Pregnancy, Human Development

zygote

fertilized egg; undergoes cleavage and eventually leads to blastocyst formation

morula

solid ball of blastomeres formed after a zygote undergoes cleavage multiple times; compact structure of 16 to 32 cells

blastocyst

stage of early embryonic development; product of cleavage; contains the trophoblast and embryoblast

implantation

the process where a blastocyst attaches to the uterine lining (endometrium) to establish pregnancy; occurs about 6 to 12 days after ovulation

embryoblast

accumulation of cells in the blastocyst from which the embryo develops; becomes the proper embryonic disc

trophoblast

outer sphere of cells of the blastocyst; critical for implantation and placental development; erodes endometrium and promotes interactions with vasculature and structure of uterine wall; secretes hCG which rescues corpus luteum and facilitates pregnancy hormone secretion

placenta

temporary organ formed from both fetal and maternal tissues that provides nutrients and oxygen to the developing fetus, carries away fetal metabolic wastes, and produces hormones of pregnancy (hCG, estrogen, progesterone, relaxin)

human chorionic gonadotropin

produced by the placenta, supports corpus luteum; rises after fertilization, peaks around 8 weeks, then falls and levels off around 20 weeks

amniotic fluid

clear, slightly yellowish fluid that surrounds and protects a developing fetus within the amniotic sac during pregnancy

gastrulation

formation fo three major germ layers from embryonic disc

ectoderm

outer germ layer of the embryo; forms the nervous system and the epidermis of the skin and its derivatives

endoderm

inner germ layer of the embryo; forms epithelial lining of digestive and respiratory systems and epithelial cells of associated glands

mesoderm

middle germ layer of the embryo; forms all other organ systems and tissues not formed by endoderm and ectoderm

chorion

outermost fetal membrane, helps form the placenta; consists of trophoblasts and extra-embryonic mesoderm

lactation

production and secretion of milk; stimulated by prolactin from the pituitary

parturition

culmination of pregnancy; giving birth; stimulated by oxytocin

lactiferous ducts

ducts that converge and form a branched system connecting the nipple to the lobules of the mammary gland

alveoli

reside in the breast lobules and produce milk

relaxin

hormone produced by corpus luteum and placenta; calms the uterus and relaxes pelvic ligaments; eases passage of baby during birth

prolactin

hormone produced by the pituitary to promote milk production

What happens during implantation?

1. cells of zygote divide to form the morula, which tumbles down the oviduct and passes into the cavity of the uterus 3 or 4 days later

2. 3 or 4 days after fertilization, morula becomes blastocyst

3. one week after fertilization, blastocyst implants into endometrium of the uterus

   1. trophoblast cells bind to epithelium of endometrium and begin secreting enzymes to help penetrate the endometrium

   2. trophoblast cells proliferate, invade uterine lining, become surrounded by maternal blood

   3. endometrial cells proliferate and seal off implanted blastocyst

4. outer layer of trophoblast cells begin secreting hCG, which rescues the corpus luteum and allows it to continue secreting estrogen and progesterone

When and where does implantation occur?

one week after fertilization in the epithelium of the endometrium in the uterus; takes about five days, completed by day 26 of monthly cycle

Which type of entity undergoes implantation?

blastocyst

Why is hCG important?

it maintains the corpus luteum in the ovary, which secretes estrogen and progesterone to maintain the integrity of the uterine endometrium

useful in pregnancy tests

What does the placenta do?

• provides nutrients and oxygen to the embryo

• carries away embryonic wastes

• endocrine organ that produces estrogen and progesterone

• also produces human placental lactogen and relaxin

What are the three main products of gastrulation and why are they significant to organogenesis?

• endoderm: forms epithelial lining of many major organ systems including digestive tract, respiratory system, and various glands

• mesoderm: gives rise to many organ systems including the kidneys, gonads, heart, non-epithelial parts of the digestive tract, and various connective tissues

• ectoderm: differentiates into the epidermis and the nervous system

What are the major hormones related to pregnancy?

hCG, estrogen, and progesterone

What is the timeline of the major hormones related to pregnancy?

• first trimester: hCG secreted in increasing amounts, peaks around weeks 7 to 8 of pregnancy

• estrogen and progesterone produced in increasing amounts throughout pregnancy, rising to a peak just before term

Where do the major hormones related to pregnancy come from?

• hCG: implanted blastocyst (which becomes the placenta)

• estrogen and progesterone: corpus luteum, then placenta

What events stimulate parturition?

• fetal development: rise in cortisol initiates positive feedback loop that further increases CRH and cortisol

  • leads to increased estrogen in the placenta, which overrides quieting actions of progesterone on the myometrium

• prostaglandin production increased

  • stimulates labor

• myometrial sensitivity to oxytocin is increased due to upregulation of oxytocin receptors and increases in circulating oxytocin levels

Why is oxytocin important in childbirth?

• it stimulates the contraction of uterine smooth muscle during childbirth

• important role in milk let-down and bonding between mother and infant

• promotes nurturing, bonding, and trust

The cell type responsible for forming the placenta is the:

trophoblast

The hormone responsible for milk letdown in a woman who is lactating is:

oxytocin

What is the outermost layer of an embryo?

ectoderm

What produces hCG?

placenta