I.	Phases of Mammary Gland Growth and Development

	1.	Five distinct phases
		A.	Prenatal
		B.	Prepubertal
		C.	Postpubertal
		D.	Pregnancy
		C.	Early Lactation

 II. 	Embryonic and Fetal Period (Prenatal)

      	1.	Ectodermal origin
	2.	Early phases
		A.	Mammary band
			a.	Raised are of ectoderm on either side 
				of the midline.
			b.	Pectoral, abdominal or inguinal
		B.	Mammary streak
			a.	Narrowing of band
		C.	Mammary line
		      	a. 	Also called Milk lines
                   	b.	Divide into nodules
			c.	Each nodule will become a gland
		D.	Mammary crest
		E.	Mammary hillock
		F.	Mammary bud
                	a.	Lenticular
                   	b.	Spherical 
                   	c.	Conical
	2.	Teat and Duct Formation
                A.	Primary Mammary Cord or Sprout
                   	a.	Sprouts from apex of bud cone
                   	b.	Becomes Gland & Teat Cisterns
				i.	Canalization
				ii.	Starts at proximal end of sprout
				iii.	Cells separate and become cornified
				iii.	Streak canal is last are to 
					become canalized
                   	c.	In species with two streak canals 
				have two Cords per Bud, etc.
			d.	Delayed in males
		B.	Teat formation
			a.	Occurs from body of the bud
			b.	Tip inverts and becomes cornified opening 
				to streak canal
			c.	Male mice, rats, horses, and beavers have 
				no teats or nipples
				i.	Primary sprout pinches off from bud
                C.	Secondary & Tertiary Cords 
                   	a.	Develop into ducts
	3.	Connective Tissues
		A.	Connective tissue is derived from mesoderm
		B.	Mammary fatty pads
			a.	Connective tissue including adipose 
				is derived from mesoderm
			b.	Growth starts dorsal to bud and sprouts
			c.	Fatty pad needed for proper development
				i.	Scrotum inhibits fat pad growth 
					in inguinal located animals
					--Development stops at primary 
						sprout
		C.	Medial suspensory ligament
			a.	Mesodermal fibroblasts secrete elastin 
				and collagen to form fibers of ligament
		D.	Blood and Lymph Vessels
			a.	Also derived from mesoderm

	4.	By mid-gestation, most of the major developmental changes 
		that occur before birth have occurred

III. 	Prepubertal Mammary Growth

	1.	Birth

		A.	Teat is well developed

		B.	Sprouts
			a.	Secondary sprouts are canalized
			b.	Still have core of mitotically active 				
				cells at ends
				i.	Terminal End Buds
			c.	Continue growth and duct 						
				development
			d.	Growth limited to area 
				around gland cistern
			e.	Few tertiary sprouts

		C.	Non-secretory tissue well formed
			a.	Connective tissue
			b.	Blood vessels
			c.	Lymph vessels

		D.	Secretory or glandular parts not developed

		E.	Male similar to female, 
			but not as developed

	2.	Birth to puberty

		A.	Mammary growth is isometric (same as body) 
			approximately the 1st third 
			of this prepubertal period

			a.	Timing
				i.	First 2-3 months in cattle
				ii.	Shorter in pigs and sheep
				iii.	Longer in horses
				iv.	Much longer in humans

			b.	Udder or breast does increase in size 
				somewhat due to increased fat and
				connective tissue

			c.	No development of secretory tissues

		B.	Mammary growth then becomes allometric 
			(faster than body)

			a.	Timing
				i.	Calf--Max. growth 3-4 months 
					then levels off by 1 year

			b.	Extensive growth and development 
				of duct network which invades 
				surrounding adipose tissue or fat pad
				i.	Fat pad needed for ductal growth

IV.	Postpubertal Mammary Growth (Before Pregnancy)

      	1.	Puberty
		A.	Hormonal changes at puberty stimulate mammogenesis
		B.	Most of these effects are attributed to changes in 
			estrogen and progesterone
			a.	Ovary stimulated by FSH and LH
			a.	Estrogen and progesterone receptors appear 
				in the mammary gland at puberty
	2.	Estrous cycle effects
		A.	Acyclic animals (remain in estrus long periods 
						of time)
			a.	Extensive duct development during time 
				of estrus
			b.	Some alveoli form
		B.	Cyclic animals
			a.	Duct development
				i.	Bursts at Proestrus and Estrus 
					(Follicular Phase)
					--Estrogen effect
				ii.	Some involution/regression 
					at Metestrus and Diestrus
					(Luteal Phase)
				iii.	8% increase in duct development per 
					estrous cycle
					--Reaches a maximum if pregnancy 
						doesn't occur
			b.	Gland development
				i.	Minor growth of glands 
					during Luteal Phase
				ii.	Progesterone + Estrogen effect
		C.	Estrogen
			a.	Causes cell multiplication at TEB
			b.	Involved in lengthening and branching 
				of ducts
		D.	Progesterone
			a.	Duct and ductule cell multiplication
				--Enlargement and/or widening
			b.	Actually inhibits mammary growth 
				at high levels
		E.	Progesterone + Estrogen causes lobuloalveolar 
			development
			a.	Occurs eventually if animal doesn't become 
				pregnant
				i.	Heifers = 36 months
				ii.	Humans = 18-20 years
		F.	Prolactin and Somatotropin also needed 
			for mammary development
		G.	Autocrine and Paracrine Factors
			a.	Mammary Fat Pad "Factor"
				i.	Unknown
				ii.	Fatty acids may induce epithelial 
					growth
			b.	TEB cells
				i.	Somehow stimulate connective tissue 
					differentiation and angiogenesis
					--FGF?	
                		ii.	Formation of basement membrane
			c.	Growth factors
				i.	TGF 
					--inhibits growth prepubertal 
						and between estrous cycles
				ii.	EGF and TGF 
					--Stimulate mammary proliferation
					--Work with estrogen at TEB
				iii.	IGF-I
					--Mediates ST action
					--Permissive role in mammary 
						development
				iv.	FGF
					--Growth of connective tissues

V.	Mammary Growth during Pregnancy

	1.	The major portion of mammary growth and development
          	A.	Accelerates during pregnancy
		B.	Fastest during the later stages of pregnancy
			a.	Coincides with fetal growth
	2.	Growth and Development during pregnancy
		A.	Further duct development
            	B.	Extensive Alveoli growth appears later pregnancy
			a.	Secretory cell differentiate only 
				during pregnancy
			b.	Due to combination of E + P
				i.	Both are elevated
				ii.	Both are not elevated during 
					the estrous cycle
			c.	PRL, ST and ACTH are also needed 
				for maximal alveolar development
			d.	Placental lactogen also seems to play 
				a very important role, particularly in 
				ruminants
				i.	Synergizes with E + P
				ii.	Has PRL and ST activity	
           		e.	Other hormones involved in mammary 
				development during pregnancy
				i.	Insulin
				ii.	Thyroid Hormone
				iii.	Relaxin
				iv.	Growth factors

VI.	Mammary Development during Lactation

	1.	Mammary cell numbers continue to increase during 
		early lactation
	2.	Suckling or Milk removal needed for this increase
	3.	Related to lactation curve
	4.	In rats, growth is greater early lactation 
		than during pregnancy