Pregnancy and Human Development

 

I.                    From Egg to Embryo

A. Terms

1.   Pregnancy=events occurring from the time of fertilization (conception) until the infant is born.

2.   Conceptus=developing offspring.

3.   Gestation period=extends from the last menstrual period until birth (280 days).

4.   Pre-embryo=also simply called conceptus, first two weeks following fertilization.

5.   Embryo=from the third through the eight weeks after fertilization.

6.   Fetus=ninth week through birth.

7.   Infant=at birth.

B.  Accomplishing fertilization

1.   Fertilization=fusion of sperm and egg. Diagram

a. The egg is viable only for 12 to 24 hours after it leaves the ovary.

b.  Sperm are viable for 24 to 72 hours after ejaculation.

c.  Consequently, coitus must occur no more that three days before ovulation and no later than 24 hours after.

d.  Zygote=a fertilized egg or the first cell of the offspring.

2.  Sperm transport and Capacitation

a. Only a few hundred thousand sperm in a males ejaculate actually make it to the uterine tubes.

i.    Millions leak immediately from the vagina.

ii.   Millions are destroyed by acidity of the vagina.

iii. Thousands are phagocytized in the uterus.

iv. Only about a couple a hundred actually make it to the egg.

b.       Fertilization requires capacitation

                                                                                                                                       i.      Capacitation=the sperms membranes must become fragile so that the hydrolytic enzymes in their acrosomes can be released.

                                                                                                                                     ii.      Sperm penetration through the corona radiate and zona pellucida surrounding the oocyte is accomplished by release of hyaluronidase, acrosin, etc. by many sperm.

                                                                                                                                    iii.      Fusion of the plasma membrane of a single sperm with the oocyte membrane is followed by engulfment of the sperm by the oocyte cytoplasm and the cortical reaction.

                                                                                                                                   iv.      Cortical reaction involves the release of the contents of the oocytes cortical granules into the extracellular space preventing entry of more than one sperm=polyspermy.

                                                                                                                                     v.      Humans are monospermy organisms.

c.   Completion of Meiosis II and fertilization Diagram

i.   After a sperm enters the oocyte, it loses its tail and body and then migrates to the center of the oocyte.

ii.   Secondary oocyte now completes Meiosis II and ejects the second polar body.

iii. The ovum and the sperm nuclei swell to form the male and female pronuclei giving rise to the zygote.

C.     Pre-embryonic Development Diagram

1.       Cleavage occurs as the pre-embryo travels through the uterine tube and floats freely in the uterus.

a.       Cleavage is a period of fairly rapid mitotic divisions of the zygote following fertilization.  Hyperplasia vs. Hypertrophy

                                                                                                                                              i.      2-cell stage at about 36 hours after fertilization.

                                                                                                                                             ii.      4-cell stage at about 48 hours.

                                                                                                                                           iii.      8 cell stage at about 72 hours.

                                                                                                                                           iv.      Morula=solid ball of cell that are 16 cells big or more.

b.      Blastocyst=the morula hollows out and fills with fluid then “hatches” from the zona pellucida.

                                                                                                                                              i.      Trophoblasts=constitute the large flattened cells of the blastocyst which take part in placenta formation and secrete HCG to prompt the corpus luteum to continue secreting progesterone in order to maintain the endometrium.

                                                                                                                                             ii.      Inner cell mass=a cluster of small rounded cells that becomes the embryonic disc.

2.       Implantation Diagram

a.       When the blastocyst reaches the uterus, it floats freely in the uterine cavity for two or three days, receiving nourishment from the uterine secretions.

b.       Six or seven days after ovulation, the trophoblast cells embed into the endometrium and begin secreting digestive enzymes and growth factors against the endometrial surface.

c.       As the endometrium is eroded, the blastocyst burrows into the thick, velvety lining and is surrounded by a pool of blood leaked from degraded endometrial blood vessels.

d.       The chorion develops from the trophoblast cells starting to give rise to the placenta.

e.       The chorion then develops chorionic villi which extend into the endometrium where they are in contact with maternal blood.

