Search

Displaying 51 - 75 of 86 items.

Meiosis in Humans

Meiosis, the process by which sexually-reproducing organisms generate gametes (sex cells), is an essential precondition for the normal formation of the embryo. As sexually reproducing, diploid, multicellular eukaryotes, humans rely on meiosis to serve a number of important functions, including the promotion of genetic diversity and the creation of proper conditions for reproductive success.

Format: Articles

Subject: Processes, Reproduction

Nuclear Transplantation

Nuclear transplantation is a method in which the nucleus of a donor cell is relocated to a target cell that has had its nucleus removed (enucleated). Nuclear transplantation has allowed experimental embryologists to manipulate the development of an organism and to study the potential of the nucleus to direct development. Nuclear transplantation, as it was first called, was later referred to as somatic nuclear transfer or cloning.

Format: Articles

Subject: Processes

Julia Barlow Platt's Embryological Observations on Salamanders' Cartilage (1893)

In 1893, Julia Barlow Platt published her research on the origins of cartilage in the developing head of the common mudpuppy (Necturus maculosus) embryo. The mudpuppy is an aquatic salamander commonly used by embryologists because its large embryonic cells and nuclei are easy to see. Platt followed the paths of cells in developing mudpuppy embryos to see how embryonic cells migrated during the formation of the head. With her research, Platt challenged then current theories about germ layers, the types of cells in an early embryo that develop into adult cells.

Format: Articles

Subject: Experiments, Theories, Processes

Mechanistic Realization of the Turtle Shell

Turtle morphology is unlike that of any other vertebrate. The uniqueness of the turtle's bodyplan is attributed to the manner in which the turtle's ribs are ensnared within its hard upper shell. The exact embryological and genetic mechanisms underpinning this peculiar anatomical structure are still a matter of debate, but biologists agree that the evolution of the turtle shell lies in the embryonic development of the turtle.

Format: Articles

Subject: Processes

Embryonic Differentiation in Animals

Embryonic differentiation is the process of development during which embryonic cells specialize and diverse tissue structures arise. Animals are made up of many different cell types, each with specific functions in the body. However, during early embryonic development, the embryo does not yet possess these varied cells; this is where embryonic differentiation comes into play. The differentiation of cells during embryogenesis is the key to cell, tissue, organ, and organism identity.

Format: Articles

Subject: Processes

Hamburger-Hamilton Staging Series (1951)

In 1951 Viktor Hamburger and Howard Hamilton created an embryonic staging series from a combination of photographs and drawings from other researchers. The Hamburger-Hamilton stages are a sequence of images depicting 46 chronological stages in chick development. The images begin with a fertilized egg and end with a fully developed chick. The Hamburger-Hamilton staging series was produced in order to replace a previous chick staging series created in 1900. The earlier attempt lacked specific details and staged the chick embryo by using only morphological characteristics.

Format: Articles

Subject: Processes

Process of Eukaryotic Embryonic Development

All sexually reproducing, multicellular diploid eukaryotes begin life as embryos. Understanding the stages of embryonic development is vital to explaining how eukaryotes form and how they are related on the tree of life. This understanding can also help answer questions related to morphology, ethics, medicine, and other pertinent fields of study. In particular, the field of comparative embryology is concerned with documenting the stages of ontogeny.

Format: Articles

Subject: Processes

Bicoid

Bicoid is the protein product of a maternal-effect gene unique to flies of the genus Drosophila . In 1988 Christiane Nüsslein-Volhard identified bicoid as the first known morphogen . A morphogen is a molecule that determines the fate and phenotype of a group of cells through a concentration gradient across that developing region. The bicoid gradient, which extends across the anterior-posterior axis of Drosophila embryos, organizes the head and thorax.

