Download Pregnancy and Human development

Pregnancy and Human development
Objectives:
1. Describe the importance of capacitation to the ability of sperm to penetrate an oocyte.
2. Explain the mechanism of the fast and slow blocks of polyspermy.
3. Define fertilization.
4. Explain the process the product and cleavage.
5. Describe the process of implantation and placenta formation, and list placental functions.
6.Describe the process of gastrulation and its consequence.
7. Name and describe the formation, location, and function of the embryonic membranes.
8. Define organogenesis and indicate the important roles of the three primary germ layers in
this process.
9. Describe the unique features of the fetal circulation.
10. Explain how labor is initiated, and describe the three stages of labor.
11. Describe the changes that occur in the fetal circulation after birth.
Sperm transport: oppositions to sperm encountered once they are in the woman’s body make
them capacitated. The hydrolytic enzymes in the acrosome are free to be released.
Acrosomal reaction and sperm penetration: hundreds of acrosomes are needed to break
down the corona radiate and zona pellucida. The first sperm sacrifice themselves for the others.
 The “one” that finally makes contact with the cell membrane receptors has its nucleus pulled
into the oocyte cytoplasm.
Blocks to polyspermy: As soon as the cell membranes of the egg and sperm make contact,
sodium channels open up, the oocyte membrane depolarizes and others cannot fuse.
 Calcium is then released from the ER to prepare for the rest of meiosis.
 Granules deep to the membrane spill enzymes out in the ECF space to destroy sperm
receptors and other sperm are detached.
Preembryonic development: cleavage occurs 36 hrs after fertilization yielding 2 identical
blastomeres. 72 hrs later it is a 16 cell morula. 4-5 days later yields a blastocyst, a fluid-filled
hollow sphere with a single layer of cells called a trophoblast and an inner cell mass.
Implantation: occurs 6-7 days after fertilization. The trophoblast over the inner cell mass adheres
to endometrium, secretes enzymes and growth factors against the endothelium. Acute inflammatory
response occurs in mother. Trophoblast forms two layers that project invasively into endometrium.
Blastocyst is covered over and sealed off. This takes about a week. hCG is then secreted by
trophoblast cells and later by the chorion.
Placentation: The placenta forms from both embryonic and maternal tissues. The chorion
becomes chorionic villi. New vessels form that become the umbilical arteries and vein. Maternal
and fetal blood are separated by the membranes of the villi. The chorion is the portion of the
trophoblast that forms the placenta (fetal portion) and makes hCG.
Formation and roles of membranes
1. amnion: located under cytotrophoblast and becomes filled with fluid. Eventually
surrounds the embryo with cord in middle. For buoyancy, temperature regulation and movement.
2. yolk sac: below embryonic disc. Forms part of gut, produces earliest blood vessels and
blood cells and seeds the gonads.
3. allantois: connects embryo to placenta by umbilical cord and becomes urinary bladder.
Gastrulation: formation of germ layers. By the 3rd week the two layered embryonic disc becomes
a 3 layered embryo. A primitive streak forms on the dorsal surface and establishes head to tail. Cell
migrations occur, producing the 3 germ layers.
 Endoderm: gives rise to epithelial linings of digestive, respiratory, and urogenital systems
and associated glands.
 Mesoderm: gives rise to skeleton, muscle, kidneys, reproductive organs (except vagina),
circulatory system, dermis, and dentin.
 Ectoderm: gives rise to epidermis, nervous tissue, sense organs, enamel of teeth, lens of
eye.
Fetal circulation: umbilical vein carries freshly oxygenated and nutrient rich blood from mother,
through ductus venosus and into inferior vena cava. Foramen ovale shunts blood from right atrium
to left atrium. Ductus arteriosus carries blood from pulmonary trunk to aorta. Umbilical arteries
carry deoxygenated waste laden blood to placenta.
Initiation of labor: Fetal cortisol stimulates placenta to release large amounts of estrogen. This
stimulates myometrial cells to form oxytocin receptors and antagonizes progesterone. This causes
Braxton-Hicks contractions.
 Certain cells of fetus produce oxytocin which causes the placenta to release prostaglandins,
which play a role in the inflammatory response. When the hypothalamus is involved in
oxytocin production, it is now a positive feedback mechanism.
Stages of labor:
1. Dilation and effacement: 10 cm in diameter (cervix) and wall thins to paper thin at
100% effacement. Initial contraction 15-30 minutes apart. Baby’s head makes cervix
soften and thin.
2. Expulsion stage: full dilation to delivery, 2-3 minutes between contractions.
3. Placental stage: delivery of placenta, within 30 minutes after birth.
Apgar score: taken at 1 and 5 minutes after birth, 5 signs are assessed:
1.
2.
3.
4.
5.
heart rate
respiration
color
muscle tone
reflexes
An Apgar of 8-10 is a healthy baby.
Sex development: begin as mesonephric (Wolffian – male) ducts and paramesonephric (Müllerian
– female) ducts and undifferentiated gonads.
 In the presence of testosterone produced by the testes of the male, the Wolfian ducts
develop into the epididymus, and ductus deferens. The genital tubercle becomes the penis,
labioscrotal swellings become scrotum and testes descend down into the scrotum. AMH is
secreted by testes and Müllerian ducts degenerate.
 If testosterone is not present due to the ovaries of a female, the Müllerian ducts continue to
develops into the oviducts (uterine tubes) and uterus. The Wolffian ducts degenerate. The
clitoris comes from the genital tubercle and labioscrotal swelling becomes labia majora.