Endocrine tissues

Endocrine glands produce hormones that regulate metabolism, growth, reproduction, stress responses and other physiological processes. The adrenal glands produce steroid hormones and catecholamines involved in stress responses and metabolic regulation, while the thyroid and parathyroid glands regulate metabolism and calcium homeostasis, respectively. The pituitary gland coordinates several endocrine pathways through hormones controlling growth, reproduction and the activity of other endocrine organs, whereas the pineal gland contributes to the regulation of circadian rhythms through melatonin secretion. Although these functions are broadly conserved across mammals, differences in glandular organization, hormone production and cellular composition may influence gene expression profiles. For example, adrenal steroid production differs between primates, pigs and rodents, while the intermediate lobe of the pituitary is more prominent in rodents than in humans.

Adrenal, thyroid and pituitary glands are represented in all four mammalian species, enabling comparisons with corresponding human RNA data. Pineal gland samples are available for pig and rat, whereas the parathyroid gland is represented only in rat. These differences in coverage primarily reflect challenges in identifying and collecting sufficient material from small glands, rather than their anatomical absence in other species.

Figure 1. Tissue sample overview and the hierarchy of grouped tissues representing endocrine tissues

Dopamine beta-hydroxylase (DBH) catalyzes the conversion of dopamine to norepinephrine, a key step in catecholamine synthesis in the adrenal medulla. DBH shows conserved adrenal gland-enriched expression across the investigated mammalian species, consistent with its known function in chromaffin cells, although its precise cellular localization has not been experimentally confirmed in these samples.

Figure 3. A full section of an adrenal gland with immunohistochemical staining of the medulla using an antibody towards DBH.

Comparative transcriptomics of the mammalian endocrine tissues

The transcriptomic data is used to identify genes with elevated expression in individual tissues or groups of tissues, classified into three categories:

Tissue enriched: Genes with elevated expression in a single tissue compared with all other tissues.
Group enriched: Genes with elevated expression in a limited group of tissues compared with the remaining tissues.
Tissue enhanced: Genes with elevated expression in one or more tissues compared with the average expression across tissues (gap ratio ≥2)

Tissue specificity is evaluated at two levels: first, within each species using all available tissue types, and second, using only tissues that can be directly compared between the species and human. This distinction makes it possible to explore both tissue-specific expression within each species and the conservation of expression patterns across evolution.

The results can be explored for individual genes on the gene summary pages or searched to identify genes with specific expression patterns. To assess evolutionary conservation, pairwise overlap analysis compares each species individually with human, identifying genes that share the same tissue specificity classification in corresponding tissues and quantifying the extent of overlap between species.

Tissue	Species	Elevated	Human elevated overlap
Tissue	Species	Elevated	Complete overlap	Partial overlap
Adrenal gland	Macaque	94	22	27
	Mouse	116	10	17
	Rat	128	8	17
	Pig	150	14	21
Parathyroid gland	Rat	60	4	13
Pineal gland	Rat	306	n/a	n/a
Pineal gland	Pig	311	n/a	n/a
Pituitary gland	Macaque	155	45	37
	Mouse	270	43	38
	Rat	187	30	40
	Pig	215	38	37
Thyroid gland	Macaque	56	7	11
	Mouse	50	6	7
	Rat	52	6	12
	Pig	88	7	8

Pineal gland

The pineal gland produces melatonin and contributes to the regulation of circadian and seasonal rhythms. Its size, cellular organization and degree of calcification differ between mammals, while species-specific patterns of melatonin secretion reflect differences in activity cycles and reproductive physiology. Due to its small size and difficulties in anatomical identification during dissection, the pineal gland was successfully sampled in only two of the four species.

Species coverage

Pituitary gland

The pituitary gland contains several hormone-producing cell populations, including somatotrophs, lactotrophs, corticotrophs, thyrotrophs and gonadotrophs. Although these populations are broadly conserved, their relative abundance and spatial distribution differ between species, and the intermediate lobe is particularly prominent in rodents compared with adult humans.

Species coverage

Adrenal gland

The adrenal gland contains a cortex that produces steroid hormones and a medulla that produces catecholamines, but the organization and activity of the cortical zones differ between species. Humans, macaques and pigs primarily produce cortisol, whereas rats and mice produce corticosterone and lack a functional androgen-producing zona reticularis. Although pigs share several features of human adrenal physiology, they differ in the enzymes responsible for aldosterone synthesis, highlighting evolutionary differences in steroidogenic pathways that may be reflected in tissue-level gene expression.

Species coverage

Thyroid gland

The thyroid gland has a conserved follicular organization across mammals, with follicular cells producing thyroid hormones and parafollicular C cells producing calcitonin. However, follicle size, colloid content and epithelial activity vary with species, age and physiological state, potentially influencing the relative expression of genes associated with hormone synthesis and secretion.

Species coverage

Parathyroid gland

The parathyroid glands regulate calcium homeostasis through secretion of parathyroid hormone, primarily from chief cells. Their number, anatomical location and cellular composition vary between species, with differences in the distribution of chief cells, oxyphil cells and adipose tissue potentially influencing tissue-level expression profiles. Due to their small size and challenges associated with identification and dissection, the parathyroid glands were successfully sampled only in rat.