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Showing posts with label Reproductive. Show all posts
Showing posts with label Reproductive. Show all posts

Wednesday, May 5, 2021

Circulatory System: Blood Vessels

Circulatory System: Blood Vessels


Circulatory System: Blood Vessels
Time period: day 18 to birth
Vasculogenesis
Vasculogenesis is the formation of new blood vessels from cells that were not blood vessels before. As if by magic, blood cells and vessels appear in the early embryo. In fact, mesodermal cells are induced to differentiate into haemangioblasts, which further differentiate into both haematopoietic stem cells and angioblasts.
Haematopoietic stem cells will form all the blood cell types, and angioblasts will build the blood vessels. Separate sites of vasculogenesis may merge to form a network of blood vessels, or new vessels may grow from existing vessels by angiogenesis. When the liver forms it will be the primary source of new haematopoietic stem cells during development.

Angiogenesis
Angiogenesis is the development of new blood vessels from existing vessels. Endothelial cells detach and proliferate to form new capillaries. This process is under the influence of various chemical and mechanical factors. Although important in growth this also occurs in wound healing and tumour growth, and as such angiogenesis has become a target for anti‐cancer drugs.
Circulatory System: Blood Vessels, Angiogenesis, Primitive circulation, Aortic arches, Ductus arteriosus, Coronary arteries

Primitive circulation
Near the end of the third week blood islands form through vasculogenesis on either side of the cardiogenic field and the notochord (see Chapter 27). They merge, creating two lateral vessels called the dorsal aortae (Figure 29.1). These blood vessels receive blood from three pairs of veins, including the vitelline veins of the yolk sac (a site of blood vessel formation external to the embryo), the cardinal veins and the umbilical veins (Figure 29.1).
Circulation System: Changes At Birth

Circulation System: Changes At Birth


Circulation System: Changes At Birth
Time period: birth (38 weeks)
Foetal blood circulation
Dramatic and clinically significant changes occur to the circulatory and respiratory systems at birth. Here, we look at changes primarily of the circulatory system and how these changes prepare the baby for life outside the uterus.
If we were to follow the flow of oxygenated blood in the foetus from the placenta (Figure 31.1), we would start in the umbilical vein and track the blood moving towards the liver. Here, half the blood enters the liver itself and half is redirected by the ductus venosus directly into the inferior vena cava, bypassing the liver.
The blood remains well oxygenated and continues to the right atrium, from which it may pass into the right ventricle in the expected manner or directly into the left atrium via the foramen ovale (Figure 31.2). Blood within the left atrium passes to the left ventricle and then into the aorta.
Blood entering the right atrium from the superior vena cava and the coronary sinus is relatively poorly oxygenated. The small amount of blood that returns from the lungs to the left atrium is also poorly oxygenated. Mixing of this blood with the well‐oxygenated blood from the ductus venosus reduces the oxygen saturation somewhat.
Blood within the right ventricle will leave the heart within the pulmonary artery, but most of that blood will pass through the ductus arteriosus and into the descending aorta. Almost all of the well‐oxygenated blood that entered the right side of the heart has avoided entering the pulmonary circulation of the lungs, and has instead passed to the developing brain and other parts of the body (Figure 31.3).
Circulation System: Changes At Birth, Foetal blood circulation, Ductus venosus, Ductus arteriosus, Foramen ovale,

Ductus venosus
The umbilical arteries constrict after birth, preventing blood loss from the neonate. The umbilical cord is not cut and clipped immediately after birth, however, allowing blood to pass from the placenta back to the neonatal circulation through the umbilical vein.
Respiratory System

Respiratory System


Respiratory System
Time period: day 28 to childhood
Introduction
The development of the respiratory system is continuous from the fourth week, when the respiratory diverticulum appears, to term. The 24‐week potential viability of a foetus (approximately 50% chance of survival) is partly because at this stage the lungs have developed enough to oxygenate the blood. Limiters to oxygenation include the surface area available to gaseous exchange, the vascularisation of those tissues of gaseous exchange and the action of surfactant in reducing the surface tension of fluids within the lungs.
Development of the respiratory system includes not only the lungs, but also the conducting pathways, including the trachea, bronchi and bronchioles. Lung development can be described in five stages: embryonic, pseudoglandular, canalicular, saccular and alveolar.
Although not in use as gas exchange organs in utero, the lungs have a role in the production of some amniotic fluid.
Respiratory System, Lung bud, Respiratory tree, Alveoli,

Lung bud
The development of the respiratory system begins with the growth of an endodermal bud from the ventral wall of the developing gut tube in the fourth week (Figure 32.1).
Digestive System: Gastrointestinal Tract