Format: Articles

Subject: Processes

The Effects of Thalidomide on Embryonic Development

Embryogenesis is an intricate process that can easily be disrupted by means of teratogenic agents. Some of these agents target the embryonic period's "window of susceptibility," three to eight weeks after a pregnant woman's last menstruation, when the highest degree of sensitivity to embryonic cell differentiation and organ formation occurs. The embryonic period or critical period is when most organ systems form, whereas the fetal period, week eight to birth, involves the growth and modeling of the organ systems.

Format: Articles

Subject: Processes, Disorders

Circulatory Changes at Birth

When placental mammals are born their circulatory systems undergo radical changes as the newborns are prepared for independent life. The lungs are engaged, becoming the primary source of fresh oxygen, replacing the placental barrier as a means for blood-gas exchange.

Format: Articles

Subject: Processes

Mesenchyme

Mesenchyme is a type of animal tissue comprised of loose cells embedded in a mesh of proteins and fluid, called the extracellular matrix. The loose, fluid nature of mesenchyme allows its cells to migrate easily and play a crucial role in the origin and development of morphological structures during the embryonic and fetal stages of animal life. Mesenchyme directly gives rise to most of the body's connective tissues, from bones and cartilage to the lymphatic and circulatory systems.

Format: Articles

Subject: Processes

Somites: Formation and Role in Developing the Body Plan

Somites are blocks of mesoderm that are located on either side of the neural tube in the developing vertebrate embryo. Somites are precursor populations of cells that give rise to important structures associated with the vertebrate body plan and will eventually differentiate into dermis, skeletal muscle, cartilage, tendons, and vertebrae. Somites also determine the migratory paths of neural crest cells and of the axons of spinal nerves.

Format: Articles

Subject: Processes

Nerve Growth Factor

Nerve growth factor (NGF) is a signaling protein and growth factor implicated in a wide range of development and maintenance functions. NGF was discovered through a series of experiments in the 1950s on the development of the chick nervous system. Since its discovery, NGF has been found to act in a variety of tissues throughout development and adulthood. It has been implicated in immune function, stress response, nerve maintenance, and in neurodegenerative diseases.

Format: Articles

Subject: Processes

Hartsoeker's Homunculus Sketch from Essai de Dioptrique

This embryology image is a pencil sketch by Nicolaas Hartsoeker, published as part of his 1694 French-language paper entitled Essai de Dioptrique, a semi-speculative work describing the sorts of new scientific observations that could be done using magnifying lenses. Dioptrique was published in Paris by the publishing house of Jean Anisson. The image depicts a curled up infant-like human, now referred to as a homunculus, inside the head of a sperm cell.

Format: Articles

Subject: Theories, Processes

James G. Wilson's Six Principles of Teratology

James Graves Wilson's six principles of teratology, published in 1959, guide research on teratogenic agents and their effects on developing organisms. Wilson's six principles were inspired by Gabriel Madeleine Camille Dareste's five principles of experimental teratology published in 1877. Teratology is the study of birth defects, and a teratogen is something that either induces or amplifies abnormal embryonic or fetal development and causes birth defects.

Format: Articles

Subject: Processes, Reproduction

A Fate Map of the Chick Embryo

A 3-D fate map of the chicken (Gallus gallus) embryo with the prospective point of ingression and yolk. The area where the primitive streak will form during gastrulation is shown. The anterior- posterior axis is shown by labeling the anterior and posterio ends (A) and (P). Different colors indicate prospective fates of different regions of the epiblast after gastrulation.

Format: Graphics

Subject: Processes, Organisms, Theories

The Blastoderm in Chicks During Early Gastrulation

This image shows a chicken (Gallus gallus) embryo undergoing gastrulation in stage four (18-19 hrs after laying) according to the Hamburger-Hamilton staging series. At this point in time the chicken embryo is a blastoderm (shown in blue). The first magnification of the embryo shows that the blastoderm cell layers have thickened to form the primitive streak and Hensen's node. The primitive streak extends from the posterior (P) region to the anterior (A) region. The second rectangular magnification shows the blastoderm cross-sectioned through the primitive streak.

