Human embryonic development
Human embryonic development, or human embryogenesis, refers to the development and formation of the human embryo. It is characterised by the process 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. Fertilisation occurs when the sperm cell enters and fuses with an egg cell; the genetic material of the sperm and egg combine to form a single cell called a zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development. Human embryology is the study of this development during the first eight weeks after fertilisation; the normal period of gestation is 38 weeks. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus; the germinal stage takes around 10 days. During this stage, the zygote begins in a process called cleavage.
A blastocyst is formed and implanted in the uterus. Embryogenesis continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, the processes of neurulation and organogenesis follow. In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs; the entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation. A nearly identical process occurs in other species among chordates. Fertilization takes place when the spermatozoon has entered the ovum and the two sets of genetic material carried by the gametes fuse together, resulting in the zygote; this takes place in the ampulla of one of the fallopian tubes. The zygote contains the combined genetic material carried by both the male and female gametes which consists of the 23 chromosomes from the nucleus of the ovum and the 23 chromosomes from the nucleus of the sperm.
The 46 chromosomes undergo changes prior to the mitotic division which leads to the formation of the embryo having two cells. Successful fertilization is enabled by three processes, which act as controls to ensure species-specificity; the first is that of chemotaxis. Secondly there is an adhesive compatibility between the egg. With the sperm adhered to the ovum, the third process of acrosomal reaction takes place; the entry of the sperm causes calcium to be released. A parallel reaction takes place in the ovum called the zona reaction; this sees the release of cortical granules that release enzymes which digest sperm receptor proteins, thus preventing polyspermy. The granules fuse with the plasma membrane and modify the zona pellucida in such a way as to prevent further sperm entry; the beginning of the cleavage process is marked when the zygote divides through mitosis into two cells. This mitosis continues and the first two cells divide into four cells into eight cells and so on; each division takes from 12 to 24 hours.
The zygote is large compared to any other cell and undergoes cleavage without any overall increase in size. This means that with each successive subdivision, the ratio of nuclear to cytoplasmic material increases; the dividing cells, called blastomeres, are undifferentiated and aggregated into a sphere enclosed within the membrane of glycoproteins of the ovum. When eight blastomeres have formed they begin to develop gap junctions, enabling them to develop in an integrated way and co-ordinate their response to physiological signals and environmental cues; when the cells number around sixteen the solid sphere of cells within the zona pellucida is referred to as a morula At this stage the cells start to bind together in a process called compaction, cleavage continues as cellular differentiation. Cleavage itself is the first stage in the process of forming the blastocyst. Cells differentiate into an outer layer of an inner cell mass. With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable.
They are still enclosed within the zona pellucida. This compaction serves to make the structure watertight, containing the fluid that the cells will secrete; the inner mass of cells differentiate to polarise at one end. They form gap junctions, which facilitate cellular communication; this polarisation leaves a cavity, the blastocoel, creating a structure, now termed the blastocyst. The trophoblasts secrete fluid into the blastocoel; the resulting increase in size of the blastocyst causes it to hatch through the zona pellucida, which disintegrates. The inner cell mass will give rise to the pre-embryo, the amnion, yolk sac and allantois, while the fetal part of the placenta will form from the outer trophoblast layer; the embryo plus its membranes is called the conceptus, by this stage the conceptus has reached the uterus. The zona pellucida disappears and the now exposed cells of the trophoblast allow the blastocyst to attach itself to the endometrium, where it will implant; the formation of the hypoblast and epiblast, which ar
Around the 5th week, the intermaxillary segment arises as a result of fusion of the two medial nasal processes and the frontonasal process within the embryo. The intermaxillary segment gives rise to the primary palate; the primary palate will form the premaxillary portion of the maxilla. This small portion will contain the maxillary incisors; this article incorporates text in the public domain from page 70 of the 20th edition of Gray's Anatomy
In humans, the cartilaginous bar of the mandibular arch is formed by what are known as Meckel’s cartilages known as Meckelian cartilages. Meckel's cartilage arises from the first pharyngeal arch; the dorsal end of each cartilage is connected with the ear-capsule and is ossified to form the malleus. The intervening part of the cartilage disappears. Johann Friedrich Meckel, the Younger discovered this cartilage in 1820; the Meckelian Cartilage known as "Meckel's Cartilage", is a piece of cartilage from which the mandibles of vertebrates evolved. It was the lower of two cartilages which supported the first branchial arch in early fish, it grew longer and stronger, acquired muscles capable of closing the developing jaw. In early fish and in chondrichthyans, the Meckelian Cartilage continued to be the main component of the lower jaw, but in the adult forms of osteichthyans and their descendants, the cartilage was covered in bone – although in their embryos the jaw develops as the Meckelian Cartilage.
