Summary of thyroid gland Gross anatomy histology embryology and clinical anatomy

 

 Summary of thyroid gland Gross anatomy histology embryology and clinical anatomy 

Thyroid gland, the largest endocrine gland of our body which is located in front of trachea at the lower part of neck.

 It lobes are extends from the oblique line of thyroid cartilage to the 5th-6th tracheal cartilage ring , isthmus extends from 2^nd to 4^th tracheal ring

Vertebral level : it lies opposite to the C5, C6, C7 and T1 vertebrae

Parts of thyroid gland

Thyroid gland is consists of two  lobes connected by isthmus.

A small pyramidal lobe may present occasionally which  extend upwards from the upper border of the isthmus, slightly to the left of the midline.

Sometimes this  pyramidal lobe is connected to the hyoid bone by a strong fibromuscular band called levator glandulae thyroideae (remnant of thyroglossal duct).

Dimensions of thyroid gland: Thyroid gland weighs approximately 25 gms. Each lobe of thyroid gland is 5 cm long, 3 cm wide and 3cm thick.

Capsule of thyroid gland

Thyroid gland is enclosed by two capsules

The inner capsule is also known as true capsule which is formed  by condensation of fibrous stroma of the gland.

An outer capsule is known as false capsule which is formed by the pre-tracheal layer of the deep cervical fascia. It is thin along the posterior border of the lobes and thick on the medial surface of the lobes.

The pre-tracheal fascia is attached above to the body of the hyoid bone  and oblique line of the thyroid cartilage and  after enclosing the thyroid gland it descends down in front of the trachea and blends with fibrous pericardium.

On the medial surface of thyroid lobe, this capsule thickens to form the suspensory ligament of Berry, which attaches the thyroid lobes to the cricoid cartilage.

Gross features and relation of thyroid gland

Each lobe of thyroid gland has following relations:

Apex/Upper lobe: it  is sandwiched between the sternothyroid  and inferior constrictor muscle of pharynx. Superior thyroid artery, branch of external carotid artery and external laryngeal nerve diverge from each other close to upper pole. The artery is superficial and nerve lies deep to the upper pole.

Base/Lower pole: it is related to inferior thyroid artery and recurrent laryngeal nerve.

Lateral (superficial) surface is related to the following muscles:

Sternohyoid, sternothyroid, superior belly of omohyoid and anterior border of sternocleidomastoid.

Medial surface is related to:

Two tubes– trachea and oesophagus

Two muscles – inferior constrictor and cricothyroid

Two cartilages – cricoid and thyroid

Two nerves – external laryngeal and recurrent laryngeal

Posterolateral surface is related to:

Carotid sheath and its contents (common carotid artery, internal jugular vein and vagus nerve). Ansa cervicalis is embedded in the anterior aspect of carotid sheath.

Anterior border is thin and is related to anterior branch of superior thyroid artery.

Posterior border is thick and rounded and is related to parathyroid glands and anastomosis between superior and inferior thyroid arteries.

Isthmus of thyroid gland has following relations:

Anterior surface  is related to sternothyroid and sternohyoid muscles and anterior jugular veins.

Posterior surface is related to 2nd -4th tracheal rings.

Superior border  is related to anastomosis between anterior branches of superior thyroid arteries.

Inferior border is related to inferior thyroid veins  that leave isthmus along this border.

Blood supply of thyroid gland

Thyroid gland is highly vascular and is supplied by the following arteries.

Superior thyroid artery:

It is a branch of external carotid artery, which descends down to the upper pole of lateral lobe of thyroid gland with external laryngeal nerve (the nerve diverges from the artery near the upper pole).

At the apex of lateral lobe, it divides into anterior and posterior branches.

Anterior branch descends along anterior border and anastomoses with the its fellow of opposite side along the superior border of isthmus.

Posterior branch descends along posterior border and anastomoses with the ascending branch of inferior thyroid artery.

It supplies upper 2/3rd of the lobe and upper ½ of the isthmus.

Inferior thyroid artery:

It is a branch of thyrocervical trunk.

It first runs upwards, then passes behind the carotid sheath and descends to reach the base of thyroid gland (forms a loop with convexity upwards).

