Fertilization & Cleavage in Human Development

Union of sperm and egg, typically within 12-24 hours of ovulation.

The embryo at the stage with two cells, approx. 30 hours after fertilization.

Solid ball of 12-16 cells (blastomeres) formed about 3 days after fertilization.

Hollow ball (blastocyst) with inner cell mass and an outer layer (trophoblast); zona pellucida disappears.

Blastocyst embeds into the uterine lining (endometrium).

Outer layer of the trophoblast; multinucleated structure that erodes maternal tissue for nourishment.

Modified uterine lining after implantation (endometrium).

The placenta implants in the lower uterine segment, near the cervix opening, instead of the top of the uterus. This means the placenta will be delivered before the baby during birth.

The inner layer of the trophoblast, actively dividing to form new cells.

Cavities or spaces that form within the syncytiotrophoblast, which eventually connect to the maternal blood vessels.

Finger-like projections that arise from the cytotrophoblast, extending into the syncytiotrophoblast. These villi play a critical role in nutrient and waste exchange between the mother and the developing baby.

The early embryo is formed of two cell layers: the epiblast and the hypoblast. These layers are positioned one on top of the other, forming a disc-like structure.

The top layer of the bilaminar disc. This layer will eventually give rise to the ectoderm which is the outermost germ layer.

The lower layer of the bilaminar disc. This layer will eventually give rise to endoderm, which is one of the three primary germ layers.

Cells that arise from the epiblast and form the lining of the amniotic cavity, which protects and surrounds the developing baby.

The extraembryonic mesoderm that lines the cytotrophoblast and amnion.

The extraembryonic mesoderm that covers the yolk sac.

A thin membrane that lines the exocoelomic cavity, forming a boundary between the yolk sac and the amnion.

The ability of a stem cell to divide and create more undifferentiated stem cells, maintaining its state.

The ability of a stem cell to differentiate into various specialized cell types.

Stem cells capable of differentiating into all cell types, including the placenta.

The umbilical cord connects the fetus to the placenta, delivering oxygen and nutrients from the mother and carrying away waste products.

The placental barrier separates fetal and maternal blood, preventing mixing while allowing exchange of gases, nutrients, and waste products.

The placental membrane is initially composed of four layers: the endothelial lining of fetal vessels, primary mesoderm, cytotrophoblast, and syncytiotrophoblast.

Warton's jelly is a gelatinous substance that surrounds the blood vessels in the umbilical cord, providing cushioning and support.

The primitive umbilical cord is formed during folding and contains the body stalk, yolk stalk, and extraembryonic coelom.

The three-layered structure formed during gastrulation, consisting of ectoderm, mesoderm, and endoderm.

The process of forming the three germ layers (ectoderm, mesoderm, and endoderm) from a bilaminar disc.

A midline groove that forms on the epiblast during gastrulation, initiating cell migration and germ layer formation.

The cephalic end of the primitive streak, a crucial site for cell migration and notochord formation.

A rod-shaped structure formed from migrating cells, serving as the axis for the developing skeleton and inducing neural tube formation.

The outermost germ layer, responsible for forming tissues like skin, nervous system, and sensory organs.

The middle germ layer, forming muscles, bones, blood, and connective tissues.

The innermost germ layer, responsible for forming lining of digestive and respiratory tracts, glands, and organs.

The notochord provides structural support, induces the formation of the nervous system, and ultimately degenerates to form components of the intervertebral discs.

The epiblast (ectoderm) is the source of all three germ layers; cells migrate and differentiate to form the endoderm and mesoderm.

The process of converting the flat, trilaminar embryonic disc into a cylindrical embryo. It occurs during the 3rd and 4th weeks of development.

A type of embryonic folding that happens along the head-to-tail axis due to the rapid growth of the central nervous system.

A birth defect caused by insufficient mesoderm formation in the caudal region of the embryo, leading to abnormalities in the lower body.

A tumor that arises from remnants of the primitive streak persisting in the sacrococcygeal region.

The three primary layers of cells that develop during gastrulation: ectoderm, mesoderm, and endoderm.

The notochord, a rod-shaped structure, provides structural support, induces the formation of the nervous system, and eventually contributes to the intervertebral discs.

The ectoderm gives rise to tissues like skin, nervous system, and sensory organs.

The mesoderm is the middle layer and differentiates into muscles, bones, blood, and connective tissues.

The endoderm is the inner layer and gives rise to the lining of the digestive and respiratory tracts, glands, and other internal organs.

The embryonic period (3rd to 8th weeks) is the most sensitive stage for teratogens (substances that cause birth defects) to disrupt development and cause structural abnormalities.

