What is endocrine glands ?
Majority of vertebrates possess certain specialised glands which lack definite duct for the transportation of their secretions but discharge them into the bloodstream. These glands are, therefore, known as ductless glands or endocrine glands , Some endocrine glands (i.e. thyroid, parathyroid, pituitary, etc. are strictly ductless and have no outlet like pancreas, ovaries and testes,etc. have both external secretion via duct and internal secretions by the blood steam . The internal secretions of the endocrine glands are known as hormones (Gr. Hormas, I excite) These have marked effect over all the chemical or metabolic reactions but are strictly specific in function. The important endocrine glands are –
A. In Head Region
Origin – Pituitary gland originates from the ectoderm of the embryo .
Location– The pituitary is a small, red-grey, pea-shaped gland attached to the hypothalamus of the brain by a stalk or infundibulum in front of the pons .
Structure – The pituitary gland consists of three lobes : anterior lobe or pars anterior , also called adenohypophysis , intermediate lobe or pars intermedia , and posterior lobe or pars nervosa , also termed neurohypophysis .
Hormones – All the three lobes of the pituitary glands secrete separate hormones.
Anterior lobe of pituitary produce six hormones i.e. Follicle-stimulating hormone (FSH) , Luteinsing hormone (LH) , Thyroid-stimulating hormone(TSH) , Adrenocorticotrophic hormone (ACTH), Somatotrophic or Growth hormone (STH or GH) or Somatotrophin , Prolactin hormone or Luteotrophic (PH or LTH) .
Intermediate lobe of pituitary secretes a hormone named melanocyte-stimulating hormones (MSH) .
Posterior lobe of pituitary stores and releases two hormones : oxytocin and vasopressin or antidiuretic hormone (ADH) .
B. In Neck Region
Origin. The thyroid gland originates from the endoderm of the embryo.
Location. The thyroid gland surrounds the front of the larynx and upper part of the trachea in the neck .
Structure. The thyroid is the largest endocrine gland . It is a bilobed, highly vascular organ . The two lobes are connected by a narrow band, the isthmus . It is composed of rounded follicles held together by loose connective tissue, the stroma and enclosed by a thin fibroelastic capsule.
Hormones. The thyroid gland secretes three hormones : thyroxine or tetraiodothyronine(T4) , triiodothyronine (T3) and calcitonin .
Origin. The parathyroid glands also arise from the endoderm of the embryo.
Location. The parathyroid are situated on the posterior surface of the thyroid lobes.
Structure. The parathyroids are four small, flat, oval glands, two on each side . The cells of parathyroids are arranged in a compact mass and are of 2 types : small chief cells and large oxyphil cells. The cells are enclosed by a deicate connective tissue capsule.
Hormone. The parathyroids severe a single hormone called parathormone (PTH), also called the Collip’s hormone .
Origin. The thymus arises from the endoderm of the embryo.
Location. The thymus us situated in the upper chest near the front side of the heart.
Structure. The thymus is a soft, pinkish,bilobed mass of lymphoid tissue. At birth, it is a prominent glands, but it gradually atrophies in the adult. The thymus is composed of tiny lobules held together by connective tissue.
Hormone. Thymus secrete peptide hormones named thymosine.
5. Thyrocalcitonin It is secreted by the parafolicular cells and controls the amount of calcium in the body.
C. In Body Cavity
Origin. The pancreas develops from the endoderm of the embryo.
Location. The pancreas lies below the stomach.
Structure. The pancreas is an elongated, yellowish gland. It consists largely of lobules (acini, alveoli) that secret pancreatic juice . Interspersed at random among the acini are islands of Langerhans which produce hormone. The islands of Langerhans consists of four types of cells : fewer larger alpha cells, numerous beta cells, delta cells and F-cells.
Hormones. The islands of Langerhans secrete four hormones: insulin, glucagon, somatostatin and pancreatic polypeptide.
7. Adrenals or Suprarenals glands
Origin. Adrenals have a dila origin, mesoderm and ectoderm.
Location. The adrenals are paired glands placed on the top of the kidneys.
Structure. The adrenals are conical, yellowish bodies. Each has two distinct regions having different embryonic origin, structure and function. The two regions are called adrenal cortex and adrenal medulla.
Adrenal cortex secretes three groups of steroid hormones i.e. Mineralocorticoids , Glucocorticoid and Sexcorticoids .
Adrenal Medulla secretes two similar hormones i.e adrenaline (epinephrine) and noradrenaline (norepinephrine) that have almost the same basic effect.
Origin. The gonads arise from the mesoderm.
Hormones– Besides producing gametes , the gonads secrete sex hormones from the onset of puberty (sexual maturity) to control the reproduction organs and sexual behaviour. The testes and ovaries secrete different hormones.
Testes are located in the scrotum (outside of abdomen). They secrete male sex hormones called androgens , such as testosterone.
Ovaries are located in the abdomen . They secrete three female sex hormones : estrogen such as estradiol, progesterone and relaxin.
Origin. The kidneys originate from the mesoderm of the embryo.
Location– The kidneys are located in the abdominal cavity attached to the back muscles just below the stomach.
Appearance – The kidneys are a pair of dark-red, bean-shaped organs, one on either side.
Hormones – Kidneys secrete a hormone called renin.
10. Mucous membrane of alimentary canal ( Gastrointestinal mucosa)
Origin. The hormone-secreting mucosa of gastrointestinal tract from the endoderm.
a. Stomach. The mucosa of the pyloric stomach near the dudoenum secretes a hormone called gastrin.
b. Intestine. The intestinal mucosa secretes six hormones : secretin, cholecystokinin, enterogastrone , enterocrinin, duocrinin and vilikinin.
Heart and liver also have an endocrine role
Heart The cells, called cardiocytes, of the atrial wall of our heart secrete a very important peptide hormone called atrial natriuretic factor (ANF) in response to an increased return of the venous blood. This hormone regulates the blood volume through increased excretion of ions and water.
Liver The liver produces a protein angiotensinogen which is changed to angiotensin by an enzyme renin in the kidney. Angiotensin stimulates the adrenal cortex to produce aldosterone.