Near the base (lower pole) the artery is very close to recurrent laryngeal nerve.

It supplies the lower 1/3rd of the lobe and lower half of the isthmus.

Thyroideaima artery: It is not always present. It is a branch of brachiocephalic trunk or arch of aorta. Ascends in front of trachea to enter the isthmus.

Venous drainage of thyroid gland

It occurs mainly via 3 pairs of veins. All thyroid veins do not accompany arteries. The three pairs of veins arise from the venous plexus present deep to the true capsule of thyroid gland.

Superior thyroid veins emerge at the upper pole of thyroid gland and terminates in internal jugular vein.

Middle thyroid veins are short veins, emerges at the middle of the lobe of thyroid gland and drains in internal jugular vein.

Inferior thyroid veins emerge at the lower border of isthmus, descend in front of trachea (right and left vein communicate and form plexus in front of trachea) to drain into left brachiocephalic vein.

Vein of Kocher: Sometimes a fourth vein (Vein of Kocher) emerges between the middle and inferior thyroid veins which drains into internal jugular vein.

Lymphatic drainage of thyroid gland

Lymphatics from the upper part of thyroid gland drain into pre-laryngeal and upper deep cervical lymph nodes. From the lower part of thyroid gland, the lymphatics drain into pretracheal, paratracheal and lower deep cervical lymph nodes.

Development of thyroid gland

It  is the first of the body's endocrine glands to develop, on approximately the 24th day of intra uterine life . it is originates from two source : the primitive pharynx and the neural crest.

Stroma and follicular cells are developed from primitive pharynx and parafollicular cell develops from neural crest cells.

 

The thyroid gland forms as a proliferation of endodermal epithelial cells on the median surface of the developing pharyngeal floor. The site of this development lies between 2 structures which developed tongue, the tuberculum impar and the copula, and it is known as the foramen cecum.

This embryonic swelling develops from the first pharyngeal arch and occurs midline on the floor of the developing pharynx, eventually helping form the tongue as the two lateral lingual swellings overgrow it.

 

The foramen cecum begins rostral to the copula, also known as the hypobranchial eminence. This median embryologic swelling consists of mesoderm that arises from the second pharyngeal pouch (although the third and fourth pouches are also involved). The thyroid gland, therefore, originates from between the first and second pouches.

 

The initial thyroid precursor, the thyroid primordium, starts as a simple midline thickening and develops to form the thyroid diverticulum. This structure is initially hollow, although it later solidifies and becomes bilobed. The stem usually has a lumen, the thyroglossal duct, that does not descend into the lateral lobes. The two  lobes are located on either side of the midline and are connected via an isthmus

Histology of Thyroid gland

1.      Thyroid follicle is lined by simple cuboidal epithelium

2.     Thyroid follicles are filled with colloid.

3.     Thyroid follicle is formed by  Follicular cells and parafollicular cells

 

Clinical anatomy of thyroid gland

Why during deglutition or swallowing, thyroid swellings rise and fall.

 Once the thyroid gland is encased, the pretracheal fascia connects to the hyoid bone's body and the thyroid cartilage's oblique lines. It thickens on the thyroid lobe's medial surface to produce the Berry suspensory ligament, which joins the thyroid gland's lobe to the cricoid cartilage. Consequently, during deglutition, when the larynx and pharynx are lifted due to the contraction of the pharynx's longitudinal muscles linked to the posterior border of the thyroid cartilage, the thyroid gland also moves up and down. This aids in separating the enlargement of the thyroid from other neck swellings.

Why thyroid gland is removed along with its true capsule?

The venous plexus is situated deep into the true capsule in the case of the thyroid gland. Thus, during a thyroidectomy, the thyroid gland and true capsule are removed in order to prevent bleeding. However, in the case of the prostate gland, the venous plexus is between the genuine and false capsules; as a result, when the prostate gland is surgically removed (prostatectomy), both capsules are left behind.

Why superior thyroid artery is ligated close to thyroid gland?

