Pregnancy and Lactation Lecture Notes PDF

PREGNANCY AND LACTATION Master of Arts in Biomedical Sciences LEARNING OBJECTIVES 1. List the protein hormones secreted by the placenta and describe the role of human chorionic gonadotropin (hCG) in the maintenance of the corpus luteum. 2. Describe the different stages of the embryo (ex: morul...

PREGNANCY AND LACTATION Master of Arts in Biomedical Sciences LEARNING OBJECTIVES 1. List the protein hormones secreted by the placenta and describe the role of human chorionic gonadotropin (hCG) in the maintenance of the corpus luteum. 2. Describe the different stages of the embryo (ex: morula, blastocyst, etc.) 3. Explain the general process of implantation and placentation. 4. Explain the role of prolactin and oxytocin in mammary gland function during lactation. 5. Explain the benefits of breast feeding to both baby and mother. FROM EGG TO EMBRYO: NOMENCLATURE Pregnancy – events that occur from fertilization until the infant is born Conceptus – the developing offspring Gestation period – from the last menstrual period until birth Preembryo – conceptus from fertilization until it is two weeks old Embryo – conceptus during the third through the eighth week Fetus – conceptus from the ninth week through birth SPERM TRANSPORT Fates of ejaculated sperm ­ Destroyed by the acidic vaginal environment ­ Fail to make it through the cervix ­ Dispersed in the uterine cavity or destroyed by phagocytic leukocytes ­ Reach the uterine tubes ACCOMPLISHING FERTILIZATION The oocyte is viable for 12 to 24 hours Sperm is viable 24 to 72 hours For fertilization to occur, coitus must occur no more than: ­ Three days before ovulation ­ 24 hours after ovulation Fertilization – when a sperm fuses with an egg to form a zygote FERTILIZATION Fertilization: fusion of the male and female gametes. Where: distal third of the oviduct. Fact: in vitro fertilization resulted in the precisely ordered events constituting a “fertilization pathway”. Fertilization pathway. A and B show sperm binding and the acrosome reaction, exposing the zona pellucida to acrosomal enzymes. C depicts penetration of the enzyme-modified zona pellucida by the sperm. D shows activation of the egg, including oocyte membrane hyperpolarization and release of enzymes by the cortical granules. Modified from Wassarman PM ACROSOMAL REACTION AND SPERM PENETRATION An ovulated oocyte is encapsulated by: ­ The corona radiata and zona pellucida ­ Extracellular matrix Sperm binds to the zona pellucida and undergoes the acrosomal reaction: ­ Enzymes are released near the oocyte ­ Hundreds of acrosomes release their enzymes to digest the zona pellucida Binding is relatively species-specific and requires a complete plasma membrane (i.e., an acrosome-intact sperm). Once bound to the zona pellucida surface, the sperm undergoes a series of dynamic membrane fusions known as the acrosome reaction ACROSOMAL REACTION AND SPERM PENETRATION Once a sperm makes contact with the oocyte’s membrane: ­ Beta protein finds and binds to receptors on the oocyte membrane ­ Alpha protein causes it to insert into the membrane BLOCKS TO POLYSPERMY Only one sperm is allowed to penetrate the oocyte Two mechanisms ensure monospermy 1. 2. Fast block to polyspermy: membrane depolarization prevents sperm from fusing with the oocyte membrane Slow block to polyspermy: zonal inhibiting proteins: ­ Destroy sperm receptors ­ Cause sperm already bound to receptors to detach COMPLETION OF MEIOSIS II AND FERTILIZATION 1. Upon entry of sperm, the (A) secondary oocyte: ­ ­ A Completes meiosis II Casts out the second polar body (B) 2. The ovum nucleus swells, and the two nuclei approach each other 3. When fully swollen, the two nuclei are called pronuclei 4. Fertilization – when the pronuclei come together B EVENTS IMMEDIATELY FOLLOWING SPERM PENETRATION PREEMBRYONIC DEVELOPMENT The first cleavage produces two daughter cells called blastomeres Morula – the 16 or more cell stage (72 hours old) By the fourth or fifth day the preembryo consists of 100 or so cells (blastocyst) Morula Blastocyst PREEMBRYONIC DEVELOPMENT Blastocyst – a fluid-filled hollow sphere composed of: ­ A single layer of trophoblasts ­ An inner cell mass Trophoblasts take part in placenta formation The inner cell mass becomes the embryonic disc CLEAVAGE: FROM ZYGOTE TO BLASTOCYST Degenerating zona pellucida Inner cell mass Blastocyst cavity Blastocyst cavity (a) Zygote (fertilized egg) Fertilization (sperm meets