Chapter 15: Human Development

Embryonic Period of Development

A period of development that take place over the first eight weeks, or the first two thirds of the first trimester. Here, tremendous changes take place. Cells divide and become redistributed. Tissues and organs form as do structures that support and nourish the developing embryo.

Fetal Period of Development

A period of development that takes place from the start of the ninth week through to birth. It corresponds to the remaining thirds of the first trimester and all of the second and third trimesters. Here, the body grows rapidly, organs begin to function and coordinate to from organ systems.

Fertilization

Involves the joining of male and female gametes (sperm and egg) to form a zygote (a fertilized egg) that contains 23 chromosomes from each parent (46 Chromosomes in total) Occurs within the oviduct. Layers around the egg ensure that only one sperm attaches.

Zona Pellucida

A thin, clear layer of protein and carbohydrates that surrounds the plasma membrane of the egg.

Corona Radiatia

Several jelly-like layers of follicle cells that loosely adhere to one another covering of the zona pellucida. Source of nourishment for the egg when it was in an ovarian follicle.

Cleavage

The process of cell division without enlargement of the cells. Occurs in the zygote.

Morula

A sphere of 16 cells formed from the zygote. Reaches the uterus within 3-5 days after fertilization. At this time, it begins to fill with a fluid that diffuses from the uterus. Forms two groups of cell forms.

Undifferentiated

Not having a specialized function or structure, occurs in the initial divisions of the zygote.

Blastocyst

What the morula forms into. A hollow structure that contains two layer: the trophoblast and the inner cell mass.

Trophoblast

Forms the outer layer of the blastocyst. Will later develop into the chorion.

Chorion

What the trophoblast develops into. A membrane that later develops into the placenta. The outermost membrane, encloses all other extra embryonic membranes as well as the embryo, the fetal portion of the placenta.

Inner Cell Mass

Develops into the embryo.

Implantation

Between the 5th-7th day when the blastocyst attaches to the endometrium with the inner cell mass positioned against the endometrium. Trophoblast cells secrete enzymes to digest tissues/blood of the endometrium, the blastocyst then sinks into the uterine wall. Completes by the 10th-14th day.

Human Chorionic Gonadotropin

A hormone secreted by the trophoblast after implantation. Has the same effects as LH so it maintains the corpus luteum past the time when it would otherwise degenerate.

Postimplantation

- After implantation occurs, when the trophoblast begins secreting hCG

- hCG maintains the corpus luteum so it continues secreting estrogen and progesterone for 12 weeks

- Elevated estrogen and progesterone maintains the endometrium and prevents menstruation

- hCG declines after the 2nd months of implantation when the placenta secretes hormones necessary for pregnancy

- hCG in urine is the basis for pregnancy tests

Gastrulation

The process by which a blastula develops into a gastrula with the formation of three embryonic layers (ectoderm, endoderm, and mesoderm) Occurs during week 2.

Amniotic Cavity

A space that forms between the inner cell mass and trophoblast. Will soon fill with fluid and is the place where the baby will develop. Surrounded by the amnion.

Embryonic Disk

In the blastocyst, the inner layer of cells, which will go on to form the embryo. Supported by a short stalk that connects the blastocyst with the endometrium. Forms into the ectoderm, endoderm, and mesoderm.

Gastrula

An embryo at the stage following the blastula, when it is a hollow cup-shaped structure having three layers of cells.

Primary Germ Layers

The three layers of the embryonic disk.

Morphogenesis

The series of events that form distinct structures of a developing organism, Depends on the ability of early embryonic cells to become different types of cells (differentiation) occurs after gastrulation.

Differentiation

The cellular process that enables a cell to develop a particular shape and perform specific functions. Different cells are derived from different germ layers.

Endoderm

The inner germ layer that forms the cellular lining (respiratory tract, digestive tract, urinary bladder, urethra), liver, gallbladder, pancreas, thymus, tonsils (partial), parathyroid glands, thyroid glands.

Mesoderm

The middle germ layer that forms the dermis, cellular lining (vessels, lymphatic vessels, body cavities). muscle tissue, connective tissue (bone, cartilage, blood), adrenal cortex, heart, internal reproductive organs, kidneys, ureters, and the spleen.

Ectoderm

The outside germ layer that becomes the epidermis (hair, nails, sweat glands, mammary glands), nervous tissue/sense organs, pituitary gland, tooth enamel. adrenal medulla, and the eye lens.

Organ Formation

Occurs between weeks 3 and 8 when germ layers begin to fold and early organ systems take shape.

Week 3

Formation of the neural tube (will develop into brain and spinal cord through a process called neurulation), heart starts beating.

Notochord

Made of a thickened band of the mesoderm disk, forms a rod-like structure which will form the basic framework of the skeleton.

Week 4

Rapid growth and differentiation. Blood cell start forming and fill developing blood vessels. Lungs and kidneys take shape. Small buds for arms and legs. Distinct head, eyes, ears, and nose. The embryo is 0.6 cm long.