The superior thyroid artery is located in close proximity to the proximal portion of the external laryngeal nerve, and it diverges close to the lateral lobe apex (the nerve is deep to the apex, while the artery is superficial). To prevent damage to the external laryngeal nerve, the superior thyroid artery during a thyroidectomy should be clamped near the apex.

Why Inferior thyroid artery is ligated away from the thyroid gland.

Near the base or lower pole of the thyroid gland, the inferior thyroid artery is located extremely close to the recurrent laryngeal nerve. To prevent damage to the recurrent laryngeal nerve, the thyroidectomy site should be ligated away from the thyroid gland's inferior pole.

Goitre

"Goitre" is the term used to describe thyroid gland hypertrophy. Iodine shortage in the diet (lower levels of T3 and T4) causes endemic goiter. These goiters exhibit hypothyroidism symptoms and indicators.Excess T3 and T4 secretion causes toxic goiters, also known as thyrotoxicosis, which manifests clinically as elevated BMR, tremors, and tachycardia. Thyroid enlargement often grows either downward or backward. Individuals with big goitres exhibit the typical signs and symptoms of

Dysphonia, or voice hoarseness

Dysnoea, or trouble breathing

Dysphagia, or trouble swallowing

Retinopathy and Lymphoedema

Thyroid hypofunction in adults is referred to as myxedema, and in newborns and children as cretinism.

Partial Thyroidectomy

In partial thyroidectomy the posterior part of thyroid lobes are left behind to avoid removal of parathyroid glands which otherwise may result in tetany.

Epidermis of skin : microscopic and clinical anatomy

 For learning anatomy, please visit :

My youtube channel :  @easyhumanatomy73

My website : http://easyhumananatomy.com

My facebook page: https://www.facebook.com/easyhumanatomy/

My blog: http://www.easyhumanatomy73.blogspot.com

My blog:  Difference between : http://www.microscopicanatomybd.blogspot.com

Epidermis of skin : microscopic and clinical anatomy 

Skin : It has two  layers  

1.      Epidermis

2.     Dermis

Sometimes, hypodermis consider as third layer of skin contain subcutaneous fat.

Epidermis

It  is the superficial layer of skin which is formed by  keratinized stratified squamous epithelium.

Cells of epidermis:  It is composed of four cell types:

1.      Keratinocytes,

2.     Melanocytes,

3.     Langerhans cells, and

4.     Merkel cells.

The keratinocytes are arranged in five layers, and the remaining three cell types are interspersed between them .

 Five layers of the epidermis: deep to superficial

1. Stratum Basale :

A single layer of cuboidal to columnar cells that stand on the basement membrane. This is a region of cell division.

It also contains melanocytes and Merkel cells.

2. Stratum Spinosum: it is composed by many layers of polyhedral prickle cells bearing intercellular bridges. Mitotic activity is also present.

It also contains Langerhans cells and processes of melanocytes.

3. Stratum Granulosum : it is composed by flattened cells which contain keratohyalin granules.

It is absent as a distinct layer in thin skin.

4. Stratum Lucidum A thin, translucent layer that is also absent in thin skin.

5. Stratum Corneum it is composed of squamous cells packed with keratin. Superfi cial squamous cells  are desquamated.

 

        Location of skin cells and functions

Difference between location, functions & development of different skin cells

Cells of skin	Location	Functions	Development
Keratinocytes	New skin cells develop at the bottom layer of your epidermis (stratum basale) and travel up through the other layers as they get older.	It forms barrier against environmental damage by heat, UV radiation, dehydration, pathogenic bacteria, fungi, parasites, and viruses.	Surface ectoderm
Melanocytes	Stratum basale	Melanocytes are well known for their role in skin pigmentation, and their ability to produce and distribute melanin has been studied extensively	Neural crest
Langerhans cells	Stratum spinosum	These cells act as the outermost guard of the cutaneous immune system and are likely to induce the first reactions against pathogens encountered via the skin	Fetal Langerhans cells from Primitive yolk sac in fetal life , in adult Langerhans cells  from fetal monocytes
Merkel cells	Stratum basale	It is also known as tactile epithelial cells, which is an  oval-shaped mechanoreceptors essential for light touch sensation	Surface ectoderm

Keratinocytes, reach the outermost layer of your epidermis after about a month, where the skin cells shed from your body as new cells develop at the bottom layer.

