Biology 12 - Fertilization and Pregnancy

Gamete

A haploid reproductive cell containing 23 chromosomes that combines with another gamete during fertilization.

Egg (Ovum)

Female gamete; the largest cell in the human body. Contains a nucleus, cytoplasm, and a zona pellucida. Non-motile and moved by cilia in the fallopian tubes. Approximately 1–2 million are present at birth, 300,000 remain at puberty, and about 400 are released during a lifetime.

Sperm

Male gamete consisting of a head (DNA), midpiece (mitochondria for ATP production), and tail (flagellum for movement). Motile and produced continuously, with approximately 300 million released per ejaculation.

Zona Pellucida

Protective glycoprotein layer surrounding the egg that protects the embryo during early development, regulates sperm interaction, and prevents polyspermy after fertilization.

Acrosome

Enzyme-filled cap on the sperm head that releases digestive enzymes during the acrosome reaction, allowing the sperm to penetrate the zona pellucida.

Pathway of Sperm

Testes → Epididymis → Vas Deferens → Ejaculatory Duct → Urethra → Vagina → Cervix → Uterus → Fallopian Tube.

Pathway of Egg

Ovary → Fallopian Tube (Oviduct) → Uterus.

Fertilization

Usually occurs in the outer one-third of the fallopian tube. A sperm penetrates the secondary oocyte, triggering completion of meiosis II. The sperm and egg pronuclei (23 chromosomes each) fuse to form a diploid zygote with 46 chromosomes.

Acrosome Reaction and Polyspermy Prevention

When a sperm binds to receptors on the zona pellucida, the acrosome releases digestive enzymes that digest the zona. After one sperm enters, granules released by the egg alter the zona pellucida to prevent additional sperm from entering.

Zygote

The first diploid cell formed after fusion of the sperm and egg nuclei.

Cleavage

Rapid mitotic cell divisions of the zygote that increase cell number without increasing overall size.

Morula

A solid ball of cells produced through cleavage divisions.

Blastocyst

A hollow ball of cells formed approximately 4.5–5 days after fertilization. Consists of a trophoblast, inner cell mass, and blastocoel and enters the uterus at this stage.

Blastocoel

Fluid-filled cavity within the blastocyst.

Implantation

Occurs approximately 6–7 days after fertilization when the blastocyst secretes enzymes that allow it to embed within the endometrium and obtain nutrients.

hCG (Human Chorionic Gonadotropin)

Hormone secreted by the chorion beginning 8–12 days after fertilization. Maintains the corpus luteum, allowing continued secretion of estrogen and progesterone during the first three months of pregnancy.

Corpus Luteum

A temporary endocrine structure that secretes estrogen and progesterone during early pregnancy. Maintained by hCG and later replaced by the placenta as the primary hormone source.

Chorion

The embryonic membrane that secretes hCG and forms part of the placenta.

Placenta

A temporary organ composed of maternal and fetal tissues. Develops partly from the chorion and serves as the connection between mother and fetus. Functions include gas exchange, nutrient transport, waste removal, hormone production (estrogen and progesterone), and partial barrier protection against harmful substances.

Umbilical Cord

Connects the fetus to the placenta and contains blood vessels formed from the allantois that transport oxygen, nutrients, and wastes between the fetus and placenta.

Ectoderm

The outer germ layer that forms the nervous system, skin, hair, nails, teeth, eyes, and ears.

Gestation

The period of development from fertilization to birth. Lasts approximately 38 weeks from fertilization or 40 weeks from the last menstrual period.

First Trimester

The trimester major organs begin forming in.

Labour

The process of childbirth and delivery of the placenta. Consists of three stages: Dilation (cervix opens), Expulsion (baby is delivered), and Afterbirth (placenta is expelled).

Prolactin

Hormone responsible for stimulating milk production after birth. Its release is triggered by the drop in estrogen and progesterone following delivery of the placenta.

Trophoblast

Outer layer of the blastocyte that contributes to the placenta and embryonic membranes.

Inner Cell Mass

Inner layer of the blastocyst that develops into the embryo.

Amnion

The embryotic membrane that forms the amniotic sac and fluid that protect, insulate, and hydrate the fetus.

Allantois

The embryotic membrane that forms blood vessels in the umbilical cord.

Yolk sac

The embryotic membrane that provides nourishment during early embryonic development.

Mesoderm

The middle germ layer that forms muscles, bones, circulatory system, kidneys, and reproductive organs.

Endoderm

The inner germ layer that forms the digestive system and lungs

Second Trimester

The trimester with continued growth and development.

Third Trimester

The trimester which the baby undergoes rapid growth and preparation for birth.