chapter 29: human development

the study of prenatal development

embryology

examines changes in form and function from fertilized egg through old age

developmental biology

what can labeling something as an embryo mean?

its fertilized egg (2-cell stage) or its 16 days old and consists of the 3 primary germ layers

the first objective of a zygote is to gain mass by multiplying & dividing, how long does ths typically take?

1 weeks

about how many sperm actually get near the egg?

20 to 200

less than __% of sperm actually enter the uterus bc of narrow folds and sticky mucus

1

what must sperm undergo before it can fertilize an egg? how is this achieved?

female fluids make it go through capacitation

what is capacitation?

make sperm membrane more permeable to calcium & activate receptors for chemical attractants

sperm find the egg by way of chemotaxis, what is it attracted to?

progesterone (cumulus cells) surrounding the egg

As sperm works it way through cumulus cells, ___ from surrounding fluid stimulates sperm tail lashing

calcium

When head reaches zona pellucida (ZP), ____ ____ of sperm digests path through ZP

acrosomal enzyme

when the egg & sperm membranes fuse, sperm release their ___ and their mitochondria are tagged for distruction

nucleus

what happens with the fast block method to prevent polyspermy?

egg membrane depolarizes when first penetrated, inhibiting attachment by another sperm

what happens with the slow block method to prevent polyspermy?

cortical granules release contents via exocytosis; enzyme destroys ZP3 protein of zona pellucida, blocking attachment of another sperm

Upon fertilization, sperm and egg nuclei swell and become a ___

pronuclei

what happens when the pronuclei of the sperm and egg ruptures?

the chromosomes of the two gametes mix into a single diploid set and are considered a zygote

extends from fertilization through 12 weeks. more than half of all embryos die during this time due to chromosomal defects. conceptus is most vulnerable to stress, drugs, and nutritional deficiencies during this time

first trimester

weeks 13 through 24. organs complete most of their development. fetus looks distinctly human. chance of survival (with intensive care) if born near end of this trimester

second trimester

week 25 to birth. fetus grows rapidly and organs achieve enough cellular differentiation to support life outside of womb. at 35 weeks and 5.5 lbs, fetus is considered mature

third trimester

first 16 days of development, culminating in an embryo. involves 3 major processes: cleavage, implantation, & embryogenesis

pre-embryonic stage

mitotic divisions that occur in first 3 days while conceptus migrates down uterine tube. occurs within the first 30 hours after fertilization and the zygote splits into 2 daughter cells (blastomeres)

cleavage

By the time the conceptus arrives in the uterus, it is a ___, solid ball of 16 cells that resembles a mulberry

morula

a hollow sphere with an outer layer of squamous cells called the trophoblast and internal cavity called the blastocoel

blastocyst

the morula lies free in uterine cavity for 4 to 5 days, & divides into 100 cells or so. the ____ _____ disintegrates and releases conceptus, called blastocyst

zona pellucida

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

trophoblast of blastocyst

inner cell mass, destined to become embryo in a blastocyst

embryoblast

About two-thirds of twins. Two eggs are ovulated and both are fertilized by separate sperm forming two zygotes. No more or less genetically similar than any other siblings. Implant separately in the uterine wall and each forms its own placenta

dizygotic twins

One egg is fertilized (one zygote) but embryoblast divides into two up to 14 days after fertilization. They are of the same sex and very similar in appearance. Not quite genetically identical: genetic differences occur due to mutations that occur during gestation, and DNA methylation can cause differences in gene expression

monozygotic twins

agents that cause anatomical deformities in the fetus Fall into three major classes: Drugs and other chemicals, Infectious diseases, & Radiation such as X-rays. exposure during the first 2 weeks may cause a spontaneous abortion

teratogen

abnormal structures or positions of organs at birth resulting from a defect in prenatal development. Not all are noticeable at birth, with some being detected months to years later. In 50% to 60% of cases, the cause is unknown

congenital abnormalizes or birth defects

process of attachment to uterine wall; usually takes one week and occurs on fundus or posterior wall. it begins when the blastocyst adheres to the endometrium

implantation

during implantation, the trophoblast on the attachment side separates into 2 layers. what are they?