Digestive System: Gastrointestinal Tract


Digestive System: Gastrointestinal Tract
Time period: days 21–50
Induction of the tube
The gut tube forms when the yolk sac is pulled into the embryo and pinched off (see Figure 20.2) as the flat germ layers of the early embryo fold laterally and cephalocaudally (head to tail). Consequently, it has an endodermal lining throughout with a minor exception towards the caudal end. Epithelium forms from the endoderm layer and other structures are derived from the mesoderm.
Initially, the tube is closed at both ends, although the middle remains in contact with the yolk sac through the vitelline duct (or stalk) even as the yolk sac shrinks (Figure 33.1).
The cranial end will become the mouth and is sealed by the buccopharyngeal membrane, which will break in the fourth week, opening the gut tube to the amniotic cavity. The caudal end will become the anus and is sealed by the cloacal membrane, which will break during the seventh week.
Buds develop along the length of the tube that will form a variety of gastrointestinal and respiratory structures (see Chapter 34).
Digestive System: Gastrointestinal Tract, Mesenteries, Story of the hindgut and the cloaca, Twists of the midgut

Divisions of the gut tube
The gut is divided into foregut, midgut and hindgut sections by the region of the gut tube that remains linked to the yolk sac and by the anterior branches from the aorta that supply blood to each part (Figure 33.2).

Saturday, April 24, 2021

Associated Organs

Associated Organs


Associated Organs
Time period: day 21 to birth
Introduction
In Chapter 33 we looked at the development of the gastrointestinal tract as a tube and mentioned a number of buds that sprout from the tube and its associated mesenchyme. These develop into a number of organs (Figure 34.1).

Digestive System: Associated Organs

Lung bud
As the oesophagus develops and elongates during week 4 the respiratory diverticulum buds off from its ventral wall (Figure 34.1). To create two separate tubes a septum forms between the respiratory bud and the oesophagus called the tracheoesophageal septum (see Figure 32.1). This creates the oesophagus dorsally and the respiratory primordium ventrally (see Chapter 32).
Congenital Anomalies

Congenital Anomalies


Congenital Anomalies
Time period: birth
Facial abnormalities
A relatively common congenital abnormality is cleft lip and/or cleft palate which affects around 1 in 600–700 live births and has a collection of defects.
Cleft lip (cheiloschisis) can be incomplete (affects upper lip only) or complete (continues into the nose) and unilateral (Figure 35.1) or bilateral. It is caused by the incomplete fusion of the medial nasal prominence with the maxillary process (Figure 35.2). When these fuse normally they form the intermaxillary segment, which goes on to become the primary (soft) palate.
The secondary (hard) palate forms from outgrowths of the maxillary process called the palatine shelves. Failure of these shelves to fuse or ascend to a horizontal position causes cleft palate (palatoschisis). In very severe cases the cleft can continue into the upper jaw. Cleft palate is often accompanied by cleft lip (complete), but not always (incomplete; Figure 35.3), and can also be unilateral or bilateral.
A cleft lip is generally diagnosed at the 20‐week anomaly scan, whereas cleft palates are diagnosed after birth. Cleft lips require surgical intervention before 3 months, whereas cleft palate surgery should happen before the child reaches 12 months old. Cleft lip and palate can affect feeding and speech, but also hearing. To aid prevention of cleft lip and palate maternal dietary folic acid is recommended (see also spina bifida, Chapter 17).
Digestive System: Congenital Anomalies

Foregut abnormalities
Abnormalities in development of the foregut can include stenosis and atresia at various points along its length, and hypertrophy of the pylorus of the stomach. Depending upon the point of restriction projectile vomiting can be a symptom, and the presence or absence of bile in the vomit can help diagnose the location.
Urinary System

Urinary System


Urinary System
Time period: day 21 to birth
Introduction
The development of the urinary system is closely linked with that of the reproductive system. They both develop from the intermediate mesoderm, which extends on either side of the aorta and forms a condensation of cells in the abdomen called the urogenital ridge. The ridge has two parts: the nephrogenic cord and the gonadal ridge (see Figure 38.2).

Urinary System

Kidneys
Three structures involved in kidney development grow from intermediate mesoderm in an anterior to posterior sequence, termed the pronephros, mesonephros and metanephros.
The pronephros appears in the third week in the neck region of the embryo and disappears a week later. In humans this is a primitive, non‐functional kidney that c ial nephrons joined to an unbranched nephric duct.

Tuesday, April 20, 2021

Gonads

Gonads


Gonads
Time period: day 30 to postnatal development
Introduction
In the chapter on renal development (see Chapter 36) we talked about the development of the gonadal ridge from intermediate mesoderm, an important source of cells for the reproductive system and the location for the beginning of the development of the gonads.