In all tetrapods the cartilage ossifies at the rear end of the jaw and becomes the articular bone, which forms part of the jaw joint in all tetrapods except mammals. In some extinct mammal groups like eutriconodonts, the Meckel's cartilage still connected otherwise modern ear bones to the jaw; this article incorporates text in the public domain from page 66 of the 20th edition of Gray's Anatomy synd/2049 at Who Named It
The olfactory system, or sense of smell, is the part of the sensory system used for smelling. Most mammals and reptiles have an accessory olfactory system; the main olfactory system detects airborne substances, while the accessory system senses fluid-phase stimuli. The senses of smell and taste are referred to together as the chemosensory system, because they both give the brain information about the chemical composition of objects through a process called transduction; the peripheral olfactory system consists of the nostrils, ethmoid bone, nasal cavity, the olfactory epithelium. The primary components of the layers of epithelial tissue are the mucous membranes, olfactory glands, olfactory neurons, nerve fibers of the olfactory nerves. Odor molecules can enter the peripheral pathway and reach the nasal cavity either through the nostrils when inhaling or through the throat when the tongue pushes air to the back of the nasal cavity while chewing or swallowing. Inside the nasal cavity, mucus lining the walls of the cavity dissolves odor molecules.
Mucus covers the olfactory epithelium, which contains mucous membranes that produce and store mucus and olfactory glands that secrete metabolic enzymes found in the mucus. Transduction Olfactory sensory neurons in the epithelium detect odor molecules dissolved in the mucus and transmit information about the odor to the brain in a process called sensory transduction. Olfactory neurons have cilia containing Olfactory receptors that bind to odor molecules, causing an electrical response that spreads through the Sensory neuron to the olfactory nerve fibers at the back of the nasal cavity. Olfactory nerves and fibers transmit information about odors from the peripheral olfactory system to the central olfactory system of the brain, separated from the epithelium by the cribriform plate of the ethmoid bone. Olfactory nerve fibers, which originate in the epithelium, pass through the cribriform plate, connecting the epithelium to the brain's limbic system at the olfactory bulbs; the main olfactory bulb transmits pulses to both mitral and tufted cells, which help determine odor concentration based off the time certain neuron clusters fire.
These cells note differences between similar odors and use that data to aid in recognition. The cells are different with mitral having low firing-rates and being inhibited by neighboring cells, while tufted have high rates of firing and are more difficult to inhibit; the uncus houses the olfactory cortex which includes the piriform cortex, olfactory tubercle, parahippocampal gyrus. The olfactory tubercle connects to numerous areas of the amygdala, hypothalamus, brain stem, auditory cortex, olfactory system. *In total it has 27 inputs and 20 outputs. An oversimplification of its role is to state that it: checks to ensure odor signals arose from actual odors rather than villi irritation, regulates motor behavior brought on by odors, integrates auditory and olfactory sensory info to complete the aforementioned tasks, plays a role in transmitting positive signals to reward sensors; the amygdala processes pheromone and kairomone signals. Due to cerebrum evolution this processing is secondary and therefore is unnoticed in human interactions.
Allomones include flower scents, natural herbicides, natural toxic plant chemicals. The info for these processes comes from the vomeronasal organ indirectly via the olfactory bulb; the main olfactory bulb's pulses in the amygdala are used to pair odors to names and recognize odor to odor differences. Stria terminalis bed nuclei, act as the information pathway between the amygdala and hypothalamus, as well as the hypothalamus and pituitary gland. BNST abnormalities lead to sexual confusion and immaturity. BNST connects to the septal area, rewarding sexual behavior. Mitral pulses to the hypothalamus promote/discourage feeding, whereas accessory olfactory bulb pulses regulate reproductive and odor-related-reflex processes; the hippocampus receives all of its olfactory information via the amygdala. The hippocampus forms reinforces existing memories; the parahippocampus encodes and contextualizes scenes. The parahippocampal gyrus houses the topographical map for olfaction; the orbitofrontal cortex is correlated with the cingulate gyrus and septal area to act out positive/negative reinforcement.
