Embryo Project Encyclopedia Articles

Mesoderm is one of the three germ layers, groups of cells that interact early during the embryonic life of animals and from which organs and tissues form. As organs form, a process called organogenesis, mesoderm interacts with endoderm and ectoderm to give rise to the digestive tract, the heart and skeletal muscles, red blood cells, and the tubules of the kidneys, as well as a type of connective tissue called mesenchyme. All animals that have only one plane of symmetry through the body, called bilateral symmetry, form three germ layers. Animals that have only two germ layers develop open digestive cavities. In contrast, the evolutionary development of the mesoderm allowed in animals the formation of internal organs such as stomachs and intestines (viscera).

Diethylstilbestrol (DES) is an artificially created hormone first synthesized in the late 1930s. Doctors widely prescribed DES first to pregnant women to prevent miscarriages, and later as an emergency contraceptive pill and to treat breast cancer. However, in 1971, physicians showed a link between DES and vaginal cancer during puberty in the children of women who had taken DES while pregnant. Consequently, the US Food and Drug Administration (FDA) banned its use during pregnancy. In the late 2000s, several studies showed that the grandchildren of women who had consumed DES also suffered medical issues. By the early decades of the twenty-first century, roughly ten million people in the US had been exposed to DES, and three generations of individuals had suffered medical issues due to DES exposure. Researchers class DES as an endocrine disruptor, which affects the form and function of the hormone (endocrine) system.

Edgar Allen identified and outlined the role of female sex hormones and discovered estrogen in the early 1900s in the US. In 1923, Allen, through his research with mice, isolated the primary ovarian hormone, later renamed estrogen, from ovarian follicles and tested its effect through injections in the uterine tissues of mice. Allen’s work on estrogen, enabled researchers to further study hormones and the endocrine system.

John Hunter studied human reproductive anatomy, and in eighteenth century England, performed one of the earliest described cases of artificial insemination. Hunter dissected thousands of animals and human cadavers to study the structures and functions of organ systems. Much of his anatomical studies focused on the circulatory, digestive, and reproductive systems. He helped to describe the exchange of blood between pregnant women and their fetuses. Hunter also housed various natural collections, as well as thousands of preserved specimens from greater than thirty years of anatomy work. Hunter's work developed practices in reproductive and reparative surgery and furthered the study of human anatomy and physiology.

In 1953, Raymond Greene and Katharina Dalton, who were doctors in the UK, published The Premenstrual Syndrome in the British Medical Journal. In their article, Dalton and Greene established the term premenstrual syndrome (PMS). The authors defined PMS as a cluster of symptoms that include bloating, breast pain, migraine-headache, fatigue, anxiety, depression, and irritability. The article states that the symptoms begin one to two weeks before menstruation during the luteal phase of the menstrual cycle, and they disappear upon the onset of the menstrual period. Menstruation is the monthly series of changes a woman's body undergoes in preparation for the possibility of pregnancy. Dalton and Greene described how progesterone affected women during different phases of their menstrual cycles. The paper convinced many about the phenomenon of PMS, and docotors and scientists adopted Dalton's and Green's term. The paper furthered research about the role of hormones in physiology and of conditions linked to the reproductive system.

Estrogen is the primary sex hormone in women and it functions during the reproductive menstrual cycle. Women have three major types of estrogen: estrone, estradiol, and estriol, which bind to and activate receptors within the body. Researchers discovered the three types of estrogen over a period of seven years, contributing to more detailed descriptions of the menstrual cycle. Each type of estrogen molecule contains a slightly different arrangement or number of atoms that in turn causes some of the estrogens to be more active than others. The different types of estrogen peak and wane throughout women's reproductive cycles, from normal menstruation to pregnancy to the cessation of menstruation (menopause). As scientists better explained the effects of estrogens, they used that information to develop oral contraceptives to control pregnancy, to map the menstrual cycle, and to create hormone therapies to regulate abnormal levels of estrogen.

Carl Richard Moore was a professor and researcher at the University of Chicago in Chicago, Illinois who studied sex hormones in animals from 1916 until his death in 1955. Moore focused on the role of hormones on sex differentiation in offspring, the optimal conditions for sperm production, and the effects of vasectomy or testicular implants on male sex hormone production. Moore's experiments to create hermaphrodites in the laboratory contributed to the theory of a feedback loop between the pituitary and fetal gonadal hormones to control sex differentiation. Moore showed that the scrotal sac controls the temperature for the testes, which is necessary for sperm production. He also helped distinguish the hormones testosterone, and androsterone from testicular extracts.

A germ layer is a group of cells in an embryo that interact with each other as the embryo develops and contribute to the formation of all organs and tissues. All animals, except perhaps sponges, form two or three germ layers. The germ layers develop early in embryonic life, through the process of gastrulation. During gastrulation, a hollow cluster of cells called a blastula reorganizes into two primary germ layers: an inner layer, called endoderm, and an outer layer, called ectoderm. Diploblastic organisms have only the two primary germ layers; these organisms characteristically have multiple symmetrical body axes (radial symmetry), as is true of jellyfish, sea anemones, and the rest of the phylum Cnidaria. All other animals are triploblastic, as endoderm and ectoderm interact to produce a third germ layer, called mesoderm. Together, the three germ layers will give rise to every organ in the body, from skin and hair to the digestive tract.

Endoderm is one of the germ layers-- aggregates of cells that organize early during embryonic life and from which all organs and tissues develop. All animals, with the exception of sponges, form either two or three germ layers through a process known as gastrulation. During gastrulation, a ball of cells transforms into a two-layered embryo made of an inner layer of endoderm and an outer layer of ectoderm. In more complex organisms, like vertebrates, these two primary germ layers interact to give rise to a third germ layer, called mesoderm. Regardless of the presence of two or three layers, endoderm is always the inner-most layer. Endoderm forms the epithelium-- a type of tissue in which the cells are tightly linked together to form sheets-- that lines the primitive gut. From this epithelial lining of the primitive gut, organs like the digestive tract, liver, pancreas, and lungs develop.

The sex of a reptile embryo partly results from the production of sex hormones during development, and one process to produce those hormones depends on the temperature of the embryo's environment. The production of sex hormones can result solely from genetics or from genetics in combination with the influence of environmental factors. In genotypic sex determination, also called genetic or chromosomal sex determination, an organism's genes determine which hormones are produced. Non-genetic sex determination occurs when the sex of an organism can be altered during a sensitive period of development due to external factors such as temperature, humidity, or social interactions. Temperature-dependent sex determination (TSD), where the temperature of the embryo's environment influences its sex development, is a widespread non-genetic process of sex determination among vertebrates, including reptiles. All crocodilians, most turtles, many fish, and some lizards exhibit TSD.