Human Development and Embryology

Flashcards covering the stages of human development from fertilization to birth, including germ layer formation and hormonal regulation.

ZYGOTE

One cell (fertilized egg).

EMBRYO

Stage of development from the first mitotic cell division to $9$ weeks.

FETUS

Stage of development from $9$ weeks to birth.

BABY

The term used for the offspring when it is born.

$2n$ zygote

Produced by the fusion of a spermatozoan and an ovum (secondary oocyte) in the oviduct on day $14$-$$15$.

Acrosome reaction

The release of enzymes (acrosin) required for digestion through the zona pellucida.

Cortical reaction

The release of lysosomal enzymes.

Capacitation

The process required for the sperm to penetrate the corona radiata.

Zona reaction

A reaction that inactivates sperm receptors and prevents polyspermy.

Fusion of sperm and oocyte plasma membranes

Involves integrins (oocyte) and disintegrins (sperm), stimulates completion of meiosis II, and results in sperm mitochondria being degraded.

MORULA

A ball of undifferentiated cells formed by cleavage (mitosis) of the zygote, increasing cell number without increasing size.

BLASTOCYST

A hollow ball of undifferentiated cells formed by Day $5$ that has entered the uterus cavity.

Trophoblast

The outer layer of the blastocyst that will develop into extraembryonic tissues.

Inner cell mass

The group of cells in the blastocyst that will develop into the embryo.

Human Chorionic Gonadotropin (hCG)

A hormone secreted by the trophoblast that mimics LH to maintain the corpus luteum for the first three months of pregnancy.

GASTRULATION

The second phase of development involving a migration of blastocyst cells inward, leading to the formation of three germ layers.

Ectoderm

The outer germ layer that becomes the epidermis of the skin, hair follicles, nervous and sensory systems, pituitary gland, adrenal medulla, and teeth.

Mesoderm

The middle germ layer that becomes the skeletal and muscular systems, circulatory and lymphatic systems, excretory and reproductive systems, and the dermis.

Endoderm

The inner germ layer that becomes the epithelial lining of the digestive tract, respiratory tract, liver, pancreas, thymus, and thyroid glands.

Amnion

Extraembryonic membrane that contains amniotic fluid, which cushions and protects the embryo.

Yolk sac

The first site of blood cell formation; it also contributes to the digestive tract and produces future egg and sperm cells.

Allantois

Form of the foundation for the umbilical cord and provides blood vessels to build the placenta.

Chorion

Develops villi to provide nutrients until the placenta forms and secretes hCG to maintain estrogen and progesterone levels.

Placenta

Responsible for nutrition, waste removal, and gas exchange; it also produces estrogen, progesterone, and relaxin.

Relaxin

A hormone produced by the placenta that relaxes ligaments to loosen joints for delivery.

Umbilical Artery

Transfers wastes like $CO_2$ and urea from the fetus to the placenta.

Umbilical Vein

Transfers $O_2$ and nutrients from the placenta to the fetus.

Trimester I

Characterized by gastrulation, heart pumping, primary sex organ development, and the beginning of brain and limb formation.

Trimester II

Characterized by movement felt by the mother, a bony skeleton, and completion of organ development.

Trimester III

Characterized by rapid growth, an increase in mass, and brain maturation.

Prolactin

Hormone that activates gland tissue to turn blood into milk and inhibits GnRH, theoretically preventing ovulation during lactation.

Oxytocin

Hormone that stimulates the release of milk through ducts into the nipple.