Week 5

The embryo's head is large compared to the body. Eyes begin to form but do not have eyelids, cells in the brain differentiate very quickly. Embryo is 1.3 cm long.

Week 6

Brain continues rapid development. Limbs lengthen and flex slightly. Gonads produce hormones that influence external genitalia development.

Week 7

Organs are formed, NS coordinates the body, skeleton of cartilage, eyes are developed but closed, nostrils are developed but plugged with mucus, external genitalia are still differentiating.

Week 8

Embryo is the size of a paperclip. 90% of organs are established. Now called a fetus.

Neurulation

The folding of germ layers to form the neural tube which will develop the brain and spinal cord. At week 3. Folic acid must be present. After, the heart form and starts beating.

Folic Acid

Must be present in order for the neural tube to form properly. Inadequate amounts lead to a birth defect called spina bifida.

Afterbirth

The membranes and structures expelled from the uterus (allantois, amnion, chorion, yolk sac, placenta and umbilical cord).

Amniocentesis

A technique of prenatal diagnosis in which amniotic fluid, obtained by aspiration from a needle inserted into the uterus, is analyzed to detect certain genetic and congenital defects in the fetus.

Chorionic Villus Sampling

The sampling of placental tissue for microscopic and chemical examination to detect fetal abnormalities.

Amnion

A transparent sac that develops from the cells of the embryonic disk. Encloses the embryo completely. Penetrated only by the umbilical cord. Becomes filled with amniotic fluid which protects the embryo from trauma and temperature fluctuations. Allows limb movement and prevents them from sticking to the body.

Yolk Sac

A small sac suspended from the abdominal area. Contributes to the formation of the digestive tract and produces the first blood cells and future egg or sperm cells. Unlike other animals, it serves no nutritive function.

Chorionic Villi

These are finger like projections that form the fetal portion of the placenta. By the 8th week, chorionic villi sampling is possible.

Placenta

A disk shaped organ rich in blood vessels. Attaches the embryo/fetus to the uterine wall. Site of metabolic exchange. Fully develops in 10 weeks.

Nutritional Function of Placenta

Transports nutrients from mother to fetus and stores nutrients.

Excretory Function of Placenta

Transports wastes from fetal blood to the mother's blood.

Respiratory Function of Placenta

Transports oxygen from the mother to the fetus and CO2 from the fetus to the mother.

Endocrine Function of Placenta

Secretes hormones and allows hormones from the fetus to diffuse into the mother's blood (and vice versa).

Immune Function of Placenta

Transports antibodies from the mother to the fetus.

Teratogen

Not filtered out by the placenta, drugs, alcohol and other toxins are circulating in the mother's blood and can pass through the placenta to fetal blood. Leads to severe effects like fetal alcohol spectrum disorder.

Umbilical Cord

Formed at the end of the 8th week as the yolk sac shrinks and the amniotic sac enlarges. A rope likes structure 60 cm long and 2 cm in diameter (can be as long as 300cm and short. as a few mm). Leads from the navel area of the fetus to the centre of the placenta. Contains two arteries which transport O2 depleted blood from the fetus to the placenta and a vein which brings O2 rich blood to the fetus.

Embryonic Period

Takes place in the first eight weeks after fertilization.

First Trimester

Weeks 1-12, includes embryonic period, and the body continuing to grow, the cartilage skeleton begins to harden and external reproductive organs become distinguishable.

Second Trimester

Heartbeat is strong enough to hear with stethoscope, bones begin to form, brain grows rapidly, Nervous System starts to function, fetal legs grow and develop, mother feels movement, languo is usually shed before birth.

Third Trimester

Fetal brain cells form rapidly connecting to form networks. The testes descend into the scrotum. A layer of fat develops beneath the skin. Digestive and respiratory systems are the last to mature.

Lanugo

A fine soft hair and oily substance to protect the still developing skin. Created in the 5th month and shed before birth.

Month 1

• Spinal column and central nervous system start to form.

• Appendages are represented by small limb buds.

• Heart begins beating (around day 22).

Month 2

• Eyes form, but eyelids are fused shut.

• Brain waves are detectable.

• Nose is flat.

• Bone growth begins.

• Head is nearly as large as rest of body.

• Blood cells and major blood vessels form.

• Nostrils are evident, but plugged with

• Genitals are present, but sexes are not yet

mucus. distinguishable.

• Limb buds form paddle-like hands and form ridges

(which later separate into distinct fingers and toes).

Month 3

• Eyes are well developed, but eyelids are closed

• Limbs are well-formed, with nails on fingers

still fused.

• Nose develops bridge.

• External ears are present.

• Fetus swallows amniotic fluid.

• The fetus moves but too weakly for the mother to feel it.

• Sexes can be distinguished visually.

Month 4

• Face looks more distinctly human.

• Joints are forming.

• Body is larger in proportion to head.

• Lips exhibit sucking movements.

• Skin is bright pink.

• Heartbeat can be heard with a stethoscope.

