pediagenosis: Reproductive
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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.

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