Albinism

 It is a congenital disorder characterized by the complete or partial absence of melanin pigmentation in the skin, hair,  iris of eye, and eyelashes.

 It is an autosomal recessive/X -linked disorder. 

 Although the melanocytes are present, but genetic mutation results in lack of tyrosinase enzyme, which catalyzes the production of melanin from tyrosine.

People with albinism have skin that is very sensitive to light and sun. Sunburn is one of the most serious complications of albinism.

This allows light to shine through the irises and makes the eyes extremely sensitive to bright light.

Vitiligo

 It is s a clinical condition with presence of spots without melanin in the skin, which occurs due to partial or complete absence of melanocytes.

Carotid triangle of neck

For learning anatomy, please visit :

My youtube channel :  @easyhumanatomy73

My website : http://easyhumananatomy.com

My facebook page: https://www.facebook.com/easyhumanatomy/

My blog: http://www.easyhumanatomy73.blogspot.com

My blog:  Difference between : http://www.microscopicanatomybd.blogspot.com 

Carotid triangle

Anterior triangle of neck contains four triangles; Carotid triangle is one of the subdivisions of it. It name occurs according to the content of it. It contains all the 3 carotid arteries, viz. common carotid, internal carotid and external carotid.

Boundary:

Posteriorly: Anterior border of sternocleidomastoid (SCM) muscle

Superiorly: Posterior belly of digastric and stylohyoid muscle

Antero-inferiorly: Superior belly of omo-hyoid muscle

Roof is covered  by the investing layer of deep cervical fascia.

Contents of carotid triangle

Arteries: 

1.    Common carotid artery

2.    Internal carotid artery

3.    External carotid arteries: origin of five branches of external carotid artery 

·         Superior thyroid artery

·         Ascending pharyngeal artery

·         Occipital, lingual and

·         Facial arteries.

Veins: 

·         Internal jugular vein and its tributaries : common facial, lingual, pharyngeal, superior thyroid, middle thyroid veins. 

Nerves:  Last three cranial nerves

1.     Vagus nerve

2.    Hypoglossal nerve

3.    spinal accessory,

4.     ansa cervicalis and

5.    cervical part of the sympathetic chain.

·         Lymph nodes: Deep cervical lymph nodes.

·         Others: Carotid sinus and carotid body.

Development of testis

For learning anatomy, please visit :

My youtube channel :  @easyhumanatomy73

My website : http://easyhumananatomy.com

My facebook page: https://www.facebook.com/easyhumanatomy/

My blog: http://www.easyhumanatomy73.blogspot.com

My blog:  Difference between : http://www.microscopicanatomybd.blogspot.com

 Development of testis

1.          Intermediate mesoderm forms the urogenital ridge, a longitudinal elevation at the dorsal body wall of the fetus.
The coelomic epithelium and underlying mesoderm of the urogenital ridge protifelate to form the gonadal ridge.
The coelomic epithelium forms irregular primary sex cords and incorporative primordial germ cells, which migrate from the wall of the yolk sac.
The primary sex
The development of the testis ord extends deep into the medulla to form medullary cords, which lose their connection to the surface epithelium as the thick tunica albuginea forms in between the medullary cord and coelomic epithelium.

2.       Near the hilum medullary cord break up into network of tiny cell stands that form rete testis

3.       By the 4^th month of interuterine life medullary cord is continuous with rete testis

4.       The testis or medullary cord contain and primitive germ cells and sustentacular cell of  sertoli cell which derived from the surface epithelium of genital ridge.

