. Biology of Reproduction, Pregnancy
University of South
Dakota......Department of Biology

Biology of Reproduction

Fall 1998

click here for text only
text: Human Reproductive Biology 2nd Edition - R.E. Jones: Read pages 182-218 for this lecture


XX. Pregnancy                          back to XIX.  Gamete Interaction

	A. Zygote

		1. fertilized ovum

			a. Blastomeres formed by cleavage

		2. Morula

			a. 8 cells or more

	B. Timing of Fertilization to Implantation 


		0			Ovulation				Ovarian Follicle

		1 (end)			Fertilization			Ampullary-Isthmic Junction

		3			Morula				Fallopian Tube

		4			Early Blastocyst			into Uterus

		5			Late Blastocyst			Uterine Glands

				   zona pellucida degenerates	         secrete protease + uteroglobin

		6			sticks to epithelium			Endometrium

		7			Implantation begins			Endometrium

		10			Implantation complete

	C. Implantation

		1. Inner Cell Mass -gives rise to ® Embryonic Disc

			a. Day 8 in humans 

			b. now called an Embryo

				i. 2nd through 8th week

					(1) 9th week ® called a Fetus

		2. Deciduoma

			a. implantation becomes so complete that cells of the
			    endometrium completely cover the embryo

		3. Formation of the Amnion and other Extra-Embryonic Membranes

			a. extra-embryonic membranes 		

				i. Amnion

				ii. Yolk Sac			

					(1) germ cells

				iii. Allantois					 

				iv. Chorion 						

					(1) from cytotrophoblast

			b. Sinusoids develop 

				i. pools of blood rather than vessels

					(1) enzyme secreted by syncytiotrophoblast lyses
					    uterine blood vessels

					(2)  Day 9 in humans

			c. implantation complete on Day 10 in humans

			d. Chorionic Villi formed 

				i. outgrowths of chorion into sinusoids

					(1) Day 11 in humans

				ii. Hemo-Choriol Placenta in humans

					(1) fetus' chorion bathed directly
					   in maternal blood

			e. Day 14 = Missed Menstrual Peroid

		4. Amnion

			a. Filled with fluid

				i. 5-10 ml at 8 wks

				ii. 1000-1500 ml 38 wks

				iii. 500-1000 ml term

			b. Provides buffer against shock

			c. fetus swallows fluid

		5. Yolk Sac

			a. Produces Primordial Germ Cells

			b. Hemopoietic Organ

		6. Allantois

			a. Contributes Blood Vessels to Placenta

				i. becomes Umbilical Circulation

					(1) Vein goes to the Heart
					   (usually deoxygenated)

					(2) Arteries go away from the heart
					 (usually oxygenated)

	D. Placenta

		1. Disc shaped structure, a combination of fetal and
		   maternal tissue = discoid placenta

		2. Hemochorial Placenta

			a. maternal blood directly bathes the chorionic villi

			b. maternal blood never mixes with fetal blood

				i. membranes of the chorionic villi and capillaries
				   keep them separate

		3. Chorio-Allantoic Placenta

			a. Villi tissue from chorion

			b. vessels from allantois

		4. Delivery and Excretion

			a. delivers O2,	 removes CO2

			b. delivers amino acids, 	removes urea

			c. delivers sugars and fats

			d. delivers hormones

				i. T3 to fetus (early)

				i. Estrogen (estriol) to mother

		5. Endocrine Organ

	E. Endocrinology of Pregnancy

		1. Chorionic Gonadotropin (for humans: hCG)

			a. produced by cells of the chorion or trophoblast

				i. chorion develops from the trophoblast

			b. cytotrophoblast produces hCG
			   very early in embryonic life

				i. most early pregnancy tests confirm pregnancy
				   by binding to hCG with a specific antibody and
				   then to a color producing molecule

				ii. hCG is produced even before implantation

			c. hCG prolongs the life of the corpus luteum
			   and the production of P

			d. hCG stimulates placental production of E and P

		2. Progesterone

			a. Maternal blood supplies cholesterol

			b. Placenta converts cholesterol to Progesterone

				i. takes over following luteolysis

				ii. produces enough to support pregnancy
				      by 5-6 wks in humans

			c. necessary for endometrial support and secretion

				i. \ necessary for support of pregnancy

					(1) pro = support ... gest = gestation = pregnancy

			d. exerts negative feedback on LH and FSH

				i. ovarian follicles don't grow
				ii. no stimulation for ovarian steroid production 