A thin membrane separating the primitive mouth (stomodeum) from the foregut during early embryonic development.

A sheet of tissue that eventually forms the diaphragm, separating the thoracic and abdominal cavities.

Arrangement of body parts from head to tail, reflecting the direction of embryonic development.

Paired blocks of mesoderm tissue that give rise to vertebrae, muscles, and skin.

The bones that form the central axis of the body, including the skull, vertebral column, and ribs.

A cavity that forms within the lateral plate of mesoderm, eventually becoming the body cavities.

The outer layer of the lateral plate mesoderm, which gives rise to the muscles of the body wall.

The inner layer of the lateral plate mesoderm, which gives rise to the smooth muscles of the internal organs.

During labor, the amniotic sac bulges into the cervix creating the "fore bag of water" which helps dilate the cervix. It also serves as a lubricant and bacteriostatic agent.

A condition where there's excessive amniotic fluid (more than 1.5-2 liters). This can be caused by fetal abnormalities like anencephaly, esophageal atresia, or maternal diabetes.

A condition where there's too little amniotic fluid (less than half a liter). It can be due to fetal kidney problems (renal agenesis) or placental insufficiency.

The delivery of a baby still enclosed in the intact amniotic membrane. This is rare and usually requires prompt action.

The yolk sac, present in the early embryo, helps form parts of the digestive and respiratory systems, urinary bladder lining, and provides blood cell precursors. It also houses primordial germ cells that develop into eggs and sperm.

Implantation extending deeper than the compact layer can lead to complications, including placenta previa, placental abruption, or even uterine rupture.

The trophoblast differentiates into two layers: the inner cytotrophoblast and the outer syncytiotrophoblast.

The syncytiotrophoblast produces enzymes that erode the maternal tissues, enabling the blastocyst to burrow into the endometrium.

The trophoblast differentiates into the inner cytotrophoblast and the outer syncytiotrophoblast.

The inner cell mass differentiates into the epiblast and the hypoblast.

The final product of the first week of development is the bilaminar disc, composed of the epiblast and hypoblast.

Stem cells can be obtained from embryonic cells (limited use due to ethical concerns) and adult cells (obtained from sources like bone marrow).

A birth defect caused by insufficient mesoderm formation in the caudal region of the embryo, leading to abnormalities in the lower body, including fused legs, vertebral defects and genital abnormalities.

The ectoderm, the outermost germ layer, differentiates into tissues like skin, nervous system, and sensory organs.

The mesoderm, the middle germ layer, differentiates into muscles, bones, blood, and connective tissues.

The endoderm, the innermost germ layer, differentiates into the lining of the digestive and respiratory tracts, glands, and other internal organs.

The embryonic period, from the third to the eighth weeks of development, is when all major organs and organ systems form. It's a crucial period because it's when the foundation of the body is laid down.

During the embryonic period, cells are rapidly dividing and differentiating to form various tissues and organs. This rapid development makes the embryo highly susceptible to disruptions caused by harmful substances called teratogens.

Fetal blood in the umbilical arteries carries CO2 and waste products from the fetus to the placenta.

The placental barrier prevents mixing of fetal and maternal blood while allowing exchange of gases, nutrients, and waste products.

The primitive umbilical cord forms during folding and contains the body stalk (with blood vessels), yolk stalk (with blood vessels), and the remaining part of the extra embryonic coelom.

A small pouch in the ileum (small intestine) that forms due to the persistence of a part of the yolk sac. It's usually 2 inches long and located about 2 feet from the ileocecal junction. It occurs in about 2% of people.

A string-like structure that forms when the yolk stalk disappears. It can sometimes cause intestinal obstruction.

A small pouch that arises from the yolk sac during embryonic development and extends into the connecting stalk. Its main function is to form the umbilical blood vessels and contribute to the formation of the bladder.

A fluid-filled sac that forms when the allantois doesn't completely close off. It's located between the bladder and the umbilicus.

A small opening at the umbilicus that persists when the allantois doesn't fully close. It can sometimes lead to infection.

An abnormal connection between the bladder and the umbilicus, formed when the allantois doesn't fully obliterate. It can cause urine leakage from the umbilicus.

A fibrous cord that connects the bladder to the umbilicus, formed from the obliterated intraembryonic part of the allantois after birth.

An organ that forms during pregnancy, providing nutrients and oxygen to the developing baby and removing waste products.

A tube connecting the baby to the placenta, carrying blood vessels that transport nutrients, oxygen, and waste products.

A protective membrane that separates the baby's blood from the mother's blood while allowing the exchange of essential substances like nutrients, oxygen, and waste.