5.       Sertoli cell secrete mullarian inhibiting factor (MIF) which degenerate mullarian duct

6.       Interstitial cell of leydig derive from mesenchyme of genital ridge which derived from intermediate mesoderm ,By the 8^th week of gestation leydig cell begin to produce testosterone which influence development of genital duct and external genitalia

7.       The testis or medullary cord of testis remain solid cord until puberty , then it become canalize and known seminiferous tubule, the canal of seminiferous tubules are continuous with rete testis which inturn join with efferent ductile. The efferent ductules and other excretory part of male genital system derived from mesonephric or wolffian duct

Development of ovary

 Development of ovary 

For learning anatomy, please visit :

My youtube channel :  @easyhumanatomy73

My website : http://easyhumananatomy.com

My facebook page: https://www.facebook.com/easyhumanatomy/

My blog: http://www.easyhumanatomy73.blogspot.com

My blog:  Difference between : http://www.microscopicanatomybd.blogspot.com

1.        intermediate mesoderm form urogenital ridge, a longitudinal elevation at the dorsal body wall of fetus

2.       The coelomic epithelium and underlying mesoderm of the urogenital ridge proliferate to form the gonada ridge

3.       The coelomic epithelium forms irregular primary sex cords and incorporate primordial germ cells which migrate from the wall of yolk sac

4.       The primary sex cord extends deep to medulla and dissociate into irregular cell clusters which eventually degenerate and replace by vascular stroma, the ovarian medulla.

5.       The surface epithelium of female gonad unlike that of the male continues to proliferate

6.       In the 7^th week of development it produces second generation of cords, cortical cords, which penetrate underlying mesenchyme but remain close to the surface do not extend into medulla.

7.       In the 3^rd months, the cortical cords get fragmented and form isolated cell cluster.

8.      Each cell cluster consists of a primordial germ cell in the center surrounded by a layer of celomic epithelial cells

9.       The primordial germ cells form oogonia and coelomic epithelial cells form follicular cells. The resulting structure is called primordial follicles which remain confined in the cortex of the ovary

10.   A large number of primordial follicles are formed during fetal life

11.     No new primordial follicles are formed after birth

12.    The further development of  primordial follicles takes place after puberty

13.    Tunica albuginea formed around the testis but in case ovary tunica albuginea is not formed . the surface epithelium of ovary flattens to form a single layer of cells , called germinal epithelium which is continuous with peritoneum

14.    The connective tissue of ovary derived from mesoderm

Development of male reproductive system

 

Development of male reproductive system

For learning anatomy, please visit :

My youtube channel :  @easyhumanatomy73

My website : http://easyhumananatomy.com

My facebook page: https://www.facebook.com/easyhumanatomy/

My blog: http://www.easyhumanatomy73.blogspot.com

My blog:  Difference between : http://www.microscopicanatomybd.blogspot.com

When fertilization occurs, the genetic makeup of the male gamete (22X or 22Y) determines the sex of the developing embryo.

The process of sex differentiation is complicated and multigenic.
The most significant one is the Y chromosome gene's sex-determining region (SRY), whereas certain autosomal chromosomal genes are also involved in this process.

On the short arm of Y chromosome Yp11, the SRY gene is located. Male development is influenced by the SRY protein, whereas female development is developed in its absence.

Indifferent gonad

Up to 6 weeks of development, gonads are two longitudinal genital ridges, at the dorsal body wall of embryo which develop from intermediate mesoderm.

The genital ridge is formed by coelomic epithelium and underlying mesoderm. The coelomic epithelium forms irregular sex cords in both male and female embryos, and it is not possible to distinguish between male and female gonads at this stage, so this stage of development is known as indifferent gonads.

By the 12-week mark, the male and female structures of the external genitalia can be differentiated.

By the 20th week, the phenotypical differentiation is complete.

Source of development of gonad

1. Genital ridge: it is formed by the elevation of the intermediate mesoderm, which is covered by the coelomic epithelium. 
2. Primordial germ cells

The STY gene produces a protein known as a testis-determining factor.

TDF induces the testis to secrete FGF9, which stimulates the mesonephric duct to penetrate the gonadal ridge.

The SRY gene also produces SF1, which stimulates the differentiation of sertoli and leydig cells, which produce testosterone and Mullarrian-inhibiting factor (MIF), respectively.

In the presence of TDF, testosterone, and MIF, the indifferent gonad will produce a male phenotype.

In the absence of TDF, testosterone, and MIF, the indifferent gonad will produce a female phenotype.

