Final Development

Embryonic Period: Week 3 through Week 8

A. At the beginning of the third week of development, the embryonic period has begun.

B. Cleavage and Blastocyst Formation.

Approximately 30 hours after fertilization occurs, the zygote begins cleavage, which is a series of rapid mitotic divisions that produce small, genetically identical cells called blastomeres

c. The final stage, the fetal period, lasts from week 9 through 38 of gestation, or until birth. The conceptus is now called a fetus; it grows larger and continues to develop until its organ systems can function without assistance from the mother. After birth, the fetus becomes a newborn, and the postnatal period of life begins.

C. The Postnatal Period: Rapid developmental changes continue to occur in the postnatal period, which is broadly divided into the following five stages: neonatal period, infancy, childhood, adolescence, and adulthood.

Postnatal Changes in the Newborn and Mother

Changes in the Newborn. The first 4 weeks after birth constitute the neonatal period for the newborn, or neonate, and is a time of significant physiological adjustments to adapt to life outside the uterus.

Overview of Human Development

Human development begins at fertilization, with the union of gametes, and continues throughout life.

B. Childbirth (Parturition):

Pregnancy ends with childbirth, or parturition, when the fetus is expelled from the uterus through the vagina. The series of events that expel the fetus are collectively called labor and birth takes place during week 38 of pregnancy, but is variable.

D. Development of Extraembryonic Membranes.

The bilaminar embryonic disc and trophoblast together produce the extraembryonic membranes, which first appear during the second week of development but continue to develop during the embryonic and fetal periods.

a. Nucleotides form the 46 chromosomes, which are found in all somatic cells with a nucleus. Chromosomes are paired so that there are 23 pairs of homologous chromosomes in each nucleus.

b. Each pair of diploid (2n) homologous chromosomes contains a copy from each parent, so in total our cells have 23 total maternal chromosomes and 23 total paternal chromosomes. Each member of a pair of autosomes, chromosomes 1 through 22, carries the same genes.

c. The twenty-third pair are the sex chromosomes, which consist of one X chromosome and one Y chromosome. The X and Y chromosomes determine the sex of the individual: What sex chromosomes does a human female normally have? XX What sex chromosomes does a human male normally have? XY

d. A full set of the 23 chromosome pairs can be arranged and displayed in what is called a karyotype.

What happens to the conceptus during this stage?

grows, folds, and forms rudimentary organ systems

Developmental Biology

is the science that studies the changes in form and function from the fertilized egg through old age.

Embryology

is the study of the events that occur during the approximately 38 weeks of development inside the mother's womb.

When does the postnatal period begin?

period from birth through all life stages

List the membranes:

yolk sac, amnion, allantois, and the chorion. Collectively, these membranes protect the embryo and assist with vital functions, including nutrition, gas exchange, and storage and removal of waste

B. Changes in the Mother: The first 6 weeks following birth make up the postpartum period, when the mother's body undergoes several changes to return the body to pre-pregnancy homeostasis and to begin and then maintain the production of breast milk, the process of lactation

1. Anatomical and Physiological Changes: Many adjustments occur after birth to reverse changes made during pregnancy and to restore homeostasis to the mother's system.

C. Fetal development is characterized by the maturation of tissues and organs. Fetal growth is rapid and fetal size increases dramatically

1. Common fetal measurements of length include the crown-rump length (CRL) or crown-heel length (CHL). Summarize the most important changes that occur during the 6 months of the fetal period (from months 3-9 of the pregnancy).

B. Patterns of Inheritance: Most inherited traits are determined by multiple alleles or by the interaction of several genes. The following features the different patterns of inheritance, and the genotypes and phenotypes in which they result.

1. Dominant and Recessive Traits. The pattern of inheritance known as dominant-recessive inheritance refers to the interaction of alleles that are strictly dominant or recessive.

C. Implantation occurs approximately 4-7 days after fertilization when the blastocyst begins to attach to the endometrium of the uterus

1. The trophoblast invades the stratum functionalis, the layer of the endometrium by secreting digestive enzymes that catalyze reactions to degrade the endometrial lining. Trophoblast cells divide into two layers: the inner cytotrophoblast and the outer synctiotrophoblast.

B. Describe placentation. Placentation is the process of forming the disc-shaped placenta, which attaches to the uterine wall and to the embryo or fetus through the umbilical cord.

1. What is the function of the placenta? The placenta is a temporary organ, shed after the infant is born, that is the site of exchange of oxygen, nutrients, and waste between the mother and fetus. In addition, the placenta produces hormones to support the pregnancy.

C. Organogenesis is the process where the three primary germ layers differentiate into organs and organ systems

1. When the embryonic period ends at the end of week 8, the embryo will have recognizable organ systems. 2. Into what structures will the ectoderm differentiate? The ectoderm forms the epidermis of the skin and is responsible for forming the majority of the nervous system and sense organs. The first major event of organogenesis is neurulation, which produces the brain and spinal cord.

9. Fetal circulation and the fetal cardiovascular system change rapidly soon after birth to adjust to life without a placenta. The most conspicuous anatomical changes between the prenatal and postnatal state occur in the cardiovascular system. Unique cardiovascular structures present during prenatal development include the umbilical arteries and umbilical vein and three circulatory "shortcuts" called vascular shunts: the ductus venosus, foramen ovale, and ductus arteriosus.

10. After birth, these shunts and the umbilical vessels close, and a normal circulatory pattern begins within a year in most infants, forming the fossa ovalis from the foramen ovale, ligamentum arteriosum from the ductus arteriosus , and the ligamentum venosum from the ductus venosus.

9. Digestive System. Good nutrition is necessary during all trimesters of pregnancy for the health of the conceptus. A pregnant woman typically needs about 300 extra calories each day to supply both herself and the fetus. Specifically, protein, calcium salt, iron salt, and folic acid requirements increase from the recommended daily levels required for nonpregnant women. During the first trimester, many pregnant women suffer from morning sickness, nausea and sometimes vomiting, thought to be related to elevated hCG, estrogen, and progesterone levels, but no definitive cause has been identified.

10. Urinary System. The fetus produces metabolic wastes that the mother's kidneys must excrete. The glomerular filtration rate can rise by up to 50% during pregnancy. The uterus tends to compress the urinary bladder as it expands, sometimes leading to stress incontinence, or unintentional loss of urine.

1. The divisions occur too quickly for cells to grow; for this reason, the cell number increases, but the cell size becomes progressively smaller. Each subsequent division takes less time than the previous one, so that by the second day there are 4 cells and by the end of the third day there are 16 cells. At this point, the conceptus is known as a morula and remains covered by the zona pellucida.

2. By the time the blastocyst is ready for implantation, it has two distinct cell populations: (1) An outer layer of large, flattened cells, called trophoblast cells surround the fluid-filled cavity, participate in forming the placenta, the temporary organ that provides nutrients and oxygen to the conceptus, and also suppress the mother's immune system, preventing it from attacking the conceptus. (2) An inner cluster of rounded cells, the inner cell mass, or embryoblasts form the embryo proper—the developing body.

1. The series of events involved in initiating and regulating childbirth are as follows: The fetal adrenal cortex produces cortisol, which stimulates the placenta to secrete a high level of estrogen. High estrogen levels also override the quieting influence of progesterone on the myometrium, which becomes "irritable," leading to irregular contractions called Braxton-Hicks contractions that are referred to as false labor because they do not result in the three stages of labor. Prostaglandins dilate the cervix and, along with oxytocin, increase the strength of uterine contractions. As the head of the fetus pushes on and stretches the cervix, more oxytocin is released. As more oxytocin is released, the myometrium contracts more forcefully and the placenta secretes more prostaglandins. Both effects cause the cervix to stretch more, which stimulates the release of more oxytocin in a positive feedback loop.

2. During pregnancy, the rising level of progesterone leads to the formation of a mucus plug at the cervix. When the cervix begins to dilate, the mucus is discharged into the vagina. Labor may begin within hours after the mucus plug is discharged, or it may not occur for up to 2 weeks.

1. The blastocyst is fully implanted, the inner cell mass has differentiated into the bilaminar embryonic disc, and the extraembryonic membranes have developed. We then enter the embryonic period, which begins at the third week of development and continues through the eighth week.

2. During this five-week stage, the conceptus is referred to as an embryo.

1. In week 3, a bilaminar embryonic disc contains two cell layers: the epiblast and hypoblast. A thin groove on the dorsal surface of the epiblast develops, called the primitive streak which elongates along the cephalic-caudal (head-tail) line of the embryo, establishing the head and tail regions, the right and left sides, and the dorsal (posterior) and ventral (anterior) surfaces of the embryo.

2. Gastrulation begins after the primitive streak has completed its development and starts as cells detach from the epiblast layer and move into the primitive streak, a process called ingression. The first cells that migrate in this way become the inner germ layer, or endoderm, which replaces the hypoblast. The next cells that migrate to a position between the epiblast and endoderm, and this becomes the middle layer, the mesoderm. The remaining cells of the epiblast form the outer layer, the ectoderm.

1. Sperm cells are ejaculated in semen, which is a mixture of sperm and fluid secretions from the male reproductive glands. Several events must occur before fertilization can occur between the sperm cell and the oocyte.

2. Sperm undergo capacitation as they migrate to the oocyte. Sperm move by a whip-like movement of their flagella, or "tails."

1. The yolk sac arises from hypoblast cells. In humans, cells from the yolk sac form part of the digestive tract and are the source of the first blood cells and blood vessels. The yolk sac also produces the first germ cells, the precursors of the gametes, which will eventually migrate into the developing ovaries and testes.

2. The amnion is a transparent membrane that develops from the epiblast. It eventually encloses the embryo in the fluid-filled amnionic cavity, which is penetrated only by the umbilical cord, the vessels that will connect the growing embryo or fetus to the mother's blood supply. The amnion secretes amnionic fluid into the cavity, which protects the embryo from trauma and drying out, and helps maintain a constant temperature.

1. Pregnancy embraces all of the events from conception, when an egg is fertilized and begins to develop, until birth, and includes the anatomical and physiological changes to the mother during this time. The growing offspring is referred to as the conceptus and develops during the gestation period of pregnancy, which extends from the mother's last menstrual period until birth, which is approximately 280 days, or 40 weeks.

2. The three stages of prenatal development that occur during the 38-week time period include the pre-embryonic period, the embryonic period, and the fetal period.

2. The syncytiotrophoblast is also responsible for secreting the hormone human chorionic gonadotropin (hCG), which stimulates the corpus lutium in the ovary to secrete estrogen and progesterone and helps promote placental development.

3. Simultaneously, the inner cell mass, or developing offspring, separates from the trophoblast and differentiates into 2 layers: (1) the superior epiblast and (2) the inferior hypoblast together form a flat bilaminar embryonic disc. This disc will eventually become the three primary germ layers that differentiate to produce all tissues in the body: ectoderm, mesoderm, and endoderm.

1. The neonate must now perform all of the physiological functions that the mother had been doing, including breathing, obtaining nutrients, excreting waste, and maintaining body temperature. 2. The Apgar score, calculated to evaluate whether a newborn is adjusting appropriately, its physical status is evaluated at 1 minute and 5 minutes after birth, is based on five criteria: (1) skin color, (2) pulse rate, (3) respiratory level, (4) muscle activity, and (5) grimace (which assesses reflexes).

3. The most dramatic event after the birth is the first breath. In addition to inflating the alveoli of the lungs for the first time, the first breath also causes the blood pressure in the lungs to drop and decreases pressure in the right side of the heart. 4. Although the kidneys are almost fully developed at birth, they are less capable than adult kidneys of concentrating urine. Babies are born with a near-adult level of IgG antibodies acquired passively from the mother through the placenta.

2. What does a Punnett square predict? A Punnett square can predict the possible offspring genotypes that might occur if a man and woman have a child.

3. Traits with Incomplete Dominance. The pattern known as incomplete dominance, the heterozygous genotype results in a phenotype that is "between" the phenotypes of individuals who are homozygous dominant and homozygous recessive. What is an example of incomplete dominance in humans? The sickling gene that causes sickle-cell anemia.

2. The umbilical cord connects the center of the placenta to the fetus' umbilicus (bellybutton). The cord normally contains two umbilical arteries that carry deoxygenated fetal blood away from the fetal heart to the placenta, and one umbilical vein that carries oxygen and nutrients toward the fetal heart.

3. When the embryonic period begins, the blastocyst has implanted in the uterus and chorionic villi have penetrated uterine blood vessels to form lacunae filled with maternal blood, which merge into a single blood-filled cavity called the placental sinus

The Process of Prenatal Development: What time period does the prenatal period include?

38 week of development inside the mother's uterus

3. Genes and Alleles: homologous chromosomes have pairs of genes that code for the same trait and these are found at the same location, or locus, of that chromosome. These genes are not necessarily identical—they may have slightly different sequences of nucleotides. These variants of a gene are called alleles. If two alleles code for the same trait, they are said to be homozygous for that trait. If the two alleles are different they are heterozygous. An allele that has the ability to mask another is called dominant, and the allele that is masked is known as recessive.

4. Define genotype. An individual's genetic makeup is called the genotype. Define phenotype. The physical expression of an individual's genotype in the form of a trait is referred to as the phenotype.

3. Summarize the three stages of labor: the dilation, expulsion, and placental stages:

4. Dilation Stage: This stage is the time from the onset of labor until the cervix is fully dilated, or expanded, to 10 cm (3.9 in.) in diameter. When labor begins, contractions originate in the upper part of the uterus and move toward the vagina. Dilation is the longest and most variable stage of labor. As the fetus' head is forced against the cervix, the cervix thins and dilates. At some point during this stage, the amnion ruptures and releases the amniotic fluid, which is the event known as the "water breaking" or "rupture of membranes." Toward the end of the dilation stage, the fetus' head enters the true pelvis and the fetus rotates so that its head can pass through the pelvic outlet.

3. Occasionally, within the first 8 days after fertilization, the cells separate into two groups and develop into two individuals, called monozygotic, or identical twins.

4. Dizygotic or fraternal twins is a more common type of twinning that result from the ovulation of two secondary oocytes at the same time, which are then fertilized by two separate sperm cells.

1. First trimester: months 1-3 of pregnancy. In the first 12 weeks of development, all pre-embryonic and embryonic development is completed and fetal development begins. 2. Second trimester: months 4-6 of pregnancy. During the second trimester, the fetus continues to grow and develop, and the pregnancy usually becomes obvious as the uterus and abdomen expand. 3. Third trimester: months 7-9 of pregnancy. The fetus grows rapidly and gains a significant amount of weight.

4. Hormonal changes involve the release of hCG, relaxin, and hormones released by the placenta, including human placental lactogen (hPL), or human chorionic somatomammotropin (hCS), melanocyte-stimulating hormone (MSH), and corticotropin-releasing hormone (CRH). Other endocrine glands increase secretion during pregnancy, including the release of aldosterone, parathyroid hormone, prolactin, and oxytocin.

3. Into what structures will the mesoderm differentiate? Mesodermal cells form a structure called the notochord, which serves to support and organize the embryo around a central axis.

4. Into what structures will the endoderm differentiate? The endoderm becomes the internal epithelial layers of the digestive, respiratory, urinary, and reproductive systems. Endoderm forms the cavity of the middle ear and the auditory tube. Several glands and accessory digestive organs—including the thyroid gland; parathyroid glands; thymus; parts of the palatine tonsils; and the majority of the liver, gallbladder, and pancreas—also form from the endoderm.

2. Month 3: The fetal body lengthens as its head growth slows; the upper limbs grow to their birth length. Ossification begins in most bones. The genitals are distinguishable as male or female. 3. Month 4: Body growth is rapid. The lower limbs lengthen and joints are forming as the skeleton continues to ossify. The fetus has nipples and some hair. The kidneys are well formed, the digestive glands are forming, and the heartbeat can be heard with a stethoscope.

4. Month 5: Growth slows and the lower limbs achieve their correct proportions relative to the body. Hair grows on the head, and the skin is covered by a downy hair called lanugo and a white to gray secretion composed of shed epithelial cells and sebum known as vernix caseosa. 5. Month 6: The fetus gains significant weight. Eyebrows and eyelashes appear, and the eyelids, which have been fused since the third month, are partially open. The fetus' skin is wrinkled and translucent and the lungs begin to produce surfactant, which is key to survival if the infant is born prematurely.

3. The allantois is a small out pocketing of the yolk sac. It forms the base for the umbilical cord and ultimately will become part of the urinary bladder.

4. The chorion is the outermost extra embryonic membrane which encloses all other membranes and the conceptus. As the chorion develops, it forms outgrowths called the chorionic villi. By late week four of development, the chorionic villi are blended with the stratum functionalis layer and begin to form the principal embryonic part of the placenta.

4. Multiple-Allele Traits. Many human traits involve the interaction of multiple alleles. Describe an example of multiple-allele traits. ABO blood types exhibit the phenomenon of codominance, meaning that some alleles are equally dominant and are therefore equally expressed. IA and IB are codominant alleles, while i is recessive.

5. Sex-linked traits are those that are specifically expressed on the X chromosome or Y chromosome. The Y chromosome contains genes that determine male phenotypic traits, which explains why females have the genotype XX and males have the genotype XY. There can be alleles for genes on the X chromosome that encode harmful traits, producing what are called sex-linked, or X-linked disorders.

4. The sperm binds to the plasma membrane of the oocyte. When the first sperm cell penetrates the zona pellucida and binds to the oocyte's plasma membrane, changes in the oocyte and zona pellucida occur so that no more sperm can enter.

5. Sperm entry stimulates the cortical reaction, which destroys sperm-binding receptors. Additional sperm cells are rejected. The acrosomal reaction prevents additional sperm cells from binding.

4. Once placentation has begun, the stratum functionalis of the uterus is known as the decidua. The region of the endometrium that lies beneath the fetus becomes the decidua basalis. The region that surrounds the uterine cavity forms the decidua capsularis.

5. The placenta is a unique organ because it develops from both fetal and maternal structures: the fetal portion is formed by the chorionic villi and the maternal portion by the decidua basalis.

5. Expulsion Stage (30 minutes-1 hour): This stage is the time from full dilation to delivery of the newborn. When the first part of the fetus' head distends the vagina, crowning has occurred. The fetus' head is followed by the rest of the body, which is delivered more easily. The umbilical cord is then clamped and cut.

6. Placental Stage (30 minutes): This is the time after delivery of the newborn when the placenta and the attached fetal extraembryonic membranes, collectively called the afterbirth, are delivered. Complete removal of the placenta is important to stop uterine bleeding. A retained placenta can lead to significant blood loss for the mother and an increased risk for infection.

5. Anatomical and Physiological Changes: Many anatomical and physiological changes take place to support pregnancy and prepare a woman's body for childbirth and lactation, or breastfeeding.

6. Reproductive System. The uterus enlarges greatly over the 9-month pregnancy and eventually extends upward from its normal location in the pelvic cavity to reach the level of the xiphoid process of the sternum. By about 28 weeks, the uterus reaches above the umbilicus, and as the uterus continues to expand, the abdominal organs are displaced upward, compressing the diaphragm, and pressing on the urinary bladder. The mammary glands also undergo significant changes in preparation for lactation after the newborn is born.

6. The sperm nucleus swells to form a male pronucleus. The oocyte completes meiosis II, and the resulting ovum swells to form a female pronucleus.

7. A spindle forms between the pronuclei, and the chromosomes intermix. Spindle fibers form between the male and female pronuclei, and the pronuclear membranes break down, allowing the chromosomes to mingle.

6. Describe polygenic inheritance patterns. Most inherited traits are controlled by the combined effects of two or more genes, a condition called polygenic inheritance.

7. Describe multifactorial inheritance. Multifactorial inheritance occurs when the combined effects of many genes are further influenced by environmental factors.

6. Month 7: The eyelids open completely, and fat is deposited in subcutaneous tissue so the skin is slightly smoother, but still wrinkled and red. The fetus usually turns upside down, assuming the vertex position. 7. Months 8 and 9: In the final months, fetal neurons form networks, and organs grow and develop. Blood cells form in bone marrow, and the digestive and respiratory systems complete development during the ninth month.

8. Approximately 266 days after fertilization, a fetus is considered full-term and ready to be born. The scalp usually has hair, and the fingers and toes have well-developed nails. If, however, a fetus is born before that time, its tissues and organs may not have developed completely.

7. Cardiovascular System: The growing placenta requires increased blood volume, and as the offspring grows, it needs more oxygen and produces more waste that must be removed. The mother's blood volume and cardiac output increase while blood pressure may rise to some extent. As the uterus grows and presses on pelvic blood vessels, venous return from the lower limbs may be impaired and sometimes results in varicose veins and leg swelling, or edema.

8. Respiratory System. More oxygen is needed during pregnancy, and more carbon dioxide is produced that must be exhaled.

Pre-Embryonic Period: Fertilization through Implantation (Weeks 1 and 2)

A. Fertilization: The pre-embryonic period begins with fertilization, the fusion of a sperm cell and secondary oocyte to form a blastocyst_, and continues until the blastocyst moves into the uterus and implants in the endometrium of the uterine wall.

Pregnancy and Childbirth

A. Maternal Changes during Pregnancy: This period in the mother's life is one of dramatic anatomical and physiological changes that occur to her body during the nine month process of pregnancy. Pregnancy is subdivided into the following three, 3-month long, trimesters:

Fetal Period: Week 9 until Birth (about Week 38)

A. The fetal period of development extends from the beginning of week 9 until birth, which usually occurs near week 38. By the 12th week, the placenta is fully functional, providing nutrition to the fetus for the rest of the gestation period.

Heredity

Heredity is the transmission of genetic characteristics from parent to offspring through the genes. The study of heredity and how it is transmitted is the field of genetics. 1. Genes are segments of the DNA that code for a specific protein. 2. An individual's entire DNA is referred to as the genome.

3. The acrosomal reaction releases enzymes from the head of the sperm. An ovulated secondary oocyte is surrounded by a layer of granulosa cells called the corona radiata.

Sperm must penetrate this cell layer by a combination of sperm movements and hyaluronidase, an enzyme released from the membrane-bound sac on the head of the sperm called the acrosome. Underneath the oocyte's corona radiata is the zona pellucida. Enzymes released by the sperm catalyze reactions that digest holes in the zona pellucida.

a. The pre-embryonic period lasts for the first 2 weeks after fertilization. During this stage the zygote divides mitotically to produce a multicellular structure, the blastocyst, which implants in the endometrium of the uterus.

b. The embryonic period extends from week 3-8 of gestation.