egg) (b) 4-cell stage 2 days (a) (c) Morula 3 days (b) (d) Early blastocyst 4 days Trophoblast (e) Implanting blastocyst 6 days (c) Ovary Uterine tube (d) Oocyte (egg) Ovulation Uterus Endometrium Cavity of uterus (e) TRANSPORT OF THE FERTILIZED OVUM IN THE FALLOPIAN TUBE The fallopian tubes are lined with a rugged cryptoid surface that impedes passage of the ovum despite the fluid current. Isthmus of the fallopian tube remains spastically contracted for about the first 3 days after ovulation. After this time, the rapidly increasing progesterone secreted by the ovarian corpus luteum first promotes increasing progesterone receptors on the fallopian tube smooth muscle cells; then the progesterone activates the receptors, relaxing the tubules and allowing entry of the ovum into the uterus. This delayed transport of the fertilized ovum through the fallopian tube allows several stages of cell division to occur before the dividing ovum—now called a blastocyst, with about 100 cells—enters the uterus. During this time, the fallopian tube secretory cells produce large quantities of secretions used for nutrition of the developing blastocyst. IMPLANTATION Blastocyst takes 1-3 days to starts the implantation in the uterus. Implantation ordinarily occurs on about the fifth to seventh day after ovulation. Blastocyst obtains its nutrition from the uterine endometrial secretions, called “uterine milk” before the implantation. Implantation results from the action of trophoblast cells that develop over the surface of the blastocyst. Once implantation has taken place, the trophoblast cells and other adjacent cells (from the blastocyst and the uterine endometrium) proliferate rapidly, forming the placenta and the various membranes of pregnancy. The trophoblasts then proliferate and form two distinct layers: ­ Cytotrophoblast : cells of the inner layer that retain their cell boundaries ­ Syncytiotrophoblast : cells in the outer layer that lose their plasma membranes and invade the endometrium IMPLANTATION OF THE BLASTOCYST IMPLANTATION Viability of the corpus luteum is maintained by human chorionic gonadotropin (hCG) secreted by the trophoblasts hCG prompts the corpus luteum to continue to secrete progesterone and estrogen Chorion : developed from trophoblasts after implantation, continues this hormonal stimulus Between the second and third month, the placenta: ­ Assumes the role of progesterone and estrogen production ­ Is providing nutrients and removing wastes HUMAN CHORIONIC GONADOTROPIN Human chorionic gonadotropin (hCG) is one of the first hormones of pregnancy, produced by trophoblasts even before placenta formation. After placentation, hCG is synthesized primarily by the syncytiotrophoblasts and passes into the maternal circulation via secretion into the intervillous space. hCG can be detected in human serum or urine within a week of conception and is the most frequently used biochemical marker for pregnancy. The primary biologic role of hCG is to maintain progesterone production by the corpus luteum until this function shifts to the maturing placenta. HORMONAL CHANGES DURING PREGNANCY EARLY NUTRITION OF THE EMBRYO Progesterone secreted by the ovarian corpus luteum during the latter half of each monthly sexual cycle has an effect on the uterine endometrium, converting the endometrial stromal cells into large swollen cells containing extra quantities of glycogen, proteins, lipids, and even some minerals necessary for development of the conceptus (the embryo and its adjacent parts or associated membranes). Then, when the conceptus implants in the endometrium, continued secretion of progesterone causes the endometrial cells to swell further and to store even more nutrients. These cells are now called decidual cells, and the total mass of cells is called the decidua. The placenta also begins to provide nutrition after about the 16th day beyond fertilization. PLACENTATION Implantation constitutes the first contact between the developing blastocyst and the uterine mucosa After implantation the placenta controls fetomaternal exchange of nutrients, gasses, and waste products The human placenta is of the most invasive type characteristic of those species with interstitial implantation—that is, that in which the blastocyst burrows completely beneath the surface of the uterine epitheliu. Placental trophoblast cells invade as far as the myometrium, eroding maternal tissue so extensively that they become completely surrounded by, and