Sequence of development of reproductive system

Gonads > then genital duct formation > then formation of external genitalia

Development and migration of primordial germ cells originate from the epiblast, then migrate through the primitive streak to reach the endodermal cell of the yolk sac close to allantois by the 3^rd week. During the 4^th week of development, they pass through the dorsal mesentery of the hindgut, and by the 5^th week of development, they reach near the genital ridges and start to penetrate the genital ridge by the 6^th week.

If primordial germ cells have an inductive influence on the development of the gonad, if they fail to reach the genital ridges, the gonad will not develop.

 

Development of gonads

Development of testis

1.          Intermediate mesoderm forms the urogenital ridge, a longitudinal elevation at the dorsal body wall of the fetus.
The coelomic epithelium and underlying mesoderm of the urogenital ridge protifelate to form the gonadal ridge.
The coelomic epithelium forms irregular primary sex cords and incorporative primordial germ cells, which migrate from the wall of the yolk sac.
The primary sex
The development of the testis ord extends deep into the medulla to form medullary cords, which lose their connection to the surface epithelium as the thick tunica albuginea forms in between the medullary cord and coelomic epithelium.

2.       Near the hilum medullary cord break up into network of tiny cell stands that form rete testis

3.       By the 4^th month of interuterine life medullary cord is continuous with rete testis

4.       The testis or medullary cord contain and primitive germ cells and sustentacular cell of  sertoli cell which derived from the surface epithelium of genital ridge.

5.       Sertoli cell secrete mullarian inhibiting factor (MIF) which degenerate mullarian duct

6.       Interstitial cell of leydig derive from mesenchyme of genital ridge which derived from intermediate mesoderm ,By the 8^th week of gestation leydig cell begin to produce testosterone which influence development of genital duct and external genitalia

7.       The testis or medullary cord of testis remain solid cord until puberty , then it become canalize and known seminiferous tubule, the canal of seminiferous tubules are continuous with rete testis which inturn join with efferent ductile. The efferent ductules and other excretory part of male genital system derived from mesonephric or wolffian duct

 Relative descent of the testis

The testis originally developed within the abdomen but later descent to scrotum due to disproportionate growth of upper abdomen region away from the pelvic region

The gubernaculum testis a fibrous band extend from testis to scrotum pull the testis

The gubernaculum also pull part of peritoneum with it the processus vaginalis which obliterate later except a remnat of peritoneal sac the tunica vaginalis of the testis

12^th week , testis reach inguinal region

28^th week, inguinal canal

By 33^rd week it reach the scrotum  

Genital duct in male

There are two genital ducts, mesonephric and paramesonephric ducts  are present in both sex at the initial period of development

In case male mesonephric duct is developed but paramesonephric duct mostly degenerate, while in case female para mesonephric duct is developed but mesonephric duct mostly degenerate

Gonads influence the development of genital duct and genitalia.

The Sertoli cell secrete mullarian inhibiting factor (MIF) which inhibit the development of paramesonephric or  mullarian duct

Interstitial cell of leydig derive from mesenchyme of genital ridge which derived from intermediate mesoderm ,

By the 8^th week of gestation leydig cell begin to produce testosterone which stimulates the mesonephric ducts to form genital ducts .  Structure derived from mesonephric duct : Efferent ductules, epididymis, vas deferens, seminal vesicle and ejaculatory duct

The paramesonephric duct regress due to presence of Mullarian inhibiting factor secreted from the sertoli cell

The remnant of paramesonephric duct in male: Appendix testis , appendix epididymis,

, The remnant of mesonephric duct in male: paradidymis,

 

Development of external genitalia

Indifferent stage

In the 3^rd week of development, mesenchymal cells origining in the region of the primitive streak migrate aroung the cloacal membrane to form a pair of slightly elevated cloacal folds , cranially these folds unit and form genital tubercles , caudally the folds are subdivided into urethral folds anteriorly and anal folds posteriorly .  another pair of swellings lateral to it known as genital swellings

A the end of the 6^th week of development it is impossible to distinguish between two sexes.

Development of male external genitalia

Under influence of testosterone genital tubercle is elongated known as phallus which pull urethral folds forward

Phallus forms the (glans penis, corpora cavernosa and corpus spongiosum of penis )

Urethral folds form the ventral part of the penis

The genital swelling form the scrotum