in place, but not much support)

Relaxin

Hormone that causes the pubic symphysis to loosen up during pregnancy

• Pelvis widens (usually permanent)

~28

Average amount of pounds one might gain during pregnancy

• Dependent on weight at beginning of pregnancy

  • If you are lighter, you will need to gain more; if you are overweight, you will only need to gain slightly

~300

Amount of extra calories one will necessarily have to consume in order to maintain healthy pregnancy

Placental Growth Hormone

Hormone that replaces GH in pregnant women

• Stimulates lipolysis and glucose production for growing fetus

  • Breaks down more adipose tissue than GH

Human Placental Lactogen (hPL)

Hormone that stimulates the maturation of breasts for lactation, promotes fetal growth, and is glucose-sparing

• Tends to make parent’s body resistant to insulin → hyperglycemia

• Parent’s body cells use less glucose and is saved for the fetus

Corticotropin-releasing Hormone (CRH)

Hormone that rises only towards the end of pregnancy

• Helps during birth, leads to maturation of fetal organs

Physiological Changes of the GI System

• Surge in hCG, estrogen, and progesterone can result in nausea/vomiting → morning sickness

• Uterus invades abdomen and presses onto other organs → heartburn, constipation, etc.

Physiological Changes of the Urinary System

• Increased metabolic rate, higher blood volume, and need to dispose of extra waste form fetus → more to expel

• Uterus pushes on bladder → more frequency

Physiological Changes of the Respiratory System

• Respiratory rate unaffected, but taking deeper breaths

• Increase in tidal volume and decrease in residual volume (need more oxygen as you are breathing for two people)

• Breathing can be difficult due to the uterus pushing onto the diaphragm

Physiological Changes of the Cardiovascular System

• Total blood volume increases by AT LEAST 25% → blood pressure stays the same however (hormones stimulate vasodilation)

• Uterus pushes on blood vessels which can block blood flow, forming varicose veins and edema

Varicose Veins

Gnarled, enlarged veins that are very obvious underneath the skin and are very dark in color, normally irreversible

Increased Synthesis of Prostaglandins

As a result of a surge in estrogen → thins and softens mucus, making the infant easier to pass through

Increase in Oxytocin & Prostaglandin Receptors

As a result of a surge in estrogen in myometrium → stimulates uterine contractions

Addition of more Actin, Myosin, and Other Contractile Elements

As a result of a surge in estrogen → generates stronger contractions

Oxytocin

Hormone that is released by the posterior pituitary gland of parent that stimulates uterine contractions

• Stretching of cervix by the fetus’ head stimulates more of this to be released

Dilation Stage

Stage of labor that occurs from onset of labor until cervix is fully widened

• ~10 cm in diameter

• Variable in time

Expulsion

Stage of labor that occurs from full dilation to delivery

• Contractions occur every 2-3 minutes and last about 1 minute each

• Basically, the removal of the infant

Crowning

Occurs when the largest portion of an infant’s head distends vulva

• Part of expulsion

Vertex Position

The infant should be in this position, head first (widest part of the fetus)

• Dilates the cervix for the rest of the body to pass easier, can suction mucus from oral/nasal passage for breathing to occur sooner

Breech Position

Infant is in a butt-first position

• If too late to detect → c-section

• If early to detect → doctors will physical move around the female’s abdomen to move around the baby’s position

Placental

Stage of later that occurs ~30 minutes after the delivery of the infant

Afterbirth

Stage where the parent delivers the placenta and all of its structures

• Uterine contractions compress blood vessels and tear placenta from uterine wall → if not removed, major infection will occur as perceived as “foreign” by female immune system

Glandular Alveoli

Small, sac-like structures in lobules that produce milk during lactation

• Only occurs when pregnant, when not pregnant the mammary glands are not functional

Prolactin

Hormone that is released due to a rise in levels of progesterone, hPL, and estrogens for a few weeks after birth

• Slows down the release of estrogen, which slows down ovulation (making the person less likely to get pregnant for safety)

Colostrum

Delay in milk production → mammary glands secretes this product

• Contains less lactose than milk, very little fat, but contains much more protein, minerals, vitamin A and some antibodies

Meconium

Built up epithelial cells and other structures → the waste in the digestive system that develops during fetal development

• Breast milk serves as a natural laxative that can cleanse the infants GI tract