The OFC is the expectation of reward/punishment in response to stimuli. The OFC represents the reward in decision making; the anterior olfactory nucleus distributes reciprocal signals between the olfactory bulb and piriform cortex. The anterior olfactory nucleus is the memory hub for smell. Loss of smell is known as anosmia. Anosmia can occur on a single side. Olfactory problems can be divided into different types based on their malfunction; the olfactory dysfunction can be total, distorted, or can be characterized by spontaneous sensations like phantosmia. An inability to recognize odors despite a functioning olfactory system is termed olfactory agnosia. Hyperosmia is a rare condition. Like vision and hearing, the o
Lateral nasal prominence
By the upgrowth of the surrounding parts the olfactory areas are converted into pits, the nasal pits, which indent the frontonasal prominence and divide it into a medial nasal prominence and a lateral nasal prominence. Failure to fuse can cause a cleft lip; this article incorporates text in the public domain from page 67 of the 20th edition of Gray's Anatomy
The neck is the part of the body, on many vertebrates, that separates the head from the torso. It contains blood nerves that supply structures in the head to the body; these in humans include part of the esophagus, the larynx and thyroid gland, major blood vessels including the carotid arteries and jugular veins, the top part of the spinal cord. In anatomy, the neck is called by its Latin names, cervix or collum, although when used alone, in context, the word cervix more refers to the uterine cervix, the neck of the uterus, thus the adjective cervical may refer either to the uterine cervix. The neck contains vessels. In humans these structures include part of the esophagus, trachea and parathyroid glands, lymph nodes, the first part of the spinal cord. Major blood vessels include the jugular veins. Cervical lymph nodes surround the blood vessels; the thyroid gland and parathyroid glands are endocrine glands involved in the regulation of cellular metabolism and growth, blood calcium levels. The shape of the neck in humans is formed from the upper part of the vertebral column at the back, a series of cartilage that surrounds the upper part of the respiratory tract.
Around these sit soft tissues, including muscles, between and around these sit the other structures mentioned above. Muscles of the neck attach to the base of the skull, the hyoid bone, the clavicles, the sternum; the large platysma, sternocleidomastoid muscles contribute to the shape at the front, the trapezius and lattissimus dorsi at the back. A number of other muscles attach to and stem from the hyoid bone, facilitating speech and playing a role in swallowing. Sensation to the front areas of the neck comes from the roots of nerves C2-4, at the back of the neck from the roots of C4-5; the cervical region of the human spine is made up of seven cervical vertebrae referred to as C-1 to C-7, with cartilaginous discs between each vertebral body. The spinal cord sits within the cervical part of the vertebral column; the spinal column carries nerves that carry sensory and motor information from the brain down to the rest of the body. From top to bottom the cervical spine is curved in convex-forward fashion.
In addition to nerves coming from and within the human spine, the accessory nerve and vagus nerve both cranial nerves, travel down the neck. In the middle line below the chin can be felt the body of the hyoid bone, just below, the prominence of the thyroid cartilage called "Adam's apple", better marked in men than in women. Neck lines appear at a age as a development of skin wrinkles. Still, lower the cricoid cartilage is felt, while between this and the suprasternal notch, the trachea and the isthmus of the thyroid gland may be made out. At the side, the outline of the sternomastoid muscle is the most striking mark; the upper part of the former contains the submaxillary gland known as the submandibular glands, which lies just below the posterior half of the body of the jaw. The line of the common and the external carotid arteries may be marked by joining the sterno-clavicular articulation to the angle of the jaw; the eleventh or spinal accessory nerve corresponds to a line drawn from a point midway between the angle of the jaw and the mastoid process to the middle of the posterior border of the sterno-mastoid muscle and thence across the posterior triangle to the deep surface of the trapezius.
The external jugular vein can be seen through the skin. The anterior jugular vein is smaller, runs down about half an inch from the middle line of the neck; the clavicle or collar-bone forms the lower limit of the neck, laterally the outward slope of the neck to the shoulder is caused by the trapezius muscle. The neck supports the weight of the head and protects the nerves that carry sensory and motor information from the brain down to the rest of the body. In addition, the neck is flexible and allows the head to turn and flex in all directions. Disorders of the neck are a common source of pain; the neck has a great deal of functionality but is subject to a lot of stress. Common sources of neck pain include: Whiplash, strained a muscle or another soft tissue injury Cervical herniated disc Cervical spinal stenosis Osteoarthritis Vascular sources of pain, like arterial dissections or internal jugular vein thrombosis Cervical adenitis The neck appears in some of the earliest of tetrapod fossils, the functionality provided has led to its being retained in all land vertebrates as well as marine-adapted tetrapods such as turtles and penguins.
Some degree of flexibility is retained where the outside physical manifestation has been secondarily lost, as in whales and porpoises. A morphologically functioning neck appears among insects, its absence in fish and aquatic arthropods is notable, as many have life stations similar to a terrestrial or tetrapod counterpart, or could otherwise make use of the added flexibility. The word "neck" is sometimes used as a convenience to refer to the region behind the head in some snails, gastropod mollusks though there is no clear distinction between this area, the head area, the rest of the body. Throat Adam's apple Hickey Nape American Head and Neck Society The Anatomy Wiz. An Interactive Cross-Sectional Anatomy Atlas
The copula linguae or copula, is a swelling that forms from the second pharyngeal arch, late in the fourth week of embryogenesis. During the fifth and sixth weeks the copula becomes overgrown and covered by the hypopharyngeal eminence which forms from the third pharyngeal arch and in part from the fourth pharyngeal arch; this article incorporates text in the public domain from page 1103 of the 20th edition of Gray's Anatomy hednk-024—Embryo Images at University of North Carolina