Syncytiotrophoblast & Cytotrophoblast

Deep layer close to embryoblast

Cytotrophoblast

Superficial layer is in contact with the endometrium. it grows into the uterus like little roots, digesting endometrial cells along the way until the endometrium covers the blastocyst

Syncytiotrophoblast

what does the trophoblast secrete?

human chorionic gonadotropin (HCG)

what does HCG do?

stimulates the corpus luteum to secrete estrogen and progesterone

what does progesterone do in pregnancy?

suppresses mensuration & makes the ovaries inactive along with estrogen

Trophoblast develops into membrane called the ___, takes over role of corpus luteum making HCG unnecessary

chorion

arrangement of blastomeres into three primary germ layers in the embryoblast (ectoderm, mesoderm, and endoderm)

embryogenesis

when the embryoblast separates slightly from the trophoblast, it creates a narrow space between them, what is this space called?

amniotic cavity

begins when all three primary germ layers are present (usually Day 16). placenta forms over the next 6 weeks, becomes embryo’s primary source of nutrition & organs are present but not fully functional

embryonic stage

germ layers differentiate into organs and organ systems

organogenesis

turns into the epidermis, nervous system, lens and cornea, & internal ear

ectoderm

turns into the skeleton, muscle, cartilage, blood, lymphoid tissue, gonads and ducts, kidneys and ureters

mesoderm

turns into the gut and respiratory epithelium and glands, bladder, and urethra

endoderm

four embryonic membranes of pregnancy:

amnion, yolk sac, allantois, and chorion

transparent sac that develops from epiblast. grows to completely enclose the embryo. penetrated only by the umbilical cord. Fills with amniotic fluid

amnion

functions of the amnion

protects embryo from trauma, infections, temperature fluctuations; allows freedom of movement important to muscle development; enables symmetrical development; prevents body parts from adhering to each other; stimulates lung development as fetus “breathes” the fluid

arises from hypoblast cells opposite amnion. small sac suspended from ventral side of embryo. contributes to formation of GI tract, blood cells, and future egg or sperm cells

yolk sac

begins as an outpocketing of the yolk sac. forms the foundation for the umbilical cord. becomes part of the urinary bladder

allantois

outermost membrane enclosing all the rest of the membranes and the embryo. has shaggy outgrowths called chorionic villi around entire surface

chorion

the villi of the placental region grow and branch while the rest of them degenerate

smooth chorion

at placental attachment, forms fetal portion of the placenta

villous chorion

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

uterine milk, trophoblastic nutrition, and placental nutrition

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

uterine milk

conceptus consumes decidual cells of the endometrium. as conceptus burrows into the endometrium, the syncytiotrophoblast digests decidual cells and supplies the nutrients to the embryoblast. Only mode of nutrition for first week after implantation and remains dominant source through the end of 8 weeks

Trophoblastic nutrition

when does the placental phase start, causing placental nutrition to start?

week 9 and lasts until birth

During implantation, extensions of syncytiotrophoblast penetrate the endometrium and form early chorionic villi. as they penetrate blood vessels, they become surrounded by lacunae filled with maternal blood, these lacunae merge to form the ____ ___

placental sinus

where does the umbilical cord come from?

Mesoderm grows into villi and gives rise to blood vessels that connect to the embryo and become the umbilical cord

Pumped by the fetal heart, blood flows into the placenta by way of umbilical ____ (oxygen-poor). blood then returns to fetus by way of the umbilical ___ (oxygen rich)

arteries & veins

Chorionic villi are filled with ___ blood and surrounded by ____ blood

fetal & maternal

characteristics of an 8 week old fetus

bones have begun to calcify, heart can circulate blood, heart & liver are very large, and the head is nearly half the total body length

the first 6 to 8 hours immediately following birth when neonate must adapt to life outside of mother

transitional period

the first 4 weeks of life; infant at this stage is called a neonate

neonatal period

the infant is born with high levels of ___ antibodies from mom, but they rapidly disintegrate in the first 6 months

IgG

Breast-fed neonates acquire protection from gastroenteritis from the __ antibodies present in the colostrum

IgA

a heat-generating tissue, deposited during weeks 17 to 20 as fetus to help with thermoregulation

brown fat

Since kidneys not fully developed at birth, neonates have a high rate of ___ ___, requiring more fluid intake, relative to body weight

water loss