The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.
The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal organs, and is composed of a layer of mesothelium supported by a thin layer of connective tissue. This peritoneal lining of the cavity supports many of the abdominal organs and serves as a conduit for their blood vessels, lymphatic vessels, and nerves.
The abdominal cavity is a large body cavity in humans and many other animals that contain organs. It is a part of the abdominopelvic cavity. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the thoracic diaphragm, a thin sheet of muscle under the lungs, and its floor is the pelvic inlet, opening into the pelvis.
The coelom is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it remains undifferentiated. In the past, and for practical purposes, coelom characteristics have been used to classify bilaterian animal phyla into informal groups.
The pleural cavity, pleural space, or intrapleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication between the membranes, and also to create a pressure gradient.
The mesothelium is a membrane composed of simple squamous epithelial cells of mesodermal origin, which forms the lining of several body cavities: the pleura, peritoneum and pericardium.
In human anatomy, the mesentery, an organ that attaches the intestines to the posterior abdominal wall, comprises the double fold of the peritoneum. It helps in storing fat and allowing blood vessels, lymphatics, and nerves to supply the intestines.
An omphalocele or omphalocoele, also known as an exomphalos, is a rare abdominal wall defect. Beginning at the 6th week of development, rapid elongation of the gut and increased liver size reduces intra abdominal space, which pushes intestinal loops out of the abdominal cavity. Around 10th week, the intestine returns to the abdominal cavity and the process is completed by the 12th week. Persistence of intestine or the presence of other abdominal viscera in the umbilical cord results in an omphalocele.
The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though yolk sac is far more widely used. In humans, the yolk sac is important in early embryonic blood supply, and much of it is incorporated into the primordial gut during the fourth week of embryonic development.
The serous membrane is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane that covers internal organs is called visceral, while the one that covers the cavity wall is called parietal. For instance the parietal peritoneum is attached to the abdominal wall and the pelvic walls. The visceral peritoneum is wrapped around the visceral organs. For the heart, the layers of the serous membrane are called parietal and visceral pericardium. For the lungs they are called parietal and visceral pleura. The visceral serosa of the uterus is called the perimetrium. The potential space between two opposing serosal surfaces is mostly empty except for the small amount of serous fluid.
The cranial cavity, also known as intracranial space, is the space within the skull that accommodates the brain. The skull minus the mandible is called the cranium. The cavity is formed by eight cranial bones known as the neurocranium that in humans includes the skull cap and forms the protective case around the brain. The remainder of the skull is called the facial skeleton. Meninges are protective membranes that surround the brain to minimize damage to the brain in the case of head trauma. Meningitis is the inflammation of meninges caused by bacterial or viral infections.
The septum transversum is a thick mass of cranial mesenchyme, formed in the embryo, that gives rise to parts of the thoracic diaphragm and the ventral mesentery of the foregut in the developed human being and other mammals.
The lateral plate mesoderm is the mesoderm that is found at the periphery of the embryo. It is to the side of the paraxial mesoderm, and further to the axial mesoderm. The lateral plate mesoderm is separated from the paraxial mesoderm by a narrow region of intermediate mesoderm. The mesoderm is the middle layer of the three germ layers, between the outer ectoderm and inner endoderm.
The bilaminar embryonic disc, bilaminar blastoderm or embryonic disc is the distinct two-layered structure of cells formed in an embryo. In the development of the human embryo this takes place by day eight. It is formed when the inner cell mass, also known as the embryoblast, forms a bilaminar disc of two layers, an upper layer called the epiblast and a lower layer called the hypoblast, which will eventually form into fetus. These two layers of cells are stretched between two fluid-filled cavities at either end: the primitive yolk sac and the amniotic sac.
Human embryonic development or human embryogenesis is the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilization occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form the single cell zygote and the germinal stage of development commences. Human embryonic development covers the first eight weeks of development, which have 23 stages, called Carnegie stages. At the beginning of the ninth week, the embryo is termed a fetus. In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs.
The development of the reproductive system is the part of embryonic growth that results in the sex organs and contributes to sexual differentiation. Due to its large overlap with development of the urinary system, the two systems are typically described together as the genitourinary system.
In the development of the human embryo, the intraembryonic coelom is a portion of the conceptus forming in the mesoderm during the third week of development. During the third week of development, the lateral plate mesoderm splits into a dorsal somatic mesoderm (somatopleure) and a ventral splanchnic mesoderm (splanchnopleure). The resulting cavity between the somatopleure and splanchnopleure is called the intraembryonic coelom. This space will give rise to the thoracic and abdominal cavities. The coelomic spaces in the lateral mesoderm and cardiogenic area are isolated. The isolated coelom begins to organize into a horseshoe shape. The spaces soon join together and form a single horseshoe-shaped cavity: the intraembryonic coelom. It then separates the mesoderm into two layers.
The development of the digestive system in the human embryo concerns the epithelium of the digestive system and the parenchyma of its derivatives, which originate from the endoderm. Connective tissue, muscular components, and peritoneal components originate in the mesoderm. Different regions of the gut tube such as the esophagus, stomach, duodenum, etc. are specified by a retinoic acid gradient that causes transcription factors unique to each region to be expressed. Differentiation of the gut and its derivatives depends upon reciprocal interactions between the gut endoderm and its surrounding mesoderm. Hox genes in the mesoderm are induced by a Hedgehog signaling pathway secreted by gut endoderm and regulate the craniocaudal organization of the gut and its derivatives. The gut system extends from the oropharyngeal membrane to the cloacal membrane and is divided into the foregut, midgut, and hindgut.
In zoology, a mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms complete or incomplete partitions of the body cavity, whether that is the coelom or, as in the Anthozoa, the gastrovascular cavity.
The pleurae are the two flattened closed sacs filled with pleural fluid, each ensheathing each lung and lining their surrounding tissues, locally appearing as two opposing layers of serous membrane separating the lungs from the mediastinum, the inside surfaces of the surrounding chest walls and the diaphragm. Although wrapped onto itself resulting in an apparent double layer, each lung is surrounded by a single, continuous pleural membrane.
