Chapter 15 Human Development

Fertilization

The joining of male and female gametes in the oviduct to form a single cell that contains 23 chromosomes from each parents, for a total of 46 chromosomes.

First Stages of Development

• 1 egg and 1 sperm= Zygote

• The zygote undergoes cleavage meaning cell divisions with little growth

• After many divisions the zygote is now called a morula around the 16-32 cell stage

Ectoderm

Skin, lining of mouth/nose, brain, spinal cord, nerves

Mesoderm

Muscle, skeleton, gonads, kidneys, outer layer of digestive and respiratory tracks, circulatory systems

Endoderm

Liver, pancreas, inner layer of digestive and respiratory tracts.

Chorion

• Outer membrane of the embryo also known as trophoblast

  • secretes hCG (highest amount 1st trimester) to maintain corpus luteum

  • Fetal contribution to the placenta

    • Chorionic villi help to anchor into endometrial lining

Amnion

-inner membrane

• completely encloses embryo also known

• Contains the amniotic fluid, which protects embryo from trauma temperature fluctuations and infections

Allantois

-Becomes umbilical cord

• carries blood between embryo and placenta

• Mostly degenerates in 2nd trimester

Yolk Sac

-In other animals, provided nutrients

• small in humans (forms blood cell)

Cleavage

Many cell division but little growth in between

Blastula (Blastocyst)

The ball continues to divide when fluid from the uterus begins to fill the morula and creates a space inside resulting in two types of cell. cells

Trophoblast

Outer layer of blastocyst

• Will later develop into chorion, and then placenta

Inner cell mass

Will develop into embryo and other membranes

Blastopore

Indentation on Blastula which allows gastrulation.

Blastula function

• Enters the uterus after 7 days and begins to implant in the endometrium

  • Secretes enzymes that digest the endometrium, and the blastocyst sinks in

• Implantation is complete by day 10-14 after fertilization. the trophoblast layer begins to secrete hCG

Gastrulation

• 3rd week (Day 15 after fertilization)

• Occurs after implantation+ first type of differentiation

• A space forms between the inner cell mass and the trophoblast called the amniotic cavity.

• Inner mass flattens like a disc and divides into three types of embryonic tissues (germ Layers)

• The start of organogenesis/ organ creation and morphogenesis/ taking human shape

• Ectoderm, mesoderm, endoderm

Continuation of Gastrulation

• Between the third and eight weeks, the organs form

• Around the third week, a thick band of mesodermal cells gather along the back of the embryonic disk to form a structure called the notochord

  • Forms the frame work for the spine and skeleton

Neurulation

( Formation of the nervous system) Begins along the notochord during the third and fourth weeks of development

Placenta

• Allows for the exchange of some substances between mother and fetus

  • From the mother: nutrients, oxygen, antibodies, viruses, drugs, alcohol

  • From Fetus: Metabolic waste

  • There is no exchange of blood cells

Formation of the Placenta

• Formed when villi from chorion extend into endometrium

  • Not fully developed until 2nd trimester

• Secretes estrogen and progesterone during the 2nd and 3rd trimesters

Function of the Placenta: Nutrients

Transports and stores nutrients (from mom’s body)

Function of the Placenta: Excretory

Transports waste (urea, ammonia) to mom's blood

Function of the Placenta: Respiratory

Transports oxygen to the fetus and CO2 away

Function of the Placenta: Endocrine

Secretes hormones like hCG, estrogen, and progesterone

Function of the Placenta: Immune

Transports antibodies from mom to baby

First Trimester ( Weeks 1-12 )

• Embryonic development week 1-8 Implantation, gastrulation, neurulation, Blood vessels and blood cells form, lungs and kidney take form

• The chorion is secreting hCG

  • To maintain the corpus luteum in the ovary to keep secreting progesterone and estrogen for endometrial development

• The time that the embryo is MOST SUSCEPTIBLE to environmental factors.

Teratogens

• A substance that increases the chance of birth defects ex. alcohol

Second Trimester (Weeks 13-24)

Focus is on the growth and refinement of tissues

• Organs grow in size, cartilage skeleton replace by bone

• Fetus can survive outside womb at the end of 6th month (with medical support)

• Placenta secretes progesterone/estrogen

Third Trimester (Weeks 25-40)

Focus is on growth and fat deposition

• Organ systems increase in size

• Respiratory and circulatory systems allow for air breathing.

• Fetus grow Rapidly

Relaxin

Hormones Involved in Labor and Birth

• produced by placenta, relax ligaments in pelvis for easy passage of baby

Oxytocin

Hormones Involved in Labor and Birth

• Produced by the hypothalamus and stored in posterior pituitary, positive feedback cause uterine contractions

Prostaglandis

Hormones Involved in Labor and Birth

• local area hormone (produced by uterus) that works with oxytocin causing uterine contractions

Prolactin

Hormones Involved in Labor and Birth

Produced by anterior pituitary, causes mammary glands to produce milk.

Breastfeeding

• Hormones control the onset of lactation

• High levels of estrogen and progesterone suppress the production of prolactin in the pituitary

• After the placenta is gone, estrogen and progesterone drop, allowing prolactin to be produced.

• Before regular milk production begins, colostrum is formed ( a thin, yellowish fluid that contains more protein, antibodies and less fat.

Chorionic Villi Sampling

PreNatal Screening Technologies

• Needle removes fetal cells from chorion/placenta for genetic testing

  • Detects abnormalities in chromosomes like Down syndrome, cystic fibrosis, etc.

  • Can be performed after the 9th week of pregnancy

Amniocentesis

PreNatal Screening Technologies

• Needle withdraws amniotic fluid sample containing fetal cells

  • Genetic testing (Karyotyping) can detect the same things as CVS

  • Can be performed afterthe 14th week

Ultrasound Imaging

PreNatal Screening Technologies

• Sound waves sent through body are sent to a computer to visualize internal and external structures.