Reproductive System: Gonads

Gonads
Gonads are formed from three sources of cells: the intermediate mesoderm, the mesodermal epithelium that lines the developing urogenital ridge and germ cells.

Thursday, April 15, 2021

Endocrine System

Endocrine System


Endocrine System
Time period: day 24 to birth
Introduction
The glands of the endocrine system begin to form during the embryonic period and continue to mature during the foetal period. Functional development can be detected by the presence of the various hormones in the foetal blood, generally in the second trimester of pregnancy.
The development of the gonads, pancreas, kidneys and placenta are covered elsewhere in this book.
Endocrine System, Pituitary gland, Hypothalamus, Pineal body, Adrenal glands, Thyroid gland, Parathyroid glands,

Pituitary gland
Also known as the hypophysis, the pituitary gland develops from two sources. An out pocketing of oral ectoderm appears in week 3 in front of the buccopharyngeal membrane (Figure 39.1). This forms the hypophysial diverticulum (or Rathke’s pouch), which will become the anterior lobe.
Head And Neck: Arch I

Head And Neck: Arch I


Head And Neck: Arch I
Time period: day 21 onwards
Introduction
Pharyngeal (or branchial) arches are paired structures that develop in the ventrolateral parts of the head of the embryo (Figures 40.1 and 40.2). Six arches will form and contribute to the development of head and neck structures, although arch V is ignored as it fails to appear in human embryos. In this chapter we concentrate on arch I and its derivatives.
Each pharyngeal arch is a bud, or bar of mesenchymal tissue, with clefts separating the arches externally, and pouches separating them internally (Figure 40.3). Pharyngeal pouches develop internally as blebs of the foregut at the level of the pharynx.
Each pharyngeal arch consists of mesenchyme from paraxial and lateral plate mesoderm and receives an influx of neural crest cells. Neural crest cells from rhombomeres 1 and 2 (see Chapter 45 and Figure 45.4) migrate into the first pharyngeal arch. Hox genes, important in the organisation of the segmentation of vertebrates and in setting up the anteroposterior axis, are also important in neural crest cell migration here.
Each arch has its own nerve, artery, connective tissue cells and muscle cells (Figure 40.4).
Head And Neck: Arch I, Ligaments , Muscles, Nerve

Arch I
In week 4 a depression in the surface ectoderm of the embryo forms in the future face, the stomodeum (Figure 40.2). It is continuous with the gut tube and will become the mouth. It forms the centre of the face early in development, and surrounding it are the first pair of pharyngeal arches.
Head And Neck: Arch II

Head And Neck: Arch II


Head And Neck: Arch II
Time period: day 21 onwards
Introduction
The second arch forms caudally to the first arch (Figure 41.1). Pharyngeal arches I and II are bigger than III and IV. Arch II grows rapidly and inferiorly to cover the smaller arches forming the s growth forms a ‘lid’ over the other arches and creates the smooth covering of the neck.
Head And Neck: Arch II, Pouch II

Arch II
Highlighting the overlap between arches I and II at the ear, the stapes bone is formed from the connective tissue element of the second arch, whereas the malleus and incus bones develop from the first arch. Likewise, the tensor tympani muscle of the ear forms from the first arch but the stapedius muscle is derived from the second arch.

Tuesday, April 13, 2021

Central Nervous System

Central Nervous System


Central Nervous System
Time period: day 22 to postnatal development
Introduction
Ectoderm is induced by the notochord to form neuroectoderm during neurulation (see Chapter 17). This neuroectoderm in turn produces the neural tube and neural crest cells from which the central nervous system develops. The central nervous system comprises the brain and spinal cord.

Central Nervous System

Spinal cord
The caudal end of the neural tube continues to elongate and form the spinal cord. A lumen through the centre of the spinal cord, the neurocoel (or neural canal), forms by week 9 and will become the central canal. The neurocoel is lined with thickening layers of neuroepithelia known as the ventricular zone (Figure 44.1) or ependymal layer.
Peripheral Nervous System

Peripheral Nervous System


Peripheral Nervous System
Time period: day 27 to birth
Introduction
The peripheral nervous system develops in tandem with the brain and spinal cord. It connects the central nervous system to structures of the body as they form and includes the spinal nerves, cranial nerves and autonomic nervous system.
This process begins with neurulation (see Chapter 17), when ectoderm is induced by the notochord to form neuroectoderm. This neuroectoderm in turn produces neuroblasts (primitive neurons) and neural crest cells.
Spinal nerves