in direct contact with, maternal blood. This type of placentation is called hemochorial PLACENTATION Formation of the placenta from: ­ Embryonic trophoblastic tissues ­ Maternal endometrial tissues The chorion develops fingerlike villi, which: ­ Become vascularized ­ Extend to the embryo as umbilical arteries and veins ­ Lie immersed in maternal blood Decidua basalis : part of the endometrium that lies between the chorionic villi and the stratum basalis PLACENTATION Decidua capsularis – part of the endometrium surrounding the uterine cavity face of the implanted embryo The placenta is fully formed and functional by the end of the third month Embryonic placental barriers include: ­ The chorionic villi ­ The endothelium of embryonic capillaries The placenta also secretes other hormones – human placental lactogen, human chorionic thyrotropin, and relaxin THE MATERNOFETAL BARRIER Maternofetal barrier is what separates maternal and fetal blood within the intervillous space and placental vasculature, respectively. The barrier is formed and is composed of the following layers: 1. An outermost, continuous layer of syncytiotrophoblast covering the villi and thus lining the intervillous space. 2. A second layer of cytotrophoblast cells, which is continuous in the first trimester. As growth and branching of villi begin to offset cytotrophoblast proliferation during the second trimester, this layer becomes increasingly discontinuous, such that these cells are comparatively rare by term. 3. A basal lamina upon which the trophoblast layers rest. 4. Connective tissue, fibroblasts, and Hofbauer cells derived from the extraembryonic mesoderm. 5. The innermost fetal endothelium, which by the last trimester is surrounded by an endothelial basal lamina. RESPONSE OF THE MOTHER’S BODY TO PREGNANCY Most apparent among the many reactions of the mother to the fetus and to the higher levels of pregnancy hormones is the increased size of the various sexual organs. For example, the uterus increases from about 50 to 1100 grams, and the breasts approximately double in size. At the same time, the vagina enlarges and the introitus opens more widely. Also, the various hormones can cause marked changes in a pregnant woman’s appearance, sometimes resulting in the development of edema, acne, and masculine or acromegalic features. PROLACTIN PROMOTES LACTATION Estrogen and progesterone are essential for physical development of the breasts during pregnancy, Prolactin has the opposite effect and promotes milk secretion. Prolactin is secreted by the mother’s anterior pituitary gland, and its concentration in her blood rises steadily from the fifth week of pregnancy until birth of the baby, at which time it has risen to 10 to 20 times the normal nonpregnant level. BREAST FEEDING: IMPACT ON MOTHER Requires adequate nutrition Production of ~25 oz milk/day (~ 3 cups) Energy requirement: +330 kcal/d 1st 6 mo; + 400 kcal/d 2nd 6 months Increased need for fluids Nutritional deficiency affects quantity (vs quality) of milk; missing nutrients are pulled from mother’s stores TABLE 15-4 PAGE 531 LACTATION Secretory unit of the breast is the alveolus ­ Drain into ductules which then drain into a lactiferous duct. ­ Lactiferous ducts carry secretions to the outside Prolactin- initiates milk production from the alveolus. Oxytocin- aides in contraction of myoepithelial cells, leads to ejection of milk. BENEFITS OF BREAST MILK Immunological protection: ­ Colostrum ­ Produced first 2-3 days ­ Contains antibodies that help protect infant from infections ­ Bifidus factors ­ Found in colostrum and breast milk ­ Favor growth of lactobacillus bifidus to help control growth of undesirable intestinal bacteria Lactoferrin: helps infant absorb Fe (49% vs 4%) Binds Fe, bacteria can’t use, inhibits bacterial growth, protective against diarrhea Breast milk is sterile Decreases incidence of allergic reactions Special bonding time for mother and baby Always ready at right temperature, right mixture Decrease in ear infections, BBTD, protects against necrotizing entercolitis SUBSTANCE USE AND BREAST MILK Substances can enter breast milk Alcohol ­ Infants drink less breast milk ­ 1-1/2 oz may reduce milk production Drugs ­ Most do enter breast milk COLIC AND BREAST MILK Caffeine Irritability and wakefulness in infant Moderate consumption; ex. 1 -2 cups coffee/d Strong spices Individual variations Garlic may enhance suckling Fresh cow’s milk; may use cheese, yogurt, etc. Thank you