Format: Graphics

Subject: Processes, Organisms, Theories

Estrogen

The figure depicts three different molecular structures of estrogen found in mammals’ that differ by the arrangement of bonds and side groups. The molecular structures of the three estrogen molecules differ by the arrangement of chemical bonds and side groups attached to the core steroid structure, cholesterol, which contains three cyclohexane rings and one cyclopentane ring.

Format: Graphics

Subject: Theories, Processes, Reproduction

Mitochondria

Mitochondria are organelles found in the cytoplasm of eukaryotic cells. They are composed of an outer membrane and an inner membrane. The outer membrane faces the cellular cytoplasm, while the inner membrane folds back on itself multiple times, forming inner folds, called cristae. The space between the two membrane layers is called the intermembrane space, and the space within the inner membrane is called the matrix.

Format: Graphics

Subject: Theories, Processes

Jelly Fish and Green Fluorescent Protein

The crystal jellyfish, Aequorea victoria, produces and emits light, called bioluminescence. Its DNA codes for sequence of 238 amino acids that forms a protein called Green Fluorescent Protein (GFP). FP is folded so that a part of the protein, called the chromophore, is located in the center of the protein. The chemical structure of the chromophore emits a green fluorescence when exposed to light in the range of blue to ultraviolet.

Format: Graphics

Subject: Theories, Processes, Organisms, Technologies

DNA and X and Y Chromosomes

Y-chromosomes exist in the body cells of many kinds of male animals. Found in the nucleus of most living animal cells, the X and Y-chromosomes are condensed structures made of DNA wrapped around proteins called histones. The individual histones bunch into groups that the coiled DNA wraps around called a nucleosome, which are roughly 10 nano-meters (nm) across. The histones bunch together to form a helical fiber (30 nm) that spins into a supercoil (200 nm). During much of a cell's life, DNA exists in the 200 nm supercoil phase.

Format: Graphics

Subject: Theories, Processes

Chloroplasts

Chloroplasts are the organelles in plant and algal cells that conduct photosynthesis. A single chloroplast has an outer membrane and an inner membrane, with an intermembrane space in between. Within the inner membrane, interconnected stacks of thylakoids, called granum, float in a protein rich fluid called the stroma. These thylakoid stacks contain chlorophyll, a pigment which converts sunlight into usable energy for plants and free oxygen from water. The stacks are sites of light reactions within a plant cell.

Format: Graphics

Subject: Theories, Processes

Neurospora crassa Life Cycle

This diagram shows the life cycle of Neurospora crassa, a mold that grows on bread. N. crassa can reproduce through an asexual cycle or a sexual cycle. The asexual cycle (colored as a purple circle), begins in this figure with (1a) vegetative mycelium, which are strands of mature fungus. Some of the strands form bulbs (2a) in a process called conidiation. From those bulbs develop the conidia, which are spores. Next, (3a) a single conidium separates from its strand and elongates until it forms mycelium.

Format: Graphics

Subject: Organisms, Processes, Theories

The Process of Gastrulation in Frog Embryos

Illustration of the movement of the three hemispheres of cells, the animal cap (dark green) the marginal zone (lime green) and the ventral cap (yellow) during frog gastrulation. The external view column (images a.1-a.6) shows gastrulation as it occurs on the outside of the embryo. The cross-section view column (images b.1-b.6) shows the internal view of gastrulation. The cross-sections are through the middle of the embryo.

Format: Graphics

Subject: Processes, Organisms, Theories

Frog Embryo in the Blastula Stage

Illustration of the animal-vegetal gradient in Xenopus laevis ( African clawed frog) eggs after fertilization. During fertilization, the sperm s point of entry determines the future dorsal side (shaded) and ventral side (unshaded) of the embryo. The prospective ventral side of the embryo forms on the side where the sperm enters while the prospective dorsal side forms opposite the sperm s point of entry.

Format: Graphics

Subject: Processes, Organisms, Theories