CH. 29 - Fetal Development

Embryo

term has varied meanings

• Some say the fertilized egg or the two-cell stage is an embryo

• Others say that an individual becomes an embryo when it is 16 days old and consists of three primary germ layers

When does the egg have to become fertilized in order to survive?

Egg must be fertilized within 12 to 24 hours of ovulation, if it is to survive

Where must sperm encounter the egg? How many make it the egg and why?

• Sperm must encounter the egg somewhere in the distal one-third of the uterine tube

• Vast majority of sperm do not make it to egg

  • Destroyed by vaginal acid or drain out of vagina

  • Fail to penetrate the mucus of the cervical canal

  • Destroyed by WBCs in the uterus

  • Half go up wrong uterine tube

• Of the 300 million that were ejaculated, about 200 spermatozoa reach the vicinity of the egg

How long are sperm viable after ejaculation?

Sperm are viable for up to 6 - 7 days after ejaculation

• Conception is optimal when sperm are deposited a few days before ovulation to 14 hours after

What happens when the sperm encounters an egg?

When sperm encounters an egg, it releases enzymes needed for one sperm to penetrate the egg

• Penetrates granulosa cells, then zona pellucida

Fertilization

Combines the haploid chromosomes from the male and female gamete to form a diploid cell

• is synonymous with conception

2 Mechanisms to Prevent Polyspermy

Fast Block

• sperm binds to egg which opens up Na+ channels and change the voltage of membrane

• This change in polarity electrically repels other sperm from gaining access

Slow Block

• involves cortical granules that get exocytsized and form a thick membrane that additional sperm cannot pass through

Meiosis ll

The secondary oocyte begins meiosis II before ovulation, but completes it only once it is fertilized (sperm makes contact with egg)

• the final polar body wont be ejected until sperm comes in contact with secondary oocyte

• once that final polar body is ejected, it’s an egg for a short time

• once the male and female genome mix into one genome, its a zygote

• The fertilized egg, now called the zygote, is ready for its first mitotic division

First Trimester

from fertilization through 12 weeks

• More than half of all embryos die from natural causes in the first trimester

• Conceptus is most vulnerable to stress, drugs and nutritional deficiencies during this time

Second Trimester

weeks 13 through 24

• Organs complete most of their development

• Fetus looks human

• Chance of survival (with intensive care) if born near end of this trimester

Third Trimester

week 25 to birth

• Fetus grows rapidly and organs achieve enough cellular differentiation to support life outside of womb

• success rate is pretty high

• At 35 weeks and 5.5 lb, fetus is considered mature

Pre-embryonic Stage

First 16 days of development (starting at beginning of last menstrual period), ending with the existence of an embryo

• Involves 3 major processes

  • Cleavage

  • Implantation

  • Embryogenesis

Cleavage

multiple rounds of divisions that occur in the first 3 days while conceptus migrates down the uterine tube

• First cleavage occurs within 30 hours after fertilization

• Zygote splits into two daughter cells (blastomeres)

• By the time the conceptus arrives in the uterus, it reaches the morula stage

  • About 72 hours after ovulation

  • Morula stage—solid ball of 16 cells

  • Sill no larger than the zygote

• Cleavage produces smaller and smaller blastomeres

• Morula lies free in uterus for 4 to 5 days

  • Divides into 100 cells or so

  • Zona pellucida disintegrates and releases the conceptus, called the blastocyst

Blastocyst

a hollow sphere

• made up of the trophoblast and embryoblast

•  is what actually implants in the lining of endometrium

• attaches to uterine wall 6 days after ovulation

  • Usually on the fundus or posterior wall of the uterus

Trophoblast

outer layer of squamous cells

• Destined to form the placenta and play a role in nourishment of the embryo

Embryoblast

inner cell mass

• Destined to become the embryo

Implantation

process of attachment to uterine wall

• Begins when blastocyst adheres to endometrium

• cells from blastocyst start to grow into lining of endometrium,

  • allows for nutritional uptake (endometrium eats up some of the endometrium for nutrients)

Embryogenesis

The arrangement of blastomeres into 3 primary germ layers in the embryoblast

• Ectoderm, mesoderm, and endoderm

What do trophoblasts secrete?

they secrete human chorionic gonadotropin (HCG)

• HCG stimulates the corpus luteum to secrete estrogen and progesterone

• Progesterone suppresses menstruation

  • keeps endometrium from shedding off by keeping estrogen and progesterone high

• HCG levels rise in mother’s blood until end of second month

What does the trophoblast develop into?

