Presentation on theme: "Conception & Fetal Development Lecture 2. REVIEW Gametogenesis: production of gametes. Male gamete (sperm) produced in seminiferous tubules of testes."— Presentation transcript:
Conception & Fetal Development Lecture 2
REVIEW Gametogenesis: production of gametes. Male gamete (sperm) produced in seminiferous tubules of testes during spermatogenesis ejacula. Female gamete (ovum) produced in graafian follicule of ovary during oogenesis. At birth, each ovary has 2 mil. immature oocytes, occurs 1 st 5 mos. of development. Chromosomes divide (meiosis) from 46 → 23 before fertilization. 2 meiotic divisions in both sperm & ovum 1 spermatogonium >> 4 spermatids (approximately1000 sperm per second or ~ 30 billion/year) 1 oogonium >> 1 mature ovum & 3 polar bodies Ovum: 1 st meiotic division completed before ovulation & 2 nd meiotic division completed at fertilization.
Conception: fertilization of sperm & ovum in ampulla [upper 1/3 rd ] of fallopian tube. Now “zygote”. ½ genetic material comes from each parent cell. Head of mature sperm contains chromosomes. Zona pellucida (ovum) changes chemical composition so multiple sperm cannot enter. Fertilized ovum begins mitotic cell division.
B. Cellular Multiplication Zygote undergoes mitosis (cleavage) - rapid series of cell divisions. Forms morula; solid ball of cells. Outer shell of cells with an attached inner group of cells forms, changing morula into “blastocyst”. Blastocyst consists of inner cell mass and trophoblast. Outer group of cells become membranes that nourish & protect inner group of cells (embryo). Blastocyst reaches uterus ~ 5 th day. Inside blastocyst, inner cell mass generates 3 major layers inside the sphere: ectoderm, mesoderm, endoderm. Inner cell mass develops into fetus.
C. Implantation Outer wall of blastocyst (trophoblast) attaches to endometrium (anterior or posterior fundal region) days > fertilization. Occurs 2-4 days > entering uterus. Blastocyst receives nourishment via mother's bloodstream. Embryo fully implanted by day 10. During time between implantation & 8th week, cellular differentiation occurs.. (blood cells, kidney cells, nerve cells, etc.). From 8th week until birth - “fetus”.
Yolk Sac Attached to the embryo Continuous with intestinal cavity of embryo Supplied with blood vessels which transport nutritive yolk products to developing embryo Degenerates by week 12 when placenta takes over
D. Placentation: ongoing process of fetal & maternal placental formation. Begins immediately after implantation. Endometrium now “decidua”. 3 parts: basalis, capsularis, & vera. Basalis unites with chorion to form maternal side of placenta. Capsularis surrounds chorionic sac. Decidua Vera is mucous membrane lining main cavity of pregnant uterus other than at site of implantation.
Pregnancy Hormones Developing embryo begins to produce hCG (human chorionic gonadotropin) - enables corpus luteum to continue to secrete progesterone/estrogen. In early pregnancy, steroid hormones are responsible for maintaining endometrium [uterus] rich with blood vessels so zygote can develop. > 7 th week shift to placental production of hormones begins. By 12 th week, hormone production entirely from placenta.
E. Fetal Membranes Chorion: outer fetal membrane closest to uterine wall Fingerlike projections “chorionic villi” form & invade endometruim; becomes fetal part of future placenta 2-3 weeks > fertilization. Intervillous space: space between the chorionic villi where maternal blood circulates within placenta
Complex network of embryonic vessels allows diffusion of nutrients, oxygen, & wastes bet. mother & fetus. Amnion: inner fetal membrane Chorion & amnion fuse to become one membrane, amniotic sac. Holds fetus & amniotic fluid.
F. Umbilical Cord: cm length; 2 cm diameter 2 arteries & 1 vein – “AVA” Made of Wharton’s jelly (gelatinous) Blood flows thru rate of ~ 400 ml/min. Vessels remain patent 5 wks.gestation; lifeline between placenta & fetus. Carries waste & O2 poor blood away from fetus to placenta. Carries O2 rich blood back to fetus.
G. Placenta Structure: subdivisions “cotyledons”. Fetal surface: smooth, shiny, covered by amnion. Maternal surface: red, flesh-like, subdivisions or cotyledons. "Dirty Duncan" and "Shiny Schultz" Physiology: Function: transport mechanism between mom & fetus. Lifespan depends on O2 consumption. Function depends on maternal circulation. Circulation best when mother in lateral position.