Spinal nerves
Neural crest cells migrate out from the neural tube, passing towards multiple targets throughout the embryo (see Chapter 18). Some neural crest cells only migrate a little way from the developing spinal cord, collect together and differentiate to form neurons of the dorsal root ganglia (Figure 45.1). Located bilaterally to the spinal cord, the dorsal root ganglia send afferent processes back towards the alar plate of the spinal cord (see Figure 44.1), eventually passing to the dorsal horn. The dorsal root ganglia also send processes out to run alongside processes of neurons of the ventral root. Their combined bundle of neuronal axons become the spinal nerve.
The Ear

The Ear


The Ear
Time period: 22 day to birth
Internal ear
The function of the internal ear is to receive sound waves and interpret them into nerve signals, and to identify changes in balance.

The Ear, Internal ear, Membranous labyrinth

Membranous labyrinth
At about 22 days, a thickening of ectoderm on either side of the hindbrain develops; this is the otic placode (Figure 46.1). The placode invaginates forming a pit that later becomes separated from the ectoderm, forming the otic vesicle (or otocyst) deep to the ectoderm. The otic vesicle is surrounded by mesoderm that will become the otic capsule, the cartilaginous precursor of the bony labyrinth.
The Eye

The Eye


The Eye
Time period: weeks 3–10
Introduction
The development of the eye begins around day 22 with bilateral invaginations of the neuroectoderm of the forebrain (Figure 47.1).
Optic cup and lens
As the neural tube closes these invaginations become the optic vesicles and remain continuous with the developing third ventricle (Figure 47.1). Contact of these optic vesicles with the surface ectoderm induces the formation of the lens placodes (Figures 47.1 and 47.2).
As the optic vesicle invaginates it forms a double‐walled structure, the optic cup (Figure 47.2). At the same time the lens placode invaginates and forms the lens vesicle which lies in the indent of the optic cup and is completely dissociated from the surface ectoderm. Epithelial cells on the posterior wall of the lens vesicle lengthen anteriorly and become long fibres that grow forwards. It takes about 2 weeks for these fibres to reach the anterior cell wall of the vesicle. These are primary lens fibres (Figure 47.3). Secondary lens fibres form from epithelial cells located at the equator of the lens and are continuously added throughout life along the scaffold made by the primary fibres from the centre of the lens. These cells elongate and eventually lose their nuclei to become mature lens fibres. This occurs in early adulthood.
The Eye, Optic cup and lens, Retina, Optic nerve, Meninges, Cornea, Extraocular muscles,

Retina
In the optic cup there is an outer layer that develops into the pigmented layer of the retina and an inner layer that becomes the neural layer.
Antenatal Screening

Antenatal Screening


Antenatal Screening
Introduction
Modern antenatal care is based on the assessment of risk and identification of the most appropriate care pathway for a pregnant woman. Obstetric ultrasound is a routine tool in antenatal screening for detecting foetal anomalies. Low risk women are offered ultrasound screening in the first and second trimesters, and as these are anomaly scans appropriate care and counselling should be immediately available.
Primary care practitioners will refer a pregnant woman to ante- natal care and aim for a first appointment with ultrasound scanning at approximately 10 weeks into the pregnancy from the date of the last menstrual period (8 weeks after fertilisation). A full obstetric history should be taken and it is good practice to take full gynaecological and medical histories. At the booking appointment the midwife will initiate a close relationship with the mother, and for primigravida women this is the opportunity to discuss the effects of early pregnancy and non‐specific symptoms. Tiredness, nausea and vomiting may be worrying, but hyperemesis gravidarum (excessive nausea and vomiting) should be identified and treated. The mother will be weighed at this meeting, but normally weight is not monitored throughout pregnancy.
Antenatal Screening

The first scan
The first‐trimester ultrasound scan, sometimes referred to as the ‘dating scan’, is performed at a minimum of 10 weeks’ gestation and usually before 14 weeks. Scanning at this stage will confirm foetal viability, give gestational dating information, identify multiple pregnancy, define chorionicity (see Chapter 11) and look for indicators of anomalous development (Figure 48.1). These indicators include nuchal translucency, abdominal wall defects (see Chapter 35) and brain anomalies (see Chapter 44). Nuchal translucency screening is not reliable in smaller foetuses and other anomalies may also be missed. Nuchal translucency measures the thickness of fluid between the cervical spine and skin, and is associated with a number of chromosomal anomalies such as Down syndrome, Turner syndrome, trisomy 18 and trisomy 13, and with cardiac anomalies (Figure 48.2). Skeletal dysplasias may be detectable by ultrasound in the first trimester, and in the near future cardiac defects may be screened for as the resolution of ultrasound scanning improves.

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