Trophoblast develops into membrane called the chorion

• chorion takes over the role of the corpus luteum, making HCG unnecessary

• Ovaries become inactive for remainder of pregnancy

• Estrogen and progesterone levels rise from chorion

Embryonic Stage

begins when all three primary germ layers are present (usually day 16)

• placenta starts to grow

  • forms over the next 6 weeks

    • Becomes embryo’s primary source of nutrition

• organogenesis begins

Organogenesis

germ layers differentiate into organs and organ systems

• Organs are present (but not fully functional) at 8 weeks

  • when the embryo becomes a fetus

Embryonic Folding

In embryonic stage, the flat embryonic disc is converted into a more cylindrical form

• Occurs during week 4

• Embryo grows rapidly and folds around a membrane called a yolk sac

  • yolk sac serves as early blood

• Embryo becomes C-shaped, with head and tail almost touching

• Lateral margins of the disc fold around the sides of the yolk sac to form the front surface of the embryo

• Neural groove forms in the center of this C shaped embryo - is the CNS 

• As a result of the embryonic folding, the entire surface is covered with ectoderm, and the mesoderm splits into 2 layers

When are all organs present in the fetus?

At 8 weeks

Ectoderm

Outer layer of embryo

• Gives rise to epidermis, nervous system, lens and cornea, integumentary glands, internal ear

  • basically the integumentary and nervous systems

Mesoderm

Middle layer of embryo

• 2 layers of the mesoderm

  • One adheres to the ectoderm

  • The other to the endoderm

  • Coelom exists in-between

    • is a cavity between the two layers

• gives rise to the skeleton, muscle, cartilage, blood, dermis, lymphoid tissue, gonads and ducts, kidneys and ureters

Endoderm

inner most portion of embryo

• forms the tissues and structures that line the tubes that go through our body

  • Gut and respiratory epithelium and glands, bladder, and urethra

Accessory organs that develop with embryo:

• placenta

• umbilical cord

• 4 embryonic membranes:

  • amnion

  • yolk sac

  • allantois

  • chorion

Amnion

transparent sac that completely enclose the embryo

• Penetrated only by the umbilical cord

• Fills with amniotic fluid

  • amniotic fluid is generated by the amnion in the first part of pregnancy

  • in late pregnancy, it’s produced by the baby’s urinary system

Amniotic Fluid

• Protects embryo from trauma, infections, and temperature fluctuations

• Allows freedom of movement important to muscle development

• Enables embryo to develop symmetrically

• Prevents body parts from attaching to each other

• Stimulates lung development as fetus “breathes” fluid

• At first, amniotic fluid formed from filtration of mother’s blood plasma

• Fetus contributes to fluid volume by urinating into amniotic cavity (but fetus also swallows amniotic fluid)

During gestation, the conceptus is nourished in 3 different, overlapping ways:

• Uterine milk

• Trophoblastic nutrition

• Placental nutrition

Uterine Milk

glycogen-rich secretion of the uterine tubes and endometrial glands

• Conceptus absorbs this fluid as it travels down the tube and lies free in the uterine cavity before implantation

• primary source of nutrition during pre embryonic and embryonic stage

Trophoblastic Nutrition

Conceptus consumes decidual cells of the endometrium

• Progesterone from corpus luteum stimulates decidual cells to grow

• The cells accumulate a store of glycogen, proteins, lipids

• As conceptus burrows into the endometrium, the decidual cells are digested

• Is the only mode of nutrition for first week after implantation

• Remains dominant source through the end of 8 weeks

• Decreases as placental nutrition increases

Placental Nutrition

nutrients diffuse from the mother’s blood through the placenta into the fetal blood

Placenta

disc-shaped organ attached to the uterine wall on one side

• on the other side, it is attached to the fetus by the umbilical cord

• placenta previa - when the placenta covers the external os, so C-section is needed 

Placental Phase

the period beginning at week 9

• Sole mode of nutrition from end of week 12 until birth

What is developed by the end of 8 weeks?

• All organ systems are present

• Now considered a fetus

• Bones have begun to calcify

• Skeletal muscles exhibit spontaneous contractions

  • Too weak to be felt by the mother

• Heart, beating since week 4, now circulates/pumps blood

• Heart and liver are very large, forming the prominent ventral bulge

• Head is nearly half the total body length

Fetal Development

The fetus is the final stage of prenatal development

• From the start of week 9 until birth

• Organs mature to support life outside the mother

Unique aspects of fetal circulation:

• Umbilical-placental circuit

  • arteries carry deoxygenated blood to placenta

  • vein carry oxygenated blood to fetus

• Presence of three circulatory shortcuts: shunts

  • blood bypasses lungs by flowing from right atrium to left atrium

  • blood also bypasses lungs by flowing from pulmonary trunk to aorta

  • blood also bypasses the liver

How is blood oxygenated in utero?

• blood is oxygenated by the placenta NOT the lungs

  • placenta serves as a temporary pulmonary circuit

Two Semester Summary

• Anatomy is the study of structure

• Physiology is the study of function

  • structure determines function

• We maintain internal balance (homeostasis) with negative feedback loops

• Too much, or too little of anything is harmful. Moderation is important

• Kindness is of lasting value.