• Scalp has hair.

Month 5

• Body covered with fine hair (lanugo).

• Mother can feel fetal movements.

• Skin has oily secretion to protect it from

• Skin is bright pink.

• Fetus is now bent forward into "fetal position"

because it is beginning to outgrow the amniotic sac.

Month 6

• Eyes are open.

• Skin is wrinkled, pink, and translucent.

• Eyelashes form.

Month 7

• Skin is wrinkled and red.

• Testes descend into scrotum.

• Fetus turns to an upside-down position.

• Fetus can usually survive if born prematurely.

• Bone marrow is now the only site for red blood cell formation.

Month 8

• Fatty tissue deposition gives fetus a more plump,

"babyish" appearance, with lighter, less wrinkled skin.

• Twins are usually born now.

Month 9

• More fat deposition occurs.

• Nails extend to or beyond fi ngertips.

• Lanugo is shed.

• Birth is imminent.

Cigarette Smoke

Can constrict the fetus blood vessels, preventing the fetus from getting enough oxygen. Usually leads to underweight babies, a greater risk for premature birth, stillbirths, miscarriages, and a reduced intellectual ability.

Fetal Alcohol Syndrome

A group of birth defects caused by the effects of alcohol on an unborn child.

Vitamin C

If the fetus has too much vitamin c and the dose drops, the baby can develop scurvy, bruising easily, and being prone to infection.

Parturition

The birthing process, commonly known as labour.

Breech Birth

Baby is delivered feet or buttocks first.

Caesarean Section

The delivery of the child through an incision in the maternal abdominal and uterine walls.

Prostaglandins

Modified fatty acids that are produced by a wide range of cells. Stimulate uterine contractions.

Colostrum

A specialized form of milk that delivers essential nutrients and antibodies in a form that the newborn can digest. A thin yellowish fluid that contains more protein and less fat than milk.

Relaxin

Produced by the placenta prior to birth, relaxes ligaments of the pelvis, causing the cervix to soften prior to labour.

Oxytocin and Prostaglandins

Cause strong uterine cotractions for the delivery of the baby.

Prolactin

After birth, secretes prolactin which stimulates milk production.

Oxytocin

Causes contraction within the mammary lobules. Sucking motions send signals through nerve endings to the hypothalamus, which then stimulates the pituitary to release oxytocin.

Areola

A dark-pigmented area surrounding the breast nipple.

Mammary Lobules

A cluster of mammary alveoli that makes up a breast lobe. The lobe then drains produced milk via the lactiferous duct.

Reproductive Technologies

Technologies that enhance or reduce reproductive potential.

In Vitro Fertilization

The most common assisted reproduction procedure, in which a woman's eggs are mixed with sperm in culture dishes (in vitro) and then carefully inserted into a woman's uterus. Ultrasound is used to identify and harvest an immature ova.

Sterile

Describes men and women who are unable to have any children.

Infertile

Men and women who have difficulty conceiving children and have been trying for a year or more.

Artificial Insemination

Sperm are collected and concentrated before being placed into a woman's vagina.

Gamete Intrafallopian Transfer

A variation of IVF in which eggs and sperm are brought together in the oviduct rather than in vitro. Higher success rate.

Surrogate Mother

When an infertile couple contracts another woman to carry the baby using AI or IVF.

Superovulation

The production of multiple eggs as a result of hormone treatment. Received by women who ovulate rarely or not at all. Stimulates follicle development and ovulation. Usually used in conjunction with other reproductive technologies. Use FSH.

Surgical Sterilization

A surgery to make both men and women infertile or sterile such as tubal litigation or a vasectomy.

Tubal Ligation

A sterilization procedure for women where the oviducts are cut and then the ends are tied off ensuring the ovum never encounters the sperm.

Vasectomy

A sterilization procedure for me where the ductus deferens is cut and tied. The man may still have an erection and ejaculate but the semen does not have any sperm.

Contraceptive Technolgies

Work by changing the balance of reproductive hormones within a woman's body such as oral (contraceptive or pill), injection or implant under skin. Mimic progesterone and inhibit the release of FSH and LH.

Morning After Pill

An emergency contraceptive taken within 3 days of intercourse. Delivers a high dose of synthetic estrogen and progesterone disrupting the ovarian cycle and preventing or delaying ovulation. If fertilization has already taken place, it will prevent the embryo from implanting in the uterus. Side effects include vomiting and painful cramps. Effectiveness decreases from the day of intercourse.

Physical or Chemical Barriers

Prevent sperm from reaching the ovum.

- Male/female condoms = 85%

- Latex cap over cervix = 90%

- Contraceptive sponge = 70-90%

- Spermicides - kill the sperm (jellies, foams, creams) = 75%

Natural Family Planning

When couples refrain from having sex when a woman is most fertile (a week before ovulation and a day or two after) 70% effectiveness).

allantois

The allantois is an extra-embryonic membrane that is involved in gas exchange and stores uric acid.