Exam 5 Review:  Chapter 28 Embryonic Development & Birth

embryonic period - Human prenatal development is divided into an embryonic period and a fetal period; the embryonic period begins with fertilization and ends eight weeks later; it involves the development of transient structures such as the three primary embryonic tissues which establish body plan, organogenesis and placental tissues, and their initial folding and form.; [Note:  some references define the first two weeks as the germinal period, which ends approximately 3-4 days after implantation and the embryonic period as the subsequent six weeks.]

embryo - The initial product of conception from implantation at about two weeks after fertilization through the eighth week of development.

fetal period - Human prenatal development is divided into an embryonic period and a fetal period; the second and longer fetal period (four times the embryonic period), from the ninth week to the fortieth week, approximately, or until delivery, is a time of extensive growth in size and mass as well as ongoing differentiation of organ systems established in the embryonic period.  [See details in the tables below.]

embryology - The branch of biology that deals with the formation, embryonic structure, early growth, and development of living organisms.

fetus - The unborn young from the end of the eighth week after conception to the moment of birth, as distinguished from the earlier embryo; by this stage the developing unborn individual has taken on a recognisable form with all parts in place, tissues and organs differentiating.

primary germ layers - The first three tissue types to differentiate in an early stage of embryonic development, the gastrula, at approximately day sixteen, and from which all other more specialiazed tissues are later derived; the outermost layer is the ectoderm; the innermost, the endoderm; between these two layers, and developing slightly later, lies the mesoderm; at first, the three layers are more or less uniform; soon, however, they begin to take on different characteristics.

gastrulation - the process in which a gastrula develops from a blastula by the inward migration of cells by a complex and coordinated series of cellular movements which occur at the end of the cleavage stage of development; the cells begin to differentiate into the three primary embryonic germ layers, ectoderm and endoderm first, followed shortly thereafter by mesoderm, with the cells which differentiate into endoderm migrate inward to form the primitive gut = archenteron and with subsequent cells differentiating into mesoderm within the blastocoel cavity.

amniotic cavity - The amniotic fluid-filled cavity surrounding the developing embryo/fetus within the uterus.

ectoderm - The outermost of the three primary germ layers of an embryo, which develop during the transition from blastocoele to gastrula stage, from which the epidermis and epidermal derivatives (hair, nails, tooth enamel, sweat and sebaceous glands), and the nervous system and its sensory receptor cells (retina, spiral organ, gustatory cells, etc.) develop.

endoderm - The innermost of the three primary germ layers of an animal embryo, which develop during the transition from blastocoele to gastrula stage, developing into the epithelial linings of the gastrointestinal tract, the lungs, and associated structures, e.g., all digestive glands, most endocrine glands, etc.

mesoderm - The middle embryonic germ layer, lying between the ectoderm and the endoderm, which develop during the transition from blastocoele to gastrula stage,  from which the connecitve tissues proper, bone, cartilage, dentin, cementum, all types of muscle, blood, endothelium of blood vessels, synovial membranes, serous membranes lining body cavities, the epithelium of the kidneys and urinary drainage system, and the germinal epithelium of the gonads and the linings of the reproductive tracts develop.

primitive gut = archenteron - The central cavity of the gastrula, formed by the inward migration of endodermal cells, which ultimately becomes the lining epithelium of the intestinal or digestive cavity.  aka - gastrocele

umbilical cord - The flexible cordlike structure connecting a fetus at the abdomen with the placenta and containing two umbilical arteries and one vein which transport nourishment to the fetus and remove its wastes; it is derived from the chorion and the allantois, the outer two extraembryonic membranes.

relaxin - A female protein hormone secreted by the corpus luteum which helps soften the cervix and relax the pelvic ligaments and pubic symphysis and causes appropriate smooth muscle relaxation in childbirth; in males it encourages motility of spermatozoa within semen.

oxytocin (OT) - A neurohypophyseal peptide hormone which stimulates the contraction of smooth muscle of the uterus during labor and delivery and facilitates ejection of milk from the breast during nursing.

prostaglandins - Any of the group of ~20 potent local hormones which are produced in various tissues, are derived primarily from arachidonic acid (a cell membrane phospholipid) via the cyclooxygenase pathway, and mediate a wide range of physiological functions, e.g., control of blood pressure, contraction of smooth muscle, and modulation of inflammatory and immune responses; their synthesis is inhibited by aspirin and other NSAIDs.  [See also Tripping Lightly Down the Prostaglandin Pathway.]

dilation - The period from the onset of labor until the cervix is fully dilated; it is also the longest stage and can take from 6 to 12 hours; at first, only the upper part of the uterus is actively contracting; the contractions will be 15-30 minutes apart and will last for only 10-30 seconds; as labor progresses, the contractions come more often, harder, and the lower part of the uterus becomes involved; when the infant's head starts to push against the cervix, the cervix gets softer, thinner and expands; the amnion will rupture, releasing the amniotic fluid ("breaking water").