			e. increases fat deposition

				i. stimulating appetite

				ii. diverting energy stores from sugar to fat

		3. Feto-Placental Unit  -  Estrogens

			a. Progesterone from Placenta to fetal adrenal gland

				i. through umbilical and fetal vasculature

			b. outer layers = fetal adrenal zone converts P to DHEA

			c. DHEA circulates to fetal liver ® converted to
			    16a-OH-DHEA sulfate

			d. converted to Estriol in the Placenta

				i. Estriol is the primary estrogen during pregnancy

			e. fetus and placenta cooperate to produce
			   maternal estrogens

		4. Chorionic Somatomammotropin = Placental Lactogen

			a. hCS = hPL like PRL help prime mammary glands
			    for later milk secretion

				i. with estrogens and progesterone

			b. and has effects similar to GH

			c. placenta also secretes PRL

		5. Relaxin

			a. secreted by the corpus luteum and then the placenta

			b. levels rise late in pregnancy

			c. Loosens connective tissue

				i. widens pubic symphysis so head can pass through

			d. inhibits spontaneous uterine contractions

			e. promotes cervical effacement

				i. flattening, spreading, dilation of cervical os

		6. Placenta also secretes

			a. Chorionic GnRH

			b. Chorionic Corticotropin (ACTH)

			c. Chorionic Thyrotropin (TSH)

			d. Chorionic Endorphins

		7. Change in hormone levels during pregnancy

	F. Fetal Development

		1. Growth

			a. slow to week 20, maximum weeks 30-36, then slower

			b. stimulated by maternal Growth Factors (e.g. IGFs)
			    and fetal thyroid hormones (later)

				i. not stimulated by Growth Hormone (GH)

					(1) not effective prenatally

						(a) postnatally GH works by inducing IGFs

			c. Maternal Nutrition and Health

				i. diet, calcium, vitamins

					(1) fetus usually supplied first
						(a) an undernourished fetus has an ­ chance
						     of obesity as an adult

				ii. smoking

					(1) reduced size, slowed development

				iii. alcohol

					(1) reduced neural development

				iv. any drug should be verified by a physician

		2. The Nervous System

			a. thyroid hormones necessary

			b. masculinization of the brain by T ® E2

			c. many drugs affect neural development
			   due to their lipid solubility

				i. even painkilling drugs given during labor

			d. induction  of other tissues

				i. nervous tissue (neural crest) helps direct
				   the development of other tissues

					(1) eg. endocrine system, musculature

			e. motor nerves effective by 14 weeks in humans

				i. fetal movements help develop motor nervous system

		3. Immune System

			a. the fetus has many antigens

				i. the fetus is seen as a foreign object
				   by the mother

					(1) important for the success of the pregnancy

			b. Immunological Filter

				i. syncytiotrophoblast lacks antigen activity

				ii. P and hCG act locally as immunosuppressants 

				iii. some maternal antibodies cross the placenta

					(1) IgG

					(2) confers some passive immunity
					   (including postnatally)

					(3) also delivered in maternal milk

					(4) stimulates development of fetal immune system

					(5) Rh

						(a) mixing of fetal and maternal blood during
						     birth results in Rh antibody expression

						(b) subsequent pregnancies antibodies destroy
						    fetal red blood cells

		4. Respiratory System

			a. fetus makes respiratory movements at least 1-4 h / day

				i. motor nerves necessary

			b. primitive air sacs develop by week 20

				i. vascularized by 28 wks

			c. development enhanced by cortisol from the adrenal

			d. surfactant

				i. as lung increases in size it is necessary for
				    expansion of lungs without collapse

		5. Endocrine System

			a. hCG produced early but will not remain
			    part of the fetal endocrine system

			b. Thyroid gland

				i. T3/T4 necessary for development

					(1) thyroid secretory by week 20

				ii. fetal levels may exceed maternal

				iii. oviparous animals may include T3/T4
				     in the yolk of eggs

			c. Pituitary

				i. functional throughout most of fetal life

				ii. TSH to stimulate T3/T4 

				iii. LH, FSH, and ACTH

			d. Adrenal Gland (Cortex)

				i. begins by aiding production of estriol and is
				   converted to production of cortisol as
				   development progresses

				ii. cortisol necessary for 

					(1) lung maturation

					(2) liver enzymes

					(3) pancreas insulin cell maturation

					(4) synthesis of adrenaline

					(5) hemoglobin formation

					(6) parturition
					    (controversial - stimulates labor in sheep)

						(a) postpartum stress response

XXI. Labor and Parturition

University of South
Dakota......Department of Biology