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

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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