3.       Placenta Formation

a.       Functions to exchange waste products and blood gases.

b.       Chorionic villi penetrate into the deciduas basalis of the endometrium.

c.       Placenta takes over the role of secreting HCG and also secretes relaxin=which causes the pubic symphysis to soften and become more flexible.

 

II.                 Embryonic Development

A.  The blastocyst is converted into the gastrula in which the embryonic membranes develop and the three primary germ layers form.  The process by which the embryonic tissues are formed is called gastrulation.

1.  The embryonic membranes form as the inner cell mass splits to form an upper and lower cell layer. Diagram

a.   Amnion=develop from the upper cells of the splitting inner cell mass.  This sac fills with amniotic fluid that provides a buoyant environment that protects the developing embryo.

b.   Yolk sac=forms from the lower cells of the inner cell mass.  It serves to form part of the digestive tube, produces the earliest blood cells and blood vessels, and is the source of primordial germ cells of the embryo’s gonads.

c.   Allantois=forms as a small out-pocketing at the caudal end of the yolk sac.  Acts as the structural base of the umbilical cord and becomes part of the urinary bladder.

d.   Chorion=develops from proliferating trophoblast cells and helps to give rise to the placenta.

2.  During the third week, the primary germ layers form along the embryonic disc. Diagram

a.   Ectoderm=gives rise to skin and nervous system

b.   Endoderm=gives rise to the functional linings of the digestive, respiratory, and urogenital systems as well as to associated glands.

c.   Mesoderm=Forms muscle, bone, blood vessels, kidneys and all the other components of organs (except linings).

3.  Circulation in fetus versus newborn Diagram

a.   Fetal circulation has several adaptations so that the lungs and liver are largely bypassed because they are non-functional.

i.   The umbilical vein carries oxygen- and nutrient-rich blood from the placenta to the fetus.

ii.   The umbilical arteries carry waste-laden blood from the fetus to the placenta.

                                                                                                                                           iii.      The ductus arteriosus and foramen ovale to partially bypass the lungs.

                                                                                                                                           iv.      The ductus venosus allows blood to partially bypass the liver.

b.   Changes in the cardiovascular system occur at birth.

i.    The umbilical vessels are occluded.

ii.   The ductus venosus, ductus arteriosus, and foramen ovale also occlude.

4.       Development through the end of the embryonic period. Diagram

a.       Head nearly as large as body

b.      All major brain regions present; first brain waves in brain stem

c.       Liver disproportionately large and begins to form blood cells.

d.      Limbs present; digits initially webbed become separated

e.       Ossification begins and spontaneous muscle contractions occur

f.        Cardiovascular system is fully functional

g.       All body organs/systems present though not fully developed

h.       Final approximate crown-to-rump length is 30 mm

 

III.               Events of Fetal Development

A.  9 to 12 weeks

1.   Head still dominant with brain enlargement continuing and cervical and lumbar enlargements of the spinal cord are obvious.

2.   Skin epidermis and dermis are obvious; facial features in crude form.

3.   Liver is prominent and bile being secreted; smooth muscle increasing.

4.   Blood cells formation begins in bone marrow.

5.   Notochord degenerating and ossification accelerating.

6.   Sex readily detected from the genitals.

7.   Final approximate crown-to-rump length is 90 mm.

B. 13-16 weeks

1.  Cerebellum becoming prominent; sensory organs differentiate, eyes and ears assume shape and position; sucking motions of lips occurs.