Functions of Placenta Receives maternal 02 via diffusion. Produces all hormones to sustain endometrium thus the pregnancy (HPL, estrogen, progesterone, relaxin) Supplies fetus with CHO, water, fats, proteins, minerals & inorganic salts. Carries end products of fetal metabolism into maternal circulation for excretion. Transfers passive immunity via maternal antibodies.
H. Amniotic Fluid ml. Clear, yellow fluid Contains albumin, lanugo & urea. Replaced every 3 hours; swallowed by fetus. Functions Prevents heat loss; preserves constant fetal body temperatures. Cushions fetus. Acts as excretion – collection system. Facilitates fetal growth & development
First Trimester - fetus most susceptible to damage from external sources including: teratogens (causing birth defects … ie. alcohol, some Rx & recreational drugs) infections (ie. rubella or cytomegalovirus) radiation (x-rays, radiation therapy, or accidental exposure to radiation) nutritional deficiencies
Summary of Fetal Development 4 th wk: Fetal heart begins to beat. (smaller than grain of rice). 8 wks: All body organs formed. Weighs less than grape. 1/3 baby is head. Arms & legs are tiny buds. L: 30mm ( 1in.) Wt. = 2 g. 12 wks: Fetal heart tones heard by Doppler. Baby can kick. Sex organs formed. Fingers & toes developed. Eyelids fused. L: 4 in or 11cm. W: 45 g.
Fetal Development 16 wks: Sex can be seen. Thin; looks like baby. Uterus size of grapefruit. Fetus as large as orange. Starting to suck/swallow. Nails on fingers/toes. L: 5 in or 15 cm. W: 200 g. Actively swallows amniotic fluid. Lanugo forms. 20 wks: Heartbeat heard with fetoscope. Develops regular schedule: sleeping, sucks thumb, kicking. Hands grasp.Vernix caseosa begins to form. Assumes favorite position in utero. Lanugo keeps oil on skin. L: 25 cm. W: 400 g. + fetal movement (quickening)
Fetal Development 24 wks: L: 30 cm. W: 750 g. Weighs about 1.3 lb. Increased activity. Respiratory movement begins. Skin is thin. No fat. Regular sleep time. 28 wks: Eyes open & close. Baby makes breathing motions. Surfactant begins forming. Testes descend. More fat forms. Can hiccup, cry, hear your voice. 14” long; 2.2 lbs. Baby 2/3 rd final size. L: 35 cm. W: 1200g.
Fetal Development 32 weeks: More subcutaneous fat laid down. Appears less red & wrinkled; 14” long; 4 lbs. brain growing. Lungs immature. Gains 1/2 lb/wk. L:35-38 cm. W: 2000 g. Skin pink; covered with vernix caseosa; lanugo begins to disappear. Braxton Hicks are felt. wks: Full beg. of 37 th wk. Fills uterus. Gets IgA ab from mother. Gains 2 ½ lbs; mostly fat. L: cm. or 20” long. W: g, or ~ 8 lbs.
Fetal Circulation Placenta [O2 rich blood >> maternal circulation] Enters Umbilical vein (O2 rich blood) ductus venous (2/3 rd bypasses liver) hepatic vein (small amt. blood flow) Enters inferior vena cava [IVC] 30-35% enters R. atrium >> passes through foramen ovale (shunt in fetal heart) >> L. Atrium >> L.Ventricle >> Ascending aorta to Head & upper extremities (to oxygenate where needed most)
65-70%of blood in Right atrium mixes with O2 poor blood returning from SVC >> Right Ventricle >> enters ductus arteriosis to be shunted away from lungs >> enters descending aorta >> lower extremities/trunk Only ~ 8 % enters pulmonary arterial bed through right & left pulmonary arteries and returns from lungs to left atrium via 4 pulmonary veins. Eventually all O2 poor blood leaves thru aorta >> 2 umbilical arteries >> placenta to re-oxygenate.
Fetal circulation: Preferential shunting of blood with highest O2 saturation to L side of heart. Assures adequate oxygenated blood flow to coronary & cerebral circulations—tissues with greatest need. Circulation > birth : With first breaths, larger amount of blood sent to lungs to pick up O2. Ductus arteriosus no longer needed; begins to wither & close off in 1-2 days. PDA may be heard 1 st hours of life. Circulation in lungs ^ & more blood flows into L atrium. This ^ pressure causes foramen ovale to close & blood circulates normally. Umbilical arteries/vein degenerate. Shunts & vessels > ligaments & supporting structures. l