Human Anatomy 205

Endocrine System

Pheromones are chemical signals between individuals of the same species.
Chemical messengers that affect local target cells
- Neurotransmitters [released from axon at synapse and bind to denrite receptor]
- Growth factors are peptides that regulate cells in developing tissues.
- Prostaglandins are modified fatty acids that influence nearby cells
Hormones are chemical signals transported to target organ in the blood.

Try this link to introduce endocrinology at Human Anatomy On Line
Endocrine (ductless) [hormone secretions enter bloodstream] vs Exocrine [secretions transported in ducts]
Endocrine system coordinates control with nervous system. Contrast control systems
Hormones bind to receptors on or within target cells
Receptor that binds hormone activates response in target cell.
Hormones and their modes of action - Two types
1. Peptide hormones attach to protein receptors on plasma membrane of target cells. This activates a "second messenger" in the cytoplasm of target cell which triggers a sequence of biochemical events that affects the target cell.
2. Steroid hormones penetrate the plasma membrane of the target cell. Once inside it interacts with receptor proteins usually in the nucleus. The receptor-steroid complex activates or inactivates specific genes.
Two types of endocrine cells
1. Neurosecretory cells [brain neurons that secret into blood, not a synapse]
2. cuboidal epithelial cells [typical glandular cell]
Control of hormone secretion
- Negative feedback control [high conc of hormone inhibits its output]
- Neural impulse [ANS signals gland to secrete]
- Antagonistic hormone action [One hormone opposes action of the other]

Endocrine Glands, Hormones and Target Cells

See the list of Hormones
Pituitary Gland (hypophysis) [3] and the pituitary gland hormones
- Attached to hypothalamus and located in sella turcica of sphenoid
- Two Portions (different origins and secretory cells)
  1. Adenohypophysis (anterior lobe) epithelial cell hormones
    - HGH targets bone and muscle
    - Targets endocrine gland cells [TSH - thyroid]
      [ACTH - adrenal cortex]
    - Reproductive hormones target ovary & testis [FSH and LH]
      Prolactin targets mammary glands
  2. Neurohypophysis (posterior lobe) two neurosecretory cell hormones
    1. Oxytocin targets smooth muscle (stimulates uterine contraction)
    2. ADH targets kidney (triggers reabsorption of water into blood)
- Pituitary hormone and target list
Hypothalamus [2] Located in brainstem inferior to thalamus
- hormones are Releasing factors or inhibiting factors
- Secretions released into blood by neurosecretory cells
- Releasing factors target anterior pituitary cells
Pineal Gland [1] Located on brainstem between midbrain and thalamus. Secretes melatonin
- Stimulated by darkness and inhibited by light.
- melatonin rises and falls in circadian cycle
- Influences reproduction by targeting hypothalamus

Thyroid [4] Gross structure of thyroid gland on anterior trachea, inferior to larynx
Thyroid controlled by TSH from anterior pituitary; secretes 3 hormones

Tyroxine (T₄) promotes development; in adults controls metabolic rate
Triiodothyronine (T₃) [same effect as 4]
Calcitonin (CT) antagonist to Parathyroid hormone (PH ) in calcium homeostasis

Parathyroid Glands [5] 4 patchs of gland on posterior thyroid [see lab model]

Adrenal Glands [7] are located on superior border of kidneys.

Composed of 2 structurally and functionally different regions [adrenal medulla & adrenal cortex]
Three zones of Adrenal Cortex [zona glomerulosa, zona fasiculata & zona reticularis]
All corticosteroid hormones organized into`3 groups]
Adrenal Medulla [ norepinephrine ] activated by stress and stimulates a sympathetic response.
Histology of Adrenal Medulla

Pancreatic Islets cell clusters located in pancreas [8]

Microscopic endocrine cell clusters * Pancreatic Islets on lab slide
Beta cells secrete hormone, insulin
Alpha cells secrete hormone, glucagon ,
Two hormones are antagonistic regulators of blood glucose and target liver & adipose tissue.
Insulin promotes conversion of blood glucose to glycogen in liver during absorptive phase
Glucagon promotes the breakdown of glycogen and release of glucose into blood during fasting phase

Gastrointestinal peptide Hormones [endocrine cells scatterd in epithelium]

Click on gland then find on lab model map of major endocrine glands

[ Major Endocrine Glands]

Kidney Hormone

Erythropoietin (EPO) targets bone marrow to increase the production of red blood cells.
Also: Renin is enzyme secreted by kidney which results in an increase in BP.
- Renin acts on angiotensinogen, a plasma peptide, to form angiotensin I.

Testis [9] (Interstitial cells secrete testosterone; stimulated by LH)
- See microscopic view of interstitial cells in testis
- Testosterone promotes development of male secondary sex structures
Ovary [10] two endocrine tissues develop during menstrual cycle
1. Follicle develops around ovum prior to ovulation
  - FSH stimulates follicle growth
  - Follicle produces steroid, estradiol (estrogen)
  - Estrogen promotes estrus; development and maintenance of female secondary sex characteristics.
2. Corpus Luteum develops from follicle after ovulation
  - Develops under influence of adenohypophysis hormone, LH
  - Corpus luteum produces estrogen + additional steroid, progesterone
  - Progesterone plays a role in estrus, secondary female sex characteristics and supports pregnancy
Placenta is a temporary endocrine organ
- hCG is a glycoprotein hormone of early pregnancy produced by placenta.
- Acts like LH to target Corpus luteum to sustain progesterone to maintain endometrium

Lab Models

Crosswords and quizzes on endocrine system:

Dynamic Human
at McGraw-Hill