2.   Face looks human and body beginning to outgrow head.

3.   Glands developed in GI tract; meconium is collecting.

4.   Kidneys attain typical structure.

5.   Most bones are now distinct and joint cavities are apparent.

6.   Final approximate crown-to-rump length is 140 mm.

C. 17-20 weeks

1.   Fatty secretions from sebaceous glands and silk-like hair cover body.

2.   Fetal position assumed because of space restrictions.

3.   Limbs near-final proportions.

4.   Muscular activity of fetus increases.

5.   Final approximate crown-to-rump length is 190 mm.

D. 21-30 weeks

1.   Increase in weight.

2.   Myelination of cords begins; eyes are open.

3.   Distal limb bones are beginning to ossify.

4.   Skin is wrinkled and red; fingernails and toenails are present.

5.   Body is lean and well proportioned.

6.   Bone marrow becomes sole site of blood cell formation.

7.  Testes reach scrotum in seventh month.

8.   Final approximate crown-to-rump length is 280 mm.

E. 30-40 weeks

1.   Skin whitish pink; fat laid down in subcutaneous tissue.

2.   Final approximate crown-to-rump length is 360-400 mm.

IV.              Effects of Pregnancy on the Mother Diagram

A.   Anatomical Changes

1.   Maternal reproductive organs and breasts become increasingly vascularized during pregnancy, and the breasts enlarge.

2.   The uterus eventually occupies nearly the entire abdominopelvic cavity.  Abdominal organs are pushed superiorly and encroach on the thoracic cavity, causing the ribs to flare and breathing difficult.

3.   The increased abdominal mass changes the woman’s center of gravity; lordosis and backache are common.  A woman’s waddling gait occurs as pelvic ligaments and joints are loosened by placental relaxin.

4.   A typical weight gain during pregnancy in a woman of normal weight is 29 pounds.

B.  Metabolic changes

1.   Human placental lactogen (hPL) has anabolic effects and promotes glucose sparing in the mother.

2.   Human chorionic thyrotropin (hCT) similar to thyroid-stimulating hormone in that it results in maternal hypermetabolism.

C.  Physiological changes

1.   Many women suffer morning sickness, heartburn, and constipation during pregnancy.

2.   The kidneys produce more urine, and pressure on the bladder may cause frequency, urgency, and stress incontinence.

3.   Vital capacity and respiratory rate increase, but residual volume decreases.  Dyspnea is common.

4.   Total body water and blood volume increase dramatically.  Heart rate and blood pressure rise, resulting in enhancement of cardiac output.

5.   Hypertension is also common.

 

V.                 Parturition (Birth)

A.  Initiation of labor Diagram

1.  When estrogen levels are sufficiently high, they induce oxytocin receptors to increase on the myometrial cells and inhibit progesterone secretion by the placenta.  Weak irregular contractions begin.

2.   Fetal cells produce oxytocin, which stimulates prostaglandin production by the placenta.  Both hormones stimulate contraction.

3.   Increasing stress causes the hypothalamus of the mother to cause oxytocin release by the pituitary gland (posterior).

B.  Stages of labor 

1.   Dilation stage=rhythmic contractions occur until the cervix dilates 10 cm.  The head of the fetus rotates and descends through the pelvic outlet.

2.   Expulsion stage=extends from full cervical dilation until birth of the infant.

3.   Placental stage=delivery of the afterbirth. 

C.  Lactation

1.   The breasts are prepared for lactation during pregnancy by high blood levels of estrogen, progesterone, and placental lactogen.  Oxytocin is important in stimulating the “let down” phase.

2.   Colostrum (a pre-milk fluid that is a fat-poor fluid that contains more protein, vitamin A, and minerals than true milk) is produced towards the end of the pregnancy and for the first 2-3 days after birth.

3.   True milk is produced around day 3 in response to suckling which stimulates the hypothalamus to prompt the pituitary gland to secrete even more prolactin and oxytocin.

4.   At first, ovulation and menses are absent or irregular during nursing.

 

VI.   Homeostatic Imbalances

A.  Fetal Alcohol Syndrome=typified by microcephaly, mental retardation, and abnormal growth.

B.  Dystocia=prolonged or difficult labor.

C.  Abortion=premature removal of the embryo or fetus from the uterus (may be spontaneous or induced).

D.  Eclampsia=dangerous condition of pregnancy from edema and hypertension that causes proteinuria and seizure.

E.  Ectopic pregnancy=embryo implants in any site other than the uterus typically within the uterine tubes.

F.  Placenta abruption=premature separation of the placenta from the uterine wall.

G. Placenta previa=formation of the placenta adjacent to or across the opening of the uterus which is easily torn as the uterus enlarges.