Conception & Fetal

Development

Lecture 2

REVIEW

Gametogenesis: production of gametes.Male gamete (sperm) produced in seminiferous tubules of

testes during spermatogenesis. 200-600 mil.@ ejacula.

Female gamete (ovum) produced in graafian follicule of ovary during oogenesis. At birth, each ovary has 2 mil. immature oocytes, occurs 1st 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: 1st meiotic division completed before ovulation & 2nd meiotic division completed at fertilization.

Conception: fertilization of sperm & ovum in ampulla [upper 1/3rd ] 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 ~ 5th 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)

7 - 9 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.

> 7th week shift to placental production of hormones begins.

By 12th 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:

50-55 cm length; 2 cm diameter 2 arteries & 1 vein – “AVA” Made of Wharton’s jelly (gelatinous) Blood flows thru cord @ rate of ~ 400 ml/min. Vessels remain patent Develops @ 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. PlacentaStructure: 15-20 subdivisions “cotyledons”. Fetal surface: smooth, shiny, covered by

amnion. Maternal surface: red, flesh-like, 15-20

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 800-1200ml. 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

4th 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 2/3rd 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.

38-40 wks: Full term @ beg. of 37th wk. Fills uterus. Gets IgA ab from mother. Gains 2 ½ lbs; mostly fat. L: 48-52 cm. or 20” long. W: 3000-4000g, or ~ 8 lbs.

Fetal Circulation Placenta [O2 rich blood >> maternal circulation] Enters Umbilical vein (O2 rich blood) ductus venous (2/3rd 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 1st 24-48 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.

http://www.indiana.edu/~anat550/cvanim/fetcirc/fetcirc.html