Ch. 3 - Embryogenesis and Development

5% of MCAT biology content

What is fertilization, and where does it usually occur?

Fertilization is the joining of a sperm and an ovum, and it usually occurs in the ampulla of the fallopian tube.

What role do acrosomal enzymes play in fertilization?

The sperm uses acrosomal enzymes to penetrate the corona radiata and zona pellucida of the ovum.

What happens when a sperm contacts the oocyte's plasma membrane?

When the first sperm penetrates, it establishes the acrosomal apparatus and injects its pronucleus.

What is the cortical reaction in fertilization?

Occurs when the first sperm penetrates

Causes a release of calcium ions that prevents additional sperm from fertilizing the egg

Increases the metabolic rate of the resulting diploid zygote

What are fraternal (dizygotic) and identical (monozygotic) twins?

Fraternal twins: result from the fertilization of two eggs by two different sperm

Identical twins: result from the splitting of a zygote into two

How can monozygotic twins be classified?

By the placental structures they share (monoamniotic vs. diamniotic, and monochorionic vs. dichorionic)

What is cleavage in embryonic development?

early divisions of cells in the embryo, resulting in a larger number of smaller cells without a change in overall volume

Why do a zygote become an embryo after the first cleavage?

it is no longer unicellular

What is the difference between the indeterminate and determinate cleavage?

Indeterminate cleavage: results in cells that are capable of becoming any cell in the organism

Determinate cleavage: results in cells that are committed to differentiating into a specific cell type

What is the morula in early development?

solid mass of cells

What is the blastula (blastocyst) in early development?

has a fluid-filled center called a blastocoel and contains two structures:

• trophoblast becomes placental structures

• inner cell mass becomes the developing organism

What forms the placenta?

blastula when it implants in the endometrial lining

What is the role of the chorion and chorionic villi?

Chorion contains chorionic villi that penetrate the endometrium, creating the interface between maternal and fetal blood.

How is the embryo supported before the placenta is established?

by the yolk sac

What does the allantois do in embryonic development?

involved in early fluid exchange between the embryo and the yolk sac

What is the function of the amnion?

lies inside the chorion and produces amniotic fluid

How is the developing organism connected to the placenta?

via the umbilical cord

What occurs during gastrulation?

Archenteron is formed with a blastopore at the end, and it establishes three primary germ layers (ectoderm, mesoderm, endoderm) as it grows through the blastocoel

What tissues arise from the ectoderm?

Becomes the epidermis, hair, nails, the epithelia of the nose, mouth, and anal canal, the nervous system and lens of the eye.

What tissues arise from the mesoderm?

Becomes musculoskeletal, circulatory, and excretory systems, gonads, and muscular/connective tissue layers of the digestive and respiratory systems.

What processes does the endoderm develop into?

Becomes much of the epithelial linings of the respiratory and digestive tracts, and parts of the pancreas, thyroid, bladder, and distal urinary tracts.

What begins the development of the nervous system (Neurulation)?

Begins after the formation of the three germ layers (ectoderm, mesoderm, endoderm)

What is the function of the notochord during neurulation?

induces a group of overlying ectodermal cells to form neural folds surrounding a neural groove.

How is the central nervous system (CNS) formed?

neural folds fuse to form the neural tube (becomes CNS)

What are neural crest cells?

Found at the tip of each neural fold, become the peripheral nervous system (PNS) and specific cell types in other tissues.

What are teratogens?

substances that interfere with development, causing defects or even death of the developing embryo

ex. alcohol, certain prescription drugs, viruses, bacteria, and environmental chemicals

How can maternal conditions affect embryonic development?

ex. diabetes may lead to increased fetal size and hypoglycemia after birth

ex. folic acid deficiency can result in neural tube defects

What are the two processes involved in cell specialization?

Determination: commitment to a specific cell lineage, achieved by uneven segregation of cellular material during mitosis or morphogens promoting development

Differentiation: process where a cell undergoes changes through selective transcription to take on characteristics of its cell line.

What must a cell have to respond to a specific morphogen?

Competency

What are stem cells?

Cells capable of developing into various cell types.

What are the classifications of stem cell potency?

• Totipotent: Differentiate into all cell types, including germ layers and placental structures.

• Pluripotent: Differentiate into all germ layers and derivatives.

• Multipotent: Differentiate into a specific subset of cell types

What is an inducer in cell communication?

A cell that releases factors to promote differentiation of a competent responder

What are the types of cell signaling?

• Autocrine: Acts on the same cell that released the signal.

• Paracrine: Acts on local cells.

• Juxtacrine: Direct stimulation of adjacent cells.

• Endocrine: Acts on distant tissues through the bloodstream

What are growth factors?

Peptides that promote differentiation and mitosis in certain tissues.

What is reciprocal induction?

When two tissues induce further differentiation in each other

How does signaling often occur in tissues?

via gradients

Why might cells need to migrate?

To reach their correct location.

What is apoptosis, and how does it occur?

Programmed cell death via apoptotic blebs, which are absorbed and digested by other cells

What anatomical role can apoptosis serve?

Sculpting structures, such as removing webbing between digits.

What is regenerative capacity, and how does it vary by organ?

The ability to regrow parts of the body.

ex. liver has high regenerative capacity

ex. heart has low regenerative capacity

What is senescence?

result of molecular and metabolic processes, particularly the shortening of telomeres during cell division.

How are oxygen and carbon dioxide exchanged at the placenta?

Through passive diffusion due to concentration gradients.

What is the role of fetal hemoglobin (HbF) in oxygen exchange?

HbF has a higher affinity for oxygen than adult hemoglobin (HbA), aiding oxygen transfer and retention in the fetal circulatory system.

Besides gas exchange, what immune function does the placenta provide?

It acts as a barrier against pathogens and transfers antibodies from the pregnant individual to the fetus.

What hormones are secreted by the placenta?

Estrogen, progesterone, and human chorionic gonadotropin (hCG)

What do the umbilical arteries transport? What does the umbilical vein transport?

Arteries: Deoxygenated blood from the fetus to the placenta.

Vein: Oxygenated blood from the placenta to the fetus.

What does the foramen ovale connect, and what does it bypass?

It connects the right atrium to the left atrium, bypassing the lungs.

What does the ductus arteriosus connect, and what does it bypass?

It connects the pulmonary artery to the aorta, bypassing the lungs.

What does the ductus venosus connect, and what does it bypass?

It connects the umbilical vein to the inferior vena cava, bypassing the liver.

What major process occurs in the first trimester?

Organogenesis, including the development of the heart, eyes, gonads, limbs, liver, and brain.

What are the key events of the second trimester?

Tremendous growth, initiation of movement, the face becoming distinctly human, and elongation of digits.

What are the primary developments in the third trimester?

Rapid growth, continued brain development, and transfer of antibodies to the fetus.

What happens to the cervix and the amniotic sac during birth?

The cervix thins out, and the amniotic sac ruptures.

What coordinates uterine contractions during birth?

Prostaglandins and oxytocin.

What is expelled after the birth of the fetus?

The placenta and the umbilical cord.