expulsion - The stage of labor from dilation to delivery; this stage may take 20-50 minutes, depending on the number of children delivered in the mather's life, or as much as 2 hours; contractions come every 2-3 minutes and may last for 1 minute; crowning occurs as the baby's head starts to distend the vulva (it may be necessary to perform an episiotomy, an incision from the vagina to the rectum to avoid or reduce tissue tearing); the infant usually comes out in the vertex, or head first position; the head acts as a wedge to dilate the cervix even more; breech births are more difficult since there is no head to provide a wedge, and the baby may have difficulty getting its first breath.

To see a series of photos of a caeserian section, visit "Pictures of Patrick Oliver Jones-O'Brien."

List:

1. the body tissues derived from each of the primary germ layers.

Primary Germ Layer Body Tissues Derived Therefrom
ectoderm epidermis and epidermal derivatives (hair, nails, tooth enamel, sweat and sebaceous glands), and the nervous system and its sensory receptor cells (retina, spiral organ, gustatory cells, etc.)
mesoderm connecitve tissues proper, bone, cartilage, dentin, cementum, all types of muscle, blood, endothelium of blood vessels, synovial membranes, serous membranes lining body cavities, the epithelium of the kidneys and urinary drainage system, and the germinal epithelium of the gonads and the linings of the reproductive tracts
endoderm epithelial linings of the gastrointestinal tract, the lungs, and associated structures, e.g., all digestive glands, most endocrine glands, etc.

Describe:

2. the neuroendocrine regulation of labor.

(1) rising estrogen levels signal that the fetus is reaching full term; estrogen levels also stimulate development of oxytocin receptors in uterus

(2)  oxytocin from the maternal posterior pituitary = neurohypophysis, along with small quantities from the fetus itself, initiate uterine smooth muscle contractions

(3)  oxytocin stimulates the placenta to synthesize and secrete prostaglandins which further stimulate uterine smooth muscle contractions

(4)  stretch receptors provide positive feedback to the hypothalmus which causes further output of oxytocin and oxytocin provides further positive feedback for additional output of prostaglandins

3. the three stages of labor.

Stages of Labor Events
dilation begins with the onset of rhythmic, strong contractions until the cervix is fully dilated; the head of the fetus rotates as it descends through the pelvic outlet
expulsion extends from full cervical dilation until birth of the infant
placental continues with the delivery of the placenta and attached fetal membranes (the "afterbirth")

4. some internal anatomical and physiological adjustments of the infant's physiology at birth and immediately thereafter.

After the umbilical cord is clamped, carbon dioxide accumulates in the infant's blood, causing respiratory centers in the brain stem to trigger the first inspiration.
Once the lungs are inflated, breathing is eased by the action of surfactant which decreases the surface tension of the alveolar fluid.
Inflation of the lungs causes pressure changes in the circulation; as a result, the umbilical arteries and veins, ductus venosus, ductus arteriosus, and the foramen ovale close.  The occluded blood vessels are converted to fibrous cords; the site of the foramen ovale becomes the fossa ovalis; the remnant of the ductus arteriosus becomes the ligamentum arteriosum.
The kidneys must take over the entire responsibility for waste removal and electrolyte and acid-base balance.
The nervous system must begin adaptation to and learning about the external environment; e.g., initiating suckling when placed at the nipple; adjusting to light and visual image processing; adjusting to sound and hearing processing, etc.
The neonate must begin thermoregulation by establishing and maintaining a core body temperature on its own through regulation of thermogenesis and cooling.
For the first 8 hours after birth, the infant is physiologically unstable and adjusting.  After stabilizing, the infant wakes approximately every 3-4 hours in response to hunger.
The gastrointestinal tract must begin the digestion and absorption of nutrients from colostrum and milk.

5. the differences between the fetal and adult blood circulations.

Four Shunts Exist in utero:

(1) the ductus venosus routes some oxygenated nutrient rich blood from the umbilical vein around the liver bypassing the hepatic portal vein to carry blood directly to the inferior vena cava

(2) the foramen ovale routes some oxygenated nutrient rich blood from the right atrium directly to the left atrium bypassing the pulmonary circulation to carry blood directly to the systemic circulation

(3) the interventricular shunt routes some oxygenated nutrient rich blood from the right ventricle directly to the left ventricle bypassing the pulmonary circulation to carry blood directly to the systemic circulation

(4) the ductus arteriousus routes some oxygenated nutrient rich blood from the pulmonary trunk directly to the aorta bypassing the pulmonary circulation to carry blood directly to the systemic circulation


Life Progression as Reflected in the Delivery of Nutrients in Bottles: