Developmental Biology: Embryonic Period

• Developmental biology studies the events from a secondary oocyte's fertilization by a sperm cell to adult organism formation

Stages of Development

• Embryonic Period: encompasses fertilization through the 8th week of development
• Fetal period: from week 9 until birth
• Neonatal Period: the first 28 days after birth

Embryonic Period: Week 1 - Fertilization

• Fertilization typically occurs in the uterine tubes
• Sperm needs around 7 hours post-ejaculation to be capable of fertilization
• Capacitation: sperm undergo changes in the female reproductive tract causing their tails to beat more vigorously and prepare their plasma membranes to fuse with the oocyte
• Only capacitated sperm can respond to the chemical factors produced by the ovulated oocyte's surrounding cells
• A capacitated sperm must penetrate the corona radiata and the zona pellucida using acrosomal enzymes and strong movements
• Corona radiata consists of granulosa cells left over from the mature follicle
• Zona pellucida is a clear glycoprotein layer between the corona radiata and the oocyte plasma membrane

Blocking Polyspermy

• Fusion of the sperm cell triggers events that block polyspermy, which is fertilization by more than one sperm
• Fast block to polyspermy: the oocyte cell membrane depolarizes
• Slow block to polyspermy: molecules released in the fast block hardens the zona pellucida
• After a sperm cell enters, the oocyte completes meiosis II
• The oocyte divides into a larger ovum (mature egg) and a smaller second polar body, which disintegrates

Zygote and Cleavage

• The sperm nucleus turns into a male pronucleus, while the ovum nucleus develops into a female pronucleus
• Syngamy: the pronuclei then fuse to produce a single diploid nucleus
• The zygote, the first embryonic stage, is the resulting cell
• Cleavage happens during the first week of development, consisting of rapid mitotic divisions of the zygote
• Blastomeres are progressively smaller cells produced by cleavage

Morula and Blastocyst

• A solid ball of cells, the morula, forms 3-4 days after fertilization
• One day later after that the blastocyst forms, which is a hollow ball of cells that enters the uterine cavity
• The blastocyst contains:
• Trophoblast: the outer cell layer that becomes part of the chorion
• Inner cell mass or embryoblast: becomes the embryo

Implantation

• The dividing ball of cells reaches the uterine cavity around day 4 or 5
• About 2 days after the dividing ball reaches the cavity, the blastocyst attaches to the endometrium of the uterine wall, known as implantation
• The inner cell mass directs itself toward the endometrium upon blastocyst attachment
• As the blastocyst burrows, the endometrium becomes more vascularized, and the endometrial glands enlarge forming the decidua

Blastocyst Development: Trophoblast and Inner Cell Mass

• Around day 8, the trophoblast develops into a syncytiotrophoblast and a cytotrophoblast that will both become part of the chorion as they undergo further growth
• The inner cell mass differentiates into two layers: epiblast and hypoblast which give rise to the amniotic cavity and yolk sac, respectively
• The bilaminar embryonic disc is the spot where the two layers touch, and it becomes the embryo

Chorion Development

• The chorion develops from the trophoblast and extraembryonic mesoderm
• The chorion turns into the main embryonic component of the placenta
• The chorion (and trophoblast before it) secretes human chorionic gonadotropin (hCG)
• This is a vital pregnancy hormone that prevents corpus luteum degeneration, sustaining its function

Amniotic Cavity and Yolk Sac Development

• From the epiblast, the amnion develops as a thin, protective membrane
• Initially, the amnion only overlies the bilaminar embryonic disc, but as the embryo grows, it surrounds it to create the amniotic cavity
• The amniotic cavity fills with amniotic fluid which protects the developing fetus
• The yolk sac develops from the hypoblast
• Yolk sac functions include giving nutrients to the embryo, being a source of blood cells early on, and making germ cells that become spermatogonia and oogonia

Lacunae and Maternal Sinusoids

• Syncytiotrophoblast expands on the 9th day forming lacunae and by day 12, the lacunae merge becoming maternal sinusoids
• Maternal blood provides nutrition and waste removal for the embryo

Connecting Stalk

• By the end of the second week, the bilaminar embryonic disc connects to the trophoblast by a band of extraembryonic mesoderm called the connecting stalk

Gastrulation

• Trilaminar embryonic disc: at around 15 days, gastrulation happens where the bilaminar embryonic disc transformss into the trilaminar embryonic disc consisting of 3 germ layers
• Ectoderm becomes tissues of the brain and nerves, and the epidermis of the skin
• Mesoderm becomes blood, muscles, bones, and connective tissue derivatives
• Endoderm becomes epithelial lining of digestive tract, respiratory tract, blood vessels, and organs

Gastrulation Structures

• Key structures during gastrulation:
• Notochord: a solid cylinder of cells playing a key role in induction for tissue specialization
• Oropharyngeal membrane: its degeneration connects the mouth cavity to the pharynx and the remainder of the gastrointestinal tract
• Cloacal membrane: its degeneration forms the openings of the anus and the urinary/reproductive tracts
• Allantois: an outpouching of the yolk sac that helps form the umbilical cord

Neurulation

• Around 17 days, neurulation begins, in this process the notochord induces overlying ectodermal cells to eventually become the neural tube
• In the 4th and 5th weeks, the neural tube turns into vesicles that form the brain and spinal cord
• Neural tube defects (NTDs) stem from arrested normal development and closure of the neural tube including anencephaly and spina bifida

Somites Development

• Paired cube-shaped mesoderm structures called somites develop next to the notochord and neural tube
• A total of 42-44 pairs will eventually develop with each somite having 3 parts
• Myotome: becomes muscles
• Dermatome: forms CT, incl. dermis of skin
• Sclerotome: forms vertebrae and ribs
• Other mesoderm tissue simultaneously develops becoming the pericardial, pleural, and peritoneal cavities, portions of the heart, respiratory and digestive systems, and the bones + ligaments of the limbs

Cardiovascular Development

• Angiogenesis starts at the start of the 3rd week
• The process forms blood vessels in the extraembryonic mesoderm found in the yolk sac, connecting stalk, and chorion
• On days 18 and 19, the heart is formed by mesoderm cells in the head end of the embryo
• The mesodermal cells form a pair of endocardial tubes, which fuse to form a single primitive heart

Chorionic Villi and Placenta Development

• Chorionic villi develop as projections of the cytotrophoblast that eventually gets filled with blood
• The capillaries connect to the embryonic heart through two umbilical arteries and one umbilical vein
• The placenta has a fetal part (formed by chorionic villi) and a maternal part formed by decidua basalis of the endometrium
• Placental facts:
• Exchange of nutrients and waste between maternal and fetal blood occurs without the mixing of the blood
• Acts as protection, stores nutrients, and produces hormones to maintain pregnancy being (hCG, progesterone, and estrogens)
• The umbilical cord connects placenta and embryo

Embryonic Folding

• The embryo goes from flat two-dimensional trilaminar disc to a three-dimensional cylinder, with the development of somites and neural tube continuing

Pharyngeal Arches, Clefts, and Pouches

• Several pharyngeal (branchial) arches develop on each side of the future head and neck regions
• The arches will turn into structures of the head and neck
• The otic placode is the first sign of a developing ear
• The lens placode is the first sign of a developing eye
• The upper limb buds show up in the around the middle of the fourth week and the lower limb buds at in the end of the fourth week

Rapid Development

• There’s rapid brain development and head growth during the fifth week of embryonic development
• The head grows even larger during the sixth week, limb growth is more substantial, straightening of the the neck and trunk begins,and the heart becomes four-chambered
• The different regions of the limbs turn distinct during the seventh week
• Distinctions appear and by eighth week all regions of the limbs are distinct
• The digits are distinct, eyelids come together, the externa genitals start differentiating and tail disappears

Fetal Period

• The fetal period takes up rest of the pregnancy timeframe from week 9-40
• Tissues + organs keep growing and differentiating

Vulnerability Factors of the Fetus

New structures are very few and the fetus is less vulnerable effects of drugs, radiation, and microbes

Fetal Growth

Remarkable, and during the last 2 and a half months, half the weight is added The head becomes more proportionate relative to the rest of the body

Fetal Position

By the 34th week, the fetus is positioned upside down

Teratogens

Developmental defects can occur with specific agents that cause them The placenta is a porous barrier, and if the mother ingests anything dangerous the mother can also have adverse effects on the child

Alcohol

The most serious teratogen out of all teratogens, and if one with child ingests even the smallest amount of alcohol, fetal alcohol syndrome(mental retardation

Cigarette Smoking

Smoking of cigarettes may cause low weights, heart abnormalities, and brain abnormality

Fetal Utrasongraohy

• An ultrasound gets the image of the fetus and it’s called sonogram

How Ultrasounds Are Used

Determines true fetal age, evalutates viability, identifies fetal maternal abnormalities, determines fetal positions, etc.

amniocentesis

• Medical procedure that samples amniotic fluid through the amniotic sac
• Performed at 14-16 weeks of gestation and it can be identified in the fluid

Chorionic vill sampling

• Chromosomal analysis with this process and can be evaluated earlier
• First noninvasive prenatal test was maternal alphafetoprotein, used in maternal blood to screen down syndrome, trisomy, or neural tube defects

Hormones Secretion During Pregnancy

• Estrogen, progesterone, corpus luteum gets secreted during the 3-4 months
• The chorion beings to secrete HCG,
• The HCG maintains the corpus luteum levels to fall, and levels of corpus leutem dengerate

Human Somatomammotropin’s Function

• Helps regulate and helps prepare for glands to release milk known as lactation

Corticotopin Releasing Hormone

• The placenta secretes it and is a clock that determines timing for birth and increases cortisol needed for lung and surfactant maturation

Relaxin Production

• Corpus leutem is the producer and increases flexibility and helps to dilate during labor

In Full Term Pregnancy

• The uterus fills the abdominal cavity and causes major changes which are weight gain from fluid, increase of storage in triglycerides and minerals etc.

Physical Activity During Pregnancy

• Physical activity that’s low should never kill the fetus and should have normal pregnancy and can improve emotional being
• Changes and fatigue can affect one’s body as well

Labor Stage

• The stage that brings fetus to life,
• Positive feedback mechanisms increase and high levels of estrogens are increased, it has 3 different distinct stages

Stage ! Of Labor

• Involves regular contractions of the uterus with amniotic sac, with complete dilation of the cerviex

Stage 2 Of Labor

• complete cervical dilation to delivery of the baby, as well as contractios for the baby’s head, it is broken when delivery is accomplished

Labor Stgae 3

• After delivery there’s 5-30 minutes and powerful uterine contractions lead to blood vessels’ reduction and tear

Puerperium

• After delivery in which reproductive system in maternal returns and uterus undergoes remarkable size alterations

Adjustments Of the Infant At Birth

• Changes in cardiorespiratory occur, lungs inflate for first time and there’s also change in cardiovascular
• The foramen ovale closes when there’s deoxygenation between the atrial

Physiology of Lactation

• The milk production and release, hormone released is prolactin the pregnancy progresses for milk proaction can only be done with the removals of progesterone
• Hypothalamus is stimulated to release oxytocin when signals of touches in nipple gets activated

Inheritance

Is the passage of hereditary traits that’s genetic and health related when offering advice

Chromosome Gene Review

• All chromosome genes are except in gametes when there’s 23 pairs
• Both pairs that contains genes that control the same hereditary traits are chromosomes
• Mutation also creates a gene variant

Genotype Phenotype

• Outward expression is phenotype and allele inhibits trait of gene
• When both homozygus trait are presented it’s an expression
• When there’s different alleles it’s heterozygus

Imprinting Genes

• Genes that are inherited and are more expressed by mother or the father with parental origin
• Punnett squares determines fertilized eggs through top or left of sides of sperm or alleles

Aneuoploidy

• There are anomolaires when meiosis errors, aneuploidy is when more or less chromosome

Mononsomy

• When cells are missing a chromosome and trisomy is when cells have chromosome 21
• Meta syndrom can be involved in disorders

Alleles Code

• Not always dominatn when coding such six fingers and most code recssice
• Complex and inherited effects happen with many variations

Dominant Reessive Inheritance

• Patterns which dominant genes interact, multiple is multiple alternatice, inherited effects

Multi Allele Inheritance

Is a codominant pair or recssiceness

Medical Terminology

• Butt checks lower or breasts is Breech presentaations
• Morning sickness is Emesirs Gradirarum
• Fetus calculated age is gestational and karyotyope is size
• All arranged in the Karyote with size

This section covers the first week of embryonic development, starting with fertilization in the uterine tubes. It describes capacitation, the process by which sperm become capable of fertilization. It also details how capacitated sperm penetrate the corona radiata and zona pellucida.